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Physics Frontier Center

About the PFC @ JQI

The Physics Frontier Center is devoted to leading-edge experimental and theoretical investigation of ways to control and process quantum coherence and entanglement: the physics of quantum information. It is funded through a cooperative agreement with the National Science Foundation (NSF) and operated within the Joint Quantum Institute (JQI), a partnership between the University of Maryland (UMD) and the National Institute of Standards and Technology (NIST), with additional support from the Laboratory for Physical Sciences.

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  • boson spin-hall thumb
  • Interfering Waves
  • Two-toned light pattern creates steep quantum walls for atoms
  • A new landscape promises to bring ultracold atomic neighbors closer than ever before
  • March 13, 2018

Exotic physics can happen when quantum particles come together and talk to each other. Understanding such processes is challenging for scientists, because the particle interactions can be hard to glimpse and even harder to control. Moreover, modern computer simulations struggle to make sense of all the intricate dynamics going on in a large group of particles. Luckily, atoms cooled to near zero temperatures can provide insight into this problem.

Lasers can make cold atoms mimic the physics seen in other systems—an approach that is familiar terrain for atomic physicists. They regularly use intersecting laser beams to capture atoms in a landscape of rolling hills and valleys called an optical lattice. Atoms, when cooled, don’t have enough energy to walk up the hills, and they get stuck in the valleys. In this environment, the atoms behave similarly to the electrons in the crystal structure of many solids, so this approach provides a straightforward way to learn about interactions inside real materials.

But the conventional way to make optical lattices has some limitations. The wavelength of the laser light determines the location of the hills and valleys, and so the distance between neighboring valleys—and with that the spacing between atoms—can only be shrunk to half of the light’s wavelength. Bringing atoms closer than this limit could activate much stronger interactions between them and reveal effects that otherwise remain in the dark.

Now, a team of scientists from the Joint Quantum Institute (JQI), in collaboration* with researchers from the Institute for Quantum Optics and Quantum Information in Innsbruck, Austria, has circumvented the wavelength limit by leveraging the atoms’ inherent quantum features, which should allow atomic lattice neighbors to get closer than ever before. The new technique manages to squeeze the gentle lattice hills into steep walls separated by only one-fiftieth of the laser’s wavelength—25 times narrower than possible with conventional methods. The work, which is based on two prior theoretical proposals**, was recently published in Physical Review Letters.

In most optical lattices, atoms are arranged by repeating smooth dips in the intensity of laser light—a mechanism that also works with non-quantum objects like bacteria or even glass beads. But this ignores many inherent quantum characteristics of the atoms. Unlike glass beads, atoms, prompted by laser light of certain colors, can internally switch between different quantum versions of themselves, called states. The team exploits this property to build lattices that effectively replace the rolling hills with spiky features.

“The trick is that we don’t rely on the light’s intensity by itself,” explains Yang Wang, a postdoctoral researcher at the JQI and the lead author of the paper. “Instead, we use light as a tool to facilitate a quantum mechanical effect. And that creates the new kind of landscape for the atoms.”

To create this lattice, the researchers ensnare the atoms in a two-toned light pattern. Each color is chosen so that it can change an atom’s internal state on its own, but when the two colors overlap, the more intense color at each spot takes charge and decides which internal state the atom lands in. But this pattern is not smooth—there are vast valleys where the atom prefers one state, interrupted by thin strips where it should switch. The rules of quantum mechanics dictate that every time an atom changes its state, the atom must pay a price in the form of energy, just like climbing a hill. While a smooth transition may appear as a Sunday stroll to the atom, large changes over shorter distances quickly evolve into an increasingly steep hike. In the experiment, the thin strips inside the light pattern are so narrow, that they look like insurmountable walls to the atom, so it avoids them and gets stuck in between.

These sharp walls are an important first step in the quest to bring atoms even closer. The new technique still provides plenty of room for atoms to travel within the wide, flat plains, but researchers plan to reduce this freedom by adding more barriers. “As we take steps to confine the atoms further and further, quantum effects between the atoms should become increasingly important,” says Trey Porto, a JQI Fellow and an author of the paper. “This has a practical side effect, because it also increases the temperature that we need to be at to see weird quantum behavior. Cooling is quite difficult, so this would make the physics that we’re after more easily attainable.”

The research team says that this tool may also be useful for future quantum chemistry experiments, allowing scientists to bring atoms close enough to engage in a small-scale, highly-controlled reaction.

Written by Nina Beier
* The research was also done in collaboration with the Joint Center for Quantum Information and Computer Science at the University of Maryland, the Jagiellonian University in Kraków, Poland, and the University of Innsbruck in Austria.
** The work was based on the following proposals: Phys. Rev. Lett. 117, 233001 (2016) and Phys. Rev. A 94, 063422 (2016)
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  • New hole-punched crystal clears a path for quantum light
  • Photonic chip guides single photons, even when there are bends in the road.
  • February 12, 2018 Activity 2

Optical highways for light are at the heart of modern communications. But when it comes to guiding individual blips of light called photons, reliable transit is far less common. Now, a collaboration of researchers from the Joint Quantum Institute (JQI), led by JQI Fellows Mohammad Hafezi and Edo Waks, has created a photonic chip that both generates single photons, and steers them around. The device, described in the Feb. 9 issue of Science, features a way for the quantum light to seamlessly move, unaffected by certain obstacles.

"This design incorporates well-known ideas that protect the flow of current in certain electrical devices," says Hafezi. "Here, we create an analogous environment for photons, one that protects the integrity of quantum light, even in the presence of certain defects."

The chip starts with a photonic crystal, which is an established, versatile technology used to create roadways for light. They are made by punching holes through a sheet of semiconductor. For photons, the repeated hole pattern looks very much like a real crystal made from a grid of atoms. Researchers use different hole patterns to change the way that light bends and bounces through the crystal. For instance, they can modify the hole sizes and separations to make restricted lanes of travel that allow certain light colors to pass, while prohibiting others.

Sometimes, even in these carefully fabricated devices, there are flaws that alter the light’s intended route, causing it to detour into an unexpected direction. But rather than ridding their chips of every flaw, the JQI team mitigates this issue by rethinking the crystal’s hole shapes and crystal pattern. In the new chip, they etch out thousands of triangular holes in an array that resembles a bee’s honeycomb. Along the center of the device they shift the spacing of the holes, which opens a different kind of travel lane for the light. Previously, these researchers predicted that photons moving along that line of shifted holes should be impervious to certain defects because of the overall crystal structure, or topology. Whether the lane is a switchback road or a straight shot, the light’s path from origin to destination should be assured, regardless of the details of the road.

The light comes from small flecks of semiconductor—dubbed quantum emitters—embedded into the photonic crystal. Researchers can use lasers to prod this material into releasing single photons. Each emitter can gain energy by absorbing laser photons and lose energy by later spitting out those photons, one at time. Photons coming from the two most energetic states of a single emitter are different colors and rotate in opposite directions. For this experiment, the team uses photons from an emitter found near the chip’s center.

The team tested the capabilities of the chip by first changing a quantum emitter from its lowest energy state to one of its two higher energy states. Upon relaxing back down, the emitter pops out a photon into the nearby travel lane. They continued this process many times, using photons from the two higher energy states. They saw that photons emitted from the two states preferred to travel in opposite directions, which was evidence of the underlying crystal topology.

To confirm that the design could indeed offer protected lanes of traffic for single photons, the team created a 60 degree turn in the hole pattern. In typical photonic crystals, without built-in protective features, such a kink would likely cause some of the light to reflect backwards or scatter elsewhere. In this new chip, topology protected the photons and allowed them to continue on their way unhindered.

“On the internet, information moves around in packets of light containing many photons, and losing a few doesn’t hurt you too much”, says co-author Sabyasachi Barik, a graduate student at JQI. “In quantum information processing, we need to protect each individual photon and make sure it doesn't get lost along the way. Our work can alleviate some forms of loss, even when the device is not completely perfect.”

The design is flexible, and could allow researchers to systematically assemble pathways for single photons, says Waks. "Such a modular approach may lead to new types of optical devices and enable tailored interactions between quantum light emitters or other kinds of matter."

Written by E. Edwards

*Mohammad Hafezi is an Associate Professor in the University of Maryland (UMD) Departments of Electrical and Computer Engineering and Physics. Edo Waks is a Professor in the UMD Department of Electrical and Computer Engineering.

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  • Light may unlock a new quantum dance for electrons in graphene
  • January 12, 2018

A team of researchers has devised a simple way to tune a hallmark quantum effect in graphene—the material formed from a single layer of carbon atoms—by bathing it in light. Their theoretical work, which was published recently in Physical Review Letters, suggests a way to realize novel quantum behavior that was previously predicted but has so far remained inaccessible in experiments.

"Our idea is to use light to engineer these materials in place," says Tobias Grass, a postdoctoral researcher at the Joint Quantum Institute (JQI) and a co-author of the paper. "The big advantage of light is its flexibility. It’s like having a knob that can change the physics in your sample."

The proposal suggests a method to alter a physical effect that occurs in flat materials held at very low temperatures and subjected to extremely strong magnets—at least a thousand times stronger than a fridge magnet. Under these circumstances, electrons zipping around on a two-dimensional landscape start to behave in an unusual way. Instead of continuously flowing through the material, they get locked into tight circular orbits of particular sizes and energies, barely straying from their spots. Only a certain number of electrons can occupy each orbit. When orbits are partially filled—which gives electrons some room to breathe—it activates new kinds of interactions between the charged particles and leads to a complex quantum dance.

Electrons carry out this choreography—known as the fractional quantum Hall effect—in graphene. Interestingly, tuning the interactions between electrons can coax them into different quantum Hall dance patterns, but it requires a stronger magnet or an entirely different sample—sometimes with two layers of graphene stacked together.

The new work, which is a collaboration between researchers at JQI and the City College of New York, proposes using laser light to circumvent some of these experimental challenges and even create novel quantum dances. The light can prod electrons into jumping between orbits of different energies. As a result, the interactions between the electrons change and lead to a different dance pattern, including some that have never been seen before in experiments. The intensity and frequency of the light alter the number of electrons in specific orbits, providing an easy way to control the electrons’ performance. "Such a light-matter interaction results in some models that have previously been studied theoretically," says Mohammad Hafezi, a JQI Fellow and an author of the paper. "But no experimental scheme was proposed to implement them."

Unlocking those theoretical dances may reveal novel quantum behavior. Some may even spawn exotic quantum particles that could collaborate to remain protected from noise—a tantalizing idea that could be useful in the quest to build robust quantum computers.

Written by Nina Beier

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  • Former JQI researcher wins Chilean L'Oréal-UNESCO Award For Women in Science
  • January 12, 2018

Carla Hermann Avigliano, a former postdoc with JQI Fellow Paul Lett, is one of two women scientists to receive the Chilean L'Oréal-UNESCO Award For Women in Science. She was selected for the prize out of 77 applications and cited for her research achievements during her early career. The award is part of a larger program that aims to internationally recognize women researchers in science and operates throughout the world. In Chile, 21 women from various areas of science such as physics, chemistry, biology, nursing, geology, forestry, biotechnology and ecology, among others, have received the prize since 2007.

“Getting this award is an honor for me. And this wouldn’t have been possible without the constant support of teachers throughout my career,” says Hermann Avigliano. “I wouldn’t be what I am today without the guidance of these incredible people.” Hermann Avigliano currently works as a postdoctoral researcher in the Optics Group of Rodrigo Vicencio at the University of Chile, where she explores light propagation in photonic lattices. She plans to use some of the award’s funds to coordinate visits to her collaborations and to support student internships.

Throughout her career, Hermann Avigliano has been conducting research around the world. After studying physics at the University of Concepción in Chile, she moved to Paris where she completed her Ph.D. in the Cavity Quantum Electrodynamics group of Nobel Laureate Serge Haroche. While working towards her Ph.D., Hermann Avigliano, together with other researchers, showed for the first time that atom chips are useable for studying the behavior of highly excited atoms known as Rydberg atoms. She also participated in other work probing the strong interactions between such Rydberg atoms. Parallel to her experimental work, Hermann Avigliano did theoretical work on the generation of Schrödinger cat states under the co-supervision of Jean-Michel Raimond and Carlos Saavedra.

After her PhD, Hermann Avigliano worked at the JQI as a postdoctoral researcher with JQI Fellow Paul Lett, where her research focused on non-linear optics. During her time at the JQI, she was involved in the construction of a precise phase measuring device which uses non-linear processes instead of passive elements. Such devices are useful tools for precision metrology. In addition to this, she worked towards the direct observation of quantum effects that could reduce optical noise and lead to more precise measurements with CCD detectors.

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  • Narrow glass threads synchronize the light emissions of distant atoms
  • December 4, 2017

If you holler at someone across your yard, the sound travels on the bustling movement of air molecules. But over long distances your voice needs help to reach its destination—help provided by a telephone or the Internet. Atoms don’t yell, but they can share information through light. And they also need help connecting over long distances.

Now, researchers at the Joint Quantum Institute (JQI) have shown that nanofibers can provide a link between far-flung atoms, serving as a light bridge between them. Their research, which was conducted in collaboration with the Army Research Lab and the National Autonomous University of Mexico, was published last week in Nature Communications. The new technique could eventually provide secure communication channels between distant atoms, molecules or even quantum dots.

An excited atom—that is, one with some extra energy—emits light when it loses energy. Usually atoms spit this light out in random directions and at different times. But this random process can be tamed if excited atoms are bunched up close together. In that case, atoms can sync up their light emissions, like the rhythmic clapping of an appreciative audience. However, this synchronization effect, which is caused by light of different atoms joining together, doesn’t reach very far because the strength of light weakens drastically over short distances. While your neighbor might hear you yelling over several meters, atoms need to be really close to interact with each other—typically closer than one micron, which is a hundred times smaller than the width of a human hair.

Now, physicists have extended the range over which atoms can synchronize their light emission by using an optical nanofiber. In an experiment, the researchers immerse a nanofiber in a cloud of cold rubidium atoms and excite the atoms with a laser beam. As atoms in the cloud move around, they sometimes get very close to the fiber. If an atom emits light near the fiber, the glass thread can capture the light and pipe it to another atom, even if the atoms are far apart.

The team observed a group of atoms emitting light pulses at different rates than their ordinary, unsynchronized selves—one signature of these far-reaching interactions. The effect persisted even when physicists cleaved the atomic cloud in two so that atoms in separate clouds could only connect through the fiber, and not through other atoms in the cloud.

The atoms in this experiment are only separated by distances of a few pieces of paper, but the authors say that longer distances—meters or even kilometers—should be doable. “We have shown that optical nanofibers are excellent for connecting atoms that are quite far apart—if the atoms were the size of people, it would be a distance of more than 300 kilometers,” says Pablo Solano, the lead author of the paper and a former JQI graduate student.* “The question now is not whether the atoms interact, but how far can we push their optical-fiber-mediated connections.” On the scale of atoms even a few meters is an enormous distance. But the authors say that a combination of optical nanofibers and regular fiber optics—technologies already deployed for long-distance phone calls, cable TV and the Internet—could extend the range of these atomic connections even farther.

* Solano is now a postdoctoral researcher at the MIT-Harvard Center for Ultracold Atoms.

Written by N. Beier

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  • Quantum simulators wield control over more than 50 qubits
  • Atoms provide a robust platform for observing quantum magnets in action.
  • November 29, 2017 Activity 3

Two independent teams of scientists, including one from the Joint Quantum Institute, have used more than 50 interacting atomic qubits to mimic magnetic quantum matter, blowing past the complexity of previous demonstrations. The results appear in this week’s issue of Nature.

As the basis for its quantum simulation, the JQI team deploys up to 53 individual ytterbium ions—charged atoms trapped in place by gold-coated and razor-sharp electrodes. A complementary design by Harvard and MIT researchers uses 51 uncharged rubidium atoms confined by an array of laser beams. With so many qubits these quantum simulators are on the cusp of exploring physics that is unreachable by even the fastest modern supercomputers. And adding even more qubits is just a matter of lassoing more atoms into the mix.

“Each ion qubit is a stable atomic clock that can be perfectly replicated,” says JQI Fellow Christopher Monroe*, who is also the co-founder and chief scientist at the startup IonQ Inc. “They are effectively wired together with external laser beams. This means that the same device can be reprogrammed and reconfigured, from the outside, to adapt to any type of quantum simulation or future quantum computer application that comes up.” Monroe has been one of the early pioneers in quantum computing and his research group’s quantum simulator is part of a blueprint for a general-purpose quantum computer.

Quantum hardware for a quantum problem

While modern, transistor-driven computers are great for crunching their way through many problems, they can screech to a halt when dealing with more than 20 interacting quantum objects. That’s certainly the case for quantum magnetism, in which the interactions can lead to magnetic alignment or to a jumble of competing interests at the quantum scale.

“What makes this problem hard is that each magnet interacts with all the other magnets,” says research scientist Zhexuan Gong, lead theorist and co-author on the study. “With the 53 interacting quantum magnets in this experiment, there are over a quadrillion possible magnet configurations, and this number doubles with each additional magnet. Simulating this large-scale problem on a conventional computer is extremely challenging, if at all possible.”

When these calculations hit a wall, a quantum simulator may help scientists push the envelope on difficult problems. This is a restricted type of quantum computer that uses qubits to mimic complex quantum matter. Qubits are isolated and well-controlled quantum systems that can be in a combination of two or more states at once. Qubits come in different forms, and atoms—the versatile building blocks of everything—are one of the leading choices for making qubits. In recent years, scientists have controlled 10 to 20 atomic qubits in small-scale quantum simulations.

Currently, tech industry behemoths, startups and university researchers are in a fierce race to build prototype quantum computers that can control even more qubits. But qubits are delicate and must stay isolated from the environment to protect the device’s quantum nature. With each added qubit this protection becomes more difficult, especially if qubits are not identical from the start, as is the case with fabricated circuits. This is one reason that atoms are an attractive choice that can dramatically simplify the process of scaling up to large-scale quantum machinery.

An atomic advantage

Unlike the integrated circuitry of modern computers, atomic qubits reside inside of a room-temperature vacuum chamber that maintains a pressure similar to outer space. This isolation is necessary to keep the destructive environment at bay, and it allows the scientists to precisely control the atomic qubits with a highly engineered network of lasers, lenses, mirrors, optical fibers and electrical circuitry.

“The principles of quantum computing differ radically from those of conventional computing, so there’s no reason to expect that these two technologies will look anything alike,” says Monroe.

In the 53-qubit simulator, the ion qubits are made from atoms that all have the same electrical charge and therefore repel one another. But as they push each other away, an electric field generated by a trap forces them back together. The two effects balance each other, and the ions line up single file. Physicists leverage the inherent repulsion to create deliberate ion-to-ion interactions, which are necessary for simulating of interacting quantum matter.

The quantum simulation begins with a laser pulse that commands all the qubits into the same state. Then, a second set of laser beams interacts with the ion qubits, forcing them to act like tiny magnets, each having a north and south pole. The team does this second step suddenly, which jars the qubits into action. They feel torn between two choices, or phases, of quantum matter. As magnets, they can either align their poles with their neighbors to form a ferromagnet or point in random directions yielding no magnetization. The physicists can change the relative strengths of the laser beams and observe which phase wins out under different laser conditions.

The entire simulation takes only a few milliseconds. By repeating the process many times and measuring the resulting states at different points during the simulation, the team can see the process as it unfolds from start to finish. The researchers observe how the qubit magnets organize as different phases form, dynamics that the authors say are nearly impossible to calculate using conventional means when there are so many interactions.

This quantum simulator is suitable for probing magnetic matter and related problems. But other kinds of calculations may need a more general quantum computer with arbitrarily programmable interactions in order to get a boost.

“Quantum simulations are widely believed to be one of the first useful applications of quantum computers,” says Alexey Gorshkov**, JQI Fellow and co-author of the study. “After perfecting these quantum simulators, we can then implement quantum circuits and eventually quantum-connect many such ion chains together to build a full-scale quantum computer with a much wider domain of applications.”

As they look to add even more qubits, the team believes that its simulator will embark on more computationally challenging terrain, beyond magnetism. “We are continuing to refine our system, and we think that soon, we will be able to control 100 ion qubits, or more,” says Jiehang Zhang, the study’s lead author and postdoctoral researcher. “At that point, we can potentially explore difficult problems in quantum chemistry or materials design.”

Written by E. Edwards

* Christopher Monroe is a fellow of the Joint Quantum Institute and the Joint Center for Quantum Information and Computer Science. He is a Distinguished University Professor & Bice Seci-Zorn Professor in the UMD Physics Department.

**Alexey Gorshkov is a fellow of the Joint Quantum Institute and the Joint Center for Quantum Information and Computer Science. He is an Adjunct Professor in the UMD Physics Department.

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  • Ion qubits offer early glimpse of quantum error detection
  • November 8, 2017

Computers based on quantum physics promise to solve certain problems much faster than their conventional counterparts. By utilizing qubits—which can have more than just the two values of ordinary bits—quantum computers of the future could perform complex simulations and may solve difficult problems in chemistry, optimization and pattern-recognition.

But building a large quantum computer—one with thousands or millions of qubits—is hard because qubits are very fragile. Small interactions with the environment can introduce errors and lead to failures. Detecting these errors is not straightforward, since quantum measurements are a form of interaction and therefore also disrupt quantum states. Quantum physics presents another wrinkle, too: It’s not possible to simply copy a qubit for backup.

Scientists have come up with clever ways to detect errors and keep them from spreading. But so far, a complete error detection protocol has not been tested in experiments, partly due to the difficulty of creating controlled interactions between all of the necessary qubits.

Now, in a recent article published in Science Advances, researchers at the Joint Quantum Institute tested a full procedure for encoding a qubit and detecting some of the errors that occur during and after the encoding. They applied a scheme that distributed the information of one qubit among four trapped ytterbium ions—themselves also qubits—using a fifth ion qubit to read out whether certain errors had occurred. Ions provide a rich set of interactions, which allowed scientists to link the fifth ion qubit with the other four at will—a common requirement of error detection or correction schemes. With this approach, the scientists detected nearly all of the single-ion errors, performing more than 5000 runs of the full encoding and measurement procedure for a number of different quantum states. Additionally, the encoding itself didn’t appear to introduce errors on multiple ions at the same time, a feature that could have spelled doom for error detection and correction in ions.

Although the result is an early step toward larger quantum memories and quantum computers, the authors say it demonstrates the potential of qubit protection schemes with trapped ions and paves the way toward error detection and eventually error correction on a larger scale.

Written by Nina Beier

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  • Turning ions into quantum cats
  • A new technique spreads single-ion "cat states" 300 nanometers apart.
  • September 27, 2017

In Schrödinger's famous thought experiment, a cat seems to be both dead and alive—an idea that strains credulity. These days, cats still don't act this way, but physicists now regularly create analogues of Schrödinger's cat in the lab by smearing the microscopic quantum world over longer and longer distances.

Such "cat states" have found many homes, promising more sensitive quantum measurements and acting as the basis for quantum error-correcting codes—a necessary component for future error-prone quantum computers.

With these goals in mind, some researchers are eager to create better cat states with single ions. But, so far, standard techniques have imposed limits on how far their quantum nature could spread.

Recently, researchers at the Joint Quantum Institute developed a new scheme for creating single-ion cat states, detailing the results this week in Nature Communications. Their experiment places a single ytterbium ion into a superposition—a quantum combination—of two different states. Initially, these states move together in their common environment, sharing the same motion. But a series of carefully timed and ultrafast laser pulses apply different forces to the two ion states, pushing them in opposite directions. The original superposition persists, but the states end up oscillating out of phase with each other. 

Using this technique, the JQI team managed to separate the states by a distance of almost 300 nanometers, roughly twelve times further than previously possible. There's still just one ion, but its quantum nature now extends over a distance more than a thousand times larger than its original size. Such long-range superpositions are highly sensitive, and could enable precise atom interferometry measurements or robust quantum cryptographic techniques.

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  • Sensing atoms caught in ripples of light
  • September 26, 2017

Optical fibers are ubiquitous, carrying light wherever it is needed. These glass tunnels are the high-speed railway of information transit, moving data at incredible speeds over tremendous distances. Fibers are also thin and flexible, so they can be immersed in many different environments, including the human body, where they are employed for illumination and imaging.

Physicists use fibers, too, particularly those who study atomic physics and quantum information science. Aside from shuttling laser light around, fibers can be used to create light traps for super-chilled atoms. Captured atoms can interact more strongly with light, much more so than if they were moving freely. This rather artificial environment can be used to explore fundamental physics questions, such as how a single particle of light interacts with a single atom. But it may also assist with developing future hybrid atom-optical technologies.

Now, researchers from the Joint Quantum Institute and the Army Research Laboratory have developed a fast-acting, non-invasive way to use fiber light to reveal information about fiber traps. This technique is reminiscent of biomedical and chemical sensors that use fibers to detect properties of nearby molecules. Fiber sensors are an attractive measurement tool because they can often extract information without totally disrupting interesting phenomena that may be going on. The research appeared as an Editor’s Pick in the journal Optics Letters. The team also published a review article about optical nanoscale fibers in the most recent volume of Advances in Atomic, Molecular, and Optical Physics.

Typical optical fibers, like the ones used in communications and medicine, have only a tiny amount of light near the outside surface, and that is not enough to capture atoms from a surrounding gas. Physicists can push more light to the outside by reshaping the fiber to look like a tiny hourglass instead of a tunnel. The waist of the hourglass is hundreds of nanometers, a few times the width of a human hair and too small to contain light waves that are propagating along the inside of the fiber. But instead of just stopping at the constriction, the light squeezes to the outside surface. When physicists inject light into both ends of such a fiber, the light waves combine together to form a stationary ripple around the constriction. Atoms will be attracted to dips in the wave and line up like a row of eggs in a carton.

This trapping is an example of how light affects atoms, drawing them in. But the light-atom relationship is reciprocal: The presence of atoms can alter the light, too. Light waves, sent into one end of a nanoscale fiber, will pick-up information about the atoms in the vicinity of the fiber, and then convey it to a detector at the opposite end of the fiber.

Each trapped atom acts like a marble in a glass bowl. When pushed, a marble will roll up the side of the bowl, back down, and then up the other side. The speed of this cycle is related to the bowl’s curvature: Steeper walls cause faster cycles. Now imagine shining a flashlight through one side of the bowl. As it goes back and forth the marble will keep passing through the flashlight beam. The beam signal will blink on and off at the rate at which the marble was moving in the bowl. In other words, the information about the marble motion, and therefore the bowl’s shape, is encoded onto the flashlight beam.

In this research, the team uses laser light as the probe, analogous to the flashlight. A mere 70 nanowatts in power is injected into the fiber, gently kicking the atoms into motion. Similar to marble wobbles, the atoms rock back and forth in their bowl traps. Instead of causing the probe light to blink on and off, the atom motion affects the direction that the light waves oscillate. The speed of the atom rocking, which is directly related to the atom trap shape, will be imprinted on the light as faster or slower changes.

When the light waves complete their journey and exit the fiber, the team catches them with a detector to continuously monitor the atom-light oscillations. The process is fast, taking only a fraction of a millisecond, and it can be seamlessly integrated into an experimental sequence.

When it comes to measuring these atom trap properties, physicists want to avoid disturbances. This can be difficult to do because one of the most effective ways to probe atoms involves blasting them with light, which can heat and even release them from their traps. This conventional method is acceptable because scientists can just recool and recapture the atoms. In contrast, the JQI-ARL technique uses very little light and is done in-situ, meaning it collects information while minimizing disruptions. This appealing alternative promises to streamline atom-fiber experiments.

by E. Edwards/JQI

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  • Long-range interactions leave a quantum reminder
  • September 1, 2017

Given enough time, a forgotten cup of coffee will lose its appeal and cool to room temperature. One way of telling this tepid tale involves a stupendous number of coffee molecules colliding like billiard balls with themselves and colder molecules in the air above. Those constant collisions siphon energy away from the coffee, bit by bit, in a process that physicists call thermalization.

But this story doesn’t mention quantum physics, and scientists think that thermalization must ultimately have a precursor at the quantum level. Recently, scientists have sketched out some of the ways that small quantum systems thermalize, sometimes even when they are almost completely isolated.

Last week, in Science Advances, a team of researchers from JQI and Indiana University reported finding a new kind of effect on the road to thermalization—one in which a chain of up to 22 trapped ions, all initially with their quantum spins aligned, can retain a memory of a flipped spin long after it begins to roam through the chain.

Unlike previous results in which imperfections trapped such flips near their starting spot, the memory in this experiment comes from the long-range communication of the ions and confirms a theoretical prediction by two of the paper’s authors.

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  • Atomic cousins team up in early quantum networking node
  • Researchers use different ion species for storage and communication.
  • July 12, 2017

Large-scale quantum computers, which are an active pursuit of many university labs and tech giants, remain years away. But that hasn’t stopped some scientists from thinking ahead, to a time when quantum computers might be linked together in a network or a single quantum computer might be split up across many interconnected nodes.

A group of physicists at the University of Maryland, working with JQI Fellow Christopher Monroe, are pursuing the second goal, attempting to wire up isolated modules of trapped atomic ions with light. They imagine many modules, each with a hundred or so ions, linked together to form a quantum computer that is inherently scalable: If you want a bigger computer, simply add more modules to the mix.

In a paper published recently in Physical Review Letters, Monroe and his collaborators reported on putting together many of the pieces needed to create such a module. It includes two different species of ions: an ytterbium ion for storing information and a barium ion for generating the light that communicates with other nodes.

This dual-species approach isolates the storage and communication tasks of a network node. With a single species, manipulating the communication ion with a laser could easily corrupt the storage ion. In several experiments, the researchers demonstrated that they could successfully isolate the two ions from each other, transfer information between them and capture light generated by both ions. 

The light from the barium communication ion could eventually be routed through fiber optic cables to a reconfigurable sensor, where it would meet light from other nodes. To demonstrate that the module could produce this communication light, the team carefully excited the barium ion with a laser—leaving the ytterbium ion untouched—and captured the light emitted as it decayed. By observing both this emitted light and the ion, the team determined that the two were entangled, a requirement if the light is to carry messages in a quantum network.

The team also transferred information between the two ions, using their mutual electrical push and the resulting motion to intermingle the ions’ internal quantum characteristics. Using lasers to excite specific motion, the team showed how to swap information from one ion to the other and even entangle the two ions. Entangling the storage ion with the communication ion and the communication ion with outgoing light are the main ingredients needed for a node in a quantum network.

Using two different species came with some challenges, though. One problem to overcome was a size mismatch. Since ions give each other an electrical push, they wobble in a coordinated way when they are trapped next to each other. But ytterbium is heavier than barium, creating a mismatch in this motion that slows down the rate that information can be transferred from the ytterbium memory to the barium interface.

By analyzing this coupled motion, the team realized that using motion along the line connecting the two ions—something that is typically slower because ions aren’t as tightly confined in this direction—would speed up the information transfer.

The team has added memory ions to their module since the experiments they report in this work. But their main focus going forward will be to wire more modules together, with the eventual goal being a large-scale, modular quantum computer.

The paper has four authors in addition to Monroe: lead author and JQI graduate student Volkan Inlek, JQI graduate student Clayton Crocker, JQI postdoctoral researcher Marty Lichtman and JQI graduate student Ksenia Sosnova.

Story by Chris Cesare

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  • Neural networks take on quantum entanglement
  • Techniques that enable computers to learn can also describe complex quantum systems.
  • June 12, 2017 Activity 1

Machine learning, the field that’s driving a revolution in artificial intelligence, has cemented its role in modern technology. Its tools and techniques have led to rapid improvements in everything from self-driving cars and speech recognition to the digital mastery of an ancient board game.

Now, physicists are beginning to use machine learning tools to tackle a different kind of problem, one at the heart of quantum physics. In a paper published recently in Physical Review X, researchers from JQI and the Condensed Matter Theory Center (CMTC) at the University of Maryland showed that certain neural networks—abstract webs that pass information from node to node like neurons in the brain—can succinctly describe wide swathes of quantum systems.

Dongling Deng, a JQI Postdoctoral Fellow who is a member of CMTC and the paper’s first author, says that researchers who use computers to study quantum systems might benefit from the simple descriptions that neural networks provide. “If we want to numerically tackle some quantum problem,” Deng says, “we first need to find an efficient representation.”

On paper and, more importantly, on computers, physicists have many ways of representing quantum systems. Typically these representations comprise lists of numbers describing the likelihood that a system will be found in different quantum states. But it becomes difficult to extract properties or predictions from a digital description as the number of quantum particles grows, and the prevailing wisdom has been that entanglement—an exotic quantum connection between particles—plays a key role in thwarting simple representations.

The neural networks used by Deng and his collaborators—CMTC Director and JQI Fellow Sankar Das Sarma and Fudan University physicist and former JQI Postdoctoral Fellow Xiaopeng Li—can efficiently represent quantum systems that harbor lots of entanglement, a surprising improvement over prior methods.

What’s more, the new results go beyond mere representation. “This research is unique in that it does not just provide an efficient representation of highly entangled quantum states,” Das Sarma says. “It is a new way of solving intractable, interacting quantum many-body problems that uses machine learning tools to find exact solutions.”

Neural networks and their accompanying learning techniques powered AlphaGo, the computer program that beat some of the world’s best Go players last year (and the top player this year). The news excited Deng, an avid fan of the board game. Last year, around the same time as AlphaGo’s triumphs, a paper appeared that introduced the idea of using neural networks to represent quantum states, although it gave no indication of exactly how wide the tool’s reach might be. “We immediately recognized that this should be a very important paper,” Deng says, “so we put all our energy and time into studying the problem more.”

The result was a more complete account of the capabilities of certain neural networks to represent quantum states. In particular, the team studied neural networks that use two distinct groups of neurons. The first group, called the visible neurons, represents real quantum particles, like atoms in an optical lattice or ions in a chain. To account for interactions between particles, the researchers employed a second group of neurons—the hidden neurons—which link up with visible neurons. These links capture the physical interactions between real particles, and as long as the number of connections stays relatively small, the neural network description remains simple.

Specifying a number for each connection and mathematically forgetting the hidden neurons can produce a compact representation of many interesting quantum states, including states with topological characteristics and some with surprising amounts of entanglement.

Beyond its potential as a tool in numerical simulations, the new framework allowed Deng and collaborators to prove some mathematical facts about the families of quantum states represented by neural networks. For instance, neural networks with only short-range interactions—those in which each hidden neuron is only connected to a small cluster of visible neurons—have a strict limit on their total entanglement. This technical result, known as an area law, is a research pursuit of many condensed matter physicists. 

These neural networks can’t capture everything, though. “They are a very restricted regime,” Deng says, adding that they don’t offer an efficient universal representation. If they did, they could be used to simulate a quantum computer with an ordinary computer, something physicists and computer scientists think is very unlikely. Still, the collection of states that they do represent efficiently, and the overlap of that collection with other representation methods, is an open problem that Deng says is ripe for further exploration.

By Chris Cesare,

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  • Trapped ions and superconductors face off in quantum benchmark
  • April 13, 2017 Activity 3

The race to build larger and larger quantum computers is heating up, with several technologies competing for a role in future devices. Each potential platform has strengths and weaknesses, but little has been done to directly compare the performance of early prototypes. Now, researchers at the JQI have performed a first-of-its-kind benchmark test of two small quantum computers built from different technologies.

The team, working with JQI Fellow Christopher Monroe and led by postdoctoral researcher Norbert Linke, sized up their own small-scale quantum computer against a device built by IBM. Both machines use five qubits—the fundamental units of information in a quantum computer—and both machines have similar error rates. But while the JQI device relies on chains of trapped atomic ions, IBM Q uses the movement of charges in a superconducting circuit.

To make their comparison, the JQI team ran several quantum programs on the devices, each of which solved a simple problem using a series of logic gates to manipulate one or two qubits at a time. Researchers accessed the IBM device using an online interface, which allows anyone to try their hand at programming IBM Q.

Both computers have strengths and weaknesses. For example, the superconducting platform has quicker gates and may be easier to mass produce, but its man-made qubits are all slightly different and have shorter lifetimes. Monroe says that the slower gates of ions might not be a major hurdle, though. "Because there is time,” Monroe says. “Trapped ion qubit lifetimes are way longer than any other type of qubit. Moreover, the ion qubits are identical, and they can be better replicated without error."

When put to the test, researchers found that the trapped-ion module was more accurate for programs that involved many pairs of qubits. Linke and Monroe attribute this to the simple fact that every qubit in their device is connected to every other—meaning that a logic gate can connect any pair of qubits. IBM Q has fewer than half the connections of its JQI counterpart, and in order to run some programs it had to shuffle information between qubits—a step that introduced errors into the calculation. When this shuffling wasn’t necessary, the two computers had similar performance.  “As we build larger systems, connectivity between qubits will become even more important,” Monroe says.

The new study, which was recently published in Proceedings of the National Academy of Sciences, provides an important benchmark for researchers studying quantum computing. And such head-to-head comparisons will become increasingly important in the future. “If you want to buy a quantum computer, you’ll need to know which one is best for your application,” Linke says. “You’ll need to test them in some way, and this is the first of this kind of comparison.”

By Erin Marshall

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  • JQI undergraduate researcher Eliot Fenton receives Goldwater Scholarship
  • April 10, 2017

Three University of Maryland students have been awarded scholarships by the Barry M. Goldwater Scholarship and Excellence in Education Foundation, which encourages students to pursue advanced study and careers in the sciences, engineering and mathematics. The Goldwater Foundation also recognized a fourth UMD student with an Honorable Mention.

Eliot Fenton, along with Christopher Bambic and Prayaag Venkat were among the 240 Barry Goldwater Scholars selected from 1,286 students nominated nationally this year. Natalie Livingston was recognized with an Honorable Mention. The four students, all currently UMD Juniors, plan to pursue doctoral degrees in their areas of study and to become university professors or researchers at government laboratories.

Fenton—a physics major and JQI undergraduate researcher who is also a member of the College Park Scholars’ Science, Discovery and the Universe program and a UMD President’s Scholar—plans to study atomic, molecular, and optical physics in graduate school. He intends to pursue an academic career in quantum mechanics and quantum computing.

Working with his faculty mentor, Physics Professor and JQI Fellow Luis Orozco, Fenton’s research has yielded significant technical breakthroughs in the production and characterization of high-quality optical transmission nanofibers used to trap atoms for quantum physics and computing studies. His achievements earned him an invitation to the Niels Bohr Institute in Copenhagen, Denmark in the summer of 2016, where he worked with experienced scientists to refine nanofiber fabrication techniques.

In January 2017, Fenton co-authored a peer-reviewed publication in the journal Optica that described the measurement of a nanofiber with sub-Angstrom precision—less than the diameter of an atom. Fenton will spend 9 weeks in the summer of 2017 as an undergraduate researcher at CERN in Geneva, Switzerland, where he will gain hands-on experience with subatomic physics research.

“Eliot is a deep and fast thinker who understands the concepts explained to him, and immediately generalizes them to situations in the laboratory,” Orozco said. “He shows a rare level of intellectual leadership and a questioning mind that that is the key to a successful career in science.”

Click here for the full CMNS news article. Written by Matthew Wright, CMNS.

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  • Ions sync up into world's first time crystal
  • Researchers forge a new state of matter using atoms and lasers.
  • March 8, 2017 Activity 3

Consider, for a moment, the humble puddle of water. If you dive down to nearly the scale of molecules, it will be hard to tell one spot in the puddle from any other. You can shift your gaze to the left or right, or tilt your head, and the microscopic bustle will be identical—a situation that physicists call highly symmetric.

That all changes abruptly when the puddle freezes. In contrast to liquid water, ice is a crystal, and it gains a spontaneous rigid structure as the temperature drops. Freezing fastens neighboring water molecules together in a regular pattern, and a simple tilt of the head now creates a kaleidoscopic change.

In 2012, Nobel-prize winning physicist Frank Wilczek, a professor at the Massachusetts Institute of Technology, proposed something that sounds pretty strange. It might be possible, Wilczek argued, to create crystals that are arranged in time instead of space. The suggestion prompted years of false starts and negative results that ruled out some of the most obvious places to look for these newly named time crystals.

Now, five years after the first proposal, a team of researchers led by physicists at the Joint Quantum Institute and the University of Maryland have created the world's first time crystal using a chain of atomic ions. The result, which finally brings Wilczek's exotic idea to life, was reported in Nature on March 9.

Much like freezing destroys the symmetry of liquid water, a time crystal disturbs a regularity in time. This is somewhat surprising, says lead author and JQI postdoctoral researcher Jiehang Zhang, since nature usually responds in sync to things that change in time. "The earth rotates around the sun once a year, and the seasons have the same period," Zhang says. "That’s what you would naturally expect."

A time crystal doesn't follow the lead, instead responding with a slower frequency—like a bell struck once a second that rings every other second. The atomic ions in the Maryland experiment, which researchers manipulated using laser pulses, responded exactly half as fast as the sequence of pulses that drove them.

Zhang, JQI Fellow Christopher Monroe and a group of experimentalists at UMD teamed up with a theory group at the University of California, Berkeley to create their time crystal. The Berkeley group, led by physicist Norman Yao, had previously proposed a way to create time crystals in the lab. For a chain of atomic ions, the challenge came down to finding the right sequence of laser pulses, along with assembling the sea of mirrors and lenses that ensured the lasers impinged on the ions in the right way.

To create their time crystal, researchers activated three types of laser-driven behavior in a chain of ten ytterbium ions. First, each ion was bombarded with its own individual laser beam, flipping an internal quantum property called spin by roughly 180 degrees with each pulse. Second, the ions were induced to interact with each other, coupling their internal spins together like two neighboring magnets. Finally, random disorder—essentially noise—was sprinkled onto each ion, a feature known from previous experiments to prevent the spins from jostling and heating up the chain.

Altogether, this sequence twisted around the ions' spins, and researchers kept track of the orientation of each spin after many repetitions of the sequence. When all three laser-driven behaviors were turned on, the spins of each ion synced up, and they would rhythmically return to their original direction at half the speed of the laser sequence.

But a time crystal is more than mere repetition, and this alone would not be enough to claim the creation of a time crystal, Zhang says. A crystal also needs to be rigid. "If you put a bunch of billiard balls on a pool table separated by exactly 10 centimeters, is that a crystal?" Zhang says.  "Not really, because if you shake the table a little bit it will fall apart."

Zhang and his colleagues demonstrated that their ions had this rigidity by attempting to artificially "melt" the time crystal. By modifying one of the laser pulses—essentially shaking the table—they observed that the rhythm remained stable, up to a point. Past a certain amount of heating, the time crystal dissolved away, just as an ice cube can melt back into a small puddle of water. But with weak shaking, it remained stable, a fact that provided the key evidence that they had created a time crystal.

This rigidity makes time crystals a potential ingredient for clocking complex quantum systems that have inherent defects and are hard to control. They could have applications to future quantum computers, which will also need to be robust. But such applications are still a long way off, especially since the time crystal that Zhang and collaborators produced lasted less than a millisecond.

"This bizarre state of matter results from a complex interplay between many quantum controls at the individual atomic level," says Monroe. "But time crystals can also emerge in certain solid-state devices, so a general understanding of this phenomenon could help bring such systems into future quantum devices."

In the same issue of Nature, a group of researchers from Harvard University, also working with Berkeley’s Yao, reported the creation of a time crystal using just such a solid-state system. Instead of ions, they used natural defects found in diamond to set up their crystal.

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  • Destabilized solitons perform a disappearing act
  • In the presence of impurities, dark solitons accelerate and vanish from sight
  • February 24, 2017 Activity 3

When your heart beats, blood courses through your veins in waves of pressure. These pressure waves manifest as your pulse, a regular rhythm unperturbed by the complex internal structure of the body. Scientists call such robust waves solitons, and in many ways they behave more like discrete particles than waves. Soliton theory may aid in the understanding of tsunamis, which—unlike other water waves—can sustain themselves over vast oceanic distances.

Solitons can arise in the quantum world as well. At most temperatures, gas atoms bounce around like billiard balls, colliding with each other and rocketing off into random directions. Near absolute zero, however, certain kinds of atoms suddenly start behaving according to the very different rules of quantum mechanics, and begin a kind of coordinated dance. Under pristine conditions, solitons can emerge inside these ultracold quantum fluids, surviving for several seconds.

Curious about how solitons behave in less than pristine conditions, scientists at NIST’s Physical Measurement Laboratory, in collaboration with researchers at the Joint Quantum Institute (JQI), have added some stress to a soliton’s life. They began by cooling down a cloud of rubidium atoms. Right before the gas became a homogenous quantum fluid, a radio-frequency magnetic field coaxed a handful of these atoms into retaining their classical, billiard ball-like state. Those atoms are, in effect, impurities in the atomic mix. The scientists then used laser light to push apart atoms in one region of the fluid, creating a solitary wave of low density—a “dark” soliton.

In the absence of impurities, this low-density region stably pulses through the ultracold fluid. But when atomic impurities are present, the dark soliton behaves as if it were a heavy particle, with lightweight impurity atoms bouncing off of it. These collisions make the dark soliton’s movement more random. This effect is reminiscent of Einstein’s 1905 predictions about randomized particle movement, dubbed Brownian motion.  

Guided by this framework, the scientists also expected the impurities to act like friction and slow down the soliton. But surprisingly, dark solitons do not completely follow Einstein’s rules. Instead of dragging down the soliton, collisions accelerated it to a point of destabilization. The soliton’s speed limit is set by the speed of sound in the quantum fluid, and upon exceeding that limit it exploded into a puff of sound waves.

This behavior made sense only after researchers changed their mathematical perspective and treated the soliton as though it has a negative mass. This is a quirky phenomenon that arises for certain collective behaviors of many-particle systems. Here the negative mass is manifested by the soliton’s darkness—it is a dip in the quantum fluid rather than a tall tsunami-like pulse.  Particles with negative mass respond to friction forces opposite to their ordinary cousins, speeding up instead of slowing down.

"All those assumptions about Brownian motion ended up going out the window—none of it applied,” says Hilary Hurst, a graduate student at JQI and lead theorist on the paper. "But at the end we had a theory that described this behavior very well, which is really nice.”

Lauren Aycock, lead author on the paper, lauded what she saw as particularly strong feedback between theory and experiment, adding that “it’s satisfying to have this kind of successful collaboration, where measurement informs theory, which then explains experimental results.”

Solitons in the land of ultracold atoms are intriguing, say Aycock and Hurst, because they are as close as you can get to observing the interface between quantum effects and the ordinary physics of everyday life. Experiments like this may help answer a deep physics riddle: where is the boundary between classical and quantum? In addition, this result may cast light on a similar problem with solitons in optical fibers, where random noise can disrupt the precise timing needed for communication over long distances.

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  • Probe for nanofibers has atom-scale sensitivity
  • January 19, 2017

Optical fibers are the backbone of modern communications, shuttling information from A to B through thin glass filaments as pulses of light. They are used extensively in telecommunications, allowing information to travel at near the speed of light virtually without loss.

These days, biologists, physicists and other scientists regularly use optical fibers to pipe light around inside their labs. In one recent application, quantum research labs have been reshaping optical fibers, stretching them into tiny tapers (see JQI News on Nanofibers and designer light traps). For these nanometer-scale tapers, or nanofibers, the injected light still makes its way from A to B, but some of it is forced to travel outside the fiber’s exterior surface. The exterior light, or evanescent field, can capture atoms and then carry information about that light-matter interaction to a detector (See JQI News on Thermometry using optical nanofibers).  

Fine-tuning such evanescent light fields is tricky and requires tools for characterizing both the fiber and the light. To this end, researchers from JQI, the Army Research Laboratory (ARL), and the Naval Research Laboratory (NRL) have developed a novel method to measure how light propagates through a nanofiber, allowing them to determine the nanofiber’s thickness to a precision less than the width of an atom. The technique, described in the January 20, 2017 issue of the journal Optica, is direct, fast and, unlike the standard imaging method, preserves the integrity of the fiber. As a result, the probe can be used in-situ with the nanofiber fabrication equipment, which will streamline implementation in quantum optics and quantum information experiments. Developing reliable and precise tools for this platform may enable nanofiber technology for sensing and metrology applications.

Light waves have a characteristic size called the wavelength. For visible light, the wavelength is roughly 100 times smaller than a human hair. Light can also have the appearance of different shapes, such a solid circle, ring, clover and more (see image below). Fibers restrict the way light waves can travel and twisting or bending a fiber will alter the light’s characteristics. Nanofibers are made by reshaping a normal fiber into an hourglass-like design, which further affects the guided light waves.

Examples of light shapes. Each panel shows a 3D (top) and 2D (bottom) intensity profile. The red (blue) areas indicate more (less) light intensity. The effect of the fiber appears in the 3D images as a sharp cutout; in 2D the fiber interface looks like a ring-shaped edge. (Images are calculations courtesy of P. Solano and L. Orozco)

In this experiment, researchers inject a combination of light shapes into a nanofiber. The light passes down a thinning taper, squeezes through a narrow waist, and then exits out the other side of the taper. The changing fiber size distorts the light waves, and multiple patterns emerge from the interfering light shapes (See JQI News on Collecting lost light). This is analogous to musical notes, or sound waves, beating together to form a complex chord.

The researchers make direct measurements of the interference patterns (beats). To do this, they employ a second micron-sized fiber that acts as a non-invasive sensor. The nanofiber is on a moving stage and crosses the probe fiber at an oblique angle. At the touching point, a tiny fraction of nanofiber light evanescently enters the second fiber and travels to a detector. As they scan the probe along the length of the nanofiber, the probe detector collects information about the evolving patterns of nanofiber light. The researchers simultaneously monitor the light transmitting through the nanofiber to ensure that the probe process is harmless.

The team can achieve a high level of precision with this technique because they are not imaging the fiber with a camera, which would have a spatial resolution limited by the collected light’s wavelength. UMD graduate student Pablo Solano explains, “We are actually seeing the different light modes mix together and that sets the limits on determining the fiber waist—in this case sub-angstrom.” A standard tool known as scanning electron microscopy (SEM) can also measure fiber dimensions with nanoscale resolution. This, however, has a comparative disadvantage, says Eliot Fenton, a UMD undergraduate student working on the project, “With our new method, we can avoid using SEM, which destroys the fiber with imaging chemicals and heating.” Other techniques involve collecting randomly scattered light from the fiber, which is less direct and susceptible to errors. Solano summarizes how researchers can benefit from this new tool, “By directly and sensitively measuring the interference (beating) of light without destroying the fiber, we can know exactly the kind of electromagnetic field that we would apply to atoms.”

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  • Atomic beltway could solve problems of cosmic gravity
  • November 14, 2016

When is a traffic jam not a traffic jam? When it's a quantum traffic jam, of course. Only in quantum physics can traffic be standing still and moving at the same time.

A new theoretical paper from scientists at the National Institute of Standards and Technology (NIST) and the University of Maryland suggests that intentionally creating just such a traffic jam out of a ring of several thousand ultracold atoms could enable precise measurements of motion. If implemented with the right experimental setup, the atoms could provide a measurement of gravity, possibly even at distances as short as 10 micrometers—about a tenth of a human hair's width.

While the authors stress that a great deal of work remains to show that such a measurement would be attainable, the potential payoff would be a clarification of gravity's pull at very short length scales. Anomalies could provide major clues on gravity’s behavior, including why our universe appears to be expanding at an accelerating rate.

In addition to potentially answering deep fundamental questions, these atom rings may have practical applications, too. They could lead to motion sensors far more precise than previously possible, or serve as switches for quantum computers, with 0 represented by atomic gridlock and 1 by moving atom traffic.

The authors of the paper are affiliated with the Joint Quantum Institute and the Joint Center for Quantum Information and Computer Science, both of which are partnerships between NIST and the University of Maryland.

Over the past two decades, physicists have explored an exotic state of matter called a Bose-Einstein condensate (BEC), which exists when atoms overlap one another at frigid temperatures a smidgen of a degree away from absolute zero. Under these conditions, a tiny cloud of atoms can essentially become one large quantum “superatom,” allowing scientists to explore potentially useful properties like superconductivity and superfluidity more easily.

Theoretical physicists Stephen Ragole and Jake Taylor, the paper’s authors, have now suggested that a variation on the BEC idea could be used to sense rotation or even explore gravity over short distances, where other forces such as electromagnetism generally overwhelm gravity's effects. The idea is to use laser beams—already commonly used to manipulate cold atoms—to string together a few thousand atoms into a ring 10 to 20 micrometers in diameter.

Once the ring is formed, the lasers would gently stir it into motion, making the atoms circulate around it like cars traveling one after another down a single-lane beltway. And just as car tires spin as they travel along the pavement, the atoms' properties would pick up the influence of the world around them—including the effects of gravity from masses just a few micrometers away.

The ring would take advantage of one of quantum mechanics' counterintuitive behaviors to help scientists actually measure what its atoms pick up about gravity. The lasers could stir the atoms into what is called a "superposition," meaning, in effect, they would be both circulating about the ring and simultaneously at a standstill. This superposition of flow and gridlock would help maintain the relationships among the ring's atoms for a few crucial milliseconds after removing their laser constraints, enough time to measure their properties before they scatter.

Not only might this quantum traffic jam overcome a difficult gravity measurement challenge, but it might help physicists discard some of the many competing theories about the universe—potentially helping clear up a longstanding traffic jam of ideas.

One of the great mysteries of the cosmos, for example, is why it is expanding at an apparently accelerating rate. Physicists have suggested an outward force, dubbed “dark energy,” causes this expansion, but they have yet to discover its origin. One among many theories is that in the vacuum of space, short-lived virtual particles constantly appear and wink out of existence, and their mutual repulsion creates dark energy's effects. While it's a reasonable enough explanation on some levels, physicists calculate that these particles would create so much repulsive force that it would immediately blow the universe apart. So how can they reconcile observations with the virtual particle idea?

"One possibility is that the basic fabric of space-time only responds to virtual particles that are more than a few micrometers apart," Taylor said, "and that's just the sort of separation we could explore with this ring of cold atoms. So if it turns out you can ignore the effect of particles that operate over these short length scales, you can account for a lot of this unobserved repulsive energy. It would be there, it just wouldn't be affecting anything on a cosmic scale."

The research appears in the journal Physical Review Letters.

This story, originally published as a news item by NIST, was writen by Chad T. Boutin.

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  • A closer look at Weyl physics
  • October 14, 2016 Activity 1

This is part two of a two-part series on Weyl semimetals and Weyl fermions, newly discovered materials and particles that have drawn great interest from physicists at JQI and the Condensed Matter Theory Center at the University of Maryland. The second part focuses on the theoretical questions about Weyl materials that Maryland researchers are exploring. Part one, which was published last week, introduced their history and basic physics. If you haven’t read part one, we encourage you to head there first before getting into the details of part two.

The 2015 discovery of a Weyl semimetal—and the Weyl fermions it harbored—provoked a flurry of activity from researchers around the globe. A quick glance at a recent physics journal or the online arXiv preprint server testifies to the topic’s popularity. The arXiv alone has had more than 200 papers on Weyl semimetals posted in 2016.

Researchers at JQI and the Condensed Matter Theory Center (CMTC) at the University of Maryland have been interested in Weyl physics since before last summer’s discovery, publishing 18 papers on the topic over the past two years. In all, more than a dozen scientists at Maryland have been working to understand the fundamental properties of these curious new materials.

In addition to studying specific topics, researchers are also realizing that the physics of Weyl fermions—particles first studied decades ago in different setting—might have wider reach. They may account for the low-energy behavior of certain superconductors and other materials, especially those that are strongly correlated—that is, materials in which interactions between electrons cannot be ignored.

"Weyl physics should be abundant in many, many correlated materials," says Pallab Goswami, a theorist and postdoctoral researcher at JQI and CMTC. "If we can understand the exotic thermodynamic, transport and electrodynamic properties of Weyl fermions, we can probably understand more about low temperature physics in general," Goswami says.

Taking a wider approach

Goswami is not only interested in discovering new Weyl semimetals. He also wants to find strongly interacting materials where Weyl semimetal physics can help explain unresolved puzzles. That behavior is often related to the unique magnetic properties of Weyl fermions.

Recently, he and several colleagues examined a family of compounds known as pyrochlore iridates, formed from iridium, oxygen and a rare earth element such as neodymium or praseodymium. While most of these are insulators at low temperatures, the compound with praseodymium is an exception. It remains a metal and, intriguingly, has an anomalous current that flows without any external influence. This current, due to a phenomenon called the Hall effect, appears in other materials, but it is usually driven by an applied magnetic field or the magnetic properties of the material itself. In the praseodymium iridate, though, it appears even without a magnetic field and despite the fact that the compound has no magnetic properties that have been seen by experiment.

Goswami and his colleagues have argued that Weyl fermions can account for this puzzling behavior. They can distort a material’s magnetic landscape, making it look to other particles as if a large magnetic field is there. This effect is hard to spot in the lab, though, due to the challenge of keeping samples at very cold temperatures. The team has suggested how future experiments might confirm the presence of Weyl fermions through precise measurements with a scanning tunneling microscope.

On the surface

Parallel to Goswami’s efforts to expand the applications of Weyl physics, Johannes Hofmann, a former JQI and CMTC theorist who is now at the University of Cambridge in the UK, is diving into the details of Weyl semimetals. Hofmann has studied Weyl semimetals divorced from any real material and predicted a generic behavior that electrons on the surface of a semimetal will have. It’s a feature that could ultimately find applications to electronics and photonics.

In particular, he studied undulating charge distributions on the surface of semimetals, created by regions with more electrons and regions with less. Such charge fluctuations are dynamic, moving back and forth in response to their mutual electrical attraction, and in Weyl semimetals they support waves that move in only one direction.

The charge fluctuations generate electric and magnetic fields just outside the surface. And on the surface, positive and negative regions are packed close together—so close, in fact, that their separation can be much smaller than the wavelength of visible light. Since these fluctuations occur on such a small scale, they can also be used to detect small features in other objects. For instance, bringing a sample of some other material near the surface will modify the distribution of charges in a way that could be measured. Getting the same resolution with light would require high-energy photons that could destroy the object being imaged. Indeed, researchers have already shown that this is a viable imaging technique, as demonstrated in experiments with ordinary metals.

On the surface of Weyl semimetals one-way waves can travel through these charge fluctuations. Ordinary metals, too, can carry waves but require huge magnetic fields to steer them in only one direction. Hofmann showed that in a Weyl semimetal, it’s possible to create these waves without a magnetic field, a fact that could enable applications of the materials to microscopy and lithography. 

Too much disorder?

Although many studies imagine that Weyl materials are perfectly clean, such a situation rarely occurs in real experiments. Contaminants inevitably lodge themselves into the ideal crystal structure of any solid. Consequently, JQI scientists have looked at how disorder—the dirt that prevents samples from behaving like perfect theoretical models—affects the properties of Weyl materials. Their work has settled an argument theorists have been having for years.

One camp thought that weak disorder—dirt that doesn’t cause big changes—was essentially harmless to Weyl semimetals, since tiny wobbles in the material's electrical landscape could safely be ignored. The other camp argued that certain fluctuations, though weak, affect a wide enough area of the landscape that they cannot be ignored.

Settling the dispute took intense numerical study, requiring the use of supercomputing resources at Maryland. "It was very hard to do this," says Jed Pixley, a postdoctoral researcher at JQI and CMTC who finally helped solve the disorder conundrum. "It turns out that the effects of large local fluctuations of the disorder are weak, but they’re there."

Pixley’s calculations found that large regions of weak disorder create a new type of low-energy excitation, in addition to Weyl fermions. These new excitations live around the disordered regions and divert energy away from the Weyl fermion quasiparticles. The upshot is that the quasiparticles have a finite lifetime, instead of the infinite lifetime predicted by previous studies. The result has consequences for the stability of Weyl semimetals in a technical sense, although the lifetime of the quasiparticles is still quite long. In typical experiments, the effects of large areas of disorder would be tough to spot, although experiments on Weyl semimetals are still in their early days.

Research into Weyl materials shows little sign of slowing down. And the broader role that Weyl fermions play in condensed matter physics is still evolving and growing, with many more surprises likely in the future. As more and more experimental groups join the hunt for exotic physics, theoretical investigations, like those of the scientists at JQI and CMTC, will be crucial to identifying new behaviors and suggesting new experiments, steering the study of Weyl physics toward new horizons.

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  • L'Oréal-UNESCO award goes to former JQI student researcher
  • October 13, 2016

Karina Jiménez-García, a former visiting graduate student who worked with JQI Fellow Ian Spielman, was one of 30 young women scientists to receive a 2016 L'Oréal-UNESCO For Women in Science fellowship. She was selected from a pool of more than 1,000 applicants and received the award for her ongoing research on the quantum behavior of ultra-cold atoms.

"This is a recognition that I owe to all those that have guided and inspired me and those who have supported me throughout my professional career, especially my family," says Jiménez-García, who is currently a postdoctoral researchers at the Kastler Brossel Laboratory at the Collège de France in Paris. She plans to use the funds from the fellowship to build a handful of physics demonstrations that will appeal to young students and to fund travel to conferences in Mexico, where she hopes to start her own research group in the future.

The award, which launched in 2007, has given fellowships to more than 140 women in France who are either studying toward a Ph.D. in the life or physical sciences or working as postdoctoral researchers. The criteria for selection include a proven academic track record and the ability to inspire the next generation of scientists. For the first time since the fellowship launched, L'Oréal organized a public event, held on October 12, that included lectures and interviews with this year's winners.

While at JQI, Jiménez-García worked on creating synthetic electric and magnetic fields for ultra-cold clouds of atoms. In a series of papers, she and a team of experimental colleagues showed that lasers could coax atoms without an electric charge into behaving like charged particles in magnetic and electric fields. The work is still a fertile area of research for Spielman and could enrich the toolkit for atomic physicists interested in simulating other quantum systems with clouds of atoms.

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  • A warm welcome for Weyl physics
  • October 6, 2016 Activity 1

This is part one of a two-part series on Weyl semimetals and Weyl fermions, newly discovered materials and particles that have drawn great interest from researchers at JQI and the Condensed Matter Theory Center at the University of Maryland. The first part focuses on the history and basic physics of these materials. Part two focuses on theoretical work at Maryland.

For decades, particle accelerators have grabbed headlines while smashing matter together at faster and faster speeds. But in recent years, alongside the progress in high-energy experiments, another realm of physics has been taking its own exciting strides forward.

That realm, which researchers call condensed matter physics, studies chunks of matter moving decidedly slower than the protons in the LHC. In fact, the materials under study—typically solids or liquids—are usually sitting still. That doesn't make them boring, though. Their calm appearance can often hide exotic physics that arises from their microscopic activity.

"In condensed matter physics, the energy scales are much lower," says Pallab Goswami, a postdoctoral researcher at JQI and the Condensed Matter Theory Center (CMTC) at the University of Maryland. "We want to go to lower energies and find new phenomena, which is exactly the opposite of what is done in particle physics."

Historically, that's been a fruitful approach. The field has explained the physics of semiconductors—like the silicon that makes computer chips—and many superconductors, which generate the large magnetic fields required for clinical MRI machines.

Over the past decade, that success has continued. In 2004, researchers at the University of Manchester in the UK discovered a way to make single-atom-thick sheets of carbon by sticking Scotch tape onto graphite and peeling it off. It was a shockingly low-tech way to make graphene, a material with stellar electrical properties and incredible strength, and it led quickly to a Nobel Prize in physics in 2010.

A few years later, researchers discovered topological insulators, materials that trap their internal electrons but let charges on the surface flow freely. It’s a behavior that requires sophisticated math to explain—math that earned three researchers a share of the 2016 Nobel Prize in physics for theoretical discoveries that ultimately explain the physics of these and other materials.

In 2012, experimentalists studying the junction between a superconductor and a thin wire spotted evidence for Majorana fermions, particles that behave like uncharged electrons. Originally studied in the context of high-energy physics, these exotic particles never showed up in accelerators, but scientists at JQI predicted that they might make an appearance at much lower energies.

Last year, separate research groups at Princeton University, MIT and the Chinese Academy of Sciences discovered yet another exotic material—a Weyl semimetal—and with it yet another particle: the Weyl fermion. It brought an end to a decades-long search that began in the 1930s and earned acclaim as a top-10 discovery of the year, according to Physics World.

Like graphene, Weyl semimetals have appealing electrical properties and may one day make their way into electronic devices. But, perhaps more intriguingly for theorists, they also share some of the rich physics of topological insulators and have provoked a flurry new research. Scientists working with JQI Fellow Sankar Das Sarma, the Director of CMTC, have published 18 papers on the subject since 2014.

Das Sarma says that the progress in understanding solid state materials over the past decade has been astonishing, especially the discovery of phenomena researchers once thought were confined to high-energy physics. “It shows how clever nature is, as concepts and laws developed in one area of physics show up in a completely disparate area in unanticipated ways,” he says.

An article next week will explore some of the work on Weyl materials at JQI and CMTC. This week's story will focus on the fundamental physics at play in these unusual materials.

Spotted at long last

Within two weeks last summer, three research groups reported evidence for Weyl semimetals. Teams from the US and China measured the energy of electrons on the surface of tantalum arsenide, a metallic crystal that some had predicted might be a semimetal. By shining light on the material and capturing electrons ejected from the sample, researchers were able to map out a characteristic sign of Weyl semimetals—a band of energies that electrons on the surface inhabit, known as a Fermi arc. It was a feature predicted only for Weyl semimetals.

Much of the stuff on Earth, from wood and glass to copper and water, is not a semimetal. It's either an insulator, which does a bad job of conducting electricity, or a conductor, which lets electrical current flow with ease.

Quantum physics ultimately explains the differences between conductors, insulators, semiconductors and semimetals. The early successes of quantum physics—like explaining the spectrum of light emitted by hydrogen atoms—revolved around the idea that quantum objects have discrete energy levels. For instance, in the case of hydrogen, the single electron orbiting the nucleus can only occupy certain energies. The pattern of light emanating from hot hydrogen gas matches up with the spacing between these levels.

In a solid, which has many, many atoms, electrons still occupy a discrete set of energies. But with so many electrons come many more levels, and those levels tend to bunch together. This leads to a series of energy bands, where electrons can live, and gaps, where they can't. The figure below illustrates this.

Electrons pile into these bands, filling up the allowed energies and skipping the gaps. Depending on where in the band structure the last few electrons sit, a material will have dramatically different electrical behavior. Insulators have an outer band that is completely filled up, with an energy gap to a higher empty band. Metals have their most energetic electrons sitting in a partially filled band, with lots of slightly higher energies to jump to if they are prodded by a voltage from a battery.

A Weyl semimetal is a different beast. There, electrons pile in and completely fill a band, but there is no gap to the higher, unfilled band. Instead, the two touch at isolated points, which are responsible for some interesting properties of Weyl materials.

Quasiparticles lead the charge

We often think of electrons as carrying the current in a wire, but that’s not the whole story. The charge carriers look like electrons, but due to their microscopic interactions they behave like they have a different mass. These effective charge carriers, which have different properties in different materials, are called quasiparticles.

By examining a material's bands and gaps, it's possible to glean some of the properties of these quasiparticles. For a Weyl semimetal, the charge carriers satisfy an equation first studied in 1929 by a German mathematician named Hermann Weyl, and they are now called Weyl fermions.

But the structure of the bands doesn't capture everything about the material, says Johannes Hofmann, a former postdoctoral researcher at JQI and CMTC who is now at the University of Cambridge. "In a sense, these Weyl materials are very similar to graphene," Hofmann says. "But they are not only described by the band structure. There is a topological structure as well, just as in topological insulators."

Hofmann says that although the single-point crossings in the bands play an important role, they don't tell the whole story. Weyl semimetals also have a topological character, which means that the overall shape of the bands and gaps, as well as the way electrons spread out in space, affect their properties. Topology can account for these other properties by capturing the global differences in these shapes, like the distinction between an untwisted ribbon and a Moebius strip.

The interplay between the topological structure and the properties of Weyl materials is an active area of research. Experiments, though, are still in the earliest stages of sorting out these questions.

A theorist’s dream

Researchers at JQI and elsewhere are studying many of the theoretical details, from the transport properties on the surfaces of Weyl materials to the emergence of new types of order. They are even finding Weyl physics useful in tackling condensed matter quandaries that have long proved intractable.

Jed Pixley, a postdoctoral researcher at CMTC, has studied how Weyl semimetals behave in the presence of disorder. Pixley says that such investigations are crucial if Weyl materials are to find practical applications. "If you are hoping semimetals have these really interesting aspects,” he says, “then things better not change when they get a little dirty."

Please return next week for a sampling of the research into Weyl materials underway at JQI and CMTC. Written by Chris Cesare with illustrations and figures by Sean Kelley.

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  • Programmable ions set the stage for general-purpose quantum computers
  • A new quantum computer module combines proven techniques with advances in hardware and software.
  • August 3, 2016

Quantum computers promise speedy solutions to some difficult problems, but building large-scale, general-purpose quantum devices is a problem fraught with technical challenges.

To date, many research groups have created small but functional quantum computers. By combining a handful of atoms, electrons or superconducting junctions, researchers now regularly demonstrate quantum effects and run simple quantum algorithms—small programs dedicated to solving particular problems.

But these laboratory devices are often hard-wired to run one program or limited to fixed patterns of interactions between their quantum constituents. Making a quantum computer that can run arbitrary algorithms requires the right kind of physical system and a suite of programming tools. Atomic ions, confined by fields from nearby electrodes, are among the most promising platforms for meeting these needs.

In a paper published as the cover story in Nature on August 4, researchers working with Christopher Monroe, a Fellow of the Joint Quantum Institute and the Joint Center for Quantum Information and Computer Science at the University of Maryland, introduced the first fully programmable and reconfigurable quantum computer module. The new device, dubbed a module because of its potential to connect with copies of itself, takes advantage of the unique properties offered by trapped ions to run any algorithm on five quantum bits, or qubits—the fundamental unit of information in a quantum computer.

“For any computer to be useful, the user should not be required to know what’s inside,” Monroe says. “Very few people care what their iPhone is actually doing at the physical level. Our experiment brings high-quality quantum bits up to a higher level of functionality by allowing them to be programmed and reconfigured in software.”

The new module builds on decades of research into trapping and controlling ions. It uses standard techniques but also introduces novel methods for control and measurement. This includes manipulating many ions at once using an array of tightly-focused laser beams, as well as dedicated detection channels that watch for the glow of each ion.

"These are the kinds of discoveries that the NSF Physics Frontiers Centers program is intended to enable," says Jean Cottam Allen, a program director in the National Science Foundation’s physics division. “This work is at the frontier of quantum computing, and it’s helping to lay a foundation and bring practical quantum computing closer to being a reality.”

The team tested their module on small instances of three problems that quantum computers are known to solve quickly. Having the flexibility to test the module on a variety of problems is a major step forward, says Shantanu Debnath, a graduate student at JQI and the paper’s lead author. “By directly connecting any pair of qubits, we can reconfigure the system to implement any algorithm,” Debnath says. “While it’s just five qubits, we know how to apply the same technique to much larger collections.”

At the module’s heart, though, is something that’s not even quantum: A database stores the best shapes for the laser pulses that drive quantum logic gates, the building blocks of quantum algorithms. Those shapes are calculated ahead of time using a regular computer, and the module uses software to translate an algorithm into the pulses in the database.

Putting the pieces together

Every quantum algorithm consists of three basic ingredients. First, the qubits are prepared in a particular state; second, they undergo a sequence of quantum logic gates; and last, a quantum measurement extracts the algorithm’s output.

The module performs these tasks using different colors of laser light. One color prepares the ions using a technique called optical pumping, in which each qubit is illuminated until it sits in the proper quantum energy state. The same laser helps read out the quantum state of each atomic ion at the end of the process. In between, a separate laser strikes the ions to drive quantum logic gates.

These gates are like the switches and transistors that power ordinary computers. Here, lasers push on the ions and couple their internal qubit information to their motion, allowing any two ions in the module to interact via their strong electrical repulsion. Two ions from across the chain notice each other through this electrical interaction, just as raising and releasing one ball in a Newton’s cradle transfers energy to the other side.

The re-configurability of the laser beams is a key advantage, Debnath says. “By reducing an algorithm into a series of laser pulses that push on the appropriate ions, we can reconfigure the wiring between these qubits from the outside,” he says. “It becomes a software problem, and no other quantum computing architecture has this flexibility.”

To test the module, the team ran three different quantum algorithms, including a demonstration of a Quantum Fourier Transform (QFT), which finds how often a given mathematical function repeats. It is a key piece in Shor’s quantum factoring algorithm, which would break some of the most widely-used security standards on the internet if run on a big enough quantum computer.

Two of the algorithms ran successfully more than 90% of the time, while the QFT topped out at a 70% success rate. The team says that this is due to residual errors in the pulse-shaped gates as well as systematic errors that accumulate over the course of the computation, neither of which appear fundamentally insurmountable. They note that the QFT algorithm requires all possible two-qubit gates and should be among the most complicated quantum calculations.

The team believes that eventually more qubits—perhaps as many as 100—could be added to their quantum computer module. It is also possible to link separate modules together, either by physically moving the ions or by using photons to carry information between them.

Although the module has only five qubits, its flexibility allows for programming quantum algorithms that have never been run before, Debnath says. The researchers are now looking to run algorithms on a module with more qubits, including the demonstration of quantum error correction routines as part of a project funded by the Intelligence Advanced Research Projects Activity.

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  • Federal report urges commitment to quantum research
  • August 3, 2016

A government report, authored by experts from a variety of federal agencies, has recommended that the US treat quantum information science as a national priority.

The 16-page report, prepared by the president’s National Science and Technology Council and released to the public on July 26, suggested steps to increase the nation’s commitment to its quantum portfolio. These included ensuring stable federal funding of basic quantum research and investing in targeted, short-term projects that could address timely technical hurdles. It also painted in broad strokes the technological advances that quantum science may one day enable. It specifically mentioned sensors for precision measurement, more secure communication and quantum simulation and computation.

The report highlighted JQI and QuICS as successful examples of federal investment in the field, focusing on the close collaboration of the centers and their role in training the next generation of academic and industry leaders. It praised the productivity of two National Science Foundation Physics Frontiers Centers, one of which is the PFC@JQI.

A post published to the White House blog summarized the report’s significance and findings. It also mentioned two other recent reports—by the Department of Energy and the National Strategic Computing Initiative—related to the future of quantum information science.

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  • Ultra-cold atoms may wade through quantum friction
  • June 24, 2016 Activity 3

Theoretical physicists studying the behavior of ultra-cold atoms have discovered a new source of friction, dispensing with a century-old paradox in the process. Their prediction, which experimenters may soon try to verify, was reported recently in Physical Review Letters.

The friction afflicts certain arrangements of atoms in a Bose-Einstein Condensate (BEC), a quantum state of matter in which the atoms behave in lockstep. In this state, well-tuned magnetic fields can cause the atoms to attract one another and even bunch together, forming a single composite particle known as a soliton.

Solitons appear in many areas of physics and are exceptionally stable. They can travel freely, without losing energy or dispersing, allowing theorists to treat them like everyday, non-quantum objects. Solitons composed of photons—rather than atoms—are even used for communication over optical fibers.

Studying the theoretical properties of solitons can be a fruitful avenue of research, notes Dmitry Efimkin, the lead author of the paper and a former JQI postdoctoral researcher now at the University of Texas at Austin. “Friction is very fundamental, and quantum mechanics is now quite a well-tested theory,” Efimkin says. “This work investigates the problem of quantum friction for solitons and marries these two fundamental areas of research.”

Efimkin, along with JQI Fellow Victor Galitski and Johannes Hofmann, a physicist at the University of Cambridge, sought to answer a basic question about soliton BECs: Does an idealized model of a soliton have any intrinsic friction?

Prior studies seemed to say no. Friction arising from billiard-ball-like collisions between a soliton and stray quantum particles was a possibility, but the mathematics prohibited it. For a long time, then, theorists believed that the soliton moved through its cloudy quantum surroundings essentially untouched.

But those prior studies did not give the problem a full quantum consideration, Hofmann says. “The new work sets up a rigorous quantum-mechanical treatment of the system,” he says, adding that this theoretical approach is what revealed the new frictional force.

It’s friction that is familiar from a very different branch of physics. When a charged particle, such as an electron, is accelerated, it emits radiation. A long-known consequence is that the electron will experience a friction force as it is accelerated, caused by the recoil from the radiation it releases.

Instead of being proportional to the speed of the electron, as is friction like air resistance, this force instead depends on the jerk—the rate at which the electron’s acceleration is changing. Intriguingly, this is the same frictional force that appears in the quantum treatment of the soliton, with the soliton’s absorption and emission of quantum quasiparticles replacing the electron’s emission of radiation.

At the heart of this frictional force, however, lurks a problem. Including it in the equations describing the soliton’s motion—or an accelerated electron’s—reveals that the motion in the present depends on events in the future, a result that inverts the standard concept of causality. It’s a situation that has puzzled physicists for decades.

The team tracked down the origin of these time-bending predictions and dispensed with the paradox. The problem arises from a step in the calculation that assumes the friction force only depends on the current state of the soliton. If, instead, it also depends on the soliton’s past trajectory, the paradox disappears.

Including this dependence on the soliton’s history leads to nearly the same equations governing its motion, and those equations still include the new friction. It’s as if the quantum background retains a memory of the soliton’s path.

Hofmann says that BECs provide a pristine system to search for the friction. Experimenters can apply lasers that set the atomic soliton in motion, much like a marble rolling around a bowl—although the bowl is tightly squeezed in one dimension. Observing the frequency and amplitude of this motion, as well as how it changes over time, could reveal the friction’s signature. “Using some typical experimental parameters, we think that the magnitude of this force is large enough to be observable in current experiments,” Hofmann says.

Infographic credit: S. Kelley/NIST and C. Cesare/JQI

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  • Disorder grants a memory to quantum spins
  • June 6, 2016 Activity 3

Nature doesn’t have the best memory. If you fill a box with air and divide it in half with a barrier, it’s easy to tell molecules on the left from molecules on the right. But after removing the barrier and waiting a short while, the molecules get mixed together, and it becomes impossible to tell where a given molecule started. The air-in-a-box system loses any memory of its initial conditions.

The universe has been forgetting its own initial state since the Big Bang, a fact linked to the unrelenting forward march of time. Systems that forget where they started are said to have thermalized, since it is often—but not always—an exchange of heat and energy with some other system that causes the memory loss. For example, a melting ice cube forgets its orderly arrangement of water molecules when heat from its surroundings splits the cube’s crystal bonds. In some sense, the initial information about the ice cube—the structure of the crystal, the distance between molecules, etc.—leaks away.

The opposite case is localization, where information about the initial arrangement sticks around. Such a situation is rare, like an ice cube that never melts, but one example is Anderson localization, in which particles or waves in a crystal are trapped near impurities. They tend to bounce off defects in the crystal and scatter in random directions, yielding no net movement. If there are enough impurities in a region, the particles or waves never escape.

Since the discovery of Anderson localization in 1958, it has been an open question whether interacting collections of quantum particles can also localize, a phenomenon known as many-body localization. Now, researchers working with JQI and QuICS Fellow Christopher Monroe have directly observed this localization in a system of 10 interacting ions, trapped and zapped by electric fields and lasers. Their findings are one of the first direct observations of many-body localization in a quantum system, and they open up the possibility of studying the phenomenon with more ions. The results were published June 6 in Nature Physics.

Although it is possible to simulate the behavior of 10 ions with an ordinary computer, the experiment is an important step toward studying many-body localization with dozens of ions. At that point, an accurate simulation is out of reach for even the fastest of today’s machines. It could also shed light on a question that has vexed physicists since the early days of the quantum theory: When does a given quantum system thermalize?

“The transition of quantum systems from thermalized to localized represents a boundary between states governed at long times by quantum mechanics and ones that follow classical physics,” says Jake Smith, a graduate student at JQI and the first author of the paper. “It’s important to know if a given quantum system will thermalize because if it does you can use techniques from classical physics to predict its long-time behavior. That doesn’t require full knowledge of the system and is easier than making predictions with quantum mechanics.”

Smith and his colleagues searched for signs of localization by introducing some disorder into a chain of 10 ytterbium ions. The suspicion was that this disorder would act like the crystal impurities responsible for Anderson localization, preventing information about each ion from dispersing throughout the chain.

In this case, the information is the initial state of each ion’s quantum spin, a property that makes it act like a tiny magnet. This spin, which can point up, down or anywhere in between, and the ion can absorb and emit photons from a laser, changing the direction its spin points. Lasers with the right color and power also let spins interact with each other in a controllable way. By applying the right lasers, researchers could dial in the strength of this interaction, as well as how far it reached. This controllability is one of the major advantages of studying many-body localization with trapped ions.

By focusing a powerful laser to a diameter of just over a micron, the team also applied a random shift to the magnetic environment of each spin, creating the necessary disorder. Then, they tuned the strength of the interactions relative to the size of this disorder and traced the emergence of localization. They performed many experiments with random amounts of disorder, preparing each spin to point either up or down and then measuring all of the spins after a certain amount of time to see where they pointed.

Without disorder, the spins rapidly lost any signature of their initial direction. At the end of the disorder-free runs, each spin was just as likely to point up as it was to point down. However, as they cranked up the disorder, the spins started to retain information about their initial state. If a spin started out pointing up, it was more likely to be measured pointing up, and the same was true for spins initially pointing down. This local memory of the initial condition even persisted through the time it took for several spin interactions to occur.

“This work is a major advance in quantum simulation as our platform can be scaled to dozens of ions, where detailed modeling becomes impossible due to the complexity of many-body quantum states,” Smith says. “Moreover, the high degree of control in our experiment opens the possibility of using these many-body localized states as potential quantum information memories.”

In addition to Smith and Monroe, several other authors contributed to the paper: Aaron Lee, a graduate student at JQI; Phil Richerme, an assistant professor at Indiana University; Brian Neyenhuis, a postdoctoral researcher at JQI; Paul Hess, a postdoctoral researcher at JQI; Philipp Hauke, a postdoctoral researcher at the University of Innsbruck; Markus Heyl, a researcher at the Technical University of Munich; and David Huse, a professor at Princeton University.

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  • Quantum cycles power cold-atom pump
  • May 23, 2016

The idea of a pump is at least as old as the ancient Greek philosopher and scientist Archimedes. More than 2000 years ago, Archimedes allegedly invented a corkscrew pump that could lift water up an incline with the turn of a handle. Versions of the ancient invention still bear his name and are used today in agriculture and industry.

Modern pumps have achieved loftier feats. For instance, in the late 1990s, NIST developed a device that could pump individual electrons, part of a potential new standard for measuring capacitance.

While pumps can be operated mechanically, electrically or via any other source of energy, they all share the common feature of being driven by a periodic action. In the Archimedean pump, that action is a full rotation of the handle, which draws up a certain volume of water. For the NIST electron pump, it is a repeating pattern of voltage signals, which causes electrons to hop one at a time between metallic islands.

But physicists have sought for decades to build a different kind of pump—one driven by the same kind of periodic action but made possible only by the bizarre rules of quantum mechanics. Owing to their physics, these pumps would be immune to certain imperfections in their fabrication.

Now, a team of physicists working in collaboration with JQI Fellow Ian Spielman and NIST postdoctoral researcher Hsin-I Lu has created just such a pump. By periodically jostling many individual atoms, the researchers were able to shift an entire atomic cloud without any apparent overall motion by its constituents. The team is the first to test this predicted behavior, which arises in what they call a geometric charge pump. The work follows close on the heels of two recent papers that examined topological charge pumps, which demonstrate a distinct but related effect. The new result was published May 20 in Physical Review Letters.

The experiment builds on other work by Spielman and his team that involves the precise manipulation of cold atomic clouds known as Bose-Einstein condensates (BECs), in which each atom occupies the same quantum state. “These cold atoms are a useful platform to study theoretical ideas that can’t be tested with ordinary condensed matter systems,” says Spielman. For example, he explains, it’s not obvious how to experiment with periodic changes in a solid crystal like sodium chloride—common table salt—that shares its two-part structure with the BEC. The organization of the crystal is too rigid to be easily modified in place.

The idea of a quantum pump dates back to 1983, when a physicist at the University of Washington asked a theoretical question: What would happen if quantum particles, confined in a one-dimensional array, saw their potential energy landscape—the backdrop that influences their motion—gently deformed and then returned to its original shape? From the point of view of the particles, everything would look the same at the beginning and the end of this cycle, so it’s natural to expect that nothing would happen. However, due to a quantum phenomenon known as Berry’s phase, the paper predicted that the particles’ motion could actually be modified.

Berry’s phase is a consequence of quantum physics, but it has a direct analogy to motion on a curved surface. Imagine an arrow pointing straight up, grazing the equator of a sphere. If it is transported along a path toward the north pole, then brought back down a different path to the equator and finally returned to its original spot, it will no longer be pointing straight up. The arrow will be rotated by an amount that depends on the path it took. This mismatch between the initial direction that the arrow points and the direction it ends up pointing is due to the curvature of the sphere. Berry’s phase is the imprint a quantum particle gains as it is “transported” along a curved path, although this movement isn’t necessarily through physical space.

In fact, in the JQI pump, the curvature comes from energy differences that atoms experience over time. Each atom in the BEC sees an energy landscape with a left and a right well—troughs that the atoms tend to fall into. The repeating pattern of wells, all arranged in a line, is generated by interfering lasers at just the right frequency and power to trap rubidium atoms with the right quantum properties.

In the new experiment, each atom initially sits in its left well, which starts out as a deeper trough. As time increases, the left well becomes shallower and the right well gets deeper. This tends to tip atoms from the left well to the right well. To complete the cycle, the change happens in the other direction, and the atoms tip back into the left well. In the end each atom again sees the same energy landscape, and the density within each well is the same as when it started. Despite this, the entire cloud of atoms has been shifted by a small amount—smaller, in fact, than the distance between lattice sites. (Click here to see a short animation of several pump cycles.)

By adjusting the power of the lasers and an applied magnetic field, the researchers traced out different paths through this see-sawing energy space, allowing the atoms to sample different amounts of curvature and thus different Berry’s phases. This allowed experimenters to adjust the amount that the cloud was displaced in each cycle, an effect they measured by transferring a small percentage of the atoms into a different quantum state and measuring the amount of light they absorbed.

Here, Lu, Spielman and colleagues realized a novel aspect of a 1980s theoretical ideal, and Spielman says that the same cold atom platform could also be used to study condensed matter effects that appear at the boundaries of materials in different phases, like superconductors and semiconductors. Moving forward, he notes, it might be possible to create novel geometric and topological charge pumps with cutting-edge condensed matter systems, such as topological superconductors.

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  • Measuring the magnetization of wandering spins
  • March 30, 2016 Activity 1

The swirling field of a magnet—rendered visible by a sprinkling of iron filings—emerges from the microscopic behavior of atoms and their electrons. In permanent magnets, neighboring atoms align and lock into place to create inseparable north and south poles. For other materials, magnetism can be induced by a field strong enough to coax atoms into alignment.

In both cases, atoms are typically arranged in the rigid structure of a solid, glued into a grid and prevented from moving. But the team of JQI Fellow Ian Spielman has been studying the magnetic properties of systems whose tiny constituents are free to roam around—a phenomenon called “itinerant magnetism." 

“When we think of magnets, we usually think of some lattice,” says graduate student Ana Valdés-Curiel. Now, in a new experiment, Valdés-Curiel and her colleagues have seen the signatures of itinerant magnetism arise in a cold cloud of rubidium atoms.

The team mapped out the magnetic properties of their atomic cloud, probing the transition between unmagnetized and magnetized phases. Using interfering lasers, the researchers dialed in magnetic fields and observed the atoms’ responses. The experiment, which was the first to directly observe magnetic properties that result from the particles’ motion, was reported March 30 in Nature Communications.

Physicists often study phase transitions, which illuminate the large-scale consequences of microscopic behavior. For instance, liquid water looks very different after it’s frozen or boiled—a result of how temperature effects the motion of atoms. Similarly, permanent magnets lose their magnetic properties when they heat up; the energy that atoms draw from their hot surroundings can overwhelm the bonds that keep their tiny magnetic poles aligned.

To explore magnetism in a cloud of rubidium, the JQI researchers first cooled their atoms down. Because the atoms were so cold, they inhabited only three low-lying quantum energy levels, or states. These states, labeled by a quantum property called spin, interacted differently with magnetic fields. Two of the three states tried to align with or against an applied field, while the third completely ignored it.

The team illuminated the cloud with two interfering lasers—similar to previous experiments in which neutral atoms were made to act like they had a charge—and created an effective magnetic field in the twisting shape of a helix. They then varied the intensity and frequency of the lasers and observed how the atoms responded. After tuning their lasers and allowing the atoms to settle into stable states, the researchers let them fall and observed how the different spin states separated. This allowed them to measure the fraction of atoms that were in magnetic states, a signature of how magnetized the cloud had become.

Three distinct phases emerged, corresponding to different settings for the two laser parameters. When one laser’s frequency was shifted higher and both lasers had relatively low intensities, the atoms sat in their non-magnetic state, unperturbed by the fields. As the frequency shift was turned down and eventually flipped—so that the second laser’s frequency was higher—atoms preferred to fall into one of the two magnetic states, leading to an increase in the atoms’ motion. Atoms even grouped together by state, leading to magnetic domains similar to those that appear in ordinary magnets.

But this magnetic ordering collapsed when the laser intensity was ramped up. In that case, the atoms occupied all three states, but they didn’t bunch into aligned patches as in the magnetized case. Instead, their spins pointed along the local direction of the effective magnetic field created by the lasers.

By scanning many parameter paths, the researchers mapped the magnetic and non-magnetic phases of the rubidium cloud, and they found that the experimental results closely matched theoretical predictions about how rubidium atoms would behave.

The results here open the door to a more detailed study of the magnetic phase transitions in neutral atoms, as well as experiments that study the interactions between mobile magnetic particles. “This experiment is the first example of a magnetic system where the motion of the particles—here atoms—was essential for the magnetic physics,” Spielman says. "Our measurements of spin-orbit coupled bosons pave the way for similar experiments with fermions—mimicking electrons in materials—that might one day help to create new types of magnetic materials."

Infographic credit: S. Kelley/JQI

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  • Rogue rubidium leads to atomic anomaly
  • Unexpected high-energy atoms illuminate the physics of potential quantum processors
  • March 16, 2016

The behavior of a few rubidium atoms in a cloud of 40,000 hardly seems important. But a handful of the tiny particles with the wrong energy may cause a cascade of effects that could impact future quantum computers.

Some proposals for quantum devices use Rydberg atoms—atoms with highly excited electrons that roam far from the nucleus—because they interact strongly with each other and offer easy handles for controlling their individual and collective behavior. Rubidium is one of the most popular elements for experimenting with Rydberg physics.

Now, a team of researchers led by JQI Fellows Trey Porto, Steven Rolston and Alexey Gorshkov have discovered an unwanted side effect of trying to manipulate strongly interacting rubidium atoms: When they used lasers to drive some of the atoms into Rydberg states, they excited a much larger fraction than expected. The creation of too many of these high-energy atoms may result from overlooked “contaminant” states and could be problematic for proposals that rely on the controlled manipulation of Rydberg atoms to create quantum computers. The new results were published online March 16 in Physical Review Letters.

“Rydberg atoms interact strongly, which is good for quantum applications,” Porto says. “However, such strong interactions with atoms in the wrong state can impact their usefulness, and our work takes a closer look at this Rydberg physics.”

Playing with atoms

Manipulating atoms is a delicate game. That’s because electrons, bound by their charge to protons in the nucleus, orbit with discrete energy levels determined by quantum physics. Electrons can ascend or descend through these levels as if on an energy ladder, but only when an atom absorbs or emits a packet of light, known as a photon, that is tuned to the energy difference between rungs.

For a single atom, the spacing between energy levels is sharply defined, and photons with slightly different energies—those that don’t quite match the gap between rungs—will excite an atom weakly or not at all. But when many atoms interact, the definite spacing gets smeared out—an example of an effect called broadening. This broadening allows a wider band of energies to excite the atoms, which is precisely the issue that the researchers ran into.

The initial discovery was an accident. “I kept changing the energy and intensity of our lasers, and I just kept seeing the same thing,” says Elizabeth Goldschmidt, a post-doctoral researcher at JQI and the first author of the paper. “You just can’t beat it.”

In an experiment to learn more about this broadening, Goldschmidt and her colleagues began by cooling a cloud of rubidium atoms and trapping them in a 3D grid using interfering beams of light. Then, they used lasers to bathe the entire cloud in photons whose energies could bridge the gap between particular low- and high-energy rungs. Even when varying the laser intensity and the density of atoms over many experiments, the researchers found that they continued to create more Rydberg atoms than expected, an indication that the atomic transition had been broadened.

A possible explanation

The team looked to causes of broadening typically encountered in the lab, such as short-range interactions between atoms or imperfections in laser beams, to explain what they were seeing. But nothing captured the magnitude of the effect.

Although the experiment did not provide direct evidence for the cause, the team suspects that a small fraction of atoms in other Rydberg levels contaminated the excitation process by interacting with clean atoms and broadening their transitions. The first few contaminants, created by stray photons in the environment, led to additional excited atoms and more contaminants, a process that quickly leads to many more excited atoms.

It’s as if the atoms are sampling different shifts to their energy levels because of the changing configuration of these unavoidable contaminants, Goldschmidt says. This causes atoms that wouldn’t otherwise get excited to absorb photons and hop up to the higher energy level, creating more Rydberg atoms.

Such broadening is a challenge for some Rydberg atom-based proposals. Many proposals call for using Rydberg atoms trapped in a lattice to create quantum computers or general purpose quantum simulators that could be programmed to mimic complex physics ordinarily too hard to study in a lab. Rydberg atoms are a favorite platform because they have strong interactions and they don’t need to be right next to each other to interact.

But the broadening discovered here prompts a closer look at these proposals. Some that don’t use Rydberg states directly, but instead use weakly excited Rydberg states to gain some of their advantages and avoid some drawbacks, could also face challenges. “Even with weak lasers that barely excite to the Rydberg state, you still get these contaminants,” Goldschmidt says. “A better understanding of this broadening is important for trying to build Rydberg-based devices for quantum information processing.”

Infographic credit: S. Kelley/JQI

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  • Characterizing quantum Hall light zooming around a photonic chip
  • February 26, 2016 Activity 2

When it comes to quantum physics, light and matter are not so different. Under certain circumstances, negatively charged electrons can fall into a coordinated dance that allows them to carry a current through a material laced with imperfections. That motion, which can only occur if electrons are confined to a two-dimensional plane, arises due to a phenomenon known as the quantum Hall effect.

Researchers, led by Mohammad Hafezi, a JQI Fellow and assistant professor in the Department of Electrical and Computer Engineering at the University of Maryland, have made the first direct measurement that characterizes this exotic physics in a photonic platform. The research was published online Feb. 22 and featured on the cover of the March 2016 issue of Nature Photonics. These techniques may be extended to more complex systems, such as one in which strong interactions and long-range quantum correlations play a role.

Symmetry and Topology

Physicists use different approaches to classify matter; symmetry is one powerful method. For instance, the microscopic structure of a material like diamond looks the same even while shifting your gaze to a new spot in the crystal. These symmetries – the rotations and translations that leave the microscopic structure the same – predict many of the physical properties of crystals.

Symmetry can actually offer a kind of protection against disruptions. Here, the word protection means that the system (e.g. a quantum state) is robust against changes that do not break the symmetry. Recently, another classification scheme based on topology has gained significant attention. Topology is a property that depends on the global arrangement of particles that make up a system rather than their microscopic details. The excitement surrounding this mathematical concept has been driven by the idea that the topology of a system can offer a stability bubble around interesting and even exotic physics, beyond that of symmetry. Physicists are interested in harnessing states protected by both symmetry and topology because quantum devices must be robust against disturbances that can interfere with their functionality.

The quantum Hall effect is best understood by peering through the lens of topology. In the 1980s, physicists discovered that electrons in some materials behave strangely when subjected to large magnetic fields at extreme cryogenic temperatures. Remarkably, the electrons at the boundary of the material will flow along avenues of travel called ‘edge states’, protected against defects that are most certainly present in the material. Moreover, the conductance--a measure of the current--is quantized. This means that when the magnetic field is ramped up, then the conductance does not change smoothly. Instead it stays flat, like a plateau, and then suddenly jumps to a new value. The plateaus occur at precise values that are independent of many of the material’s properties. This hopping behavior is a form of precise quantization and is what gives the quantum Hall effect its great utility, allowing it to provide the modern standard for calibrating resistance in electronics, for instance.

Researchers have engineered quantum Hall behavior in other platforms besides the solid-state realm in which it was originally discovered. Signatures of such physics have been spotted in ultracold atomic gases and photonics, where light travels in fabricated chips. Hafezi and colleagues have led the charge in the photonics field.

The group uses a silicon-based chip that is filled with an array of ring-shaped structures called resonators. The resonators are connected to each other via waveguides (figure). The chip design strictly determines the conditions under which light can travel along the edges rather than through the inner regions. The researchers measure the transmission spectrum, which is the fraction of light that successfully passes through an edge pathway. To circulate unimpeded through the protected edge modes, the light must possess a certain energy. The transmission increases when the light energy matches this criteria. For other parameters, the light will permeate the chip interior or get lost, causing the transmission signal to decrease. The compiled transmission spectrum looks like a set of bright stripes separated by darker regions (see figure). Using such a chip, this group previously collected images of light traveling in edge states, definitively demonstrating the quantum Hall physics for photons.

In this new experiment Hafezi’s team modified their design to directly measure the value of the topology-related property that characterizes the photonic edge states. This measurement is analogous to characterizing the quantized conductance, which was critical to understanding the electron quantum Hall effect. In photonics, however, conductance is not relevant as it pertains to electron-like behavior. Here the significant feature is the winding number, which is related to how light circulates around the chip. Its value equals to the number of available edge states and should not change in the face of certain disruptions.

To extract the winding number, the team adds 100 nanometer titanium heaters on a layer above the waveguides. Heat changes the index of refraction, namely how the light bends as it passes through the waveguides. In this manner, researchers can controllably imprint a phase shift onto the light. Phase can be thought of in terms of a time delay. For instance, when comparing two light waves, the intensity can be the same, but one wave may be shifted in time compared to the other. The two waves overlap when one wave is delayed by a full oscillation cycle—this is called a 2π phase shift.

On the chip, enough heat is added to add a 2π phase shift to the light. The researchers observe an energy shift in the transmission stripes corresponding to light traveling along the edge. Notably, in this chip design, the light can circulate either clockwise (CW) or counterclockwise (CCW), and the two travel pathways do not behave the same (in contrast to an interferometer). When the phase shift is introduced, the CW traveling light hops one direction in the transmission spectrum, and the CCW goes the opposite way. The winding number is the amount that these edge-state spectral features move and is exactly equivalent to the quantized jumps in the electronic conductance.

Sunil Mittal, lead author and postdoctoral researcher explains one future direction, “So far, our research has been focused on transporting classical [non-quantum] properties of light--mainly the power transmission. It is intriguing to further investigate if this topological system can also achieve robust transport of quantum information, which will have potential applications for on-chip quantum information processing.” 

This text was written by E. Edwards/JQI

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  • Jay Deep Sau Receives Sloan Research Fellowship
  • February 23, 2016

Jay Deep Sau, an assistant professor of physics at the University of Maryland and fellow of the Joint Quantum Institute, was awarded a Sloan Research Fellowship for 2016. This award, granted by the Alfred P. Sloan Foundation, identifies 126 early-career scientists based on their potential to contribute fundamentally significant research to a wider academic community.

Sau, a theoretical condensed matter physicist interested in applying topological principles to create protected solid-state and cold-atomic systems for quantum information processing, will use the fellowship to further his research focus on predicting phenomena that could help pave the way for topological quantum computation.

“Receiving the Sloan Research fellowship, to me, represents validation of my work from some rather distinguished members of the condensed matter physics community and is therefore a great honor,” said Sau. “This fellowship encourages me to continue my pursuit to predict truly macroscopic quantum systems and phenomena, in collaboration with experimental colleagues at Maryland who elucidate the beautiful physics of topological field theory.”

While quantum mechanics naturally operates at excruciatingly tiny length scales—such as those found in a single atom—physicists are also interested in examining much larger quantum systems where the individual quantum pieces can interact through many pathways. In this case, stabilizing the associated quantum phenomena can be exceedingly difficult due to the detrimental influence of the unavoidable interaction of the large system with its surroundings. One possible approach to creating and studying such macroscopic quantum phenomena is based on recently discovered topological phases in condensed matter systems, which for fundamental reasons are effectively protected from the environment.

Sau’s research aims to investigate the rich variety of static and dynamical phenomena that arise from the interplay of novel topological phases with conventional physics, such as electrostatic interactions, crystal lattice vibrations and material impurities. Recent experiments indicate that the physics of topological systems cannot be understood without considering these conventional ingredients. In addition, exploring the physics resulting from this interplay will likely lead to the discovery of new phenomena, which could influence the design of quantum computers.

Sau has authored more than 75 peer-reviewed journal publications. Before joining the UMD faculty in 2013, Sau worked as a postdoctoral researcher in physics at Harvard University and UMD, where he did some of his most important work. He earned his bachelor’s degree in electrical engineering from the Indian Institute of Technology in Kanpur, India, and his doctoral degree in physics from the University of California, Berkeley.

Sau joins the list of 49 current UMD College of Computer, Mathematical, and Natural Sciences faculty members who have received Sloan Research Fellowships.

Each 2016 Sloan Research Fellow is awarded a two-year $55,000 grant to support his or her research interests. Administered and funded by the Sloan Foundation, the fellowships are awarded in eight scientific fields—chemistry, computer science, economics, mathematics, computational and evolutionary molecular biology, neuroscience, ocean sciences, and physics. Winners are selected through close cooperation with the scientific community. To qualify, candidates must first be nominated by their fellow research scientists and are subsequently selected by an independent panel of senior scholars.

“Getting early-career support can be a make-or-break moment for a young scholar,” said Paul L. Joskow, president of the Alfred P. Sloan Foundation.  “In an increasingly competitive academic environment, it can be difficult to stand out, even when your work is first rate.  The Sloan Research Fellowships have become an unmistakable marker of quality among researchers.  Fellows represent the best-of-the-best among young scientists.” 

Courtesy of CMNS

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  • Nanoscale cavity strongly links quantum particles
  • Single photons can quickly modify individual electrons embedded in a semiconductor chip and vice versa
  • February 8, 2016 Activity 2

Scientists have created a crystal structure that boosts the interaction between tiny bursts of light and individual electrons, an advance that could be a significant step toward establishing quantum networks in the future.

Today’s networks use electronic circuits to store information and optical fibers to carry it, and quantum networks may benefit from a similar framework. Such networks would transmit qubits – quantum versions of ordinary bits – from place to place and would offer unbreakable security for the transmitted information. But researchers must first develop ways for qubits that are better at storing information to interact with individual packets of light called photons that are better at transporting it, a task achieved in conventional networks by electro-optic modulators that use electronic signals to modulate properties of light.

Now, researchers in the group of Edo Waks, a fellow at JQI and an Associate Professor in the Department of Electrical and Computer Engineering at the University of Maryland, have struck upon an interface between photons and single electrons that makes progress toward such a device. By pinning a photon and an electron together in a small space, the electron can quickly change the quantum properties of the photon and vice versa. The research was reported online Feb. 8 in the journal Nature Nanotechnology.

“Our platform has two major advantages over previous work,” says Shuo Sun, a graduate student at JQI and the first author of the paper. “The first is that the electronic qubit is integrated on a chip, which makes the approach very scalable. The second is that the interactions between light and matter are fast. They happen in only a trillionth of a second – 1,000 times faster than previous studies.”


The new interface utilizes a well-studied structure known as a photonic crystal to guide and trap light. These crystals are built from microscopic assemblies of thin semiconductor layers and a grid of carefully drilled holes. By choosing the size and location of the holes, researchers can control the properties of the light traveling through the crystal, even creating a small cavity where photons can get trapped and bounce around.

”These photonic crystals can concentrate light in an extremely small volume, allowing devices to operate at the fundamental quantum limit where a single photon can make a big difference,” says Waks.

The results also rely on previous studies of how small, engineered nanocrystals called quantum dots can manipulate light. These tiny regions behave as artificial atoms and can also trap electrons in a tight space. Prior work from the JQI group showed that quantum dots could alter the properties of many photons and rapidly switch the direction of a beam of light.

The new experiment combines the light-trapping of photonic crystals with the electron-trapping of quantum dots. The group used a photonic crystal punctuated by holes just 72 nanometers wide, but left three holes undrilled in one region of the crystal. This created a defect in the regular grid of holes that acted like a cavity, and only those photons with only a certain energy could enter and leave.

Inside this cavity, embedded in layers of semiconductors, a quantum dot held one electron. The spin of that electron – a quantum property of the particle that is analogous to the motion of a spinning top – controlled what happened to photons injected into the cavity by a laser. If the spin pointed up, a photon entered the cavity and left it unchanged. But when the spin pointed down, any photon that entered the cavity came out with a reversed polarization – the direction that light’s electric field points. The interaction worked the opposite way, too: A single photon prepared with a certain polarization could flip the electron’s spin.

Both processes are examples of quantum switches, which modify the qubits stored by the electron and photon in a controlled way. Such switches will be the coin of the realm for proposed future quantum computers and quantum networks.


Those networks could take advantage of the strengths that photons and electrons offer as qubits. In the future, for instance, electrons could be used to store and process quantum information at one location, while photons could shuttle that information between different parts of the network.

Such links could enable the distribution of entanglement, the enigmatic connection that groups of distantly separated qubits can share. And that entanglement could enable other tasks, such as performing distributed quantum computations, teleporting qubits over great distances or establishing secret keys that two parties could use to communicate securely.

Before that, though, Sun says that the light-matter interface that he and his colleagues have created must create entanglement between the electron and photon qubits, a process that will require more accurate measurements to definitively demonstrate.

“The ultimate goal will be integrating photon creation and routing onto the chip itself,” Sun says. “In that manner we might be able to create more complicated quantum devices and quantum circuits.”

In addition to Waks and Sun, the paper has two additional co-authors: Glenn Solomon, a JQI fellow, and Hyochul Kim, a post-doctoral researcher in the Department of Electrical and Computer Engineering at the University of Maryland.

"Creating a quantum switch" credit: S. Kelley/JQI

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  • Beating the heat
  • Ultrafast sensing and quantum control
  • January 6, 2016

Harnessing quantum systems for information processing will require controlling large numbers of basic building blocks called qubits. The qubits must be isolated, and in most cases cooled such that, among other things, errors in qubit operations do not overwhelm the system, rendering it useless. Led by JQI Fellow Christopher Monroe, physicists have recently demonstrated important steps towards implementing a proposed type of gate, which does not rely on super-cooling their ion qubits. This work, published as an Editor’s Suggestion in Physical Review Letters, implements ultrafast sensing and control of an ion's motion, which is required to realize these hot gates. Notably, this experiment demonstrates thermometry over an unprecedented range of temperatures--from zero-point to room temperature.

Graduate student and first author Kale Johnson explains how this research could be applied, “Atomic clock states found in ions make the most pristine quantum bits, but the speed at which we have been able to access them in a useful way for quantum information processing is slower than it could be. We are changing that by making each operation on the qubit faster while eliminating the need to cool the ion to the ground state after each operation.”

In the experiment the team begins with a single trapped atomic ion. The ion can be thought of as a bar magnet that can be oriented with its north pole ‘up’ or ‘down’ or any combination between the two poles (pointing horizontally along an imaginary equator is up + down).  Physicists can use lasers and microwave radiation to control this orientation. The individual laser pulses are a mere ten picoseconds in length—a time scale that is a tiny fraction of how long it takes for the ion to undergo appreciable motion in the trap. Operating in this regime is precisely what allows researchers to have superior sensing and ultimately control over the ion motion. The speed enables the team to extract the motional behavior of an ion using a technique that works independently of the energy in the motion itself.  In other words, the measurement is equally sensitive to a fast or very slow atom.

The researchers use a method that is based on Ramsey interferometry, named for the Nobel Laureate Norman Ramsey who pioneered it back in 1949. Known then as his “method of separated oscillatory fields,” it is used throughout atomic physics and quantum information science.   

Laser pulses are carefully divided and then reunited to achieve control over the ion’s spin and motion. The researchers call these laser-ion interactions ‘spin-dependent kicks’ (SDK) because each series of specially tailored laser pulses flips the spin, while simultaneously giving the ion a push (this is depicted in the illustration below). With each fast kick, the atom’s quantum wave packet is split into two parts in under three nanoseconds. Those halves are then re-combined at different points in space and time, and the signal from the unique overlap pattern reveals how the population is distributed between the two spin states. In this experimental sequence, that distribution depends on parameters such as the number of SDKs, the time between kicks, and the initial position and speed of the ion. The team repeats this experiment to extract the average motion of the ion, or its effective temperature.


In order to realize proposed two-ion quantum gates that do not require cooling the system into its quantum mechanical ground state, multiple spin dependent kicks must be employed with high accuracy such that errors remain manageable. Here the team was able to clearly demonstrate the necessary high-quality spin dependent kicks. More broadly, this protocol shows that adding ultrafast pulsed laser technology to the ion-trapping toolbox gives physicists ultimate quantum control over what can be a limiting, noise-inducing parameter: the motion.

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  • Controlling the Thermodynamics of Light
  • The concept of chemical potential can apply to light
  • December 17, 2015

The concept of temperature is critical in describing many physical phenomena, such as the transition from one phase of matter to another.  Turn the temperature knob and interesting things can happen.  But other knobs might be just as important for studying some phenomena.  One such knob is chemical potential, a thermodynamic parameter first introduced in the nineteenth century by scientists for keeping track of potential energy absorbed or emitted by a system during chemical reactions.

In these reactions different atomic species rearranged themselves into new configuration while conserving the overall inventory of atoms.  That is, atoms could change their partners but the total number of identity of the atoms remained invariant. 

Chemical potential is just one of many examples of how flows can be described.  An imbalance in temperature results in a flow of energy.  An imbalance in electrical potential results in a flow of charged particles.  Meanwhile, an imbalance in chemical potential results in a flow of particles; and specifically an imbalance in chemical potential for light would result in a flow of photons.

Can the concept of chemical light apply to light?  At first the answer would seem to be no since particles of light, photons, are regularly absorbed when then they interact with regular matter.  The number of photons present is not preserved.  But recent experiments have shown that under special conditions photon number can be conserved, clearing the way for the use of chemical potential for light. 

Now three JQI scientists offer a more generalized theoretical description of chemical potential (usually denoted by the Greek letter mu) for light and show how mu can be controlled and applied in a number of physics research areas.

A prominent experimental demonstration of chemical potential for light took place at the University of Bonn (*) in 2010.  It consisted of quanta of light (photons) bouncing back and forth inside a reflective cavity filled with dye molecules.  The dye molecules, acting as a tunable energy bath (a parametric bath), would regularly absorb photons (seemingly ruling out the idea of photon number being conserved) but would re-emit the light.  Gradually the light warmed the molecules and the molecules cooled the light until they were all at thermal equilibrium.  This was the first time photons had been successfully “thermalized” in this way.  Furthermore, at still colder temperatures the photons collapsed into a single quantum state; this was the first photonic Bose-Einstein condensate (BEC).

In a paper published in the journal Physical Review B the JQI theorists describe a generic approach to chemical potential for light. They illustrate their ideas by showing how a chemical-potential protocol can be implemented a microcircuit array. Instead of crisscrossing a single cavity, the photons are set loose in an array of microwave transmission lines. And instead of interacting with a bath of dye molecules, the photons here interact with a network of tuned circuits.

“One likely benefit in using chemical potential as a controllable parameter will be carrying out quantum simulations of actual condensed-matter systems,” said Jacob Taylor, one of the JQI theorists taking part in the new study.  In what some call a prototype for future full-scale quantum computing, quantum simulations use tuned interactions in a small microcircuit setup to arrive at a numerical solution to calculations that (in their complexity) would defeat a normal digital computer.

In the scheme described above, for instance, the photons, carefully put in a superposition of spin states, could serve as qubits. The qubits can be programmed to perform special simulations. The circuits, including the transmission lines, act as the coupling mechanism whereby photons can be respectively up- or down-converted to lower or higher energy by obtaining energy from or giving energy to excitations of the circuits.

(*) J. Klaers, J. Schmitt, F. Vewinger, and M. Weitz, Nature 468, 545 (2010)

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  • Shaking Bosons into Fermions
  • December 3, 2015 Activity 1

Particles can be classified as bosons or fermions. A defining characteristic of a boson is its ability to pile into a single quantum state with other bosons. Fermions are not allowed to do this. One broad impact of fermionic anti-social behavior is that it allows for carbon-based life forms, like us, to exist. If the universe were solely made from bosons, life would certainly not look like it does. Recently, JQI theorists* have proposed an elegant method for achieving transmutation--that is, making bosons act like fermions. This work was published in the journal Physical Review Letters.

This transmutation is an example of emergent behavior, specifically what’s known as quasiparticle excitations—one of the concepts that make condensed matter systems so interesting. Particles by themselves have mostly well-defined characteristics, but en masse, can work together such that completely distinctive, even exotic phenomena appear. Typically collective behaviors are difficult to study because the large numbers of real particles and all of their interactions are computationally challenging and in many cases prohibitive. JQI Fellow Victor Galitski explains, “The whole idea of emergent excitations is that the quasiparticles are fundamentally different from the actual individual particles. But this actually doesn’t happen that often.” In this case, it turns out that the boson-to-fermion transmutation leads to an interesting phase of matter. Galitski continues, “ Here, the bosons don’t condense--they instead form a state without long-range order. This is an example of a long sought after state of matter called a Bose liquid.

In this research, the authors propose a method for realizing and observing such unusual excitations--here the fermionic quasiparticles. The experiment harnesses the strengths of atom-optical systems, such as using bosons (which are easier to work with), a relatively simple lattice geometry (made from lasers that are the workhorses of atomic physics), and established measurement techniques. Galitski continues, “In some sense this was motivated by an experiment where researchers shook a one-dimensional lattice, and it appears that the experiment we propose here is not beyond the capabilities of current work.”

Here, the central technique also involves taking an optical lattice made from laser light and shaking it back-and-forth. An atom-optical lattice system, analogous to a crystal, has a periodic structure. Laser beams criss-cross to form standing waves of light that resemble an egg carton. Atoms interact with the light such that they are drawn to the valleys of the egg carton.  Like a true solid, this system has an accompanying band-structure, which describes the allowed energies that atoms within the lattice can take on. Without the lattice present, trapped ultracold bosons form a state of matter called a Bose-Einstein condensate. Not much changes when a typical optical lattice is turned on--the bosons will still collect into the lowest energy state and still be in this condensate form. For the simplest lattice configurations, this state corresponds to a single point on a nearly-flattened parabola in the band structure. This configuration is actually the starting point of many atomic physics experiments. Physicists are interested in modifying the energy bands to perhaps uncover more complex phases of matter. To do this, lattice properties must be altered.

In this work, the authors seek to achieve transmutation, and are among those that have previously shown that one way to accomplish this is to construct a lattice whose band structure looks like a moat. (The word moat here means what it did in medieval times--a trench around a structure.)

Lead author and postdoctoral researcher Tigran Sedrakyan explains the significance of the moat, "The moat is instrumental in achieving this statistical transmutation because it appears that the fermions in a moat-band may actually have lower energy than condensed bosons have, enforcing the constituent bosons to transmute."

It turns out that getting the requisite moat to appear has not been so easy. Surprisingly, in this new work, the team found that if, instead of modifying the lattice geometry itself, they take a simple two-dimensional lattice and shake it back and forth, then a moat appears in what was otherwise an unremarkable, almost flat band structure. The rate of shaking is specially chosen such that the bands undergo this transformation.

The particles themselves do not actually change from bosons to fermions. What’s happening is that the environment of the lattice is modifying the bosonic behavior. When the lattice is quivering periodically at a specially determined frequency, the bosons act as if they are governed by fermionic statistics. In the new band structure, the bosons do not form a condensate.

*Research affiliations: Victor Galitski is a JQI Fellow and also a member of UMD’s Condensed Matter Theory Center (CMTC). Tigran Sedrakyan is a postdoctoral researcher split between the Physics Frontier Center at JQI and the University of Minnesota. Alex Kamenev is also at University of Minnesota.

Written by E. Edwards/JQI

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  • Quantum Insulation
  • Intemperate atoms can't come to equilibrium
  • November 25, 2015

Two physical phenomena, localization and ergodicity-breaking, are conjoined in new experimental and theoretical work.  Before we consider possible implications for fundamental physics and for prospective quantum computing, let’s first look at these two topics in turn.  It will bear providing some specific examples before getting to the quantum details.


When electrons pass through a material they encounter various degrees of resistance, causing them to lose energy along their journey.  In the 1950s physicist Philip Anderson, predicted that in some disordered materials (such as a semiconductors) electrons---or more specifically the electrons viewed as a series of quantum waves---could get trapped.  They become immobilized not by losing all their energy but by an interference effect by which  the waves become bottled up in a certain region.

This assertion, later demonstrated in experiments, is at odds with conventional thermodynamics.  Electrons, at one temperature (in effect) entering a material at a different energy, ought to “thermalize,” that is, come to a common temperature.  But localization seems to sidestep this: the electrons waves remain intact but segregated.  They don’t come to the temperature of their surroundings.

Many-body localization (MBL) has become a hot topic in physics.  In 2006 only three journal articles mentioned MBL; in 2015 the number was 190.  In November 2015 the Kavli Institute for Theoretical Physics held a special meeting devoted to the subject (*)


The term ergodic dates back to the nineteenth century and was coined by Ludwig Boltzmann to describe statistically how a system of particles evolves over time.

Throw a thousand identical dice and record the numerical results.  Then throw a single similar die a thousand times.  The average showing should be very similar.  This is an example of an ergodic system.  One hallmark is that space and time averages of the system should be similar.  The average die values for the “dice system” taken singly over a long time or with multiple dice at one instant.

Open the stopper of a perfume bottle in a closed room and come back after a long time.  There will be an equal likelihood of a perfume molecule being in all the parts of the room.  This is another ergodic example.  A more technical way of saying this is that the total description of the ensemble of molecules explores all possible configurations of the molecules.  “Anything that can happen will happen.”  One possible state of the system includes the chance that all the molecules will return to the bottle whence than had come.  But since there are trillions of other configurations where this does not happen in practice our observation is of molecules all around the room.  At the end we have no sense, sampling the molecules, that they were once all in the bottle.  The system no longer remembers its origin.

What about non-ergodic systems?  Consider one person sitting in a restaurant selecting from a menu of items.  She visits the restaurant 100 times.  Compare her choices to those of one hundred people at one time ordering menu items.  Here the average statistics for ordered items will be very different?  Why?  Because humans are more choosey than dice.


In experiments conducted at the Max Planck Institute in Garching, Germany and at the Joint Quantum Institute at the University of Maryland in the U.S., confined atoms displayed localization and behavior that was non-ergodic.  In the Max Planck work, neutral atoms are stored in an optical lattice; in the JQI setup, a string of ions is stored.  Instead of electrons moving through a solid material, the atoms, each with its own characteristic spin orientation, reside in a laser-driven crate environment.   Here the disorder (imposed upon the confining laser beams) imposes localization   In the German experiment, particles (the atoms) are localized. In the U.S. experiment, it is the spins of ions that are localized.

To be more specific about the JQI experiment: special modulated laser beams  introduce disorder into the system of ions.  Instead of the spins all interacting witheach other, thereby losing  their original collective spin configuration, the disorder has the effect of localizing the spins in their abstract spin “space.”  Without the disorder localization does not occur.  When the disorder climbs above a critical value, localization does occur; the atoms do not mix up their spins; they do not “thermalize.  “They are stuck near to their initial spin configuration,” says Jacob Smith, one of the JQI experimenters.  The atomic spins retain a sense of their origin.  They are behaving non-ergodically.


So, do localization and non-ergodicity go together?  Not necessarily says a new report by four JQI theorists published in Physical Review Letters. 

Xiaopeng Li, the lead author on the new theory paper, commented on this bizarre behavior where particles could be de-localized (they keep moving; they are not confined) and yet be non-ergodic in nature---which is say that they do not thermalize.  “Our theory points to a possible physical picture that some particles are inert but others are active. An analogue for the case of dice would be if even numbers were equally likely but odd ones were forbidden. This exotic phase of matter provides one scenario for the localization transition of a quantum system.”

And since thermalization is one of the leading causes of quantum decoherence, exploiting non-ergodic systems---whether the constituent particles were localized or extended---might help in the storage of quantum information.   Non-ergodic systems might not be implemented in the form of conventional solid matter, but might be possible in the form of trapped atoms, as the experiments mentioned above indicated.   

Sankar das Sarma, the leader of team of JQI theorists working on this problem, describes non-ergodic in terms of temperature.  “We take it for granted that all systems left to themselves attain a temperature; that is, they achieve thermodynamic equilibrium.  But is this always true?  In the simplest term, ergodicity assures (almost always) the achievement of a temperature.  Non-ergodic systems are not in thermal equilibrium---ever!---and cannot be characterized by a temperature.  Isolated localized systems are always non-ergodic since there is no way to transport energy from one point to another to achieve equilibrium.”

That a body of particles could be un-localized and also non-ergodic at the same time came as a surprise to the theorists, who modeled the interactions among the particles using extensive computer simulations.  “We have to be cautious,” said das Sarma.  “I believe our results are correct for what we do, but whether it applies in the thermodynamic limit of a macroscopic system is still an open question of great interest.  But it might contribute to the effort to fight against intrinsic decoherence.  It could help create quantum insulating systems---heat insulators.”   


The Joint Quantum Institute is operated jointly by the National Institute of Standards and Technology in Gaithersburg, MD and the University of Maryland in College Park. 

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  • Sylvain Ravets awarded DIM Nano-K thesis prize
  • October 30, 2015

Sylvain Ravets has recently been awarded the DIM Nano-K prize for his thesis “Development of tools for quantum engineering using individual atoms: optical nanofibers and controlled Rydberg interactions.” Awarded annually by C’Nano IdF (a French organization promoting nanoscience research), the prize recognizes him for “research at the interface between nanosciences and cold atoms.” The DIM Nano-K gathers IFRAF (Île-de-France Cold Atom Research Institute) and C’Nano IdF (centre of competences in nanosciences for the Ile-de-France region) under the label of “Domain of Major Interest” (DIM) of the Paris Region.

While a student at the Institut d’Optique in the group of Antoine Browaeys, Ravets received fellowships from the Fulbright and the Monahan foundations to study at the JQI from 2011 to 2013. At the JQI he worked in Luis Orozco's group with Jonathan Hoffman on the manufacture and characterization of optical nanofibers for the “Atoms on SQUIDS” project. On returning to the Institut d’Optique, Ravets worked on performing quantum simulations with excited Rydberg states. His thesis discusses the work done at both institutions.

Ravets is currently a postdoc in the group of Atac Imamoglu at ETH Zürich, where he studies two-dimensional electron systems in optical cavities.

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  • At the edge of a quantum gas
  • JQI physicists observe skipping orbits in the quantum Hall regime
  • September 29, 2015 Activity 1

From NIST-PML--JQI scientists have achieved a major milestone in simulating the dynamics of condensed-matter systems – such as the behavior of charged particles in semiconductors and other materials – through manipulation of carefully controlled quantum-mechanical models.

Going beyond their pioneering experiments in 2009 (the creation of “artificial magnetism”), the team has created a model system in which electrically neutral atoms are coaxed into performing just as electrons arrayed in a two-dimensional sheet do when they are exposed to a strong magnetic field.

The scientists then showed for the first time that it is possible to tune the model system such that the atoms (acting as electron surrogates) replicate the signature “edge state” behavior of real electrons in the quantum Hall effect (QHE), a phenomenon which forms the basis for the international standard of electrical resistance.* The researchers report their work in the 25 September issue of the journal Science.

“This whole line of research enables experiments in which charge-neutral particles behave as if they were charged particles in a magnetic field,” said JQI Fellow Ian Spielman, who heads the research team at NIST’s Physical Measurement Laboratory.

“To deepen our understanding of many-body physics or condensed-matter-like physics – where the electron has charge and many important phenomena depend on that charge – we explore experimental systems in which the components have no electrical charge, but act in ways that are functionally equivalent and can be described by the same equations,” Spielman said.

Such quantum simulators are increasingly important, as electronic components and related devices shrink to the point where their operation grows increasingly dependent on quantum-mechanical effects, and as researchers struggle to understand the fundamental physics of how charges travel through atomic lattices in superconductors or in materials of interest for eventual quantum information processing.

Quantum effects are extremely difficult to investigate at the fundamental level in conventional materials. Not only is it hard to control the numerous variables involved, but there are inevitably defects and irregularities in even carefully prepared experimental samples. Quantum simulators, however, offer precise control of system variables and yield insights into the performance of real materials as well as revealing new kinds of interacting quantum-mechanical systems.

“What we want to do is to realize systems that cannot be realized in a condensed-matter setting,” Spielman said. “There are potentially really interesting, many-body physical systems that can’t be deployed in other settings.”

To do so, the scientists created a Bose-Einstein condensate (BEC, in which ultracold atoms join in a single uniform, low-energy quantum state) of a few hundred thousand rubidium atoms and used two intersecting lasers to arrange the atoms into a lattice pattern. 

Then a second pair of lasers, each set to a slightly different wavelength, was trained on the lattice, creating "artificial magnetism" — that is, causing the electrically neutral atoms to mimic negatively charged electrons in a real applied magnetic field.

Depending on the tuning of the laser beams, the atoms were placed in one of three different quantum states representing electrons that were either in the middle of, or at opposite edges of, a two-dimensional lattice.

By adjusting the properties of the laser beams, the team produced dynamics characteristic of real materials exhibiting the QHE. Specifically, as would be expected of electrons, atoms in the bulk interior of the lattice behaved like insulators. But those at the lattice edges exhibited a distinctive "skipping"motion.

In a real QHE system, each individual electron responds to an applied magnetic field by revolving in a circular (cyclotron) orbit. In atoms near the center of the material, electrons complete their orbits uninterrupted. That blocks conduction in the system’s interior, making it a “bulk insulator.” But at the edges of a QHE system, the electrons can only complete part of an orbit before hitting the edge (which acts like a hard wall) and reflecting off. This causes electrons to skip along the edges, carrying current.

Remarkably, the simulation's electrically neutral rubidium atoms behaved in exactly the same way: localized edge states formed in the atomic lattice and atoms skipped along the edge. Moreover, the researchers showed that by tuning the laser beams – that is, modifying the artificial magnetic field – they could precisely control whether the largest concentration of atoms was on one edge, the opposite edge, or in the center of the lattice.

“Generating these sorts of dynamical effects was beyond our abilities back in 2009, when we published our first paper on artificial magnetism,” Spielman said. “The field strength turned out to be too weak. In the new work, we were able to approach the high-field limit, which greatly expands the range of effects that are possible to engineer new kinds of interactions relevant to condensed-matter physics.”

* The Hall effect occurs when a current traveling in a two-dimensional plane moves through a magnetic field applied perpendicular to the plane. As electrons interact with the field, each begins to revolve in a circular (cyclotron) orbit. That collective motion causes them to migrate and cluster on one edge of the plane, creating a concentration of negative charge. As a result, a voltage forms across the conductor, with an associated resistance from edge to edge.

Much closer, detailed examination reveals the quantum Hall effect (QHE): The resistance is exactly quantized across the plane; that is, it occurs only at specific discrete allowed values which are known to extreme precision. That precision makes QHE the international standard for resistance.

An additional distinctive property of QHE systems is that they are “bulk insulators” that allow current to travel only along their edges.

This article was written by C. Suplee. Modifications to the original article were made, with permission, by E. Edwards. Animation made be S. Kelley/PML/JQI.

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  • Beyond Majorana: Ultracold gases as a platform for observing exotic robust quantum states
  • September 17, 2015 Activity 1

The quantum Hall effect, discovered in the early 1980s, is a phenomenon that was observed in a two-dimensional gas of electrons existing at the interface between two semiconductor layers. Subject to the severe criteria of very high material purity and very low temperatures, the electrons, when under the influence of a large magnetic field, will organize themselves into an ensemble state featuring remarkable properties.

Many physicists believe that quantum Hall physics is not unique to electrons, and thus it should be possible to observe this behavior elsewhere, such as in a collection of trapped ultracold atoms. Experiments at JQI and elsewhere are being planned to do just that. On the theoretical front, scientists* at JQI and University of Maryland have also made progress, which they describe in the journal Physical Review Letters. The result, to be summarized here, proposes using quantum matter made from a neutral atomic gas, instead of electrons. In this new design, elusive exotic states that are predicted to occur in certain quantum Hall systems should emerge. These states, known as parafermionic zero modes, may be useful in building robust quantum gates.

For electrons, the hallmark of quantum Hall behavior includes a free circulation of electrical charge around the edge but not in the interior of the sample. This research specifically relates to utilizing the fractional quantum Hall (FQH) effect, which is a many-body phenomenon. In this case, one should not consider just the movement of individual electrons, but rather imagine the collective action of all the electrons into particle-like “quasiparticles.” These entities appear to possess fractional charge, such as 1/3.

How does this relate to zero modes? Zero modes, as an attribute of quantum Hall systems, come into their own in the vicinity of specially tailored defects. Defects are where quasiparticles can be trapped. In previous works, physicists proposed that a superconducting nanowire serve as a defect that snags quasiparticles at either end of the wire. Perhaps the best-known example of a composite particle associated with zero-mode defects is the famed Majorana fermion.

Author David Clarke, a Postdoctoral Research Scholar from the UMD Condensed Matter Theory Center, explains, “Zero modes aren’t particles in the usual sense. They’re not even quasiparticles, but rather a place that a quasiparticle can go and not cost any energy.”

Aside from interest in them for studying fundamental physics, these zero modes might play an important role in quantum computing. This is related to what’s known as topology, which is a sort of global property that can allow for collective phenomena, such as the current of charge around the edge of the sample, to be impervious to the tiny microscopic details of a system. Here the topology endows the FQH system with multiple quantum states with exactly the same energy. The exactness and imperturbability of the energy amid imperfections in the environment makes the FQH system potentially useful for hosting quantum bits. The present report proposes a practical way to harness this predicted topological feature of the FQH system through the appearance of what are known as parafermionic zero-modes.

These strange and wonderful states, which in some ways go beyond Majoranas, first appeared on the scene only a few years ago, and have attracted significant attention. Now dubbed ‘parafermions,’ they were first proposed by Maissam Barkeshli and colleagues at Stanford University. Barkeshli is currently a postdoctoral researcher at Microsoft Station Q and will be coming soon to JQI as a Fellow. Author David Clarke was one of the early pioneers in studying how these states could emerge in a superconducting environment. Because both parafermions and Majoranas are expected to have unconventional behaviors when compared to the typical particles used as qubits, unambiguously observing and controlling them is an important research topic that spans different physics disciplines. From an application standpoint, parafermions are predicted to offer more versatility than Majorana modes when constructing quantum computing resources.

What this team does, for the first time, is to describe in detail how a parafermionic mode could be produced in a gas of cold bosonic atoms. Here the parafermion would appear at both ends of a one-dimensional trench of Bose-Einstein Condensate (BEC) atoms sitting amid a larger two-dimensional formation of cold atoms displaying FQH properties. According to first author and Postdoctoral Researcher Mohammad Maghrebi, “The BEC trench is the defect that does for atoms what the superconducting nanowire did for electrons.”

Some things are different for electrons and neutral atoms. For one thing, electrons undergo the FQH effect only if exposed to high magnetic fields. Neutral atoms have no charge and thus do not react strongly to magnetic fields; researchers must mimic this behavior by exposing the atoms to carefully designed laser pulses, which create a synthetic field environment. JQI Fellow Ian Spielman has led this area of experimental research and is currently performing atom-based studies of quantum Hall physics.

Another author of the PRL piece, JQI Fellow Alexey Gorshkov, explains how the new research paper came about: “Motivated by recent advances in Spielman's lab and (more recently) in other cold atom labs in generating synthetic fields for ultracold neutral atoms, we show how to implement in a cold-atom system the same exotic parafermionic zero modes proposed originally in the context of condensed-matter systems.”

“We argue that these zero modes, while arguably quite difficult to observe in the condensed matter context, can be observed quite naturally in atomic experiments,” says Maghrebi. “The JQI atmosphere of close collaboration and cross-fertilization between atomic physics and condensed matter physics on the one hand and between theory and experiment on the other hand was at the heart of this work.”

“Ultracold atoms play by a slightly different set of rules from the solid state,” says JQI Fellow Jay Sau. Things which come easy in one are hard in the other. Figuring out the twists in getting a solid state idea to work for cold atoms is always fun and the JQI is one of the best places to do it.”

(*)  The PRL paper has five authors, and their affiliations illustrate the complexity of modern physics work.  Mohammad Maghrebi, Sriram Ganeshan, Alexey Gorshkov, and Jay Sau are associated with the Joint Quantum Institute, operated by the University of Maryland and the National Institute for Standards and Technology.  Three of the authors---Ganeshan, Clarke, and Sau---are also associated with the Condensed Matter Theory Center at the University of Maryland physics department.  Finally, Maghrebi and Gorshkov are associated with the Joint Center for Quantum Information and Computer Science (usually abbreviated QuICS), which is, like the JQI, a University of Maryland-NIST joint venture.

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  • JQI Physicists Show ‘Molecules’ Made of Light May Be Possible
  • September 9, 2015 Activity 2

From NIST TechBeat--It’s not lightsaber time, not yet. But a team including theoretical physicists from JQI and NIST has taken another step toward building objects out of photons, and the findings hint that weightless particles of light can be joined into a sort of “molecule” with its own peculiar force. Researchers show that two photons, depicted in this artist’s conception as waves (left and right), can be locked together at a short distance. Under certain conditions, the photons can form a state resembling a two-atom molecule, represented as the blue dumbbell shape at center.

The findings build on previous research that several team members contributed to before joining JQI and NIST. In 2013, collaborators from Harvard, Caltech and MIT found a way to bind two photons together so that one would sit right atop the other, superimposed as they travel. Their experimental demonstration was considered a breakthrough, because no one had ever constructed anything by combining individual photons—inspiring some to imagine that real-life lightsabers were just around the corner.

Now, in a paper forthcoming in Physical Review Letters, the team has showed theoretically that by tweaking a few parameters of the binding process, photons could travel side by side, a specific distance from each other. The arrangement is akin to the way that two hydrogen atoms sit next to each other in a hydrogen molecule.

“It’s not a molecule per se, but you can imagine it as having a similar kind of structure,” says JQI Fellow Alexey Gorshkov. “We’re learning how to build complex states of light that, in turn, can be built into more complex objects. This is the first time anyone has shown how to bind two photons a finite distance apart."

While the new findings appear to be a step in the right direction—if we can build a molecule of light, why not a sword?—Gorshkov says he is not optimistic that Jedi Knights will be lining up at NIST’s gift shop anytime soon. The main reason is that binding photons requires extreme conditions difficult to produce with a roomful of lab equipment, let alone fit into a sword’s handle. Still, there are plenty of other reasons to make molecular light—humbler than lightsabers, but useful nonetheless.

 “Lots of modern technologies are based on light, from communication technology to high-definition imaging,” Gorshkov says. “Many of them would be greatly improved if we could engineer interactions between photons.”For example, engineers need a way to precisely calibrate light sensors, and Gorshkov says the findings could make it far easier to create a “standard candle” that shines a precise number of photons at a detector. Perhaps more significant to industry, binding and entangling photons could allow computers to use photons as information processors, a job that electronic switches in your computer do today.

Not only would this provide a new basis for creating computer technology, but it also could result in substantial energy savings. Phone messages and other data that currently travel as light beams through fiber optic cables has to be converted into electrons for processing—an inefficient step that wastes a great deal of electricity. If both the transport and the processing of the data could be done with photons directly, it could reduce these energy losses. Gorshkov says it will be important to test the new theory in practice for these and other potential benefits.

“It’s a cool new way to study photons,” he says. “They’re massless and fly at the speed of light. Slowing them down and binding them may show us other things we didn’t know about them before.”

This news item was written by Chad Boutin, NIST.

M.F. Maghrebi, M.J. Gullans, P. Bienias, S. Choi, I. Martin, O. Firstenberg, M.D. Lukin, H.P. Büchler and A. V. Gorshkov. Coulomb Bound States of Strongly Interacting Photons. Physical Review Letters, forthcoming September 2015

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  • Thermometry using an optical nanofiber
  • August 21, 2015

Experimental quantum physics often resides in the coldest regimes found in the universe, where the lack of large thermal disturbances allows quantum effects to flourish. A key ingredient to these experiments is being able to measure just how cold the system of interest is. Laboratories that produce ultracold gas clouds have a simple and reliable method to do this: take pictures! The temperature of a gas depends on the range of velocities among the particles, namely the size of the difference between the slowest- and the fastest-moving particles. If all the atoms evolve for the same amount of time, the velocity distribution gets imprinted in the position of the atoms. This is analogous to a marathon where all the runners start together so you cannot immediately tell whom is the fastest, but after some time you can discern by eye whom is faster or slower based on their location.

In some experiments, however, the cloud is so well-hidden that snapshots are near impossible. A new technique developed by JQI researchers and published in Physical Review A as an Editor’s Suggestion, circumvents this issue by inserting an optical nanofiber (ONF) into a cold atomic cloud.

ONFs are like the normal optical fibers that form the global telecommunications network, except that they are much thinner – only a few hundred nanometers in diameter (about 200th the width of a human hair). This small size allows ONFs to be integrated with another, much larger system without disturbing it. Moreover, light can actually couple into the ONF through its so-called evanescent field. When an electromagnetic field, like laser light, cannot propagate from one media (e.g. air or glass) to another it does not just reflect or disappear at the interface. The field must be continuous and it gradually turns off as it flows into the new media--this spatially decaying field is called the evanescent field. Evanescent fields occur in nature, such as when an ocean a wave breaks into the shore and it slowly propagates just so far into the sand. Notably, due to its narrow size, light traveling down an ONF has significant fraction of its energy residing outside the fiber in the form of an evanescent field. Additionally, the laws of physics do not forbid the reverse process from happening, so light that originates outside the ONF can couple back into and propagate along the ONF.

In this experiment, laser-cooled atoms slowly move around the ONF and “blink” randomly as they absorb and reemit photons from a laser. The probability of such a photon coupling into the ONF depends directly on the amplitude of the evanescent field, and hence the position of the atom relative to the fiber. Once a photon enters the ONF, it travels down the optical fiber and is recorded with sensitive single-photon detector as a “click.” Tallying up how many times two clicks occur in different time windows gives the authors a picture of how the atoms move near the ONF. The width of the resulting signal is a measure of the average amount of time the atoms interact with the ONF, so that narrower (wider) signals correspond to faster (slower) atoms. Using these times, the authors were able to calculate the temperature of the cloud. When they compared it to the well-known method of taking pictures, they found good agreement.

This technique could be applied to systems where access for traditional imaging systems is limited or even impossible, such as in some types of hybrid quantum systems. One example would be experiments that seek to trap a cloud of rubidium atoms near a superconducting device, all housed within a dilution refrigerator. Operation of the dilution refrigerator requires careful shielding of optical and thermal radiation, preventing the use of the standard imaging temperature measurement. Additionally, other types of nanophotonic systems that use evanescent waves to link to atoms may also benefit from this type of thermometry.

This research summary was written by authors J. Grover and P. Solano. 

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  • Interacting Ion Qutrits
  • Enlisting symmetry to protect quantum states from disruptions
  • July 27, 2015 Activity 1

Symmetry permeates nature, from the radial symmetry of flowers to the left-right symmetry of the human body. As such, it provides a natural way of classifying objects by grouping those that share the same symmetry. This is particularly useful for describing transitions between phases of matter. For example, liquid and gas phases have translational symmetry, meaning the arrangement of molecules doesn’t change regardless of the direction from which they are observed. On the other hand, the density of atoms in a solid phase is not continuously the same — thus translational symmetry is broken.

In quantum mechanics, symmetry describes more than just the patterns that matter takes — it is used to classify the nature of quantum states. These states can be entangled, exhibiting peculiar connections that cannot be explained without the use of quantum physics. For some entangled states, the symmetry of these connections can offer a kind of protection against disruptions.

Here, the word protection indicates that the system is robust against non-symmetry breaking changes. Like an island in the middle of an ocean, there is not a direct road leading to a symmetry-protected phase or state. This means that the only way to access the state is to change the symmetry itself. Physicists are interested in exploring these classes of protected states because building a useful quantum device requires its building blocks to be robust against outside disturbances that may interfere with device operations.

Recently, JQI researchers under the direction of Christopher Monroe have used trapped atomic ions to construct a system that could potentially support a type of symmetry-protected quantum state. For this research they used a three-state system, called a qutrit, and demonstrated a proof-of-principle experiment for manipulating and controlling multiple qutrits. The result appeared in Physical Review X, an online open-access journal, and is the first demonstration of using multiple interacting qutrits for doing quantum information operations and quantum simulation of the behavior of real materials.

To date, almost all of the work in quantum information science has focused on manipulating "qubits," or so-called spin-1/2 particles that consist of just two energy levels.  In quantum mechanics, multilevel systems are analogous to the concept of "spin," where the number of energy levels corresponds to the number of possible states of spin. This group has used ion spins to explore a variety of topics, such as the physics of quantum magnetism and the transmission speed of quantum information across a spin-crystal. Increasingly, there is interest in moving beyond spin-½ to control and simulations of higher order spin systems, where the laws of symmetry can be radically altered. “One complication of spin-1 materials is that the added complexity of the levels often makes these systems much more difficult to model or understand. Thus, performing experiments in these higher [spin] dimensional systems may yield insight into difficult-to-calculate problems, and also give theorists some guidance on modeling such systems, ” explains Jake Smith, a graduate student in Monroe’s lab and author on the paper.

To engineer a spin-1 system, the researchers electromagnetically trapped a linear crystal of atomic ytterbium (Yb) ions, each atom a few micrometers from the next. Using a magnetic field, internal states of each ion are tailored to represent a qutrit, with a (+) state, (-) state and (0) state denoting the three available energy levels (see figure). With two ions, the team demonstrated the basic techniques necessary for quantum simulation: preparing initial states (placing the ions in certain internal states), observing the state of the system after some evolution, and verifying that the ions are entangled, here with 86% fidelity (fidelity is a measure of how much the experimentally realized state matches the theoretical target state).

To prepare the system in certain initial states, the team first lowers the system into its ground state, the lowest energy state in the presence of a large effective magnetic field. The different available spin chain configurations at a particular magnetic field value correspond to different energies. They observed how the spin chain reacted or evolved as the amplitude of the magnetic field was lowered. Changing the fields that the ions spins are exposed to causes the spins to readjust in order to remain in the lowest energy configuration.  

By adjusting the parameters (here laser amplitudes and frequencies) the team can open up and follow pathways between different energy levels. This is mostly true, but for some target states a simple trajectory that doesn’t break symmetries or pass through a phase transition does not exist. For instance, when the team added a third ion, they could not smoothly guide the system into its ground state, indicating the possible existence of a state with some additional symmetry protections.

“This result is a step towards investigating quantum phases that have special properties based on the symmetries of the system,” says Smith. Employing these sorts of topological phases may be a way to improve coherence times when doing quantum computation, even in the face of environmental disruptions. Coherence time is how long a state retains its quantum nature. Quantum systems are very sensitive to outside disturbances, and doing useful computation requires maintaining this quantum nature for longer than the time it takes to perform a particular calculation.

Monroe explains, "These symmetry-protected states may be the only way to build a large-scale stable quantum computer in many physical systems, especially in the solid-state.  With the exquisite control afforded atomic systems such as trapped ions demonstrated here, we hope to study and control how these very subtle symmetry effects might be used for quantum computing, and help guide their implementation in any platform."

To further investigate this protected phase, the researchers next intend to address the problem of creating antisymmetric ground states. Smith continues, “The next steps are to engineer more complicated interactions between the effective spins and implement a way to break the symmetries of the interactions.”

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  • Gretchen Campbell receives IUPAP Young Scientist Prize
  • July 27, 2015

JQI Fellow and NIST Scientist Gretchen Campbell has recently been announced as the IUPAP 2015 Young Scientist Prize recipient in the field of Atomic, Molecular, and Optical Physics. The organization cited her "outstanding contributions in toroidal Bose-Einstein condensates and its application to "atomtronic" circuits." 

The International Union of Pure and Applied Physics (IUPAP) was established in 1922 in Brussels with 13 Member countries and the first General Assembly was held in 1923 in Paris. More about the prize can be found at

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  • Qubit Chemistry
  • Controlling interactions between distant qubits
  • July 22, 2015

A big part of the burgeoning science of quantum computation is reliably storing and processing information in the form of quantum bits, or qubits.  One of the obstacles to this goal is the difficulty of preserving the fragile quantum condition of qubits against unwanted outside influence even as the qubits interact among themselves in a programmatic way. 

Spin qubits are one of the most promising candidates for the purpose.  Besides being charged, electrons possess spin, a kind of magnetic axis that can only assume specific quantized values.  An atom with a single unpaired electron can serve as a qubit if that electron can be tickled into residing in both of two allowed quantum states (usually called spin up and down) at the same time.  Likewise, a carefully contrived small puddle of electrons known as a quantum dot can also serve as a qubit.  The dot’s spin consists of the aggregate spin of the small number of electrons (two, three, four, etc.) residing in the dot.  In this way the dot acts as a sort of artificial atom.

JQI scientists, publishing in Physical Review Letters, show how both of these qubit types — atomic spins and quantum dot spins — can be combined into a workable quantum system.  According to Jacob Taylor, the leader of the JQI work, their suggestion for a new qubit interaction protocol uses the dot’s spin to turn on what is essentially a chemical reaction between two atomic spins which sit as much as 50 or 100 nm apart.   In the proposed scheme, the atoms designated for qubit duty are phosphorus impurities sitting in a silicon crystal.


Why chemistry?  Well, according to Taylor a large part of chemistry is the reaction of two chemical species via their outer electrons by forming a temporary or stable chemical bond governed by the Pauli exclusion principle.  This principle says that two identical electrons cannot occupy the same position.  This aversion — a purely quantum property of nature — acts in addition to the ordinary electrostatic repulsion between like-charged particles. 

The direct electrical connection between otherwise identical electrons, usually called the exchange interaction, depends on the relative spin orientation of the participating electrons.  The Pauli exclusion effect will lead to greater repulsion for electrons with the same spin.  The repulsion, the inability of electrons to interpenetrate, accounts for much of the “stiffness” of matter.  For instance, you can’t push your hand through a solid desk because, at the microscopic level, electrons are repelling other electrons.

“Much of chemistry operates via the exchange interaction,” says Taylor.  “But this interaction is too tenuous to operate effectively between the distant atomic spins.  The compromise is to use a largish quantum dot — acting like at atom but having a size corresponding to the distance between the impurity atoms — to link up the atoms.”

The central metaphor for this scheme, Taylor says, is Newton’s cradle, a device consisting of several identical balls suspended from thin wires.   Lift one ball up, let it go, and it will collide with the neighboring ball.  The kinetic energy and momentum of the first ball, passed on to the second, will be passed smoothly to the rightmost ball, which flies up.  This ball, in turn, will swing back down, imparting the energy and momentum in the other direction.

By analogy, an electron striking an ensemble of electrons from one side can influence an electron on the far side of the ensemble.   The intervening ensemble is referred to as a Fermi sea since electrons are fermions, particles with a spin value of ½.  And just as in the mechanical case one ball cannot penetrate the other balls but can impart energy and momentum to a ball on the far side, so one electron can smack into a Fermi sea and impart spin information to an electron on the far side.  This transfer of quantum information across a Fermi sea is the heart of the proposed qubit-interaction plan.

In the JQI qubit scheme, repeated interactions between the spins in the two impurity atoms (corresponding to the outer balls of Newton’s cradle) will be mediated by the spins in the quantum dot (which correspond to the balls in between).  Of course, atoms and electrons are not mere balls.  They possess charge and spin, and their interactions will still be a combination of electric and magnetic forces.  And instead of gravity operating to power the slamming balls, electrostatic forces applied by electrodes at the surface of the sample are what keep the spins knocking away at each other.


A sprinkling of phosphorus atoms lies in crystal of silicon atoms, which provides a semiconducting environment for the atoms relatively free from disturbances.  Just above the Si layer is a layer of silicon dioxide.  In between the two layers is a two-dimensional sea of electrons  (a Fermi sea) whose movements can be manipulated by the electrodes mentioned above.  With just the right applied voltages the electrodes can pinch off regions of the lake, where only a few or even no electrons are allowed.

This gate-controlled pinching is how many transistors work in electronic devices like computers.  It is also how the size and shape of quantum dots can be fashioned entirely by electrostatic forces.  Those same electrodes manipulate the spins of the impurity atoms and the electrons in the quantum dot.

How many electrons are needed in the dot?   The first author on the JQI paper, postdoc Vanita Srinivasa, was surprised to learn that the atom-dot scheme works with as few as two electrons in the dot.   Communication between the two P atoms can consequently be thought of as a Newton’s-cradle juggling of four electrons: the two operative electrons in the dot and the available electrons in the two impurity atoms. 


To operate as a quantum computer, qubits have to be entangled.  Here the entanglement of the two atomic spins — making them part of a single quantum entity — is brought about by a method analogous to the process by which the atomic qubits themselves are prepared.

When  microwaves of the right frequency strike an atom, the atom can be excited from its ground state to a higher level.  If the microwaves continue to be applied, the atom will be de-excited back to its ground state.  The probability of the atom being in the excited state will vary sinusoidally with time.  If the microwaves are carefully turned off at just the right moment (halfway through the first peak) the atom will have an equal likelihood of being in the ground and excited state.   This is what makes the atom a qubit: it exists in two forms simultaneously.

Now consider the dynamic ensemble of four electrons in the JQI theoretical scheme.  A proper sequence of voltage pulses can rock the atom-dot-atom ensemble back and forth in Newton’s-cradle fashion, creating a corresponding flip-flop oscillation between the atomic spins.   The rate of this oscillation can be tuned simply by adjusting the voltages that determine how easily electrons are able to (virtually) move between the atoms and the dot. If the flip-flopping is stopped at some point, the spin states of the two phosphorus atoms will be coordinated, while the state of the two dot electrons will remain separate. 

In other words, the two P-atom qubits are entangled.  Without necessarily knowing the exact status of the qubits, if some operation is performed on one of the qubits (such as flipping its spin state) the other qubit will undergo a comparable operation.

Two qubits entangled in this way represent a fundamental logic gate for building a quantum processor. The fidelity of this gate — how well an operation on the qubits matches the desired operation — can be optimized simply by tuning the voltages, which can lead to very high theoretical fidelities (up to 99.8%) even when electrical noise is taken into account and for relatively small interaction strengths. 

“Fundamentally, the work suggests that a two-electron system represents the smallest possible Fermi sea,” said Srinivasa.  “By moving from one to just two electrons in the mediator dot, we find a more robust way to link the qubits. From a practical standpoint, current experiments on hybrid atom-dot systems suggest the exciting prospect of implementing dot-mediated atomic spin coupling in the near future.”

Reference report: “Tunable Spin-Qubit Coupling Mediated by a Multielectron Quantum Dot,”

V. Srinivasa, H. Xu, and J. M. Taylor,” Physical Review Letters, 114, 226803 (2015), 5 June 2015;

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  • Collecting Lost Light
  • Rayleigh scattering reveals light propagation in optical nanfibers
  • June 12, 2015

Optical fibers are hair-like threads of glass used to guide light. Fibers of exceptional purity have proved an excellent way of sending information over long distances and are the foundation of modern telecommunication systems. Transmission relies on what’s called total internal reflection, wherein the light propagates by effectively bouncing back and forth off of the fiber’s internal surface. Though the word “total” implies light remains entirely trapped in the fiber, the laws of physics dictate that some of the light, in the form of what’s called an evanescent field, also exists outside of the fiber. In telecommunications, the fiber core is more than ten times larger than the wavelength of light passing through. In this case, the evanescent fields are weak and vanish rapidly away from the fiber. Nanofibers have a diameter smaller than the wavelength of the guided light. Here, all of the light field cannot fit inside of the nanofiber, yielding a significant enhancement in the evanescent fields outside of the core. This allows the light to trap atoms (or other particles) near the surface of a nanofiber.

JQI researchers in collaboration with scientists from the Naval Research Laboratory have developed a new technique for visualizing light propagation through an optical nanofiber, detailed in a recent Optica paper. The result is a non-invasive measurement of the fiber size and shape and a real-time view of how light fields evolve along the nanofiber. Direct measurement of the fields in and around an optical nanofiber offers insight into how light propagates in these systems and paves the way for engineering customized evanescent atom traps.  

In this work, researchers use a sensitive camera to collect light from what’s known as Rayleigh scattering, demonstrating the first in-situ measurements of light moving through an optical nanofiber. Rayleigh scattering happens when light bounces, or scatters, off of particles much smaller than the wavelength of the light. In fibers, these particles can be impurities or density fluctuations in the glass, and the light scattered from them is ejected from the fiber.  This allows one to view the propagating light from the side, in much the same way as one can see a beam of sunlight through fog. Importantly, the amount of light ejected depends on the polarization, or the orientation of oscillation of the light, and intensity of the field at each point, which means that capturing this light is a way to view the field.

The researchers here are interested in understanding the propagation of the field when the light waves are comprised from what are known as higher-order modes. Instead of having a uniform spatial profile, like that of a laser pointer, these modes can look like a doughnut, cloverleaf, or another more complicated pattern. Higher-order modes offer some advantages over the lowest order or “fundamental” mode. Due to their complexity, the evanescent field can have comparatively more light intensity in the region of interest — locally just outside the fiber. These higher order modes can also be used to make different types of optical patterns. Nanofibers aren’t yet standardized and thus careful and complete characterization of both the fiber and the light passing through them is a necessary step towards making them a more practical and adaptable tool for research applications.

This research team had previously developed techniques for controlling the fiber manufacture process in order to support extremely pure higher-order modes. Mode quality depends on things like the width of the fiber core and how this width changes over the length of the fiber. Small deviations in the fiber diameter and other imperfections can cause undesirable combinations and the potential loss of certain modes. By analyzing how the transmitted light changes as the fiber is stretched into a nanofiber, they could infer how the modes change while propagating through the fiber. However, until now there was no way to directly measure the intensity of the field along the fiber, which would offer far more insight and control over how the evanescent fields are shaped at the location of the trapped atoms. This could be useful for analyzing fibers where the propagation conditions change multiple times, or in the case where a fiber undergoes strain or bending during use.

By collecting images of the Rayleigh scattering, the scientists can directly see how the field changes throughout a nanofiber and also the effects of changing the pattern of light injected into the fiber. In addition, the team was able to use the imaging information to feedback to the system and create desired combinations of modes in the nanofiber — demonstrating a high level of control. The same technique can be used to measure the profile and width of the fiber itself. In this case, they were able to estimate a fiber radius of 370 nanometers and variations in the waist down to 3 nm. Notably, this type of visualization is done in-situ with relatively standard optics and does not require destroying the fiber integrity with the special coatings that are necessary when using a scanning electron microscope. This also means these characterizing measurements can be used to optimize the fields that interact with atoms during experiments. “An advantage of this technique is that it can be applied to fibers that are already installed in an apparatus,” explains Fredrik Fatemi, a research physicist at the Naval Research Laboratory and author on the paper: “One could even probe fibers or other nanophotonic structures designed for fundamental modes by using shorter optical wavelengths.”

To further refine this approach, the researchers plan to modify the optics in order to capture the entire length of the nanofiber in a single image. Currently, the images are made by stitching several high-resolution images together, as in the image seen above.  

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  • Tightening the Bounds on the Quantum Information 'Speed Limit'
  • April 23, 2015

If you’re designing a new computer, you want it to solve problems as fast as possible. Just how fast is possible is an open question when it comes to quantum computers, but JQI physicists have narrowed the theoretical limits for where that “speed limit” is. The work implies that quantum processors will work more slowly than some research has suggested. 

The work offers a better description of how quickly information can travel within a system built of quantum particles such as a group of individual atoms. Engineers will need to know this to build quantum computers, which will have vastly different designs and be able to solve certain problems much more easily than the computers of today. While the new finding does not give an exact speed for how fast information will be able to travel in these as-yet-unbuilt computers—a longstanding question—it does place a far tighter constraint on where this speed limit could be.

Quantum computers will store data in a particle’s quantum states—one of which is its spin, the property that confers magnetism. A quantum processor could suspend many particles in space in close proximity, and computing would involve moving data from particle to particle. Just as one magnet affects another, the spin of one particle influences its neighbor’s, making quantum data transfer possible, but a big question is just how fast this influence can work.

The team’s findings advance a line of research that stretches back to the 1970s, when scientists discovered a limit on how quickly information could travel if a suspended particle only could communicate directly with its next-door neighbors. Since then, technology advanced to the point where scientists could investigate whether a particle might directly influence others that are more distant, a potential advantage. By 2005, theoretical studies incorporating this idea had increased the speed limit dramatically.

“Those results implied a quantum computer might be able to operate really fast, much faster than anyone had thought possible,” says postdoctoral researcher and lead author Michael Foss-Feig. “But over the next decade, no one saw any evidence that the information could actually travel that quickly.”

Physicists exploring this aspect of the quantum world often line up several particles and watch how fast changing the spin of the first particle affects the one farthest down the line—a bit like standing up a row of dominoes and knocking the first one down to see how fast the chain reaction takes. The team looked at years of others’ research and, because the dominoes never seemed to fall as fast as the 2005 prediction suggested, they developed a new mathematical proof that reveals a much tighter limit on how fast quantum information can propagate.

“The tighter a constraint we have, the better, because it means we’ll have more realistic expectations of what quantum computers can do,” says Foss-Feig.

The limit, their proof indicates, is far closer to the speed limits suggested by the 1970s result. The proof addresses the rate at which entanglement propagates across quantum systems. Entanglement—the weird linkage of quantum information between two distant particles—is important, because the more quickly particles grow entangled with one another, the faster they can share data. The 2005 results indicated that even if the interaction strength decays quickly with distance, as a system grows, the time needed for entanglement to propagate through it grows only logarithmically with its size, implying that a system could get entangled very quickly. The team’s work, however, shows that propagation time grows as a power of its size, meaning that while quantum computers may be able to solve problems that ordinary computers find devilishly complex, their processors will not be speed demons.

“On the other hand, the findings tell us something important about how entanglement works,” says Foss-Feig. “They could help us understand how to model quantum systems more efficiently.”

This was originally written by C. Boutin for NIST TechBeat, with modifications for JQI made by E. Edwards.

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  • Sharper Nanoscopy
  • What happens when a quantum dot looks in a mirror?
  • March 19, 2015

The 2014 chemistry Nobel Prize recognized important microscopy research that enabled greatly improved spatial resolution. This innovation, resulting in nanometer resolution, was made possible by making the source (the emitter) of the illumination  quite small and by moving it quite close to the object being imaged.   One problem with this approach is that in such proximity, the emitter and object can interact with each other, blurring the resulting image.   Now, a new JQI study has shown how to sharpen nanoscale microscopy (nanoscopy) even more by better locating the exact position of the light source.


Traditional microscopy is limited by the diffraction of light around objects.  That is, when a light wave from the source strikes the object, the wave will scatter somewhat.  This scattering limits the spatial resolution of a conventional microscope to no better than about one-half the wavelength of the light being used.  For visible light, diffraction limits the resolution to no be better than a few hundred nanometers.

How then, can microscopy using visible light attain a resolution down to several nanometers?  By using tiny light sources that are no larger than a few nanometers in diameter.  Examples of these types of light sources are fluorescent molecules, nanoparticles, and quantum dots.  The JQI work uses quantum dots which are tiny crystals of a semiconductor material that can emit single photons of light. If such tiny light sources are close enough to the object meant to be mapped or imaged, nanometer-scale features can be resolved.  This type of microscopy, called “Super-resolution imaging,” surmounts the standard diffraction limit. 


JQI fellow Edo Waks and his colleagues have performed nanoscopic mappings of the electromagnetic field profile around silver nano-wires by positioning quantum dots (the emitter) nearby.  (Previous work summarized in press release).  They discovered that sub-wavelength imaging suffered from a fundamental problem, namely that an “image dipole” induced in the surface of the nanowire was distorting knowledge of the quantum dot’s true position. This uncertainty in the position of the quantum dot translates directly into a distortion of the electromagnetic field measurement of the object.

The distortion results from the fact that an electric charge positioned near a metallic surface will produce just such an electric field as if a ghostly negative charge were located as far beneath the surface as the original charge is above it.  This is analogous to the image you see when looking at yourself in a mirror; the mirror object appears to be as far behind the mirror as you are in front.  The quantum dot does not have a net electrical charge but it does have a net electrical dipole, a slight displacement of positive and negative charge within the dot. 

Thus when the dot approaches the wire, the wire develops an “image” electrical dipole whose emission can interfere with the dot’s own emission.  Since the measured light from the dot is the substance of the imaging process, the presence of light coming from the “ image dipole” can interfere with light coming directly from the dot.  This distorts the perceived position of the dot by an amount which is 10 times higher than the expected spatial accuracy of the imaging technique (as if the nanowire were acting as a sort of funhouse mirror).

The JQI experiment successfully measured the image-dipole effect and properly showed that it can be corrected under appropriate circumstances.  The resulting work provides a more accurate map of the electromagnetic fields surrounding the nanowire.

The JQI scientists published their results in the journal Nature Communications.

Lead author Chad Ropp (now a postdoctoral fellow at the University of California, Berkeley) says that the main goal of the experiment was to produce better super-resolution imaging: “Any time you use a nanoscale emitter to perform super-resolution imaging near a metal or high-dielectric structure image-dipole effects can cause errors. Because these effects can distort the measurement of the nano-emitter’s position they are important to consider for any type of super-resolved imaging that performs spatial mapping.”

“Historically scientists have assumed negligible errors in the accuracy of super-resolved imaging,” says Ropp.  “What we are showing here is that there are indeed substantial inaccuracies and we describe a procedure on how to correct for them."

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  • Paul Julienne awarded William F. Meggers Award
  • March 19, 2015

The OSA announced JQI Fellow and NIST scientist Paul Julienne as the 2015 William F. Meggers Award recipient. The William F. Meggers Award recognizes outstanding work in spectroscopy. According to the citation, Julienne is being recognized for "seminal contributions to precision photoassociation and magnetic-Feshbach spectroscopy of ultracold atoms, and the application of these techniques to the formation of cold polar molecules." 

“OSA is greatly honored to recognize these leaders in the field of optics,” said Elizabeth Rogan, CEO, The Optical Society. “The recipients have demonstrated an expertise and leadership in their chosen field and have made significant contributions to the understanding of optics and photonics. OSA congratulates them on their outstanding achievements.” 

Philip Russell, president, The Optical Society, added: “This year’s awardees have contributed to optics and photonics in a wide variety of different ways, from outstanding accomplishments in research and development, to leading the International Year of Light. They represent the very best and brightest leaders in our field and the OSA Board is proud to recognize them for their unwavering commitment, creativity and leadership.”

Julienne obtained his undergraduate degree in chemistry from Wofford College in 1965 and his Ph. D. in Chemical Physics in 1969 from the University of North Carolina at Chapel Hill. After postdoctoral work at the National Bureau of Standards (NBS), he worked with the Plasma Physics Division at the Naval Research Laboratory between 1971-1974 before returning to NBS as a staff member in the Quantum Chemistry Group.  During his career at NBS, later renamed as the National Institute of Standards and Technology (NIST), he served as a group leader for the Quantum Processes Group, as a NIST Fellow, and as a Fellow of the Joint Quantum Institute (JQI) of NIST and the University of Maryland. After retiring in 2013, he continues to serve as a NIST Scientist Emeritus and an Emeritus Fellow and Adjunct Professor of Physics at the University of Maryland and JQI.  He is a Fellow of the American Physical Society and has served as an APS Divisional Councilor representing the Division of Atomic, Molecular, and Optical Physics. 

Award information and quotes provided by OSA.

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  • Modular Entanglement Using Atomic Ion Qubits
  • February 26, 2015

JQI researchers, under the direction of Christopher Monroe have demonstrated modular entanglement between two atomic systems, separated by one meter. Here, photons are the long distance information carriers entangling multiple qubit modules. Inside of a single module, quantized collective vibrations called phonons connect individual qubits. In the latest result, one module contains two qubits and a second module houses a single qubit. This work was published in the journal Nature Physics, along with two related results that appeared in the Physical Review journals.

The two-by-one qubit entanglement is an experimental result that follows the theoretical design by Monroe and collaborators on building a modular universal quantum computer, published earlier in 2014. This group is pursuing a modular quantum computer architecture that promises scalability to much larger numbers of qubits. The components of this architecture have individually been tested and are available, making it a practical approach. Previously, the authors presented expected performance and scaling calculations, demonstrating that their architecture is not only viable, but in some ways, preferable when compared to related schemes...Read more about this research in an interactive feature story

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  • Novel Phases for Bose Gases
  • February 26, 2015

Strongly correlated electronic systems, like superconductors, display remarkable electronic and magnetic properties, and are of considerable research interest. These systems are fermionic, meaning they are composed of a class of particle called a fermion. Bosonic systems, composed another family of particles called bosons, offer a level of control often not possible in solid state systems. Creating analogous states in bose gases is an excellent way to model the dynamics of these less tractable systems. This means engineering a gas that, when cooled down to a condensate, assumes a phase equivalent to its solid state counterpart.

JQI theorists Juraj Radic, Stefan Natu, and Victor Galitski have proposed a new magnetic phase for a bose gas. The transition to this phase is analogous to the formation of ferromagnetism in magnetic materials, like iron, and might give insight into the physics of strongly-correlated electronic systems...Read more about this research in an interactive feature story.

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  • Vladimir Manucharyan Receives CAREER Award
  • February 5, 2015

JQI Fellow and Assistant Professor of physics Vladimir Manucharyan has received a National Science Foundation CAREER Award. His proposal, entitled “Realizing the ultrastrong coupling regime of quantum electrodynamics using high-impedance Josephson superconducting circuits,” will receive five years of funding. NSF funds research in science and engineering through grants, contracts and cooperative agreements.

For more information on NSF Awards visit:

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  • Best Quantum Receiver
  • Record high data accuracy rates for phase-modulated transmission
  • November 14, 2014

We want data.  Lots of it.  We want it now.  We want it to be cheap and accurate.

 Researchers try to meet the inexorable demands made on the telecommunications grid by improving various components.  In October 2014, for instance, scientists at the Eindhoven University of Technology in The Netherlands did their part by setting a new record for transmission down a single optical fiber: 255 terabits per second

 Alan Migdall and Elohim Becerra and their colleagues at the Joint Quantum Institute do their part by attending to the accuracy at the receiving end of the transmission process.  They have devised a detection scheme with an error rate 25 times lower than the fundamental limit of the best conventional detector.  They did this by employing not passive detection of incoming light pulses.  Instead the light is split up and measured numerous times.

 The new detector scheme is described in a paper published in the journal Nature Photonics.

 “By greatly reducing the error rate for light signals we can lessen the amount of power needed to send signals reliably,” says Migdall.  “This will be important for a lot practical applications in information technology, such as using less power in sending information to remote stations.  Alternatively, for the same amount of power, the signals can be sent over longer distances.”

Phase Coding

Most information comes to us nowadays in the form of light, whether radio waves sent through the air or infrared waves send up a fiber.  The information can be coded in several ways.  Amplitude modulation (AM) maps analog information onto a carrier wave by momentarily changing its amplitude.  Frequency modulation (FM) maps information by changing the instantaneous frequency of the wave.  On-off modulation is even simpler: quickly turn the wave off (0) and on (1) to convey a desired pattern of binary bits.

 Because the carrier wave is coherent---for laser light this means a predictable set of crests and troughs along the wave---a more sophisticated form of encoding data can be used.  In phase modulation (PM) data is encoded in the momentary change of the wave’s phase; that is, the wave can be delayed by a fraction of its cycle time to denote particular data.  How are light waves delayed?  Usually by sending the waves through special electrically controlled crystals.

 Instead of using just the two states (0 and 1) of binary logic, Migdall’s experiment waves are modulated to provide four states (1, 2, 3, 4), which correspond respectively to the wave being un-delayed, delayed by one-fourth of a cycle, two-fourths of a cycle, and three-fourths of a cycle.  The four phase-modulated states are more usefully depicted as four positions around a circle (figure 2).  The radius of each position corresponds to the amplitude of the wave, or equivalently the number of photons in the pulse of waves at that moment.  The angle around the graph corresponds to the signal’s phase delay.

 The imperfect reliability of any data encoding scheme reflects the fact that signals might be degraded or the detectors poor at their job.  If you send a pulse in the 3 state, for example, is it detected as a 3 state or something else?  Figure 2, besides showing the relation of the 4 possible data states, depicts uncertainty inherent in the measurement as a fuzzy cloud.  A narrow cloud suggests less uncertainty; a wide cloud more uncertainty.  False readings arise from the overlap of these uncertainty clouds.  If, say, the clouds for states 2 and 3 overlap a lot, then errors will be rife.

 In general the accuracy will go up if n, the mean number of photons (comparable to the intensity of the light pulse) goes up.  This principle is illustrated by the figure to the right, where now the clouds are farther apart than in the left panel.  This means there is less chance of mistaken readings.  More intense beams require more power, but this mitigates the chance of overlapping blobs.

Twenty Questions

So much for the sending of information pulses.  How about detecting and accurately reading that information?  Here the JQI detection approach resembles “20 questions,” the game in which a person identifies an object or person by asking question after question, thus eliminating all things the object is not.

In the scheme developed by Becerra (who is now at University of New Mexico), the arriving information is split by a special mirror that typically sends part of the waves in the pulse into detector 1.  There the waves are combined with a reference pulse.  If the reference pulse phase is adjusted so that the two wave trains interfere destructively (that is, they cancel each other out exactly), the detector will register a nothing.  This answers the question “what state was that incoming light pulse in?” When the detector registers nothing, then the phase of the reference light provides that answer, … probably.

That last caveat is added because it could also be the case that the detector (whose efficiency is less than 100%) would not fire even with incoming light present. Conversely, perfect destructive interference might have occurred, and yet the detector still fires---an eventuality called a “dark count.”  Still another possible glitch: because of optics imperfections even with a correct reference–phase setting, the destructive interference might be incomplete, allowing some light to hit the detector.

The way the scheme handles these real world problems is that the system tests a portion of the incoming pulse and uses the result to determine the highest probability of what the incoming state must have been. Using that new knowledge the system adjusts the phase of the reference light to make for better destructive interference and measures again. A new best guess is obtained and another measurement is made.

As the process of comparing portions of the incoming information pulse with the reference pulse is repeated, the estimation of the incoming signal’s true state was gets better and better.  In other words, the probability of being wrong decreases.

Encoding millions of pulses with known information values and then comparing to the measured values, the scientists can measure the actual error rates.  Moreover, the error rates can be determined as the input laser is adjusted so that the information pulse comprises a larger or smaller number of photons.  (Because of the uncertainties intrinsic to quantum processes, one never knows precisely how many photons are present, so the researchers must settle for knowing the mean number.) 

A plot of the error rates shows that for a range of photon numbers, the error rates fall below the conventional limit, agreeing with results from Migdall’s experiment from two years ago. But now the error curve falls even more below the limit and does so for a wider range of photon numbers than in the earlier experiment. The difference with the present experiment is that the detectors are now able to resolve how many photons (particles of light) are present for each detection.  This allows the error rates to improve greatly.

For example, at a photon number of 4, the expected error rate of this scheme (how often does one get a false reading) is about 5%.  By comparison, with a more intense pulse, with a mean photon number of 20, the error rate drops to less than a part in a million.

The earlier experiment achieved error rates 4 times better than the “standard quantum limit,” a level of accuracy expected using a standard passive detection scheme.  The new experiment, using the same detectors as in the original experiment but in a way that could extract some photon-number-resolved information from the measurement, reaches error rates 25 times below the standard quantum limit.

“The detectors we used were good but not all that heroic,” says Migdall.  “With more sophistication the detectors can probably arrive at even better accuracy.”

The JQI detection scheme is an example of what would be called a “quantum receiver.”  Your radio receiver at home also detects and interprets waves, but it doesn’t merit the adjective quantum.  The difference here is single photon detection and an adaptive measurement strategy is used.  A stable reference pulse is required. In the current implementation that reference pulse has to accompany the signal from transmitter to detector.

Suppose you were sending a signal across the ocean in the optical fibers under the Atlantic.  Would a reference pulse have to be sent along that whole way?  “Someday atomic clocks might be good enough,” says Migdall, “that we could coordinate timing so that the clock at the far end can be read out for reference rather than transmitting a reference along with the signal.”

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spin-hall bosons
  • Restoring Order
  • A spin Hall effect without all the fuss
  • October 20, 2014 Activity 1

Every electrical device, from a simple lightbulb to the latest microchips, is enabled by the movement of electrical charge, or current. The nascent field of ‘spintronics’ taps into a different electronic attribute, an intrinsic quantum property known as spin, and may yield devices that operate on the basis of spin-transport.

Atom-optical lattice systems offer a clean, well-controlled way to study the manipulation and movement of spins because researchers can create particle configurations analogous to crystalline order in materials. JQI/CMTC* theorists Xiaopeng Li, Stefan Natu, and Sankar Das Sarma, in collaboration with Arun Paramekanti from the University of Toronto, have been developing a model for what happens when ultracold atomic spins are trapped in an optical lattice structure with a “double-valley” feature, where the repeating unit resembles the letter “W”. This new theory result, recently published in the multidisciplinary journal Nature Communications, opens up a novel path for generating what’s known as the spin Hall effect, an important example of spin-transport.

Behavior in double-valley lattices has previously been studied for a collection of atoms that all have the same value of spin. In this new work, theorists consider two-component atoms--here, the spin state each atom has can vary between “spin-up” and “spin-down” states. Atomic spins in a lattice can be thought of as an array of tiny bar magnets. In the single-component case, the atom-magnets are all oriented the same direction in the lattice. In this case, the magnets can have a tendency to favor only one of the wells of the double-valley. In the two-component system studied here, each atom-magnet can have its’ north-pole pointed either up or down, with respect to a particular magnetic field. Adding this kind of freedom to the model leads to some very curious behavior – the atoms spontaneously separate, with the spin-up atoms collecting into one well of the double-valley, and spin-down atoms in the other. Theorists have dubbed this new state a “spontaneous chiral spin superfluid” (for further explanation of what a superfluid is, see here).

This sort of spin-dependent organization is of great interest to researchers, who could employ it to study the spin Hall effect, analogous to the Hall effect for electrons. Normally, this effect is seen as a result of spin-orbit coupling, or the association of an atom’s spin with its motion. In fact, this has long been the approach for producing a spin Hall effect – apply a current to material with spin-orbit coupling, and the spins will gather at the edges according to their orientation. Ian Spielman’s group at the JQI has pioneered laser-based methods for realizing both spin-orbit coupling and the spin Hall effect phenomenon in ultracold atomic gases. In contrast, for the superfluid studied here, spin-sorting is not the result of an applied field or asymmetric feature of the system, but rather emerges spontaneously. It turns out this behavior is driven by tiny, random quantum fluctuations, in a paradoxical phenomenon known as quantum order by disorder.

Quantum order by disorder

Generally speaking, the transition from disorder to order is a familiar one. Consider water condensing into ice: this is a disordered system, a liquid, transitioning to a more ordered one, a solid. This phase transformation happens because the molecules become limited in their degrees of freedom, or the ability to move in different ways. Conversely, adding noise to a system, such as heating an ice cube, generally leads to a more disordered state, a pool of water. Amazingly, noise or fluctuations in a system can sometimes drive a system into a more ordered state.

The theory team showed that this is indeed the case for certain kinds of atoms loaded into a double-valley optical lattice. While this system is a quiet, mostly non-thermal environment, noise still lurks in the form of quantum mechanical fluctuations. In this system, the spin-up and spin-down atoms can potentially be configured in four different, but energetically identical arrangements. This is known as degeneracy and can be indicative of the amount of order in a system--the more equal energy states, the more disordered a system. It turns out that these arrangements have different amounts of quantum noise and these fluctuations play a crucial role. Surprisingly, the quantum fluctuations will break up the degeneracy, thus restoring order.

What’s the upshot? In this system, the resulting lowest energy configuration--a chiral spin superfluid-- is preferred independent of the type of double-valley lattice geometry, indicating a type of universal behavior. With this in mind, the theorists examine a number of lattice structures where this phenomenon might be realized. For instance, if the fluid is placed into a hexagonal lattice configuration, similar to the structure of graphene, they expect the characteristic spin currents of the spin Hall effect to emerge, as depicted in the graphic, above. In the publication, the team points out that optical lattice systems are a flexible, pristine platform for examining the effect of these tiny variations in quantum fluctuations, which are often masked in real materials. Outside of exploring novel forms of matter like the one found here, research into spin and atom manipulation has applications in emerging electronic-like technologies, such as spintronics, valleytronics and atomtronics.

*JQI (Joint Quantum Institute) and CMTC (Condensed Matter Theory Center)

This news item was written by S. Kelley, JQI.

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Thermal interference
  • Getting sharp images from dull detectors
  • Operating in the fuzzy area between classical and quantum light
  • October 10, 2014

Observing the quantum behavior of light is a big part of Alan Migdall’s research at the Joint Quantum Institute.  Many of his experiments depend on observing light in the form of photons---the particle complement of light waves---and sometimes only one photon at a time, using “smart” detectors that can count the number of individual photons in a pulse.  Furthermore, to observe quantum effects, it is normally necessary to use a beam of coherent light, light for which knowing the phase or intensity for one part of the beam allows you to know things about distant parts of the same beam.

In a new experiment, however, Migdall and his JQI colleagues perform an experiment using incoherent light, where the light is a jumble of waves.  And they use what Migdall calls “stupid” detectors that, when counting the number of photons in a light pulse, can really only count up to zero, as anything more than zero befuddles these detectors and is considered as number that is known only to be more than zero.

Basically the surprising result is this: using incoherent light (with a wavelength of 800 nm) sent through a double-slit baffle, the JQI scientists obtain an interference pattern with fringes (the characteristic series of dark and light stripes denoting respectively destructive and constructive interference) as narrow as 30 nm.

This represents a new extreme in the degree to which sub-wavelength interference (to be defined below) has been pushed using thermal light and small-photon-number light detection.  The physicists were surprised that they could so easily obtain such a sharp interference effect using standard light detectors.  The importance of achieving sub-wavelength imaging is underscored by the awarding of the 2014 Nobel Prize for chemistry to scientists who had done just that.

The results of Migdall’s new work appear in the journal Applied Physics Letters (1).  Achieving this kind of sharp interference pattern could be valuable for performing a variety of high-precision physics and astronomy measurements.


When they pass through a hole or past a material edge, light waves will diffract---that is, a portion of the light will fan out as if the edge were a source of waves itself.  This diffraction will limit the sharpness of any imaging performed by the light.  Indeed, this diffraction limitation is one of the traditional features of classical optical science dating back to the mid 19th century.  What this principle says is that in using light with a certain wavelength (denoted by the Greek letter lambda) an object can in general be imaged with a spatial resolution roughly no finer than lambda.  One can improve resolution somewhat by increasing lens diameters, but unless you can switch to light of shorter lambda, you are stuck with the imaging resolution you’ve got.  And since all the range of available wavelengths for visible light covers only a range of about 2, gaining much resolution by switching wavelengths requires exotic sources and optics. 

The advent of quantum optics and the use of “nonclassical light” dodged the diffraction limit.  It did this, in certain special circumstances, by considering light as consisting of particles and using the correlations between those particles 

The JQI experiment starts out with a laser beam, but it purposely degrades the coherence of the light by sending it through a moving disk of ground glass.  Thereafter the light waves propagating toward the measuring apparatus downstream originate from a number of places across the profile of the rough disk and are no longer coordinated in space and time (in contrast to laser light).  Experiments more than a decade ago, however, showed that “thermal” light (not unlike the light emitted haphazardly by an incandescent bulb) made this way, while incoherent over long times, is coherent for times shorter than some value easily controlled by the speed of the rotating ground glass disk.

Why should the JQI researchers use such thermal light if laser light is available?  Because in many measurement environments (such as light coming from astronomical sources) coherent light is not available, and one would nevertheless like to make sharp imaging or interference patterns.  And why use “stupid” detectors?  Because they are cheaper to use. 


In the case of coherent light, a coordinated train of waves approach a baffle with two openings (figure, top).  The light waves passing through will interfere, creating a characteristic pattern as recorded by a detector, which is moved back and forth to record the arrival of light at various points.  The interference of coherent light yields a fixed pattern (right top in the figure).   By contrast, incoherent light waves, when they pass through the slits will also interfere (lower left), but will not create a fixed pattern.  Instead the pattern will change from moment to moment. 

In the JQI experiment, the waves coming through the slits meets with a beam splitter, a thin layer of material that reflects roughly half the waves at an angle of 90 degrees and transmits the other half straight ahead.  Each of these two portions of light will strike movable detectors which scan across sideways.  If the detectors could record a whole pattern, they would show that the pattern changes from moment to moment.  Adding up all these patterns washes out the result.  That is, no fringes would appear.

Things are different if you record not just the instantaneous interference pattern but rather a correlation between the two movable detectors.  Correlation, in this case, means answering this question: when detector 1 observes light at a coordinate x1 how often does detector 2 observe light at a coordinate x2?

Plotting such a set of correlations between the two detectors does result in an interference-like pattern, but it is important to remember that this is not a pattern of light and dark regions.  Instead, it is a higher order effect that tells you the probability of finding light “here” given that you found it “over there.”  Because scientists want to record those correlations over a range of separations between “here” and “over there” that includes separations that pass through zero, there is a problem. If the two locations are too close, the detectors would run into each other.

To avoid that a simple partially silvered mirror, commonly called a beam splitter, effectively makes two copies of the light field.  That way the two detectors can simultaneously sample the light from virtual positions that can be as close as desired and even pass through each other.  

And what about the use of stupid detectors, those for which each “click” denoting an arrival tells us only that more than zero photons have arrived? However, here the time structure of the incoming light pulse becomes important in clarifying the measurement. If we look at a short enough time, we can arrange that the probability of more than one photon is very low, so a click tells us that with good accuracy that indeed just one photon has arrived. But then if we design the light so that its limited coherence time is larger than the recovery time of our stupid detectors, it is possible for the detector to tell us that a specific number of photons were recorded, perhaps 3 or 10, not just the superfluous  “more than zero” answer.  “In this way, we get dumb detectors to act in a smart way,” says Migdall.

This improved counting the number of photons, or equivalently the intensity of the light at various places at the measuring screen, ensures that the set of correlations between the two detectors does result in an interference-like pattern in those correlations.  Not only that, but the fringes of this correlation pattern---the distance between the successive peaks---can be as small as 30 nm.

So while seeing an interference pattern could not be accomplished with dumb detectors, it could be accomplished by engineering the properties of the light source to accommodate the lack of ability of the detectors and then accumulating a pattern of correlation between two detectors.

Considering that the incoming light has a wavelength of 800 nm, the pattern is sharper by a factor of 20 or more from what you would expect if the diffraction limitation were at work.  The fact that the light used is thermal in nature, and not coherent, makes the achievement more striking.

This correlation method is not the same as imaging an object.  But the ease and the degree to which the conventional diffraction resolution limit could be surmounted will certainly encourage a look for specific applications that might take advantage of that remarkable feature. 

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Superfluid interference
  • A cold-atom ammeter
  • A superfluid current is only as strong as its weak link
  • October 8, 2014

In certain exotic situations, a collection of atoms can transition to a superfluid state, flouting the normal rules of liquid behavior. Unlike a normal, viscous fluid, the atoms in a superfluid flow unhindered by friction. This remarkable free motion is similar to the movement of electron pairs in a superconductor, the prefix ‘super’ in both cases describing the phenomenon of resistanceless flow. Harnessing this effect is of particular interest in the field of atomtronics, since superfluid atom circuits can recreate the functionality of superconductor circuits, with atoms zipping about instead of electrons. Now, JQI scientists have added an important technique to the atomtronics arsenal, a method for analyzing a superfluid circuit component known as a ‘weak link’. The result, detailed in the online journal Physical Review X, is the first direct measurement of the current-phase relationship of a weak link in a cold atom system.

“What we have done is invented a way to characterize a particular circuit element [in a superfluid atomtronic circuit],” says Stephen Eckel, lead author of the paper. “This is similar to characterizing a component in an ordinary electrical circuit, where one measures the current that flows through the component vs. the voltage across it.”

Properly designing an electronic circuit means knowing how each component in the circuit affects the flow of electrons. Otherwise, your circuit won’t function as expected, and at worst case will torch your components into uselessness. This is similar to the plumbing in a house, where the shower, sink, toilet, etc. all need the proper amount of water and water pressure to operate. Measuring the current-voltage relationship, or how the flow of current changes based on a voltage change, is an important way to characterize a circuit element. For instance, a resistor will have a different current-voltage relationship than a diode or capacitor. In a superfluid atom circuit, an analogous measurement of interest is the current-phase relationship, basically how a particular atomtronic element changes the flow of atoms.

Interferometric Investigations

The experiment, which took place at a JQI lab on the NIST-Gaithersburg campus, involves cooling roughly 800,000 sodium atoms down to an extremely low temperature, around a decidedly chilly hundred billionths of a degree above absolute zero. At these temperatures, the atoms behave as matter waves, overlapping to form something called a Bose-Einstein condensate (BEC). The scientists confine the condensate between a sheet-like horizontal laser and a target shaped vertical laser. This creates two distinct clouds, the inner one shaped like a disc and the outer shaped like a ring. The scientists then apply another laser to the outer condensate, slicing the ring vertically. This laser imparts a repulsive force to the atoms, driving them apart and creating a low density region known as a weak link (Related article on this group's research set-up).

The weak link used in the experiment is like the thin neck between reservoirs of sand in an hourglass, constricting the flow of atoms across it. Naturally, you might expect that a constriction would create resistance. Consider pouring syrup through a straw instead of a bottle -- this would be a very impractical method of syrup delivery. However, due to the special properties of the weak link, the atoms can flow freely across the link, preserving superfluidity. This doesn’t mean the link has no influence: when rotated around the ring, the weak link acts kind of like a laser ‘spoon’, ‘stirring’ the atoms and driving an atom current.

After stirring the ring of atoms, the scientists turn off all the lasers, allowing the two BECs to expand towards each other. Like ripples on a pond, these clouds interfere both constructively and destructively, forming intensity peaks and valleys. The researchers can use the resulting interference pattern to discern features of the system, a process called interferometry.

Gleaning useful data from an interference pattern means having a reference wave. In this case, the inner BEC serves as a phase reference. A way to think of phase is in the arrival of a new day. A person who lives on the other side of the planet from you experiences a new day at the same frequency as you do, once every 24 hours. However, the arrival of the day is offset in time, that is to say there is a phase difference between your day and the other person's day.

As the two BECs interfere, the position of the interference fringes (peaks in the wave) depends on the relative phase between the two condensates. If a current is present in the outer ring-shaped BEC, the relative phase is changing as a function of the position of the ring, and the interference fringes assume a spiral pattern. By tracing a single arm of the spiral a full 360 degrees and measuring the radial difference between the beginning and end of the trace, the researchers can extract the magnitude of the superfluid current present in the ring.

They now know the current, so what about the phase across the weak link? The same interferometry process can be applied to the two sides of the weak link, again yielding a phase difference. When coupled with the measured current, the scientists now have a measure of how much current flows through the weak link as a function of the phase difference across the link, the current-phase relationship. For their system, the group found this dependence to be roughly linear (in agreement with their model).

A different scenario, where the weak link has a smaller profile, might produce a different current response, one where non-linear effects play a larger role. Extending the same methods makes it possible to characterize these weak links as well, and could be used to verify a type of weak link called a Josephson junction, an important superconducting element, in a cold atom system. Characterizing the current-phase relationship of other atomtronic components should also be possible, broadening the capabilities of researchers to analyze and design new atomtronic systems.

This same lab, led by JQI fellow Gretchen Campbell, had recently employed a weak link to demonstrate hysteresis, an important property of many electronic systems, in a cold atom circuit. Better characterizing the weak link itself may help realize more complex circuits.  “We’re very excited about this technique,” Campbell says, “and hope that it will help us to design and understand more complicated systems in the future."

This article was written by S. Kelley/JQI.

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  • On-chip Topological Light
  • First measurements of transmission and delay
  • August 21, 2014 Activity 2

Topological transport of light is the photonic analog of topological electron flow in certain semiconductors. In the electron case, the current flows around the edge of the material but not through the bulk. It is “topological” in that even if electrons encounter impurities in the material the electrons will continue to flow without losing energy.

In the photonic equivalent, light flows not through and around a regular material but in a meta-material consisting of an array of tiny glass loops fabricated on a silicon substrate. If the loops are engineered just right, the topological feature appears: light sent into the array easily circulates around the edge with very little energy loss (even if some of the loops aren’t working) while light taking an interior route suffers loss.

Mohammad Hafezi and his colleagues at the Joint Quantum Institute have published a series of papers on the subject of topological light. The first pointed out the potential application of robustness in delay lines and conceived a scheme to implement quantum Hall models in arrays of photonic loops. In photonics, signals sometimes need to be delayed, usually by sending light into a kilometers-long loop of optical fiber. In an on-chip scheme, such delays could be accomplished on the microscale; this is in addition to the energy-loss reduction made possible by topological robustness (see Miniaturizing Delay Lines below).

The next paper reported on results from an actual experiment. Since the tiny loops aren’t perfect, they do allow a bit of light to escape vertically out of the plane of the array (see Topological Light below). This faint light allowed the JQI experimenters to image the course of light. This confirmed the plan that light persists when it goes around the edge of the array but suffers energy loss when traveling through the bulk.

The third paper, appearing now in Physical Review Letters, and highlighted in a Viewpoint, actually delivers detailed measurements of the transmission (how much energy is lost) and delay for edge-state light and for bulk-route light (see reference publication below). The paper is notable enough to have received an “editor’s suggestion” designation. “Apart from the potential photonic-chip applications of this scheme,” said Hafezi, “this photonic platform could allow us to investigate fundamental quantum transport properties.”

Another measured quality is consistency. Sunil Mittal, a graduate student at the University of Maryland and first author on the paper, points out that microchip manufacturing is not a perfect process. “Irregularities in integrated photonic device fabrication usually result in device-to-device performance variations,” he said. And this usually undercuts the microchip performance. But with topological protection (photons traveling at the edge of the array are practically invulnerable to impurities) at work, consistency is greatly strengthened.

Indeed, the authors, reporting trials with numerous array samples, reveal that for light taking the bulk (interior) route in the array, the delay and transmission of light can vary a lot, whereas for light making the edge route, the amount of energy loss is regularly less and the time delay for signals more consistent. Robustness and consistency are vital if you want to integrate such arrays into photonic schemes for processing quantum information.

How does the topological property emerge at the microscopic level? First, look at the electron topological behavior, which is an offshoot of the quantum Hall effect. Electrons, under the influence of an applied magnetic field can execute tiny cyclonic orbits. In some materials, called topological insulators, no external magnetic field is needed since the necessary field is supplied by spin-orbit interactions -- that is, the coupling between the orbital motion of electrons and their spins. In these materials the conduction regime is topological: the material is conductive around the edge but is an insulator in the interior.

And now for the photonic equivalent. Light waves do not usually feel magnetic fields, and if they do it is very weak. In the photonic case, the equivalent of a magnetic field is supplied by a subtle phase shift imposed on the light as it circulates around the loops. Actually the loops in the array are of two kinds: resonator loops designed to exactly accommodate light at a certain frequency, allowing the waves to circle the loop many times. Link loops, by contrast, are not exactly suited to the waves, and are designed chiefly to pass the light onto the neighboring resonator loop.

Light that circulates around one unit cell of the loop array will undergo a slight phase change, an amount signified by the letter phi. That is, the light signal, in coming around the unit cell, re-arrives where it started advanced or retarded just a bit from its original condition. Just this amount of change imparts the topological robustness to the global transmission of the light in the array.

In summary, documented on-chip light delay and a robust, consistent, low-loss transport of light has now been demonstrated. The transport of light is tunable to a range of frequencies and the chip can be manufactured using standard micro-fabrications techniques.

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  • Spin Diagnostics
  • MRI for a quantum simulation
  • July 31, 2014

Magnetic resonance imaging (MRI), which is the medical application of nuclear magnetic resonance spectroscopy, is a powerful diagnostic tool. MRI works by resonantly exciting hydrogen atoms and measuring the relaxation time -- different materials return to equilibrium at different rates; this is how contrast develops (i.e. between soft and hard tissue). By comparing the measurements to a known spectrum of relaxation times, medical professionals can determine whether the imaged tissue is muscle, bone, or even a cancerous growth. At its heart, MRI operates by quantum principles, and the underlying spectroscopic techniques translate to other quantum systems.

Recently physicists at the Joint Quantum Institute* led by JQI Fellow Christopher Monroe have executed an MRI-like diagnostic on a crystal of interacting quantum spins. The technique reveals many features of their system, such as the spin-spin interaction strengths and the energies of various spin configurations. The protocol was published recently in the journal Science (DOI:10.1126/science.1251422). Previously, such methods existed for an array of only three spins--here, the JQI team performed proof-of-principle experiments with up to 18 spins. They predict that their method is scalable and may be useful for validating experiments with much larger ensembles of interacting spins.

‘Spin’ models are a vital mathematical representation of numerous physical phenomena including magnetism. Here, the team implements an Ising spin model, which has two central features. The spins themselves have only two options for orientation (“up” or “down”), and the interactions happen between pairs of spins, much like the interaction between bar magnets. The Ising model can be generalized to many seemingly disparate systems where there are binary choices. For instance, this model was used to study how ideas spread through social networks. In this application, spin-spin interactions represented connections between people in a network, analogous to interaction energies between magnetic spins. Here, the extent of the human connection affected how opinions spread through a social network population.

Back in the quantum laboratory, physicists have the ability to precisely study and calculate everything about a single or a small collection of essentially “textbook” spin particles within various physical platforms. Yet gaining a complete understanding of the behavior of many interacting spins is a daunting task, for both experimentalists and theorists. Ion traps are a leader in experimental studies of quantum physics, and thus well-poised for tackling this challenge.

The sheer numbers involved in large spin systems give insight into the difficulty of studying them. Consider that for N number of particles there are N(N-1)/2 two-body interactions. The interactions give rise to an energy spectrum containing 2N individual spin configurations. Here, the team does a complete analysis with 5 spins, and so there are 10 two-body interactions and 32 different spin chain configurations. Conventional computers can work with these modest numbers, but for as few as 30 spins the number of states pushes past a billion, which begins to be prohibitively complicated, particularly when the 435 separate interactions are all distinct. Physicists hope that quantum simulators can help bridge this gap.

The Ion Trap Quantum Spin Simulator

Quantum Simulation is a term that broadly describes the use of one controllable quantum system to study a second analogous, but less experimentally feasible quantum phenomenon. A full-scale quantum computer does not yet exist and classical computers often cannot solve large-scale quantum problems, thus a “quantum simulator” presents an attractive alternative for gaining insight into complex problems.

In the experiment described here, laser-cooled ytterbium atoms confined inside an ion trap are configured to simulate an array of spins. Each spin is made from two of the ion’s internal energy levels that are separated by a microwave frequency of 12.642819 GHz (billion vibrations/second). When radiation having this frequency interacts with the ion, its spin flip-flops between the two spin states, “up” and “down”.

The ions also have a vibrational frequency determined by the trap that confines them--typically around 1 MHz or 1 million vibrations/second. In the quantum regime, the quanta of vibration called a phonon can be controllably added and removed from the system with precisely controlled external laser forces. These phonons act as communication channels for the spins, and when combined with the gigahertz radiation, are used to generate a rich variety of interactions.

The simulation begins with the spins initialized into a well-known spin configuration (e.g. all of the spins in the “up” configuration). Then, the physicists apply a probe, which is a tiny oscillating electromagnetic field generated from the laser. They scan this probe to find the special “resonant” frequencies that cause the spin crystal to undergo transitions to different configurations (see Figure 1 in gallery). This energy/frequency is directly related to how the spins are interacting with each other. If the spins are interacting weakly, with only their nearest neighbors, then the transition energy will be different than when the interactions are more extended. To assemble a complete energy spectrum and measure all configurations the team must repeatedly probe the ion spins over a range of frequencies. A crucial component of this protocol is the imaging system, which allows the team to directly measure each individual ion spin in the crystal for every probe frequency.

The JQI team hopes this new tool will ease the way towards simulating larger systems and possibly other spin models.  Says Crystal Senko, JQI graduate student and lead author of this work, “Quantum simulation experiments will eventually be studying physics questions that can’t be answered in any other way, so we might not know how to tell if the experiment isn’t doing quite what we expected. That means it will be important to have many diagnostics, so that when we see something strange and interesting we can be confident that it’s interesting physics instead of just a bug in the experiment.”

Significantly, this protocol is not limited to trapped ions, and can be tailored to different simulation platforms. Just as MRI is an indispensable tool in modern medicine, this new verification technique may prove essential to the realm of quantum simulation.

This article was written by S. Kelley and E. Edwards @ JQI. 

*This research was performed at JQI in the group of Christopher Monroe, in collaboration with JQI Alum and UCLA Professor, Wes Campbell.

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  • Making Quantum Connections
  • The speed of information in a spin network
  • July 9, 2014

In quantum mechanics, interactions between particles can give rise to entanglement, which is a strange type of connection that could never be described by a non-quantum, classical theory. These connections, called quantum correlations, are present in entangled systems even if the objects are not physically linked (with wires, for example). Entanglement is at the heart of what distinguishes purely quantum systems from classical ones; it is why they are potentially useful, but it sometimes makes them very difficult to understand.

Physicists are pretty adept at controlling quantum systems and even making certain entangled states. Now JQI researchers*, led by theorist Alexey Gorshkov and experimentalist Christopher Monroe, are putting these skills to work to explore the dynamics of correlated quantum systems. What does it mean for objects to interact locally versus globally? How do local and global interactions translate into larger, increasingly connected networks? How fast can certain entanglement patterns form? These are the kinds of questions that the Monroe and Gorshkov teams are asking. Their recent results investigating how information flows through a quantum many-body system are published this week in the journal Nature (10.1038/nature13450), and in a second paper to appear in Physical Review Letters.

Researchers can engineer a rich selection of interactions in ultracold atom experiments, allowing them to explore the behavior of complex and massively intertwined quantum systems. In the experimental work from Monroe’s group, physicists examined how quickly quantum connections formed in a crystal of eleven ytterbium ions confined in an electromagnetic trap. The researchers used laser beams to implement interactions between the ions. Under these conditions, the system is described by certain types of ‘spin’ models, which are a vital mathematical representation of numerous physical phenomena including magnetism. Here, each atomic ion has isolated internal energy levels that represent the various states of spin.

In the presence of carefully chosen laser beams the ion spins can influence their neighbors, both near and far. In fact, tuning the strength and form of this spin-spin interaction is a key feature of the design. In Monroe's lab, physicists can study different types of correlated states within a single pristine quantum environment (Click here to learn about how this is possible with a crystal of atomic ions).

To see dynamics the researchers initially prepared the ion spin system in an uncorrelated state. Next, they abruptly turned on a global spin-spin interaction. The system is effectively pushed off-balance by such a fast change and the spins react, evolving under the new conditions.The team took snapshots of the ion spins at different times and observed the speed at which quantum correlations grew.

The spin models themselves do not have an explicitly built-in limit on how fast such information can propagate. The ultimate limit, in both classical and quantum systems, is given by the speed of light. However, decades ago, physicists showed that a slower information speed limit emerges due to some types of spin-spin interactions, similar to sound propagation in mechanical systems. While the limits are better known in the case where spins predominantly influence their closest neighbors, calculating constraints on information propagation in the presence of more extended interactions remains challenging. Intuitively, the more an object interacts with other distant objects, the faster the correlations between distant regions of a network should form. Indeed, the experimental group observes that long-range interactions provide a comparative speed-up for sending information across the ion-spin crystal. In the paper appearing in Physical Review Letters, Gorshkov’s team improves existing theory to much more accurately predict the speed limits for correlation formation, in the presence of interactions ranging from nearest-neighbor to long-range.

Verifying and forming a complete understanding of quantum information propagation is certainly not the end of the story; this also has many profound implications for our understanding of quantum systems more generally. For example, the growth of entanglement, which is a form of information that must obey the bounds described above, is intimately related to the difficulty of modeling quantum systems on a computer. Dr. Michael Foss-Feig explains, “From a theorist’s perspective, the experiments are cool because if you want to do something with a quantum simulator that actually pushes beyond what calculations can tell you, doing dynamics with long-range interacting systems is expected to be a pretty good way to do that. In this case, entanglement can grow to a point that our methods for calculating things about a many-body system break down.”

Theorist Dr. Zhexuan Gong states that in the context of both works, “We are trying to put bounds on how fast correlation and entanglement can form in a generic many-body system. These bounds are very useful because with long-range interactions, our mathematical tools and state-of-the-art computers can hardly succeed at predicting the properties of the system. We would then need to either use these theoretical bounds or a laboratory quantum simulator to tell us what interesting properties a large and complicated network of spins possess. These bounds will also serve as a guideline on what interaction pattern one should achieve experimentally to greatly speed up information propagation and entanglement generation, both key for building a fast quantum computer or a fast quantum network.”

From the experimental side, Dr. Phil Richerme gives his perspective, “We are trying to build the world’s best experimental platform for evolving the Schrodinger equation [math that describes how properties of a quantum system change in time]. We have this ability to set up the system in a known state and turn the crank and let it evolve and then make measurements at the end. For system sizes not much larger than what we have here, doing this becomes impossible for a conventional computer.” 

This news item was written by E. Edwards/JQI. 

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  • Advanced Light
  • Sending entangled beams through fast-light materials
  • May 27, 2014

Michael Lewis’s bestselling book Flash Boys describes how some brokers, engaging in high frequency trading, exploit fast telecommunications to gain fraction-of-a-second advantage in the buying and selling of stocks. But you don’t need to have billions of dollars riding on this-second securities transactions to appreciate the importance of fast signal processing. From internet to video streaming, we want things fast.

Paul Lett and his colleagues at the Joint Quantum Institute (1) specialize in producing modulated beams of light for encoding information. They haven’t found a way to move data faster than c, the speed of light in a vacuum, but in a new experiment they have looked at how light traveling through so called “fast-light” materials does seem to advance faster than c, at least in one limited sense. They report their results (online as of 25 May 2014) in the journal Nature Photonics (2).

Seeing how light can be manipulated in this way requires a look at several key concepts, such as entanglement, mutual information, and anomalous dispersion. At the end we’ll arrive at a forefront result.


Much research at JQI is devoted to the processing of quantum information, information coded in the form of qubits. Qubits, in turn are tiny quantum systems---sometimes electrons trapped in a semiconductor, sometimes atoms or ions held in a trap---maintained in a superposition of states. The utility of qubits increases when two or more of them can be yoked into a larger quantum arrangement, a process called entanglement. Two entangled photons are not really sovereign particles but parts of a single quantum entity.

The basis of entanglement is often a discrete variable, such as electron spin (whose value can be up or down) or photon polarization (say, horizontal or vertical). The essence of entanglement is this: while the polarization of each photon is indeterminate until a measurement is made, once you measure the polarization of one of the pair of entangled photons, you automatically know the other photon’s polarization too.

But the mode of entanglement can also be vested in a continuous variable. In Lett’s lab, for instance, two whole light beams can be entangled. Here the operative variable is not polarization but phase (how far along in the cycle of the wave you are) or intensity (how many photons are in the beam). For a light beam, phase and intensity are not discrete (up or down) but continuous in variability.


Biologists examining the un-seamed strands of DNA can (courtesy of the correlated nature of nucleic acid constituents) deduce the sequence of bases along one strand by examining the sequence of the other strand. So it is with entangled beams. A slight fluctuation of the instantaneous intensity of one beam (such fluctuations are inevitable because of the Heisenberg uncertainty principle) will be matched by a comparable fluctuation in the other beam.

Lett and his colleagues make entangled beams in a process called four-wave mixing. A laser beam (pump beam) enters a vapor-filled cell. Here two photons from the pump beam are converted into two daughter photons proceeding onwards with different energies and directions. These photons constitute beams in their own right, one called the probe beam, the other called the conjugate beam. Both of these beams are too weak to measure directly. Instead each beam enters a beam splitter (yellow disk in the drawing below) where its light can be combined with light from a local oscillator (which also serves as a phase reference). The ensuing interference patterns provide aggregate phase or intensity information for the two beams.

When the beam entanglement is perfect, the mutual correlation is 1. When studying the intensity fluctuations of one beam tells you nothing about those of the other beam, then the mutual correlation is 0.


In a famous experiment, Isaac Newton showed how incoming sunlight split apart into a spectrum of colors when it passed through a prism. The degree of wavelength-dependent dispersion for a material that causes this splitting of colors is referred to as its index of refraction.

In most materials the index is larger than 1. For plain window glass, it is about 1.4; for water it is 1.33 for visible light, and gradually increases as the frequency of the light goes up. At much higher frequency (equivalent to shorter wavelength), though, the index can change its value abruptly and go down. For glass, that occurs at ultraviolet wavelengths so you don’t ordinarily see this “anomalous dispersion” effect. In a warm vapor of rubidium atoms, however, (and especially when modified with laser light) the effect can occur at infrared wavelengths, and here is where the JQI experiment looks.

In the figure above notice that the conjugate beam is sent through a second cell, filled with rubidium vapor. Here the beam is subject to dispersion. The JQI experiment aims to study how the entanglement of this conjugate beam with the probe beam (subject to no dispersion) holds up.

When the refraction is “normal”---that is, when index of refraction causes ordinary dispersion---the light signal is slowed in comparison with the beam which doesn’t undergo dispersion. For this set of conditions, the cell is referred to as a “slow-light” material. When, however, the frequency is just right, the conjugate beam will undergo anomalous dispersion. When the different frequency components that constitute a pulse or intensity fluctuation reformulate themselves as they emerge from the cell, they will now be just slightly ahead of a pulse that hadn’t gone through the cell. (To make a proper measurement of delay one needs two entangled beams---beams whose fluctuations are related.)


No, the JQI researchers are not saying that any information is traveling faster than c. The figure above shows that the peak for the mutual information for the fast-light-material is indeed ahead of the comparable peaks for an unscattered beam or for a beam emerging from a slow-light material. It turns out that the cost of achieving anomalous dispersion at all has been that additional gain (amplification) is needed, and this amplification imposes noise onto the signal.

This inherent limitation in extracting useful information from an incoming light beam is even more pronounced with beams containing (on average) one or less-than-one photon. Such dilute beams are desirable in many quantum experiments where measurement control or the storage or delay of quantum information is important.

“We did these experiments not to try to violate causality, said Paul Lett, “but because we wanted to see the fundamental way that quantum noise “enforces” causality, and working near the limits of quantum noise also lets us examine the somewhat surprising differences between slow and fast light materials when it comes to the transport of information.”

(1) The Joint Quantum Institute is operated jointly by the National Institute of Standards and Technology in Gaithersburg, MD and the University of Maryland in College Park.

(2) See related publication below.

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  • JQI papers featured as New Journal of Physics "Highlights of 2013"
  • Zitterbewegung, Topology, and more
  • May 14, 2014

Papers from the groups of Ian Spielman and Jake Taylor were recently chosen as "Highlights of 2013" by the New Journal of Physics. The articles are listed below To see more highlights, visit

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  • JQI undergraduate researcher Geoffrey Ji receives Goldwater Scholarship
  • Three UMD Students Win Goldwater Scholarships, Another Earns Honorable Mention
  • March 25, 2014

From CMNS at UMD

Three University of Maryland students have been awarded scholarships by the Barry M. Goldwater Scholarship and Excellence in Education Foundation, which encourages students to pursue advanced study and careers in the sciences, engineering and mathematics. A fourth student received honorable mention.

UMD juniors Geoffrey Ji, Michael Mandler and Rafael Setra were among the 283 Barry Goldwater Scholars selected from 1,166 students nominated nationally this year. Junior Daniel Farias received honorable mention. The four students, who are all members of the UMD Honors College, plan to pursue doctoral degrees in their areas of study and to become university professors.

Geoffrey Ji, a JQI undergraduate researcher supported by the PFC, is majoring in physics, mathematics, economics and computer science. He has been conducting quantum science research for two years in the laboratory of JQI Fellow and Bice Zorn Professor of Physics, Chris Monroe.

“Geoffrey has almost single-handedly outfitted advanced digital and analog electronic control circuits, in addition to writing impressive computer code that will soon be adopted by most of our other projects,” said Monroe.

Ji also conducted theoretical nuclear physics research with Paulo Bedaque, associate professor of physics, which resulted in co-authorship of a peer-reviewed publication in the journal Physical Review D.

Mandler, a double major in chemistry and biological sciences, published a first-author peer-reviewed paper in the journal Organic Letters in January 2014. This paper joins three other peer-reviewed publications on which Mandler is a co-author. For his research, Mandler develops novel synthesis pathways for organic catalysts that may reduce the time and/or cost of their commercial production for drug development and other applications. 

“Michael is one of the most talented undergraduate students that I have mentored in my 46-year career, and that would place him among past undergraduate students who are now internationally known professors at top-ranked universities and colleges, as well as those who are prominent executives in industry,” said Chemistry and Biochemistry Professor Michael Doyle, who is Mandler’s mentor.

Setra, a double major in mathematics and electrical engineering, conducts research with Thomas Murphy, electrical and computer engineering professor and director of the Institute for Research in Electronics and Applied Physics; Rajarshi Roy, physics professor and director of the Institute for Physical Science and Technology; and Wojciech Czaja, mathematics professor.  Setra placed second nationally in the Siemens Competition in Math, Science and Technology in 2010.

“The project we gave to Rafael was related to overcoming a nonlinear signal scattering problem that is pervasive in optical fibers, and the project was in a research direction that had never been previously tested or initiated,” said Murphy. “In the span of just 10 weeks, Rafael taught himself about fiber optic instrumentation, measurement automation, splicing, and spectrometry, and he designed, purchased and constructed an experiment to test his hypotheses.”

Honorable mention recipient Farias is a triple major in computer science, electrical engineering, and mathematics. He has conducted research projects with Daniel Butts, assistant professor of biology, and Neil Spring, associate professor in computer science with an appointment in the University of Maryland Institute for Advanced Computer Studies. With Butts, Farias adapted a model that was developed to describe neural signal processing in the visual midbrain to work in the auditory midbrain.

The Goldwater Scholarship program was created in 1986 to identify students of outstanding ability and promise in science, engineering and mathematics, and to encourage their pursuit of advanced study and research careers. The Goldwater Foundation has honored 47 University of Maryland winners since the program’s first award was given in 1989. Prior Goldwater scholars and nominees from UMD have continued their impressive academic and research pursuits at leading institutions around the world and have garnered additional recognition as:

  • National Science Foundation graduate research fellows
  • Gates Cambridge and Churchill Scholars (University of Cambridge, United Kingdom)
  • A Clarendon Fund Scholar (University of Oxford, United Kingdom)

Colleges and universities may submit up to four nominations annually for these awards. Goldwater scholars receive one- or two-year scholarships that cover the cost of tuition, fees, books, and room and board up to $7,500 per year. These scholarships are a stepping-stone to future support for their research careers. 

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  • How do you build a large-scale quantum computer?
  • Physicists propose a modular quantum computer architecture that offers scalability to large numbers of qubits
  • February 25, 2014

How do you build a universal quantum computer? Turns out, this question was addressed by theoretical physicists about 15 years ago. The answer was laid out in a research paper and has become known as the DiVincenzo criteria [See Gallery Sidebar for information on this criteria]. The prescription is pretty clear at a glance; yet in practice the physical implementation of a full-scale universal quantum computer remains an extraordinary challenge.

To glimpse the difficulty of this task, consider the guts of a would-be quantum computer. The computational heart is composed of multiple quantum bits, or qubits, that can each store 0 and 1 at the same time. The qubits can become “entangled,” or correlated in ways that are impossible in conventional devices. A quantum computing device must create and maintain these quantum connections in order to have a speed and storage advantage over any conventional computer. That’s the upside. The difficulty arises because harnessing entanglement for computation only works when the qubits are almost completely isolated from the outside world. Isolation and control becomes much more difficult as more and more qubits are added into the computer. Basically, as quantum systems are made bigger, they generally lose their quantum-ness.  

In pursuit of a quantum computer, scientists have gained amazing control over various quantum systems. One leading platform in this broad field of research is trapped atomic ions, where nearly 20 qubits have been juxtaposed in a single quantum register. However, scaling this or any other type of qubit to much larger numbers while still contained in a single register will become increasingly difficult, as the connections will become too numerous to be reliable.

Physicists led by ion-trapper Christopher Monroe at the JQI have now proposed a modular quantum computer architecture that promises scalability to much larger numbers of qubits. This research is described in the journal Physical Review A (reference below), a topical journal of the American Physical Society. The components of this architecture have individually been tested and are available, making it a promising approach. In the paper, the authors present expected performance and scaling calculations, demonstrating that their architecture is not only viable, but in some ways, preferable when compared to related schemes.

Individual qubit modules are at the computational center of this design, each one consisting of a small crystal of perhaps 10-100 trapped ions confined with electromagnetic fields. Qubits are stored in each atomic ion’s internal energy levels. Logical gates can be performed locally within a single module, and two or more ions can be entangled using the collective properties of the ions in a module.

One or more qubits from the ion trap modules are then networked through a second layer of optical fiber photonic interconnects. This higher-level layer hybridizes photonic and ion-trap technology, where the quantum state of the ion qubits is linked to that of the photons that the ions themselves emit. Photonics is a natural choice as an information bus as it is proven technology and already used for conventional information flow. In this design, the fibers are directed to a reconfigurable switch, so that any set of modules could be connected. The switch system, which incorporates special micro-electromechanical mirrors (MEMs) to direct light into different fiber ports, would allow for entanglement between arbitrary modules and on-demand distribution of quantum information.

The defining feature of this new architecture is that it is modular, meaning that several identical modules composed of smaller registers are connected in a way that is inherently scalable.  Modularity is a common property of complex systems, from social networks to biological function, and will likely be a necessary component of any future large-scale quantum computer. Monroe explains,"This is the only way to imagine scaling to larger quantum systems, by building them in smaller standard units and hooking them together. In this case, we know how to engineer every aspect of the architecture."

In conventional computers, modularity is routinely exploited to realize the massive interconnects required in semiconductor devices, which themselves have been successfully miniaturized and integrated with other electronics and photonics. The first programmable computers were the size of large rooms and used vacuum tubes, and now people have an incredible computer literally at their fingertips. Today’s processors have billions of semiconductor transistors fabricated on chips that are only about a centimeter across.

Similar fabrication techniques are now used to construct computer chip-style ion-traps, sometimes with integrated optics. The modular quantum architecture proposed in this research would not only allow many ion-trap chips to be tied together, but could also be exploited with alternative qubit modules that couple easily to photons such as qubits made from nitrogen vacancy centers in diamond or ultracold atomic gases (the neutral cousin of ion-traps). 

For more on ion traps other research related to this work, see related links below and also visit 

This article was written by E. Edwards/JQI

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  • Stirring-up atomtronics in a quantum circuit
  • What’s so ‘super’ about this superfluid
  • February 12, 2014

Atomtronics is an emerging technology whereby physicists use ensembles of atoms to build analogs to electronic circuit elements. Modern electronics relies on utilizing the charge properties of the electron. Using lasers and magnetic fields, atomic systems can be engineered to have behavior analogous to that of electrons, making them an exciting platform for studying and generating alternatives to charge-based electronics.

Using a superfluid atomtronic circuit, JQI physicists, led by Gretchen Campbell, have demonstrated a tool that is critical to electronics: hysteresis. This is the first time that hysteresis has been observed in an ultracold atomic gas. This research is published in the February 13 issue of Nature magazine, whose cover features an artistic impression of the atomtronic system.

Lead author Stephen Eckel explains, “Hysteresis is ubiquitous in electronics. For example, this effect is used in writing information to hard drives as well as other memory devices.  It’s also used in certain types of sensors and in noise filters such as the Schmitt trigger.” Here is an example demonstrating how this common trigger is employed to provide hysteresis.  Consider an air-conditioning thermostat, which contains a switch to regulate a fan. The user sets a desired temperature. When the room air exceeds this temperature, a fan switches on to cool the room. When does the fan know to turn off? The fan actually brings the temperature lower to a different set-point before turning off. This mismatch between the turn-on and turn-off temperature set-points is an example of hysteresis and prevents fast switching of the fan, which would be highly inefficient.

In the above example, the hysteresis is programmed into the electronic circuit. In this research, physicists observed hysteresis that is an inherent natural property of a quantum fluid. 400,000 sodium atoms are cooled to condensation, forming a type of quantum matter called a Bose-Einstein condensate (BEC), which has a temperature around 0.000000100 Kelvin (0 Kelvin is absolute zero). The atoms reside in a doughnut-shaped trap that is only marginally bigger than a human red blood cell. A focused laser beam intersects the ring trap and is used to stir the quantum fluid around the ring.

While BECs are made from a dilute gas of atoms less dense than air, they have unusual collective properties, making them more like a fluid—or in this case a superfluid.  What does this mean? First discovered in liquid helium in 1937, this form of matter, under some conditions, can flow persistently, undeterred by friction. A consequence of this behavior is that the fluid flow or rotational velocity around the team’s ring trap is quantized, meaning it can only spin at certain specific speeds. This is unlike a non-quantum (classical) system, where its rotation can vary continuously and the viscosity of the fluid plays a substantial role.

Because of the characteristic lack of viscosity in a superfluid, stirring this system induces drastically different behavior. Here, physicists stir the quantum fluid, yet the fluid does not speed up continuously. At a critical stir-rate the fluid jumps from having no-rotation to rotating at a fixed velocity. The stable velocities are a multiple of a quantity that is determined by the trap size and the atomic mass.

This same laboratory has previously demonstrated persistent currents and this quantized velocity behavior in superfluid atomic gases. Now they have explored what happens when they try to stop the rotation, or reverse the system back to its initial velocity state. Without hysteresis, they could achieve this by reducing the stir-rate back below the critical value causing the rotation to cease. In fact, they observe that they have to go far below the critical stir-rate, and in some cases reverse the direction of stirring, to see the fluid return to the lower quantum velocity state.

Controlling this hysteresis opens up new possibilities for building a practical atomtronic device. For instance, there are specialized superconducting electronic circuits that are precisely controlled by magnetic fields and in turn, small magnetic fields affect the behavior of the circuit itself. Thus, these devices, called SQuIDs (superconducting quantum interference devices) are used as magnetic field sensors. “Our current circuit is analogous to a specific kind of SQuID called an RF-SQuID”, says Campbell. “In our atomtronic version of the SQuID, the focused laser beam induces rotation when the speed of the laser beam “spoon” hits a critical value. We can control where that transition occurs by varying the properties of the “spoon”. Thus, the atomtronic circuit could be used as an inertial sensor.”

This two-velocity state quantum system has the ingredients for making a qubit. However, this idea has some significant obstacles to overcome before it could be a viable choice. Atomtronics is a young technology and physicists are still trying to understand these systems and their potential. One current focus for Campbell’s team includes exploring the properties and capabilities of the novel device by adding complexities such as a second ring.

This research was supported by the NSF Physics Frontier Center at JQI. 

This article was written by E. Edwards/JQI

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  • Making Color
  • When two red photons make a blue photon
  • January 29, 2014

Color is strange, mainly due to perception. Setting aside complex brain processes, what we see is the result of light absorption, emission, and reflection. Trees appear green because atoms inside the leaves are emitting and/or reflecting green photons. Semiconductor LED brake lights emit single color light when electrical current passes through the devices.

Here’s a question: Can scientists generate any color of light? The answer is not really, but the invention of the laser in 1960 opened new doors for this endeavor. An early experiment injected high-power laser light through quartz and out popped a different color. This sparked the field of nonlinear optics and with it, a new method of color generation became possible: frequency conversion. 

Not all crystals can perform this trick and only through careful fabrication of certain materials is frequency conversion possible. In a result published in Nature Communications (DOI: 10.1038/ncomms4109), scientists* demonstrate a new microstructure that does what’s called second harmonic generation (SHG), where the output light has twice the frequency as the input. This new device is a factor of 1000 smaller than previous frequency converters.

You can’t really get something from nothing here. Physics demands that both energy and momentum are conserved in the frequency-doubling process. The energy of light is directly related to its frequency through a fundamental constant, thus this conservation law is automatically satisfied.  Two photons of fixed energy pass into the conversion crystal and the output photon has a frequency, thus energy, equal to their sum.

The challenging part is momentum conservation and achieving it takes careful engineering. This difficulty arises because light has an associated direction of travel. Materials bend and delay light, and how it occurs is very material dependent. Even more, different frequencies (colors) are bent and delayed differently by a given material. This is called dispersion and is perhaps most familiar as a rainbow, where the constituent colors of sunlight are separated.

Even with dispersion, some materials have naturally occurring refractive properties that allow momentum-matching, and thus frequency conversion. Until about 20 years ago, these materials were the only option for frequency conversion. In the 1990s, scientists began to tackle the momentum conservation issue using a technique called quasi-phase matching (QPM). 

When a light wave enters and moves through a crystal its properties such as velocity are altered depending on its color. In the case of second-harmonic generation, the injection light strongly interacts with the medium and a second color, having twice the frequency, is generated. Due to dispersion, the second light wave will be delayed. In QPM, scientists vary the spacing and orientation between the internal crystal layers to compensate for the delay, such that momentum conservation between the injection and output light is conserved. This method of QPM is successful but can be difficult from a fabrication point-of-view. Moreover, miniaturizing their overall size for integration onto chips is limited. This is because the frequency conversion process depends on the physical length of the interaction medium, thus scaling down these types of crystals will lead to an inherent reduction in efficiency.

Now this team has demonstrated a new, arguably simpler way, to achieve QPM and thus frequency conversion. In the new design, gallium arsenide (GaAs) is fabricated into a micrometer-sized disk ‘whispering gallery’ cavity. Notably, GaAs has one of the largest second-harmonic frequency conversion constants measured. Previously, scientists have harnessed its extremely nonlinear properties through the layer-varying QPM method, leading to device sizes in the centimeter range. This new device is 1000 times smaller.

In the experiment, light from a tapered optical fiber is injected into the cavity. When light travels in a loop with the proper orientation, as opposed to a linear geometry, QPM, and therefore color conversion is achieved. This team skirts around the miniaturization problem because the light can interact many times with the medium by circulating around the disk, yet the overall size can remain small. Using a cavity also means that since the power builds up in the microdisk, less injection power can be used. Think of the architectural example of a whispering gallery—wherein sound waves add together such that small input signals (whispers) can be heard. This resonant enhancement also happens for light trapped inside microdisk cavities.

NIST scientist and author Glenn Solomon continues, “Through a combination of microcavity engineering and nonlinear optics, we can create a very small frequency conversion device that could be more easily integrated onto optical chips.” 

Lead author Paulina Kuo, who is currently doing research at NIST in the Information Technology Laboratory,adds, “I am excited because this method for phase-matching is brand new. It is amazing that the crystal itself can provide the phase-matching to ensure momentum conservation, and it's promising to see efficient optical frequency conversion in a really tiny volume.”

In terms of future quantum information applications, the simple harmonic generation process can be considered as parametric down conversion (PDC) in reverse. PDC is a method for generating entangled photon pairs and so this device could provide a new technique for accomplishing this.

Gallium arsenide (GaAs) is a common semiconductor and has added benefits such as transmitting and emitting in the infrared (IR) and near IR light, respectively. IR-colored light has applications that include telecommunications and chemical sensing. Kuo adds, “The presence of an absorbing species affects the cavity resonance conditions and, in turn, the amount of frequency conversion in the microdisk. Thus, this device could be used in novel sensing applications.”

* Author information: 

Paulina Kuo, at the time of this research, was a postdoctoral researcher at JQI. Kuo is now a staff physicist working in NIST’s Information Technology Laboratory 

Glenn Solomon is a staff physicist at NIST and a JQI Fellow

Jorge Bravo-Abad, at the time of this research, was a visiting professor to JQI. He is now a Ramón y Cajal Fellow (Tenure track) professor in the Condensed Matter Theory Department at Universidad Autónoma de Madrid. Notably, he recently received a MIT TR35-Spain Award, recognizing him as one of the top 10 Spanish innovators under the age of 35

This article was written by E. Edwards/JQI

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  • Quantum Gimbal
  • JQI scientists predict coreless vortex in ultracold atoms
  • December 2, 2013

Swirling, persistent vortices can be created in superfluid helium. Generally no atoms sit at the eye of these miniature hurricanes. Things might be different in condensates of ultracold atoms. Theorists at the Joint Quantum Institute predict that for some elements a vortex of atoms can be produced which pivots around another sample of atoms at rest in the middle. Such a quantum gimbal has been observed in condensates of two atomic species but never before in a swarm of exclusively one type of atoms in a state of lowest energy.

The JQI search for new forms of quantum reality occurs not in an actual lab. Instead it takes place in a dedicated computer which, over hundreds of hours, simulates the magnetic interactions of a million atoms or so under various conditions. When the “experiment” is over the researchers take the numerical results and plot them up as if they were maps showing the disposition of actual atoms---either their positions in space or the directions of their spins (as if the atoms were little bar magnets).

In effect the simulation probes atomic matter for new types of magnetic order. It deduces what happens when magnetic atoms, such as those of the element dysprosium, are cooled into a Bose Einstein condensate (BEC) and then subjected to laser beams that make the atoms’ interactions spin-dependent. The strength of the interaction is also proportional to the density of atoms; squeeze more and more atoms into the ensemble and a variety of patterns---so called “spin textures”---emerges in succession. The results, published in the journal Physical Review Letters (see first related publication), reveals the occurrence of coreless vortices and shows an array of five such new magnetic phases, a feat interesting enough for the JQI paper to have received an “editor’s pick” designation in the journal.


Why use dysprosium? Because it is (along with holmium) the most magnetic of the elements. Indeed, its magnetic dipole moment is ten times stronger than that for rubidium, an element used in many Bose Einstein condensate (BEC) experiments. This makes Dy a potential workhorse for experiments attempting to find new states of magnetic matter. Dy has a complement of 66 electrons which can arrange themselves into seventeen different lowest-energy (groundstate) configurations. So when Dy is chilled low enough in temperature you get not one but seventeen different BECs, all pretty much residing on top of each other in the center of an atom trap. Another name for a BEC consisting of co-existing, multiple-spin orientations is “Spinor condensate.”


This co-existence can be made more complicated in an interesting way by exposing the atoms to a pair of laser beams. This has the effect of toggling the atoms---which lie mostly in a plane---from one internal energy level to another. The effectiveness of this toggling is proportional to the velocity of the atoms. Thus an internal state of each atom (the energy levels) is coordinated with an external state of the atom, namely its velocity. This coordination is referred to as spin-orbit coupling, an expression that dates back to the early days of quantum physics, when scientists studied the important force between an electron’s spin and the magnetic field created by the fact that the electron was orbiting within the atom. In this early work, spin-orbit coupling could be viewed as a form of self-interaction.

More recently spin-orbit coupling was demonstrated for the first time (see related articles) in a JQI experiment with ultracold rubidium atoms; here the “orbit” in spin-orbit coupling refers not to the motion of electrons within an atom but the motion of the atoms themselves.


The effect of turning on those lasers is to “dress” the atoms. After the laser light is applied the energy levels of the atoms are so prominently reconfigured that only two of the original 17 ground states are still effectively retained in the atom trap for detailed study. These two remaining spin species---called for convenience spin-up and spin-down---form an ideal way of exploring strong interactions in a gas of ultracold atoms, all participating in a single gigantic quantum state which allows the atoms to be oriented in two ways---with their spins up or down.

In conventional matter, magnetism appears in many forms. Some of the more important are ferromagnetic ordering, in which domains (small volumes within the material) aligned together by an external magnetic field remain aligned even after the field is turned off. In anti-ferromagnets, the domains alternate in an aligned-antialigned checkerboard pattern (that is, neighboring domains are oppositely aligned). In paramagnetism regions (atoms or domains) will be attracted or aligned with an external applied field but will lose the alignment when the field is turned off.

In conventional materials, the types of magnetism depend on the intrinsic nature of the material and on impurities present. In ultracold atoms, by contrast, there are no impurities and the magnetism can be engineered at will by adjusting the laser beams (the light used to “dress” the atoms) and the density of the atoms. In a JQI experiment conducted several years ago laser beams were first used to create spin-dependent forces in cold atoms. The researchers, led by Ian Spielman, noticed something else: the atoms with different spin segregated themselves in space (see related articles).

With ultracold Dy atoms, new magnetic phases are expected to appear when the strength of the inter-atomic force is turned up by adding more atoms, and this is what the present JQI work addresses. Here too an expected self- segregation of atoms takes place. In the diagrams below, the two ground-state conditions (spin up and spin down) are depicted as red and blue. And like oil and vinegar separating themselves in an unshaken bottle of salad dressing, the red and blue occupy separate regions of space.


Two of the newly found magnetic phases merit particular attention since they possess topological order. In atomic research “topological” has come to refer to a system or a material whose properties don’t change even when subjected to perturbations such as the appearance of defects in the layout of the material or if the geometry of the material is distorted in various ways. (When a coffee cup is pulled into the shape of a donut, the geometry certainly changes, but the topology---how many holes does the object have?---stays the same.) When this topological invariance is manifested in the persistence of a quantum state or the flow of an electrical current or the propagation of light through a fiber without dissipation, topological immunity from perturbations is a potentially valuable attribute (solid material, photonic circuit, or swarm of ultracold atoms) for a material being considered as a platform for storing or processing information in a future quantum computer (see related articles).

Two of the notable magnetic textures observed in the JQI simulations are called the Skyrmion phase (the name arising originally from particle physics research conducted by Tony Skyrme) and the Meron phase (Greek for fractional). These phases will be described in more detail below. In the meantime, you can observe the spin texture for these phases in the following pictures, which map the average spin value of the condensate of Dy atoms across the two-dimensional plane in which the atoms lie. The deepness of the color (red for up and blue for down) shows how many atoms are pointing up or down respectively, while the length of the black arrows show the strength of the spin at that point in the plane.


One can see that the two species of dysprosium atoms (remember that the condensate consists of co-existing sub-populations with spins pointing up or down) are segregated. What this graph doesn’t show is that the center population of atoms are mostly at rest, while a band of outlying blue atoms circulates in cyclone fashion.

Where does the circulation come from? One of the authors of the JQI study, Brandon Anderson, explains as follows: “The spin-orbit coupling is what sets the atoms in motion. The atoms can have two types of angular momentum: spin or orbital (around the center of the trap). What SOC does in this case is exchange one unit of spin angular momentum for one unit of orbital angular momentum.”

The Skyrmion phase occurs when the density of atoms in the trap is relatively small, about 10^11 atoms per cubic centimeter and the magnetic interactions are relatively weak. In further simulations the density is increased, and the researchers expected that the contending magnetic forces would result in a rather uniform ferromagnetic-like pattern of aligned spins, and this is indeed what happens.

But when they increased the atom density up to a value of 1014---at which point the magnetic forces were a thousand-fold stronger than for the Skyrmion phase---a big surprise happened. Things got interesting again.

“This new magnetic phase was totally unexpected,” said Ryan Wilson, the lead author on the JQI paper. “The Skyrmion state had been predicted before, but we are the first to demonstrate that a ‘Meron’ spin texture could appear for atoms in their lowest-energy ground state. This is an emergent, collective-behavior phenomenon: you wouldn’t have guessed this structure would have appeared from knowledge of single atom-atom interactions.” In the Meron state the spins point away from the center of the trap.


The Skyrmion and Meron states have a superficial similarity: they both feature a “coreless vortex” in which a motionless blob of spin-up atoms is surrounded by a circulating band of spin-down atoms. The difference between the Skyrmion and Meron states comes out more clearly when these planar spin maps are transposed onto a sphere---a sort of reversal of the process by which a Mercator projection takes points from around the surface of a globe and plots them on a planar map which can spread out on a table. For Gerardus Mercator (1512-1594) the purpose was to provide practical navigation routes for explorers. The purpose for the mapping patterns of magnetic behavior onto a sphere is to exhibit the inherent topology of the system by showing what happens to the spin value of the condensate far from the center.

Let’s stop to consider that what these maps show. The condensate of atoms is, first of all, a gigantic quantum in a superposition of spin states. It is a qubit spread out, as it were, across a million atoms, each of which can be in a mixture of orientations, spin-up and spin-down. What we call a spin map is really a map of the wavefunction for the condensate at places point-by-point across the face of the plane. The wavefunction for a quantum object is itself a map for the likelihood that the object will possess certain values---in this case the average spin of the condensate---at all those points in space.

In these diagrams the arrow at the North Pole represents the spin of the condensate at the very center of the sample. The several other arrows on this sphere (the one on the left) proceeding down in latitude along any one longitude correspond to the spin as you proceed out radially on the planar map. The South Pole corresponds to the spin of the condensate at the far horizon of the plane---think of it as infinity. Even though the number of atoms (and the net spin of the condensate) would be very small out there it still worth knowing what the spin orientation would be.

Now look at the diagram for the Meron phase. Marching downwards in latitude over the sphere along a single longitude we come to the bottom (infinitely out in the plane) and things are very different. The spin has reclined so that it sticks out not perpendicularly but at a tangent. But if we make such a sequence of arrows for all the longitudes, the arrows, as we approached the South Pole, would all be pointing in different directions, very much different from the Skyrmion state where the spins all point downwards.

The conundrum of all the spins pointing in different directions for the Meron is nothing less than a mathematical singularity. In the Skyrmion state, if we were to climb back up in latitude, the spin arrow comes all the way around to where it started; we say that the Skyrmion state’s “topological charge,” or its winding number, has a value of one. For the Meron state, the arrow---even if it could negotiate the weirdness at the South Pole---would only come half way back to where it started at the North Pole. The Meron’s topological charge is one-half.

Meron states have been predicted to exist in spinor condensates but only for atoms in an excited state, not (as here) for atoms in a ground state. This state is also rather striking in a thermodynamic sense. Atom condensates can be set to swirling by physically rotating the atom trap in which they reside. Once spun up, these atoms could subsequently be brought to rest if and when they encountered a bath of surrounding atoms, which would appropriate energy from the circulating atoms. By contrast, for the Meron phase demonstrated in the JQI simulations, the atoms are circulating because of the intrinsic magnetic forces at work, not because they had been set into motion artificially by rotating the trap. Consequently these atoms---already in the lowest energy state possible---cannot surrender their energy to surrounding atoms. The circulation will continue.

Observing these strange new magnetic states at ultracold temperatures will probably be the goal of future experiments.

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  • Thurgood Marshall Academy teacher earns national accolades
  • Congratulations to Kena Allison
  • November 2, 2013

The JQI would like to congratulate Kena Allison, science teacher at Thurgood Marshall Academy, who recently received a Milken Educator Award for her "commitment to teaching science." Called the "Oscars of teaching," the award comes with $25,000. PFC graduate student researcher Jeff Grover has been visiting Allison's classroom over the last few years, integrating physics demonstrations into her curriculum. Thurgood Marshall Academy is a D.C. Public Charter School located in the Anacostia region. For more information see the Academy's newsroom.

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  • Nanofibers and Designer Light Traps
  • Efficient travel for higher modes
  • October 23, 2013

Light can be confined inside a reflective medium—a stream of water, a thread of glass fiber. In fiber optics, the light moves, trapped in a glass strand via the mechanism of total internal reflection. Light “totally” bounces at the surfaces, back and forth, carrying information over vast distances. Perhaps the word total should be taken a little loosely, at least at the surface. Here, there is what’s called an evanescent field. Webster says “evanescent” means “tending to dissipate, like vapor.” In physics, an evanescent wave is a vanishing vibration, occurring at an interface. They occur because nature doesn’t like discontinuity. The evanescent field isn’t the same as leaky, inefficient fibers; it is always present as light moves through the fiber.

“Dissipating, like vapor” doesn’t sound like something useful, but in fact, under certain conditions, evanescent waves can be used to isolate and probe atoms. Atoms trapped in the evanescent wave can interact with light traveling in the fiber. The information from that interaction is not lost; the light then re-enters, or “couples,” back into the fiber and can be captured on a camera.

The fibers in communications are relatively large compared to the wavelength of light, about 10 to 100 times greater depending on the length and use of the fiber. Harnessing evanescent fields requires reducing the fiber core. Here the team heats up a fiber while simultaneously stretching it—a technique reminiscent of glass blowing—down to a diameter of 350 nm. This is more than two times smaller than the light’s wavelength (780 nm), which means that somewhere along the line, the light can’t completely fit inside the fiber. Instead, its electric field is mostly outside the core—where it can interact with atoms, for instance. [see accompanying video of fiber pulling and injected light, monitored on a camera]

The cross-section of a light beam, such as those from lasers, often looks smooth, with the brightest part in the middle and decreasing intensity away from the center.  But this is only one example of what is called a spatial light “mode.” Higher modes, those that have more spatial components can also be made. These modes visually can look like a doughnut, cloverleaf, or another more complicated pattern.

Higher modes offer some advantages over the lowest order or “fundamental” mode. Due to their complexity, the evanescent field can have comparatively more light intensity in the region of interest –-locally just outside the fiber. These higher order modes can also be used to make different types of optical patterns. Generally speaking, to have complete control over an atomic gas, the researchers require maximal flexibility in the power and shape of the trapping light.

In this experiment, the team squeezes combinations of higher modes of the light into a nanofiber with unprecedented efficiency and purity. Ninety-seven percent passes through the tapered nanofiber—this is compared to previous work of around 20 percent. In contrast to a high purity fundamental mode, their design allows for 99 percent of the light to be in higher mode configurations. This kind of control would potentially translate into more control over evanescent atom traps.

This paper describes some of the first results from the hybrid project at JQI called “Atoms on SQUIDS.” This project seeks to couple cold gases to superconducting systems. Hybrid approaches are collectively seen as a necessity in developing quantum information resources. This particular project is extremely challenging because it requires coupling two platforms that use seemingly disparate technologies.

Check out this video to see the propagation of light in the team’s nanofiber. The experimental results described here took place outside of a vacuum chamber. The team also has a nanofiber under vacuum and has successfully trapped rubidium atoms from a laser-cooled gas.

This article was written by E. Edwards/JQI. Video produced by S. Kelley/E. Edwards/J. Robinson in collaboration with authors. The data/media from paper was re-used with permission of authors.

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  • Topological Light
  • Living on the edge
  • October 20, 2013 Activity 2

Topology -- the understanding of how things are connected -- remains abstract, even with the popular example of doughnuts and coffee cups. This concept, esoteric as it appears, is also neat because it is the basis for creating ultrastable quantum "playgrounds."  In topological systems, certain quantum behaviors can be carefully probed and even harnessed for all kinds of practical applications—from metrology to electronics. Topological insulators, for instance, have garnered attention because they exhibit quantum Hall physics practically off-the-shelf—without the fuss of stringent laboratory conditions. More recently, scientists have sought to go beyond these exotic materials, by designing devices whose topological features can be tuned, or even generated on-demand, like a switch.

Quantum Hall physics, inherently topological, has been seen in electronic devices and in dilute atomic ensembles. In the two-dimensional electron case, current flows along an edge/interface (“edge states”) even in the presence of defects or other physical distortions in the sample---this arises from global properties of the material. This is strange when contrasted with conventional conductors/insulators, where the transport is impeded due the presence of disorder.

In this week’s issue of Nature Photonics (AOP DOI: 10.1038/nphoton.2013.274) scientists at the Joint Quantum Institute (*) report the first observation of such topological effects for light in two dimensions. To accomplish this, they built a structure to guide infrared light over the surface of a room temperature, silicon-on-insulator chip.  Amazingly, they directly observed light racing around the boundary, impervious to defects. These photonic “edge states” are directly analogous to the quantum Hall effect for electrons.  

Since silicon is the preferred material for most electronics this novel design assists with the miniaturization of optical communication technology, bringing photons a little closer to their electronic circuit counterparts. The work is a realization of a theoretical proposal by this same group of JQI scientists and their collaborators more than a year ago.

Edge States and Ring Resonators

Electrons can occupy topological edge states because they are charged particles whose energy spectrum can be dramatically modified by large magnetic fields.  To simplify, a magnetic interaction is key for realizing quantum Hall states. The question here to ask is how researchers can design a material where photons---massless, charge-free, packets of energy--- flow as if they are being manipulated by a super strong magnet. To put it another way, how can the energy spectrum of light be modified to support robust topological states?  And what do these photonic edge states look like?

In the JQI design, the light moves through a 2D landscape consisting of nearly flat ring-shaped silicon waveguides called resonators.  By comparison, the arena for electrons is typically at the two-dimensional interface between two sheets of semiconductor. What the JQI scientists showed was that indeed light can, under the right circumstances, circulate around the edge of the silicon chip, without significant loss of energy, and do so even in the presence of defects.

The array of silicon rings is designed to only let the light waves inside-- “resonate”-- if they have the right wavelength (the circumference of the ring equaling an integral number of wavelengths).  In other words, if the light frequency matches the resonant conditions of the ring it will enter the waveguide and make many circuits.  For an off-resonance condition less light will inhabit the ring. Light with one polarization (the light’s electric field pointing up or down) will, furthermore, circulate preferentially in one direction around the ring, clockwise or anti-clockwise.  For the enthusiasts, the clockwise and anti-clockwise modes, in combination with the resonator array design, allows the photonic system to be analogous to an electron (spin) interacting with a magnet. The researchers created a photonic system that experiences a so-called synthetic or effective magnetic field [[see this link on the design proposal by this same group and this link on synthetic fields in ultracold atoms]].

This breaking of the symmetry of travel around rings is what can cause the cancelling-out of light propagation through the body of the device but not around the edge.  It is also what reduces the amount of light energy wasted when light scatters or moves backwards around the edge or meets with a defect such as a defective resonator ring.   Thus the JQI device displays the hallmark of topological behavior: persistent flow in the form of an edge state and near immunity against defects.  The scientists went out of their way to deliberately turn off some resonators, thus simulating the industrial conditions of mass production---a process prone to the presence of faulty components even in the best of fabrication circumstances.  They also demonstrated the edge flow in the presence of unpredicted defects in the device.

In all of those resonator-to-resonator transfers, at least a little bit of the light gets lost, and this wasted energy is what the researchers use to image the light paths through the device.  When the resonator array is tuned with the right frequency and temperature for general (non-topological) transmission, that’s what you get: light moves through the whole of the array.  However, when the system is tuned to facilitate edge states, sure enough, no light moves through the body of the array; it only skirts the edge of the array---in a direct analogy to electron movement in a quantum Hall state.  Notably, this scheme is a realization of the quantum spin Hall effect, where photonic (pseudo-)spins take the place of electron charge.

Possible Applications

“By tuning the resonators with temperature, we can make this topological array quite flexible,” says Jacob Taylor, one of the JQI researchers.  “The array isn’t designed for one frequency only.”  Furthermore, the architecture of the array, which can be expanded to suite the need, fits in with the expectation that components such as this will need to be scaled up for use in future quantum computers, especially those that use photons as parts of hybrid electron-photon-atom systems.

JQI scientist and lead author, Mohammad Hafezi explains why edge states for photons might have an advantage over electron edge states for certain applications: “Photonic systems are remarkably malleable since photons can be easily guided inside the waveguides. Therefore, one can think of making photonic systems with non-trivial topologies, like Mobius strip, tori etc.”

What can be done with a photonic array like this?  One immediate advantage of edge states is that the arrays can be used for producing delays in photonic chips, where it is desirable to slow down a signal without being sensitive to fabrication errors.  Other future uses: as filters and optical switches.  Furthermore, by concentrating light in only two dimensions rather than three, the JQI scientists believe they can achieve certain nonlinear quantum effects, which can only occur with intense light.

This was written by P. Schewe and E. Edwards

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  • Simulation sets atoms shivering
  • Zitterbewegung in an ultracold gas
  • September 20, 2013

In “Harry Potter and the Sorcerer’s Stone” (JK Rowling, 1997), Harry, Ron, and Hermione encounter a massive stone chessboard, one of many obstacles in their path. To advance, they must play, and win. Although the board and pieces are much larger than normal, and the circumstances a bit peculiar, one thing remains clear to them—this is a game of chess, with the same rules as always. 

Just as Rowling created a fantastical chessboard for her heroes, scientists too can change the size and shape of the board and pieces: if the game plays out by the same rules, there is no fundamental difference. This approach is the basis of quantum simulation. To gain insight into inaccessible phenomena, researchers simulate elusive quantum systems with laboratory controllable ones that share the same mathematical form as the original physical system. Recently, a JQI group used this technique to simulate the motion of a free electron, known as zitterbewegung, by assembling an analogue system of neutral atoms in a Bose-Einstein condensate.

Shivering atoms

Concisely referred to as “zitter” (and often times by the even more abbreviated “ZBW”), zitterbewegung is an ultra-fast trembling motion expected for electrons (and other fermions) moving near the speed of light. This peculiar ­­­motion is predicted by the Dirac equation, which describes relativistic motion (think Einstein—special relativity) in the framework of quantum mechanics.

One of the great paradoxes of modern physics is that our description of the universe has two sets of rules, general relativity for the very large and quantum mechanics for the very small. While this works in practice, it doesn’t offer a very satisfying picture of reality—after all, the universe certainly seems consistent across all scales. Though there is a well-established link between special relativity and quantum mechanics (in the form of the Dirac equation), it is not yet understood how to reconcile the probabilistic framework of quantum mechanics with the classical mechanics of general relativity. Examining where relativistic effects mingle with quantum ones, as with zitterbewegung, may offer deep insight into a unified picture.

Unfortunately, zitterbewegung is particularly unsuitable for laboratory study: this quiver takes place a sextillion times every second, a truly staggering frequency (one sextillion = one thousand billion billion!) and far too fast to have any hope of observing directly. To investigate zitter, researchers must swap out the relativistic electron for something that oscillates more sedately.

The JQI team began by trapping rubidium atoms and cooling them to form a Bose-Einstein condensate (BEC). Inside the trap, the cloud of atoms can exist in stable, static states. By shining laser light on the cloud, researchers place the BEC in a quantum mechanical mixture of these states, in this case a “superposition” of one spin state moving with a positive momentum and a second spin state moving with an equal and opposite momentum. When these lasers were turned off, the cloud sloshed back and forth, oscillating at a few thousand Hz—for comparison, a pesky no-see-um (midge fly) flaps its wings about a thousand times per second, a thousand Hz—and this relatively slow pace allowed researchers to essentially photograph the BEC as it traversed the trap.

In the language of atomic physics, this movement between states is a Rabi oscillation, a consequence of initially placing a system in two states at once. Analogously, zitterbewegung occurs for a relativistic electron because it is in mixture of states, oscillating between them. In fact, the electron can be described as quivering due to interference between particle and antiparticle states—matter and antimatter!

Future simulators

Always direct, Richard Feynman ended his 1981 proposal for quantum computers by exclaiming “nature isn't classical, dammit, and if you want to make a simulation of nature, you'd better make it quantum mechanical[!]”. He anticipated a ‘universal’ quantum simulator, perhaps made of “little latticeworks of spins and other things,” that, if tweaked right, could imitate any quantum system. 

Quantum simulation is often seen as the ‘younger sibling’ of quantum computing, a specialized subset of what a general quantum computer can do. However, the advent of a general quantum computer likely won’t make quantum simulation irrelevant—the two approaches are quite compatible. Although flexible, a quantum computer may be a blunt tool compared to a specifically engineered simulation: free to choose their own pieces, rather than constrained by the architecture of the quantum computer, researchers can tailor the simulation to meet their needs.

By using ultracold atoms, the research team could directly observe the trembling. While the time scales were significantly slower than electrons, the particle motion is real, and not simulated. Specifically, in previous works that use ions and photons, the Dirac particles were not represented by the actual system particles themselves. But, as always, mathematics is our language for describing the universe, and if two systems are equivalent mathematically, they can display the same physical phenomena. Regardless of the approach taken, as quantum simulators are refined, more and more of the weird, exotic stuff of the universe will be accessible to study. 

This story was written by S. Kelley, JQI.

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  • Peter Kordell is awarded undergraduate research prize
  • August 27, 2013

Peter Kordell, a UMD undergrad, was awarded the IPST Monroe Martin Prize for Undergraduate Research in Physics. Kordel, now a graduate student at University of Michigan, Ann Arbor, worked with Luis Orozco on the Atoms on SQUIDs experiment. His research paper was titled "Three Layer Optical Fiber Simulations and Analysis." 

The Atoms on SQUIDs research project is supported by the PFC @ JQI. It focuses on a hybrid QI approach, developing coupled atom-SQUID systems that can exploit the long coherence times of atomic systems for quantum memory and the fast, robust operations, control, and interconnectivity of SC qubits. In the design, the researchers will use a tapered optical fiber to trap cold atoms near a superconducting device. On the atom-fiber side of the project, the team has successfully trapped atoms on an optical fiber. 

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  • Photons a la Mode
  • Studying light pulses by counting photons
  • August 14, 2013

The photodetectors in Alan Migdall’s lab often see no light at all, and that’s a good thing since he and his JQI (*) colleagues perform physics experiments that require very little light, the better to study subtle quantum effects. The bursts of light they observe typically consist of only one or two photons--- the particle form of light---or (statistically speaking) even less than one photon. Their latest achievement is to develop a new way of counting photons to understand the sources and modes of light in modern physics experiments.

Migdall’s lab, located at the National Institute for Standards and Technology (NIST), is just outside Washington, DC in the US. The new light-measuring protocol is summarized in a recent issue of the journal Physical Review A (see reference publication). The work reported there was performed in collaboration with NIST’s Italian counterpart, the Instituto Nazionale de Ricerca Metrologica (INRIM).

Light Modes

Generating light suitable for quantum mechanical applications such as quantum computing and quantum cryptography requires exquisite control over properties such as the frequency, polarization, timing, and direction of the light emitted. For instance, probing atoms with light involves matching the frequency of the light to the atoms’ natural resonance frequency often to within one part in a billion. Moreover, communicating with light means encoding information in the arrival time or frequency or spatial position of the light, so high-speed communication means using very closely- spaced arrival times for light pulses, and pinpoint knowledge of the light frequencies and positions of the arriving light in order to fit in as much information as possible.

When a light pulse contains a mixture of light photons with different frequencies (or polarizations, arrival times, emission angles, etc.) it is said to have multiple modes. In some cases, a single light source will naturally produce such multi-mode light, whereas in others multiple modes are a signature of the presence of additional, and generally unwanted, light sources in the system. Discriminating the different modes in a light field, especially a weak light field that has very few photons, can be extremely difficult as it requires very sensitive detection that can discriminate between modes that are very close together in frequency, space, time, etc.

For instance, to study a pair of entangled photons (created by shooting light into a special crystal where one photon is converted into a pair of secondary, related photons) detection efficiency is all important; and folded into that detection efficiency is a requirement that the arrival of each of the daughter photons be matched to the arrival of the other daughter photon. In addition to this temporal alignment, the spatial alignment of detectors, (each oriented at a specific angle respect to the beamline) must be exquisite. To correct for any type of less-than-perfect alignment, it is necessary to know how many different light modes are arriving at the detector.

Photon Number

The laws of quantum mechanics ensure that light always exhibits natural intensity fluctuations. Even from an ultra-stable laser, the number of photons arriving at a detector will vary randomly in time. By recording the number of photons in each pulse of light over a long time, however, the form of the fluctuations of a particular light field will become clear. In particular, we can learn the probability of generating 0, 1, 2, 3, etc. photons in each pulse.

The handy innovation in Migdall’s lab was to develop a method to use this set of probabilities to determine the modes in a very weak light field. This method is very useful because most light detectors that can see light at the level of a single photon cannot tell the exact frequency or position of the light, which makes determining the number of modes difficult for such fields.

The JQI-INRIM experiment used a detector “tree” that counts photon number. It did this by taking the incoming light pulse, using partial mirrors to divide the pulse into four, and then allowing these to enter four detectors set up to record individual photons. If the original pulse contained zero photons then none of the detectors will fire. If the pulse contained one photon, then one of the detectors will fire, and so on.

Elizabeth A. Goldschmidt, a JQI researcher and University of Maryland graduate student, is the first author on the research paper. “By looking at just the intensity fluctuations of a light field we have shown that we can learn about the underlying processes generating the light,” she said. “This is a novel use of higher-order photon-number statistics, which are becoming more and more accessible with modern photodetection.”

Goldschmidt believes that this method of counting photons and statistically analyzing the results as a way of understanding the modes present in light pulses will help in keeping tight control over light sources that emit single photons (where, for instance, you want to ensure that unwanted photons are not being produced). And those that emit pairs of entangled photons---where the quantum relation between the two photons is exactly right, such as in “heralding” experiments, where the detection of a photon in one detector serves as an announcement for the existence of a second, related, photon in a specially staged detector nearby (see related articles).

Alan Migdall compares the photon counting approach to wine tasting. “Just as some experts can taste different flavors in a wine---a result of grapes coming from different parts of the Loire Valley---so we can tell apart various modes of light coming from a source.”

(*)The Joint Quantum Institute is operated jointly by the National Institute of Standards and Technology in Gaithersburg, MD and the University of Maryland in College Park.

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  • Resonant Exchange Qubits
  • Triple-electron qubits provide a new level of quantum reliability
  • August 6, 2013

Encoding information using quantum bits—which can be maintained in a superposition of states—is at the heart of quantum computing. Superposition states offer the advantage of massive parallelism compared to conventional computing using digital bits---which can assume only one value at a time.  

Unfortunately, qubits are fragile; they dissipate in the face of interactions with their environment. A new JQI semiconductor-based qubit design ably addresses this issue of qubit robustness. Unlike previous semiconductor qubit setups, the new device has no need of external, changing magnetic fields. This permits greater control over the qubit.

The enhanced mastery results in new reliability for processing quantum information in semiconductor qubits.  Furthermore, the necessary control features---voltages applied to nanometer-scale electrodes---are directly compatible with existing transistor-based devices. The research was carried out by scientists at JQI, Harvard, the Niels Bohr Institute at the University of Copenhagen, and the University of California at Santa Barbara.  The results are published in two papers---one theoretical, one experimental---in the journal Physical Review Letters.

This new approach expands upon efforts to use individual electron spins which point “up” or “down” to form a natural qubit (Read about "Qubit Design" in Gallery, above).  Instead of using a single electron the researchers used three electrons, confined in a three adjacent quantum dots.  Advances in fabricating arrays quantum dots have opened new possibilities for realizing robust qubits. In the last few years, scientists have demonstrated control of multiple electrons and their associated spin in such triple quantum dots (Read about "Triple Quantum Dots" in Gallery, above). Quantum dots now have many of the essential characteristics needed for quantum computing but decoherence and stability remains an outstanding issue. 

Why is using three electrons better than using one?  In the case of a single electron trapped in quantum dots, the qubit is made using the two possible orientations of an intrinsic angular momentum called spin. The spin is free to point “up (aligned)” or “down (anti-aligned)” with respect to a magnetic field. Here, flipping the state of the qubit required a large, rapidly oscillating magnetic field. Fast changes in magnetic fields, however, give rise to oscillating electric fields---this is exactly what happens in electric generators. This method presents a challenge, because such additional changing electric fields will fundamentally and dynamically modify the qubit system. It is therefore better to exert control over qubits solely with electric fields, eliminating the oscillating magnetic fields.

The researchers do exactly this.  They find a way to use an electric field rather than a magnetic field to flip the qubit from 1 to 0 (or vice versa).  While single electron spin transitions require a magnetic field (magnetic resonance-think, MRI technology), if one adds electrons to the system and constructs the energy levels based on combinations of three-spin orientations, the magnetic interaction is longer be necessary.

JQI scientist Jacob Taylor explains further why using three electrons is better than one: "Amazingly, when the three electrons are in constant, weak contact, their interaction hides their individual features from the outside world. This is, in essence, why the three-spin design protects quantum information so well.  Furthermore, even though the information is protected, a well designed 'tickling' of the device at just the right frequency has a dramatic effect on the information, as demonstrated by the quantum gates shown in the experiment."

These joint few-electron states have been studied previously and are called singlet-triplet qubit (graphical explanation in Gallery). However, while the changing magnetic fields were removed in previous works, the proposed logic gates relied on varying the tunneling rate in multiple steps, a cumbersome process that is susceptible to additional noise on the control electrodes. 

These two new papers in Physical Review Letters report improvements and simplifications in qubit construction and particularly in qubit manipulation. Instead of requiring sequential changes in the tunneling, qubit manipulation is carried out directly, allowing for easy, control of the qubit around two axes. The levels are separated in energy by microwave frequencies (GHz) and can be addressed with oscillating electric fields. Additionally, by adjusting the electrode voltages, the qubit levels are made to be far in energy from other states.

Once the researchers establish the electrode voltages, they proceed to the business of performing logic operations using resonant microwaves. What does this mean? The resonant exchange qubit, as the author’s refer to it, is embedded in a crystal awash with all pesky environmental that can endanger the integrity of qubits---phonons, other electrons, and interactions with the atoms that compose the crystal.  And yet, as a home for qubits, this environment is actually pretty stable. How stable? The experiments showed a coherence time of 20 microseconds. This is about 10,000 times longer than their reported gate time, which is good news for scaling up both number of qubits and gates. 

The JQI theory paper that accompanies the paper devoted to experimental results proposes a way of elaborating on the basic resonant-exchange qubit design to accommodate two-qubit-logic gates. Jake Taylor  "Working from a few simple concepts, this new approach to using electron spins seems obvious in retrospect. I'm particularly gratified with the ease of implementation. With this new design, we are getting fast, good, cheap quantum bits in an approach that promises enormous economies of scale.  But the two-qubit gate---crucial for the future success of this approach---is the big, open experimental challenge. I'm very curious to see which method ends up working the best in practice."

Charles Marcus, leader of the Niels Bohr Institute contingent for this experiment, agrees that the resonant exchange qubit represents a workable format for reliable qubits.  He gives Taylor credit in this way: "The idea for the resonant exchange qubit came from Jake, developed, I imagine, sitting with Jim Medford in the lab, next to the cryostat.  Jake is the only full-time theorist I know who can run a dilution refrigerator. I think his creativity is stimulated by the thumping sound of vacuum pumps. With the resonant exchange qubit, I see a big step forward for spin qubits. Challenges remain, but I can now see a clear forward path."

Written by Phillip F. Schewe and Emily Edwards

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  • Quantum Information in Low Light
  • A new photo-detection scheme makes do with few photons
  • June 18, 2013

At low light, cats see better than humans. Electronic detectors do even better, but eventually they too become more prone to errors at very low light. The fundamental probabilistic nature of light makes it impossible to perfectly distinguish light from dark at very low intensity.  However, by using quantum mechanics, one can find measurement schemes that can, at least for part of the time, perform measurements which are free of errors, even when the light intensity is very low.

The chief advantage of using such a dilute light beam is to reduce the power requirement. And this in turn means that encrypted data can be sent over longer distances, even up to distant satellites.  Low power and high fidelity in reading data is especially important for transmitting and processing quantum information for secure communications and quantum computation. To facilitate this quantum capability you want a detector that sees well in the (almost) dark. Furthermore, in some secure communications applications it is preferable to occasionally avoid making a decision at all rather than to make an error.

A scheme demonstrated at the Joint Quantum Institute does exactly this. The JQI work, carried out in the lab of Alan Migdall and published in the journal Nature Communications, shows how one category of photo-detection system can make highly accurate readings of incoming information at the single-photon level by allowing the detector in some instances not to give a conclusive answer. Sometimes discretion is the better part of valor.

Quantum Morse Code

Most digital data comes into your home or office in the form of pulsed light, usually encoding a stream of zeros and ones, the equivalent of the 19th century Morse code of dots and dashes.  A more sophisticated data encoding scheme is one that uses not two but four states---0, 1, 2, 3 instead of the customary 0 and 1.  This protocol can be conveniently implemented, for example, by having the four states correspond to four different phases of the light pulse.  However, the phase states representing values of 0, 1, 2, and 3 have some overlap, and this produces ambiguity when you try to determine which state you have received. This overlap, which is inherent in the states, means that your measurement system sometimes gives you the wrong answer.

Migdall and his associates recently achieved the lowest error rate yet for a photodetector deciphering such a four-fold phase encoding of information in a light pulse.  In fact, the error rate was some 4 times lower than what is possible with conventional measurement techniques.  Such low error rates were achieved by implementing measurements for minimum error discrimination, or MED for short.  This measurement is deterministic insofar as it always gives an answer, albeit with some chance of being wrong.

By contrast, one can instead perform measurements that are in principle error free by allowing some inconclusive results and giving up the deterministic character of the measurement outcomes. This probabilistic discrimination scheme, based on quantum mechanics, is called unambiguous state discrimination, or USD, and is beyond the capabilities of conventional measurement schemes.

In their latest result (see paper linked below), JQI scientists implement a USD of such four-fold phase-encoded states by performing measurements able to eliminate all but one possible value for the input state---whether 0, 1, 2, or 3. This has the effect of determining the answer perfectly or not at all.   Alan Migdall compares the earlier minimum error discrimination approach with the unambiguous state discrimination approach: “The former is a technique that always gets an answer albeit with some probability of being mistaken, while the latter is designed to get answers that in principle are never wrong, but at the expense of sometimes getting an answer that is the equivalent of ‘don't know.’ It’s as your mother said, ‘if you can’t say something good it is better to say nothing at all.’”

With USD you make a series of measurements that rule out each state in turn. Then by process of elimination you figure out which state it must be. However, sometimes you obtain an inconclusive result, for example when your measurements eliminate less than three of the four possibilities for the input state.

The Real World

Measurement systems are not perfect, and ideal USD is not possible. Real-world imperfections produce some errors in the decoded information even in situations where USD appears to work smoothly.  The JQI experiment, which implements USD for four quantum states encoded in pulses with average photon numbers of less than one photon, is robust against real-world imperfections.  At the end, it performs with much lower errors than what could be achieved by any deterministic measurement, including MED. This advance will be useful for quantum information processing, quantum communications with many states, and fundamental studies of quantum measurements at the low-light level.

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  • Spin Hall Effect in a Quantum Gas
  • An 'atomtronic' transistor
  • June 5, 2013

From NIST Techbeat1

JQI Researchers at the National Institute of Standards and Technology (NIST) have reported* the first observation of the "spin Hall effect" in a Bose-Einstein condensate (BEC), a cloud of ultracold atoms acting as a single quantum object. As one consequence, they made the atoms, which spin like a child's top, skew to one side or the other, by an amount dependent on the spin direction. Besides offering new insight into the quantum mechanical world, they say the phenomenon is a step toward applications in "atomtronics"—the use of ultracold atoms as circuit components.

A quantum circuit might use spins, described as "up" or "down," as signals, in a way analogous to how electric charge can represent ones and zeros in conventional computers. Quantum devices, however, can process information in ways that are difficult or impossible for conventional devices. Finding ways to manipulate spin is a major research effort among quantum scientists, and the team's results may help the spin Hall effect become a good tool for the job.The spin Hall effect is seen in electrons and other quantum particles when their motion depends on their magnetic orientation, or "spin." Previously, the spin Hall effect has been observed in electrons confined to a two-dimensional semiconductor strip, and in photons, but never before in a BEC.

The team used several sets of lasers to trap rubidium atoms in a tiny cloud, about 10 micrometers on a side, inside a vacuum chamber and then cool the atoms to a few billionths of a degree above absolute zero. Under these conditions, the atoms change from an ordinary gas to an exotic state of matter called a BEC, in which the atoms all behave identically and occupy the lowest energy state of the system. Then, the NIST team employed another laser to gently push the BEC, allowing them to observe the spin Hall effect at work. 

Spin is roughly analogous to the rotation of a top, and if the top is gently pushed straight forward, it will eventually tend to curve either to the right or left, depending on which way it is spinning. Similarly, subject to the spin Hall effect, a quantum object spinning one way will, when pushed, curve off to one side, while if it spins the other way, it will curve to the other. The BEC followed this sort of curved path after the laser pushed it. 

"This effect has been observed in solids before, but in solids there are other things happening that make it difficult to distinguish what the spin Hall effect is doing," says lead author Matthew Beeler, who just completed a postdoctoral fellowship at NIST. "The good thing about seeing it in the BEC is that we've got a simple system whose properties we can explain in just two lines of equations. It means we can disentangle the spin Hall effect from the background and explore it more easily." 

Conceptually, the laser/BEC setup can be thought of as an atom spin transistor—an atomtronic device—that can manipulate spin "currents" just as a conventional electronic transistor manipulates electrical current. (see Illustration)

Beeler says that it is unlikely to be a practical way to build a logic gate for a working quantum computer, though. For now, he says, their new window into the spin Hall effect is good for researchers, who have wanted an easier way to understand complex systems where the effect appears. It also might provide insight into how data can be represented and moved from place to place in atomtronic circuits.

1This story was written by Chad Boutin for NIST Techbeat. It was modified for JQI by E. Edwards, with permission.

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  • Entanglement in a Flash
  • June 3, 2013

JQI researchers under the direction of Chris Monroe have produced quantum entanglement between a single atom’s motion and its spin state thousands of times faster than previously reported, demonstrating unprecedented control of atomic motion. This work, which may lead to faster and better quantum computer logic gates, is described a recent issue of Physical Review Letters.

This experiment focuses on using highly energetic laser pulses to perform qubit operations. Previously, they set a record for the fastest spin flip in these systems: a mere 50 picoseconds. Here they continue their work by blasting the ion so strongly that the qubit quickly becomes linked to its motion. Such speedy operations are more typically associated with solid state systems such as electrons in semiconductors or superconductors. Here the speed of operations combined with the pristine quantum environment of atoms provide the best of both worlds.

Though subdued, trapped ions are not entirely tame. Like caged tigers pacing back and forth, they exhibit regular motion, oscillating inside an electrostatic trap at a particular frequency. This motion is used as a medium for entangling multiple ions. The motion is also the enemy, introducing noise because logic operations and/or experiments are typically completed after many trap oscillation cycles have passed. In quantum computing, speed is critical because such noise can destroy the quantum-ness of the system.

Providing a solution, this team has achieved spin-motion entanglement on an ultrafast timescale of about 3 nanoseconds, nearly a thousand of times faster than the trap oscillation time. For reference, a beam of light travels only a foot in one nanosecond. Blinking your eye is about 100 million times slower than this entangling process.

The qubit is initially prepared in a quantum coherent superposition of two states: “spin-up” and “spin-down.” Laser pulses are then used to kick spin-up in one direction and spin-down in the opposite direction. As a result, the ion is in an entangled state, which is a combination of spin-up/moving left, and spin-down/moving right.

To create this spin-dependent kick, laser pulses arrive from opposite directions, overlapping to form a diffraction grating. The ion qubit interacts with the light grating by absorbing and emitting a photon. Photons carry momentum, and so the absorption/emission cycle kicks the ion qubit. For a fixed color of light, the kick strength depends primarily on the laser power. The light spectrum also contains a frequency that induces spin flips. Thus the ion qubit “feels” this kick based on its associated spin: opposite spin states are brushed in opposite directions. This is analogous when atoms are diffracted by an optical lattice; but here the resulting diffraction pattern depends on the qubit’s spin state (up or down).

In the ion trap, the arrival of the first set of overlapping pulses creates many discrete momentum states, each tangled with an alternating spin. Subsequent pulses serve to further accumulate momentum states. The goal is to apply the grating kicks in a way that builds up a particular diffraction pattern corresponding to just two momentum states, each entangled with an opposing spin. The process of building up amplitude in an interference pattern can be thought of in terms of the not-so-quantum trampoline. Say that my daughter is jumping at a rate of 0.5 bounce per second. Some of her friends can build up amplitude by simultaneously jumping at the same time, or “in phase.” Some kids can also jump at a rate that is an integer (1X, 2X, 3X…) multiple of the other kids, which will create a kind of interference pattern of jumps. The kids will go the highest at the time when the most kids are simultaneously jumping. Similarly, if the team wants to accumulate more and more momentum, they need to apply the grating (kick) at a particular time with respect to the ion trap oscillations.

At the other extreme, the kids can all jump at random times and their bounces will cancel each other leading to reduced or even zero jumping amplitude over all. You could imagine that perfect cancellation could take place at special combinations of jumps as well. Likewise, the team can even gain back the original spin superposition, with the motion entirely removed. If they flash on the grating at the precise moment the motion packets overlap, this kick neatly disentangles the spin from the motion.

This exquisite control over the spin-motion entanglement by adjusting the delay between kicks is equivalent to an interferometer using pairs of spin dependent kicks. Interferometers have numerous applications, ranging from radio astronomy to cellular imaging. When the same type of ultrafast interferometer applied to multiple atoms, their internal spin states can become entangled, which is a fundamental building block of a quantum computer.

This article was written by S. Kelley and E. Edwards. The animation was created by S. Kelley. Permissions for re-use: Contact JQI

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  • Rajibul Islam awarded Distinguished Dissertation Award
  • May 10, 2013

Rajibul Islam was recently awarded UMDs Distinguished Dissertation Award for his thesis work on quantum magnetism with ions in Chris Monroe's Trapped Ion Quantum Information group. According to the graduate school's website, "The Distinguished Dissertation Award recognizes original work that makes an unusually significant contribution to the discipline. Both methodological and substantive quality will be judged.  Awards will be given each year in four broad disciplinary areas: 1) Mathematics, Physical Sciences, and Engineering; 2) Social Sciences; 3) Humanities and Fine Arts; and 4) Biological and Life Sciences." Islam received the award in area (1). This year 625 students across all disciplines received PhDs from UMD. 

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  • Turning on Frustration
  • 16 atomic ions simulate a quantum antiferromagnet
  • May 2, 2013

Frustration crops up throughout nature when conflicting constraints on a physical system compete with one another. The way nature resolves these conflicts often leads to exotic phases of matter that are poorly understood. This week’s issue of Science Magazine features new results from the research group of Christopher Monroe at the JQI, where they explored how to frustrate a quantum magnet comprised of sixteen atomic ions – to date the largest ensemble of qubits to perform a simulation of quantum matter.

Originating in large part with Richard Feynman’s 1982 proposal, quantum simulation has evolved into a field where scientists use a controllable quantum system to study a second, less experimentally feasible quantum phenomenon. In short, a full-scale quantum computer does not yet exist and classical computers often cannot solve quantum problems, thus a “quantum simulator” presents an attractive alternative for gaining insight into the behaviors of complex material.  Says Monroe, “With just 30 or so qubits, we should be able to study ordering and dynamics of this many-body system that cannot be predicted using conventional computers. In the future, make that a few hundred qubits and there’s simply not enough room in the universe for all the memory required to do the calculation.”

In this experiment, JQI physicists engineer a quantum magnet using lasers and ion qubits. The ion trap platform has long been a leader in the field of quantum information and is an ideal playground for quantum simulations (see image 1 in gallery of ion trap used here). Ions are charged particles that interact strongly via the Coulomb force, which is an attraction/repulsion that decreases as particles separate. When a handful of positively charged ytterbium ions are thrown together, they repel each other, and, for this oblong ion trap, form a linear crystal (see gallery image 2 of real camera images of single ions arranged in a crystal). Each ion has two internal energy states that make up a qubit. 

Laser beams can manipulate the Coulomb force to create tunable, long range magnetic-like interactions, where each ion qubit represents a tiny magnet*. Imagine that invisible springs connect the ions together. Vibrations occurring on one side of the crystal affect the entire crystal. This is called collective motion and is harnessed to generate a force that depends on how a magnet is oriented (which state the qubit is in). The team can program this state-dependent force by simultaneously applying multiple laser beams, whose colors (frequencies) are specially chosen with respect to the internal vibrations of the ion crystal. The amount of influence each magnet has on the rest of the chain primarily depends on the choice of laser frequencies (See Frequency Sidebar at the bottom of this article for more information). The crystal geometry has little to do with the interactions. In fact, for some laser configurations the ions that are farthest apart in space interact most strongly.

Phenomena due to this type of magnet-magnet interaction alone can be explained without quantum physics. An additional uniform magnetic field, (here created with yet another laser beam), is necessary for introducing quantum phase transitions and entanglement. This added magnetic field (oriented perpendicular to the direction of the interactions) induces quantum fluctuations that can drive the system into different energy levels.

In the experiment, the long-range ion-ion interaction and a large effective magnetic field are turned on simultaneously. In the beginning of the simulation the ion magnets are oriented along the direction of the effective magnetic field. In the quantum world, if a magnet is pointing along some direction with certainty, its magnetic state along any perpendicular direction is totally random.  Hence the system is in a disordered state along the perpendicular direction of magnetic [spin] interactions.

During the quantum simulation the magnetic field is reduced and the ion crystal goes from being in this disordered state, with each ion magnet pointing along a random direction, to being determined by the form of the magnetic interactions. For some cases of antiferromagnetic (AFM) interactions, the spins will end in a simple up-down-up-down-etc. configuration. With the turn of some knobs, the team can cause the AFM interactions to instead frustrate the crystal. For example, nearest neighbor AFM interactions can compete strongly with the next-nearest neighbor interactions and even the next-next-nearest neighbor constraints. The crystal can easily form various antiferromagnetic combinations, instead of the simple nearest neighbor antiferromagnet (up down up down).  In fact, with a few technical upgrades, the researchers can potentially engineer situations where the magnets can reside in an exponentially large number of antiferromagnetic states, generating massive quantum entanglement that accompanies this frustration.

Previously, this same group of researchers performed quantum simulations of a ferromagnet (all magnets oriented same direction) and of the smallest system exhibiting frustration. Their ability to utilize the collective motion allows them to explore different facets of quantum magnetism. The team can ‘at will’ modify how the different collective modes contribute to magnetic order by merely changing the laser colors and/or the ion separation. This new work demonstrates the versatility of their system, even as particles are added. As lead author Dr. Rajibul Islam explains “We have a knob that adjusts the range of the interaction, something that is unavailable in real materials.  This type of simulation could therefore help in the design of new types of materials that possess exotic properties, with potential applications to electrical transport, sensors, or transducers.”

*Physicists use mathematical spin models, such as the Ising model studied here, to understand quantum magnets, thus in this news article, for clarity the ions are called “magnets.” In the language of the Science Magazine article, they are called “spins”.


Frequency Sidebar:

This experiment is all about frequency. The ions themselves are vibrating at a frequency determined by an electrostatic trap--about 1 MHz or 1 million vibrations/second. The ion qubit is made from two internal energy levels that are separated in frequency by about 12 GHz or 12 billion vibrations/second (microwave domain). When radiation with a frequency that matches either of these frequencies shines onto the ion, then the radiation is said to be in resonance with that transition. For example, 12 GHz microwave radiation will make the ion qubit cycle between two internal states. If MHz radiation is coupled to the ion, it will begin vibrating. In the quantum regime, the quanta of vibration called a phonon can be controllably added and removed from the system. These phonons act as communication channels for the magnets, and are instrumental in generating rich varieties of interactions.

Scientists must be clever about generating frequencies. We are constantly being bathed in radiation from cellphones (GHz), infrared (terahertz or 1000 GHz), UV radiation (petahertz or 1 million GHz), and more--much of this goes unnoticed. The ion here is sensitive to only very precise frequencies. To get that qubit to flip flop between two qubit states, they need to apply a radiation at precisely 12.642819 GHz. To create spin-spin interactions, they also need to simultaneously excite its motion--it is vibrating at a frequency that is 10,000 times smaller. Lasers are the key--here at 369 nm, just barely in the ultraviolet regime. Previously, a JQI news item described how they can use a pulsed laser to generate 12.642819 GHz. Scientists control the frequency, power and the direction of light waves very precisely by hitting the laser beams with sound waves and oscillating electric fields (in devices such as the acousto- and electro-optic modulators). These devices act to add lower frequencies necessary for exciting the motion in the trap as well as fine tune the main laser beam to address certain atomic transitions. This method, called modulation, is versatile and is one of the key features that make this and other quantum physics experiments possible.

This article was written by E. Edwards at JQI; [Editorial note: After posting this news item at 2 pm EST on May 2, 2013, a couple of words were added and typos fixed for clarification at 5 pm EST.]

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  • Quantum Dot Commands Light
  • A solid state ultrafast logic gate on a photon
  • March 31, 2013

If you could peek at the inner workings of a computer processor you would see billions of transistors switching back and forth between two states. In optical communications, information from the switches can be encoded onto light, which then travels long distances through glass fiber. Researchers at the Joint Quantum Institute and the Department of Electrical and Computer Engineering are working to harness the quantum nature of light and semiconductors to expand the capabilities of computers in remarkable ways.

All computers, even the future quantum versions, use logic operations or “gates,” which are the fundamental building blocks of computational processes. JQI scientists, led by Professor Edo Waks, have performed an ultrafast logic gate on a photon, using a semiconductor quantum dot. This research is described in the March 31 Advance Online Publication of Nature Photonics.*

Photons are a proven transit system for information. In quantum devices, they are the ideal information carriers that relay messages between quantum bits (qubits) such as neutral atoms, ion traps, superconducting circuits, nitrogen vacancy centers, and of course the device used here: quantum dots. A quantum dot (QD) is a semiconductor structure that acts like an atom. This means it has allowed energy levels that can be populated and even shifted around using lasers and magnetic fields. Quantum dots are an attractive platform for quantum processing because they live inside a semiconductor material, thus the technology for integration with modern electronics already exists.

The Waks team has implemented a conditional logic gate called a Controlled-NOT (CNOT). Here’s how a generic CNOT gate works: if a control qubit is in what we will call state 1, then the gate flips the state of a second qubit. If the control qubit is in state 0, nothing happens.

Waks explains the importance of this gate, “Although this logic operation sounds simple, the CNOT gate has the important property that it is universal, which means that all computational algorithms can be performed using only this simple operation. This powerful gate can thus be seen as important step towards implementing any quantum information protocol.”

In this experiment, a quantum dot plays the role of the control qubit. The second qubit is a photon that has two polarization states. Polarization can be thought of as an orientation of the traveling light waves. For instance, polarized sunglasses can shield your eyes from light having certain orientations. Here, photons can be oriented horizontally or vertically with respect to a defined direction. Just like energy levels for a quantum dot constitute a qubit, the two available polarizations make up a photonic qubit.

Light is injected into a photonic crystal cavity (see sidebar in gallery) containing a quantum dot.  Quantum dots have been trapped in photonic crystals before, but the difference here is an added large external magnetic field. The magnetic field shifts around the energy levels of the quantum dot enabling it to simultaneously act as both a stable qubit and a highly efficient photon absorber. Due to the unique energy level structure of the system, changing the qubit state of the quantum dot can render it completely invisible to the light.

This property makes the CNOT gate possible (see figure 1 in gallery). Light trapped in a cavity that does not see a QD (QD in qubit state 1) will eventually leak out, with its polarization flipped. However, if the quantum dot is in qubit state 0, the light is strongly modified such that incoming and outgoing polarizations actually remain the same. In this case the photonic qubit is not flipped.

A sensitive camera collects a fraction of the light that leaks back out of the cavity after its polarization is analyzed using special optics. Thus, the team can see if a photon’s polarization was flipped by the QD. The state of the QD qubit is not random: the team controls it. Another key feature of this protocol is that the photons are from an external laser and are not intrinsically connected to the QD through absorption/emission processes.

“Using an external photon source has an advantage that the quantum dot state is not destroyed during the process. Currently, we use a strongly attenuated laser as the photon source, but eventually this can be replaced with true single photon sources,” says lead author Dr. Hyochul Kim.

This quantum dot-photon gate happens in a flash--picosecond or 1/ trillionth of a second. Ultrafast gates are important when increasing the number of qubits and operations so that a calculation completes before the system’s quantum behavior is lost. (This is called decoherence--scientists can shield the qubit from the disruptive environment but every so often something sneaks in and destroys the quantum states.)

The team’s proof-of-principle gate demonstration paves the way for the next generation of devices that will improve light collection and QD qubit coherence times. “To improve coherence time, we need to trap the electron or hole in the quantum dot and use their spin as a qubit. This is more challenging, and we are currently working on this,” Kim says.

Additionally, they will use truly single photons as the light source. “Quantum dots are also excellent single photon sources. We consider such a system where single photons are periodically emitted from the neighbor quantum dot, which are then connected to logic devices on the same semiconductor chip,” adds Kim.

The research described here was supported in part by the PFC @ JQI.

This news item was written by E. Edwards

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  • The Future of Ion Traps
  • Technology will continue to be a leader in the development of quantum computing architectures
  • March 7, 2013

Recently Science Magazine invited JQI fellow Chris Monroe and Duke Professor Jungsang Kim to speculate on ion trap technology as a scalable option for quantum information processing. The article is highlighted on the cover of this week’s (March 8, 2013) issue, which is dedicated to quantum information. The cover portrays a photograph of a surface trap that was fabricated by Sandia National Labs and used to trap ions at JQI and Duke, among other laboratories.

Trapped atomic ions are a promising architecture that satisfies many of the critical requirements for constructing a quantum computer. At the heart of quantum computers are qubits, systems maintained in two or more quantum states simultaneously. Here, the qubits are manifested in the internal energy levels of the ions, and are manipulated through laser and microwave radiation. These technologies are a key factor in the success of atomic ions: scientists can set the frequency of the radiation to match that of the ion’s energy level spacings with extreme precision.

The qubits have long coherence time -- meaning they can be placed in quantum states and remain that way long enough to perform calculations. The qubit’s states are not sensitive to ambient disturbances like magnetic fields, giving them inherent protection from the destructive environment.

Additionally, the ions are in a vacuum of lower than 10-11 torr. This is about 100 trillion times lower than atmospheric pressure. To visualize this daunting number, imagine light particles like hydrogen or nitrogen in a vacuum chamber. After special pumps remove most of the air, there are so few molecules left that before one molecule will collide with another, it will typically travel a distance comparable to the circumference of the earth. At atmospheric pressure, even though we can’t see them with our eyes, there are so many molecules floating about that they only travel about a hundredth the width of a human hair before they bump into a neighboring particle. 

Scientists want to go even further. Using cryogenics (cooling to near absolute zero temperature), they expect to push a few more factors of ten lower in pressure. Cooling the system is effective because it makes the molecules stick to the walls, thus removing them from the region where the ions rest.

Ion traps themselves were invented more than a half-century ago, but researchers have implemented new technologies in order to store large ion crystals and shuttle ions around as quantum operations are executed. Professionally micro-fabricated devices, like the one shown on the cover, resemble traditional computer components. Some researchers are also integrating optics on-board the traps. Although quantum logic operations in such chip traps remain elusive, the obstacles are not prohibitive. In the US, researchers at institutions such as NIST (Boulder), Sandia National Labs, Georgia Tech Research Institute, JQI, Duke, MIT, and others are now, often collaboratively, fabricating and testing these technologies.

Monroe and Kim are part of a larger collaboration called MUSIQC, which stands for Modular Universal Scalable Ion-trap Quantum Computer, and is supported by the Intelligence Advance Research Projects Activity (IARPA). This program focuses on building the components necessary for a practical quantum computer. The effort involves national labs, universities, and even private small businesses. 

"Scaling the Ion Trap Quantum Processor," C. Monroe and J. Kim, Science, March 8, 2013

This news item was written by E. Edwards

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  • An Ideal Material
  • Solving a mystery leads to the discovery of a true topological insulator
  • January 29, 2013

An old material gets a new name, and with it, topological insulators have another chance to shine. Samarium hexaboride (SmB6) has been around since the late 1960s--but understanding its low temperature behavior has remained a mystery until recently. Experimentalists* have recently confirmed that this material is the first true 3D topological insulator—as originally predicted by JQI/CMTC☨ theorists in 2010. Topological insulators have been discussed widely as a new area of material science, with the potential to study quantum Hall physics and exotic states such as Majorana fermions. While this finding provides a conclusion to one mystery, it is also the beginning of a new chapter that will certainly lead to a clearer understanding of this strange physics and even new quantum devices.

As insulators are cooled to absolute zero, their ability to insulate effectively becomes infinite. About 40 years ago, scientists observed that, under these conditions, some insulators atypically retain a tiny bit of conductivity. These materials, termed Kondo insulators, were not well-understood, until recently. 

In Kondo insulators two ingredients combine to create what are called “heavy fermions.” In materials such as SmB6, some of the electrons are effectively pinned, only having a spin degree of freedom. This is in contrast to the speedy conduction electrons, which can also move in the crystal, endowing it with metallic character. The energy-momentum relationship (band structure) is flat for the pinned electrons. The conduction electrons would normally have a quadratic ("U" shaped) energy-momentum relationship, but at low temperatures, they strongly interact with the effectively stationary electrons. The band structure reorganizes to take on more of the flattened character of the stationary electrons. This hybridization gives rise to electrons that act as if they are sluggish and is the origin of the term ‘heavy fermions.’ The transition from metal to insulator, where the electrons behave, in effect, as if they are 1000 times heavier, starts to occur as the system is cooled below 50 K (see illustration). But then something strange happens a few degrees above absolute zero.

Maxim Dzero is an expert in heavy fermion materials: “Decades ago, people went through the periodic table making all sorts of combinations of elements. With this material, the major mystery in 1970s was that it was an insulator that at low temperatures, but still retained some small residual conductivity.”

The missing piece of the puzzle lay with theory that wouldn’t been developed until recent years. Above 4 K, SmB6 appears to be an insulator; looking below 4 K, it is a metal with high resistivity. This seemed confusing until recently when condensed matter theorists who study topology claim that this is exactly what you should see in a topological insulator.

A perfect topological insulator would be insulating in the bulk, but, in 3-dimensions, allow current to pass over the surface. Predicting these materials is tricky and while scientists have some hints as to the ingredients, finding them is somewhat serendipitous. Even cold atoms interacting with lasers have been proposed as a candidate for realizing this kind of physics. In recent years, researchers have studied bismuth compounds as a topological material. Unfortunately, interactions and defects tend to destroy their bulk insulating behavior, making it difficult to study the existence of conducting surface states.

In 2010**, JQI and PFC-supported scientists at the CMTC made were able to show that the mystery surrounding Kondo insulators could be explained using topological theory. It turns out that the strong interactions create a situation where the surface conducting states are truly independent and isolated from the bulk. Recently, experimental groups have verified that SmB6 is indeed a true 3D topological insulator, and in fact, is the first compound to be classified as such.

An experiment at University of Michigan involves attaching 8 electrical contacts to a thin sample in a novel way so as to unambiguously distinguish between bulk and surface conduction. Independently, a group from the University of California at Irvine has made voltage measurements, probing the Hall effect. They observe that the Hall resistance is independent of sample thickness, which is consistent with SmB6 supporting surface conduction. If the bulk was conducting, then the resistance would increase as the sample thickness was decreased.

In a related, third experiment at the University of Maryland’s Center for Nanophysics and Advanced Materials, researchers have reported careful measurements of the Kondo-insulating behavior of SmB6 at different temperatures, which supports the presence of underlying topological physics.

The surface conducting states of a topological insulator are expected to be quite impervious to disorder. Indeed, the experiments indicate this. Dzero explains, “The transport properties are quite good in spite of the crude things that are done to the sample. It is remarkable that the surface conductivity does not change.”

Victor Galitski, in response to the experimental work that is posted currently on the open-access arXiv discusses the strength of the measurements, “There is one major qualitative prediction to distinguish the topological insulator: surface conduction. This is the first material in the world that does this. There is no other conceivable theory that will explain it besides topological insulators.”

☨Affiliations [The theory work and prediction of SmB6 as a topological insulator was done primarily in Victor Galitski's group at JQI/CMTC, with support from the NSF PFC @JQI]:

Victor Galitski is a JQI fellow and a professor at the University of Maryland Physics Department/Condensed Matter Theory Center (CMTC).

Maxim Dzero was a postdoctoral researcher at JQI and is now a professor at Kent State University

Kai Sun was a postdoctoral researcher at JQI and is now a professor at the University of Michigan--Sun is an author on the previous theoretical works, as well as the current experimental results from U. Michigan

Piers Coleman is a professor at Rutgers University

 **Theory papers:

Theory of topological Kondo insulators, Maxim Dzero, Kai Sun, Piers Coleman, and Victor Galitski, Physical Review B (2012)

Topological Kondo Insulators, Maxim Dzero, Kai Sun, Piers Coleman, and Victor Galitski, Physical Review Letters, (2010)

 *Experimental papers:

Discovery of the First Topological Kondo Insulator: Samarium Hexaboride, Steven Wolgast, Çağlıyan Kurdak, Kai Sun, J. W. Allen, Dae-Jeong Kim, Zachary Fisk, arXiv:1211.5104v2

Robust Surface Hall Effect and Nonlocal Transport in SmB6: Indication for an Ideal Topological Insulator, J. Botimer, D.J. Kim, S. Thomas, T. Grant, Z. Fisk and Jing Xia, arXiv:1211.6769

Hybridization, Correlation, and In-Gap States in the Kondo Insulator SmB6 Xiaohang Zhang, N. P. Butch, P. Syers, S. Ziemak, Richard L. Greene, and J. Paglione, arXiv:1211.5532 

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  • The First Controllable Atom SQUID
  • From PML at NIST
  • November 28, 2012

PFC supported scientists at JQI have created the first controllable atomic circuit that functions analogously to a superconducting quantum interference device (SQUID) and allows operators to select a particular quantum state of the system at will.

By manipulating atoms in a superfluid ring thinner than a human hair the investigators were able for the first time to measure rotation-induced discrete quantized changes in the atoms’ state, thereby providing a proof-of-principle design for an “atomtronic” inertial sensor.

In the nascent but fast-moving field of atomtronics, flows of atoms are used in ways analogous to the flow of electrons in conventional electronic systems. “It could be argued,” says team leader Gretchen Campbell of PML’s Quantum Measurement Division, “that this is a true atomtronics device where we have a controllable circuit element.”

SQUIDs work because of a fundamental property of superconductors: When exposed to an applied magnetic field, a ring of superconductor generates currents which produce magnetic flux exactly canceling the external field. When the current reaches a critical value (determined by a junction or weak link in the ring), it “jumps” by a discrete amount, allowing a quantized amount of flux to penetrate the ring. Measuring the effect of that action on the current across a junction allows one to measure the strength of the applied field, and SQUIDs are routinely employed to detect very weak fields such as those produced by brain waves or nerve impulses in muscle tissue.

Campbell and colleagues in the Laser Cooling and Trapping Group have long been investigating analogous behavior in toroidal Bose-Einstein condensates (BECs) – ultracold, donut-shaped ensembles of atoms that are all in the same quantum state and form a superfluid.

To create rotation, which is the superfluid counterpart to external magnetic fields in a SQUID, the team introduces a green laser beam perpendicular to and penetrating the plane of the superfluid ring, and slowly rotates the beam around the ring. (See animation.) The beam acts as a sort of optical paddle, causing the superfluid BEC atoms to rotate.

Just as a superconducting ring admits flux when the current exceeds a critical value, the ring of superfluid admits a vortex, resulting in a change in the circulation of atoms around the ring. Like everything else in the quantum world, the properties of those vortices are quantized – that is, they occur only at discrete values, and lead to quantized circulation states in the BEC. Campbell’s team was able to observe and measure those quantum increments and for the first time was able to control the onset of discrete circulation states by tuning the power and rotational speed of the green laser.

"We're just at the beginning," says SQUID expert Chris Lobb, a team member from the University of Maryland Department of Physics. “Hopefully, as we learn more about the capabilities of these devices, powerful applications will emerge. The ability to easily vary the coupling strength and current-phase relationship of the weak link is unique to this system, and will lead to new devices."

In a forthcoming paper,* the team describes the experiment in detail. Sodium atoms are first confined in a magneto-optical trap and then cooled in an optical trap until about 600,000 atoms form a BEC. Intersecting laser beams (1064 nanometer wavelength) are sent through the BEC, shaping the cloud into a torus about 40 micrometers in diameter with a density that is nearly uniform azimuthally. Perpendicular to the plane of the torus, a laser beam with a wavelength of 532 nm penetrates and disrupts the BEC, forming an approximately 8 µm thick, low-atomic-density barrier that is made to rotate around the ring at speeds ranging from 1 Hz to 4 Hz, setting the atoms in motion.

"The atomic cloud is incredibly cold and very sensitive to imperfections, so the main challenge in getting the experiment to work is smoothing and stabilizing the intensity of the laser beams,” says first author Kevin Wright. “To achieve reliable control of the atoms in the ring, the combined intensity of all of the lasers has to be smooth and remain steady to better than one percent."

To determine how the circulation state of the ring-shaped condensate varies with barrier rotation speed and beam strength, the researchers turn off the trap potential and obtain images of the atoms’ configuration after 10 milliseconds time-of-flight. The images show that at very low rotation speed the atoms remain motionless; but when the rotation speed hits a critical value, the atoms instead form a vortex at the center of the ring that expands in quantized steps as the speed increases. This vortex corresponds to a quantized current state in a SQUID. As the rotation rate increases, however, off-center vortices form as a result of differential rotation speed between the inner and outer sections of the ring.

This system is, in effect, a rotational sensor. “In a magnetic SQUID operation, you bias the magnetic field of your system very close to what you want to measure, such that even a tiny external field pushes you over the edge,” Campbell says. “We could do something similar to measure rotation. Suppose we know that our system jumps at half a hertz, and we want to measure something around 0.1 Hz. We could start rotating our system at 0.4 Hz, and then that little bit of extra rotation – say the rotation of a vehicle – would push it into a different, measurable quantum state.

“Of course, there are excellent rotation sensors already available. But because we have the ability to control our system and change the parameters at will, that might offer a new functionality for future devices.”

Co-author Bill Phillips, leader of PML's Laser Cooling and Trapping Group, says “this experiment is part of the birth of a new area of research in atomic physics—the atomtronic circuit. It also represents a new intersection of condensed matter and atomic physics in ways that were impossible just a few years ago.”


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  • First Observation of the Hall Effect in a Bose-Einstein Condensate
  • June 19, 2012

National Institute of Standards and Technology (NIST) researchers have observed for the first time the Hall effect in a gas of ultracold atoms. The Hall effect is an important interaction of magnetic fields and electric current more commonly associated with metals and semiconductors. Variations on the Hall effect are used throughout engineering and physics with applications ranging from automobile ignition systems to fundamental measures of electricity. The new discovery could help scientists learn more about the physics of quantum phenomena such as superfluidity and the quantum Hall effect.

Their paper appeared June 14, 2012, in the online version of the Proceedings of the National Academy of Sciences.*

Discovered in 1879 by Edwin Hall, the Hall effect is easiest to visualize in a rectangular conductor like a copper plate when a current is flowing along its length. A magnetic field applied at a right angle to the electric current (down into the plate) deflects the path of the charge carriers in the current (electrons, for example) by inducing a force in the third direction at right angles to both the magnetic field and the current flow. This pushes the charge carriers toward one side of the plate and induces an electrical potential, or "Hall voltage." The Hall voltage can be used to measure the hidden internal properties of electrical systems, such as the concentration of the current carriers and the sign of their charge.**

"Cold atom systems are a great platform for studying complicated physics because they are nearly free of obscuring impurities, the atoms move much more slowly than electrons in solids, and the systems are much simpler," says NIST researcher Lindsay LeBlanc. "The trick is creating the conditions that will get the atoms to behave the right way."

Measuring the Hall effect in a Bose-Einstein condensate builds upon previous NIST work generating synthetic electric and magnetic fields. First, the group uses lasers to tie the atoms’ energy to their momentum, putting two internal states into a relationship called a superposition. This causes the electrically neutral atoms to act as if they are charged particles. With the cloud of about 20,000 atoms gathered into a loose ball, the researchers then cyclically vary the trapping force—pushing the atoms in the cloud together and pulling them apart—to simulate the movement of charge carriers in an alternating current. In response, the atoms begin to move in a manner that is mathematically identical to how charged particles experiencing the Hall effect would move, i.e., at right angles to both the direction of the "current" flow and the artificial magnetic field.

According to LeBlanc, measuring the Hall effect offers another tool for studying the physics of superfluidity, a low-temperature quantum-based condition where liquids flow without friction, as well as the so-called quantum Hall effect, where the ratio of the Hall voltage and the current through the material is quantized, allowing for the determination of fundamental constants.


* See reference publication. 

** See, for example,

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Brief Reports

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About the PFC @ JQI

The Physics Frontier Center is devoted to leading-edge experimental and theoretical investigation of ways to control and process quantum coherence and entanglement: the physics of quantum information. It is funded through a cooperative agreement with the National Science Foundation (NSF) and operated within the Joint Quantum Institute (JQI), a partnership between the University of Maryland (UMD) and the National Institute of Standards and Technology (NIST), with additional support from the Laboratory for Physical Sciences.

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