RSS icon
Twitter icon
Facebook icon
Vimeo icon
YouTube icon

Physics Frontier Center

Latest News

  • boson spin-hall thumb
  • Interfering Waves
  • 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. 

Continue Reading
  • 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.

Continue Reading
  • 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.

Continue Reading
  • 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.

Continue Reading
  • 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

Continue Reading
  • 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. 

Continue Reading
  • 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.”

Continue Reading
  • 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

Continue Reading
  • 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;

Continue Reading
  • 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.  

Continue Reading
  • 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.

Continue Reading
  • 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."

Continue Reading
  • 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.

Continue Reading
  • 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

(Having problems viewing this story or have comments? Let us know!

Continue Reading
  • 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.

(Having problems viewing this story or have comments? Let us know!


Continue Reading
  • 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:

Continue Reading
  • 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.”

Continue Reading
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.

Continue Reading
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. 

Continue Reading
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.

Continue Reading
  • 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.

Continue Reading
  • 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.

Continue Reading
  • 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. 

Continue Reading
  • 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.

Continue Reading
  • 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

Continue Reading
  • 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. 

Continue Reading
  • 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

Continue Reading
  • 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

Continue Reading
  • 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

Continue Reading
  • 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.

Continue Reading
  • 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.

Continue Reading
  • 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.

Continue Reading
  • 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

Continue Reading
  • 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.

Continue Reading
  • 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. 

Continue Reading
  • 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.

Continue Reading
  • 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

Continue Reading
  • 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.

Continue Reading
  • 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.

Continue Reading
  • 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

Continue Reading
  • 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. 

Continue Reading
  • 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.]

Continue Reading
  • 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

Continue Reading
  • 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

Continue Reading
  • 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 

Continue Reading
  • 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.”


Continue Reading
  • 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,

Continue Reading

Brief Reports

PFC-supported researchers 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, to demonstrate a proof-of-principle experiment for performing quantum... Read more

Optical nanofibers are optical fibers that have a diameter smaller than the wavelength of the guided light. Here, all of the light field cannot fit inside of the fiber, yielding a significant enhancement in the evanescent fields outside of the core. This allows the light to trap atoms (or other... Read more

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 PFC supported physicists have developed a new mathematical proof that reveals a much tighter limit on how fast quantum... Read more

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... Read more

Strongly correlated electronic systems, like superconductors, display remarkable electronic and magnetic properties. Creating analogous states in Bose gases is an excellent way to model the dynamics of these systems, offering a level of control often not possible in solid state systems.

... Read more

In certain situations, a collection of atoms can transition to a superfluid state, flouting the normal rules of liquid behavior. Harnessing this effect is of particular interest in the field of atomtronics, since superfluid atom circuits can recreate the functionality of superconductor circuits... Read more

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. PFC supported theorists have been developing a model for what happens when ultracold... Read more

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... Read more

Atomtronics is an emerging technology whereby physicists use ensembles of atoms to build analogs to electronic circuit elements. Using a superfluid atomtronic circuit, PFC supported physicists have demonstrated a tool that is critical to electronics: hysteresis. This is the first time that... Read more

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... Read more

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... Read more

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... Read more

A JQI/PFC experiment establishes a new record for symmetric single-mode, single-photon, heralding efficiency for a pair of entangled photons produced during parametric downconversion. About 84% of the time they observe photon A in one detector they also observe photon B just where it should be... Read more

PFC researchers 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.

Physicists engineer a quantum magnet using lasers and ion ... Read more

All computers, even the future quantum versions, use logic operations or “gates,” which are the fundamental building blocks of computational processes. PFC scientists have performed an ultrafast logic gate on a photon, using a semiconductor quantum dot.

Quantum dots are an attractive... Read more

Last year Paul Lett and his JQI colleagues reported the ability to store a sequence of images (two letters of the alphabet) which were separated in time but overlapping in space within the volume of a gas-filled memory cell. This is random access in time. In a new experiment, by contrast, parts... Read more

The blackbody radiation shift imposed by atom traps on the energy level of the enclosed ultracold atoms will soon impose limits on the accuracy of the best atomic clocks. Although only important at a precision level of a part in 1015, accurate knowledge of this shift is more pertinent now that... Read more

Physicists at the Joint Quantum Institute (JQI) and the University of Maryland show, for the first time, that qubits can successfully exist in a topological superconductor material even in the presence of impurities in the material and strong interactions among participating electrons, courtesy... Read more

Magnetic monopoles weren’t supposed to exist. If you try to saw a bar magnet in half, all you succeed in getting are two magnets, each with a south and north pole. In recent years, however, the existence of monopole quasiparticles consisting of collective excitations among many atoms has been... Read more

A PFC-supported experiment conducted at the Joint Quantum Institute examines the role of disorder in maintaining quantum coherence. It does this by introducing disorder into a Bose-Einstein condensate of rubidium atoms held in an optical lattice to simulate the role is impurity disorder in high... Read more

PFC-supported scientists have stored not one but two letters of the alphabet in a tiny cell filled with rubidium atoms which are tailored to absorb and later re-emit messages on demand. This is the first time two images have simultaneously been reliably stored in a non-solid medium and then... Read more

PFC-supported experimentalists have developed a novel form of lattice for atoms. This lattice does not arise from a spatially varying light intensity pattern, as is the case for traditional optical lattices.  Instead, laser radiation generates an effective magnetic field that changes the... Read more

An optical switch developed at the Joint Quantum Institute (JQI) spurs the prospective integration of photonics and electronics. The JQI switch can steer a beam of light from one direction to another in only 120 ps using only about 90 attojoules of input power. At the wavelength used, in the... Read more

New PFC-supported work shows how a simple “joystick” consisting of an adjustable magnetic field can create several new phases of atomtronic matter, many of them never seen before. The field is used to tune the interaction---giving the researcher force on demand, causing the atoms to assume... Read more

PFC research at the Joint Quantum Institute (JQI) has for the first time engineered and detected the presence of effective high angular momentum collisions between atoms at temperatures close to absolute zero. Previous experiments with ultracold atoms featured essentially head-on collisions. The... Read more

Electrons carry information over tiny distances in computer circuitry. Photons are commonly used to carry information over kilometer distances. Scientists are currently developing micron-scale optical devices to replace and/or be compatible with electron-based circuit elements.

Diodes... Read more

Ultracold atomic gases trapped by laser light have become a playground for exploring quantum matter and even uncovering new phenomena not yet seen in nature.

PFC researchers at JQI have shown that this kind of optical lattice system can exhibit a never-before-seen quantum state called a... Read more

If quantum computers are ever to be realized, they likely will be made of different components sharing information with one another, just as the memory and logic circuits in today's computers do. Yet, it remains unclear how the quantum states in these different systems interact.

A team of... Read more

PFC-supported research at JQI has uncovered evidence for a long-sought-after quantum state of matter, a spin liquid. You can’t pour a spin liquid into a glass.  It’s not a material at all, at least not a material you can touch. It is more like a kind of magnetic disorder within an ordered array... Read more

Quantum spin models are powerful because they can describe many types of physical phenomena such as phase transitions in magnets. Simulations of these models can provide insights when the actual system of interest is difficult to understand theoretically or challenging to experimentally probe.... Read more

Researchers in a collaboration between the PFC at JQI and CalTech have shown that it may be possible to take a conventional semiconductor and endow it with topological properties without subjecting the material to extreme environmental conditions or fundamentally changing its solid state... Read more

In atomtronics scientists construct circuit elements using ultra-cold atomic gases where the atoms take the role of electrons. PFC scientists have developed an experiment that not only generates... Read more

PFC experimentalists in the Trapped Ion Quantum Information group have performed a gate that flips the state of a single atomic qubit in less than 50 picoseconds. The time to perform this same operation with continuous wave (CW) ... Read more

PFC-supported researchers, lead by Ian Spielman, recently demonstrated for the first time spin-orbit coupling in Bose-Einstein condensates (BECs), where the neutral atoms exhibited properties similar to electrons in material systems.

The scientists engineered a laser-atom interaction... Read more

“Frustrated" ensembles of interacting components – that is, those which cannot settle into a state that minimizes each interaction – may be the key to understanding a host of puzzling phenomena that affect systems from neural networks and social structures to protein folding and magnetism.... Read more

Random number sequences are needed for data encryption and other critical uses – yet truly random numbers are nearly impossible to come by. All classical processes such as coin flips are, in principle, predictable. But one thing that absolutely cannot be predicted is the value resulting... Read more

For the first time, scientists have employed a powerful technique of laser physics – the “optical frequency comb” – to entangle two trapped atoms. This form of control is a promising candidate for use as a logic gate for quantum computing and information-processing, and offers substantial... Read more

PFC-supported scientists have devised a new method that could be used to generate multiple pairs of “indistinguishable” photons – near-identical individual quanta of light – by fine-tuning the output from two separate quantum dots. Manipulating single photons will play a role in any eventual... Read more

Topological quantum computing (TQC), in which the data are protected against decoherence because they are stored and manipulated as shapes, is a highly desirable goal in quantum information science. Unfortunately, the only physical system in which anything approaching topological protection has... Read more

Physicists supported by the PFC at the Joint Quantum Institute have developed a new source of “entangled” photons – fundamental units of light whose properties are so intertwined that if the condition of one is measured, the condition of the other is instantaneously known, even if the photons... Read more

The behavior of quantum dots – nanometer-scale semiconductor formations that have many of the same quantized properties as atoms when interacting with light – is a subject of intense interest in condensed-matter physics. Now researchers supported by the Joint Quantum Institute’s Physics Frontier... Read more

PFC-supported scientists have demonstrated a new way to control quantum interactions that makes it possible to fine-tune the way in which the spin properties of trapped atoms couple to, and are "entangled" with, those of their neighbors -- a development with potentially important applications in... Read more

September 26, 2009 A Plan for Hybrid Entanglement

Quantum entanglement, a condition in which the states of two different objects become so inextricably linked that neither can be described separately, is an essential element of any future quantum computer. Scientists have succeeded in entangling many sorts of entities, typically identical atom... Read more

The simplest form of Bose-Einstein condensation occurs when a number of bosonic atoms (those with integer net spin) coalesce into the lowest possible energy state. Such condensates exhibit macrosopic quantum interference, persistent vortex currents and other manifestations of superfluidity. ... Read more


PFC-supported physicists have created and demonstrated a remote “quantum gate” – a key component for long-range quantum information transfer and an essential... Read more

Ian Spielman of the Joint Quantum Institute proposed a novel experimental protocol whereby neutral atoms in a Bose-Einstein condensate would behave like charged particles in a magnetic field. Physicists can use such an arrangement to create precisely tunable models to study the dynamics of... Read more

Twenty years ago, Purdue University scientists proposed a highly promising design for a “spin effect” transistor – the Datta-Das transistor, or DDT. To date, however, no one has been able to build a working model. Now JQI researchers have devised a potential solution to the problem: creating a... Read more

Neutral atoms, having no net electric charge, usually don't act very dramatically around a magnetic field. But by “dressing up” a Bose-Einstein condensate of rubidium atoms – applying two beams of laser light, thus giving the atoms an effective directional tendency, or vector potential -- PFC... Read more

February 10, 2009 Topological Insulators

Topological insulators form in certain materials that, in bulk, have the distinctive physical signature of insulators: That is, the permitted energy levels (or “bands”) in their component atomic structures are characterized by a full valence band and an empty conduction band, with a substantial... Read more

Two ions are placed in separate, unconnected traps 1 meter apart. A “message” is imprinted on Ion A via a microwave pulse. Then the ions are excited into a state in which they emit one photon each. Either photon can be one of two slightly different wavelengths. Those photons travel through... Read more

previous next

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.


PFC and JQI researchers engage the public in quantum research. Click here to request a visit from one of our scientists!

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.

PFC General Info:   Academic and Research Info: Luis Orozco | CSS 2203 | (301) 405-9740 |

Subscribe to A Quantum Bit 

Quantum physics began with revolutionary discoveries in the early twentieth century and continues to be central in today’s physics research. Learn about quantum physics, bit by bit. From definitions to the latest research, this is your portal. Subscribe to receive regular emails from the quantum world. Previous Issues...

Sign Up Now

Sign up to receive A Quantum Bit in your email!

 Have an idea for A Quantum Bit? Submit your suggestions to