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Quantum Many-Body Physics


Physicists use theoretical and experimental techniques to develop explanations of the goings-on in nature. Somewhat surprisingly, many phenomena such as electrical conduction can be explained through relatively simplified mathematical pictures — models that were constructed well before the advent of modern computation. And then there are things in nature that push even the limits of high performance computing and sophisticated experimental tools. Computers particularly struggle at simulating systems made of numerous particles--or many-bodies-- interacting with each other through multiple competing pathways (e.g. charge and motion). Yet, some of the most intriguing physics happens when the individual particle behaviors give way to emergent collective properties. In the quest to better explain and even harness the strange and amazing behaviors of interacting quantum systems, JQI physicists use experimental and theoretical tools to study the complexities of many-body physics, with an emphasis on topics such as entanglement and topology.

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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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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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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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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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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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  • March 16, 2016 Quantum Many-Body Physics, PFC

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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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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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.

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For many years rubidium has been a workhorse in the investigation of ultracold atoms.  Now JQI scientists are using Rb to cool another species, ytterbium, an element prized for its possible use in advanced optical clocks and in studying basic quantum phenomena.   Yb shows itself useful in another way: it comes in numerous available isotopes, some of which are bosonic in nature and some fermionic.

Yb-171 has proven satisfactorily amenable to cooling in the atom trap lab of Steve Rolston and Trey Porto.  First Rb-87 atoms are loaded into a magneto-optic trap---an enclosure where magnetic fields and laser beams are used to confine atoms---and then cooled until they form a Bose-Einstein condensate (BEC).  Slow-moving Yb atoms, in contact with the Rb atoms, are cooled right along with them.  Thus Yb atoms lose excess energy to warming the colder Rb atoms.

In this way Yb atoms can be chilled to a state of quantum degeneracy, a condition in which the Yb atoms in the vapor reach their lowest possible energy configuration.  It’s important to mention here that the Rb atoms are bosons: they can all occupy a single common quantum state, namely the BEC.  The Yb atoms, by contrast, are fermions: quantum interactions preclude their existing side-by-side in a single quantum state.  Instead they must occupy a ladder of energy states, form the lowest level on up. 

“Yb atoms have been chilled to a state of quantum degeneracy before by evaporation techniques,” said Varun Vaidya, one of the JQI researchers, “but mostly by way of evaporative cooling, a process that depletes up to 99.9% of the atoms.  Yb-171 atoms, however, cannot be cooled this way since they don’t interact with each other.”

Yb-171 cooling down to degeneracy was achieved in one experiment, Vaidya said, by cooling it with Yb-173 atoms.  But this left a Yb-171 sample of only about a few thousand atoms. In the JQI the complement of Yb-171 atoms remains ample: up to a quarter million atoms.

What can one do with Yb atoms chilled in this way? One thing is to insert them into an optical lattice, a sort of extra constraint inside the atom trap consisting of criss-crossing laser beams. The beams, establishing a pattern of local electric field minima and maxima, hold the atoms in a regular lattice the way eggs are held in a crate.  Then the Yb atoms can then be selectively excited into higher energy states. 

Vaidya, the lead author on a JQI paper published as an "Editor's Suggestion" in a recent issue of Physical Review A, cites two examples of what could come from excited Yb atoms in a lattice.  One would be the creation of  polarons, quasi-particles consisting of a Yb atoms surrounded by several Rb atoms drawn inwards by a weak force called the Van der Waals interaction. Another possibility is that the Yb atoms could be excited to a higher energy level and then quickly stimulated to refund that energy, falling back to their starting state.  The energy refund would come not in the form of a quantum of light (photon) as in a laser, but in the form of a quantum of acoustic energy (a phonon).  Thus you would get a sort of phononic laser.

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  • At the edge of a quantum gas
  • JQI physicists observe skipping orbits in the quantum Hall regime
  • September 29, 2015 Quantum Many-Body Physics, PFC

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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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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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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Physicists use theoretical and experimental techniques to develop explanations of the goings-on in nature. Somewhat surprisingly, many phenomena such as electrical conduction can be explained through relatively simplified mathematical pictures — models that were constructed well before the advent of modern computation. And then there are things in nature that push even the limits of high performance computing and sophisticated experimental tools. Computers particularly struggle at simulating systems made of numerous particles interacting with each other through multiple competing pathways (e.g. charge and motion). Yet, some of the most intriguing physics happens when the individual particle behaviors give way to emergent collective properties. One such example is high-temperature superconductivity, where the underlying mechanism is still under debate.

In the quest to better explain and even harness the strange and amazing behaviors of interacting quantum systems, well-characterized and controllable atomic gases have emerged as a tool for emulating the behavior of solids. This is because physicists can use lasers to force atoms in dilute quantum gases to act, in many ways, like electrons in solids. The hope is studying the same physics in the atom-laser system will help scientists understand the inner workings of different exotic materials.

JQI physicists, led by Trey Porto, are interested in quantum magnetic ordering, which is believed to be intimately related to high-temperature superconductivity and also has significance in other massively connected quantum systems. Recently, the group studied the magnetic and motional dynamics of atoms in a specially designed laser-based lattice that looks like a checkerboard. Their work was published in the journal Science.

To engineer a system that might behave like a real chunk of material, the team loads an ultracold gas of around 1000 rubidium atoms into a two-dimensional optical lattice, which is a periodic array of valleys and hills created by intersecting beams of laser light. The atoms are analogous to the electrons in a solid; the lattice geometry is defined by the pattern of laser light. The depth of the lattice can be precisely adjusted to allow for certain types of atom behaviors. For this research, atoms can move between the lattice sites via quantum tunneling — this is the motional component. Secondly, each atom can be thought of as having an orientation similar to that of a magnet*, ‘up’ and ‘down.’ Like magnets, the atoms, under certain conditions, can interact to form ordered arrangements in the lattice (e.g. up-down-up-down…).

However, observing the emergence of such magnetic states has been challenging because these particular phases of matter are only revealed when the atoms have extraordinarily low energy, here considered as an effective temperature. A typical ultracold atomic gas is around 10-100 nanokelvin. To glimpse magnetic order in an optical lattice, the atoms need to be at picokelvin temperatures, 10 to 1000 times colder.

Porto’s experiment takes a different approach — the researchers work in a regime out-of-equilibrium where magnetic dynamics can be induced at higher, more amenable temperatures. While the daunting picokelvin criteria remains, the authors believe that this methodology will open up new pathways for achieving and studying quantum magnetic matter in lower temperature, equilibrated systems.

The team uses a novel ‘checkerboard’ lattice, in which they have exquisite control over the different sites; this gives them the ability to study both the motional and magnetic behavior of the atoms. The lattice is constructed from two controllable sublattices, denoted A and B, which together form an array of ‘double-wells’ (see above graphic). The researchers can create desired magnetic order by altering the relative depth of sublattice B with respect to sublattice A. This same mechanism is also employed to drive dynamics between lattice wells. In the beginning of the experiment, all atom magnets are initialized in the ‘up’ orientation and are trapped in a two-dimensional uniformly deep optical lattice. The second step is to flip the atoms in the B lattice to ‘down’; this makes the system antiferromagnetically ordered and out-of-equilibrium. Next, the depth of sublattice B is suddenly changed in a maneuver called a ‘quench.’ Essentially, the quench kicks the system, initiating dynamics across the lattice. The atom magnets flip up and down and tunnel between sites as they relax to a configuration that is, in this case, no longer magnetized.

This experiment in some ways gets at the potential power of quantum simulations, even independent of the material science applications. Porto explains, “Here is an atom-lattice system that is challenging to calculate accurately. Yet being able to demonstrate precise control over different competing parameters in such a system and also watch it evolve over time may yield insights into the underlying physics.”

Lead author Roger Brown, now a National Research Council Postdoctoral Fellow at NIST, Boulder continues, “Our relatively extended system in two-dimensions poses an interesting theoretical challenge because numerically exact techniques are not available and traditional analytical theories require approximations. Thus experimental observations may be useful for choosing appropriate theories.”

*Physicists use mathematical models, such as the bosonic t-J model studied here, to understand quantum magnets. Thus for clarity in this news item the atoms are called “magnets.” In the language of the Science article, they are called “spins”.


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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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spin-hall bosons

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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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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