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Physics Frontier Center

About the PFC @ JQI

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  • boson spin-hall thumb
  • Interfering Waves
  • Trapped ions and superconductors face off in quantum benchmark
  • April 13, 2017

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

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

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

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

By Erin Marshall

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

The research appears in the journal Physical Review Letters.

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

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

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

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

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

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

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

Taking a wider approach

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

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

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

On the surface

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

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

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

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

Too much disorder?

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

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

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

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

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

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

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

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

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

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

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

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

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

Spotted at long last

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

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

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

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

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

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

Quasiparticles lead the charge

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

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

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

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

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

A theorist’s dream

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

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

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

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

Putting the pieces together

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Infographic credit: S. Kelley/JQI

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

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

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

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

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

Playing with atoms

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

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

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

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

A possible explanation

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

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

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

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

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

Infographic credit: S. Kelley/JQI

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

Symmetry and Topology

This text was written by E. Edwards/JQI

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

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

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

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

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

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

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

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

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

Courtesy of CMNS

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.”

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.

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.

This news item was written by Chad Boutin, NIST.

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

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  • Thermometry using an optical nanofiber
  • August 21, 2015

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Award information and quotes provided by OSA.

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

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

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

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  • Novel Phases for Bose Gases
  • February 26, 2015

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

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

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  • Vladimir Manucharyan Receives CAREER Award
  • February 5, 2015

For more information on NSF Awards visit:

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

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

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

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

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

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

Phase Coding

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

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

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

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

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

Twenty Questions

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Quantum order by disorder

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Interferometric Investigations

This article was written by S. Kelley/JQI.

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

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

The Ion Trap Quantum Spin Simulator

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

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).

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

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

(2) See related publication below.

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

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

From CMNS at UMD

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

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

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

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

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

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

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

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

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

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

  • 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)

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

This article was written by E. Edwards/JQI

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

This article was written by E. Edwards/JQI

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

* Author information:

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

This article was written by E. Edwards/JQI

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Edge States and Ring Resonators

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

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

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

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

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

Possible Applications

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

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

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

This was written by P. Schewe and E. Edwards

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

Shivering atoms

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.

This story was written by S. Kelley, JQI.

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

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

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

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

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

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

Light Modes

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

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

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

Photon Number

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Written by Phillip F. Schewe and Emily Edwards

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

Quantum Morse Code

The Real World

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  • Spin Hall Effect in a Quantum Gas
  • An 'atomtronic' transistor
  • June 5, 2013

From NIST Techbeat1

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.

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)

1This story was written by Chad Boutin for NIST Techbeat. It was modified for JQI by E. Edwards, with permission.

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  • Entanglement in a Flash
  • June 3, 2013

This experiment focuses on using highly energetic laser pulses to perform qubit operations. Previously, they set a record for the fastest spin flip in these systems: a mere 50 picoseconds. Here they continue their work by blasting the ion so strongly that the qubit quickly becomes linked to its motion. Such speedy operations are more typically associated with solid state systems such as electrons in semiconductors or superconductors. Here the speed of operations combined with the pristine quantum environment of atoms provide the best of both worlds.

Though subdued, trapped ions are not entirely tame. Like caged tigers pacing back and forth, they exhibit regular motion, oscillating inside an electrostatic trap at a particular frequency. This motion is used as a medium for entangling multiple ions. The motion is also the enemy, introducing noise because logic operations and/or experiments are typically completed after many trap oscillation cycles have passed. In quantum computing, speed is critical because such noise can destroy the quantum-ness of the system.

Providing a solution, this team has achieved spin-motion entanglement on an ultrafast timescale of about 3 nanoseconds, nearly a thousand of times faster than the trap oscillation time. For reference, a beam of light travels only a foot in one nanosecond. Blinking your eye is about 100 million times slower than this entangling process.

The qubit is initially prepared in a quantum coherent superposition of two states: “spin-up” and “spin-down.” Laser pulses are then used to kick spin-up in one direction and spin-down in the opposite direction. As a result, the ion is in an entangled state, which is a combination of spin-up/moving left, and spin-down/moving right.

To create this spin-dependent kick, laser pulses arrive from opposite directions, overlapping to form a diffraction grating. The ion qubit interacts with the light grating by absorbing and emitting a photon. Photons carry momentum, and so the absorption/emission cycle kicks the ion qubit. For a fixed color of light, the kick strength depends primarily on the laser power. The light spectrum also contains a frequency that induces spin flips. Thus the ion qubit “feels” this kick based on its associated spin: opposite spin states are brushed in opposite directions. This is analogous when atoms are diffracted by an optical lattice; but here the resulting diffraction pattern depends on the qubit’s spin state (up or down).

In the ion trap, the arrival of the first set of overlapping pulses creates many discrete momentum states, each tangled with an alternating spin. Subsequent pulses serve to further accumulate momentum states. The goal is to apply the grating kicks in a way that builds up a particular diffraction pattern corresponding to just two momentum states, each entangled with an opposing spin. The process of building up amplitude in an interference pattern can be thought of in terms of the not-so-quantum trampoline. Say that my daughter is jumping at a rate of 0.5 bounce per second. Some of her friends can build up amplitude by simultaneously jumping at the same time, or “in phase.” Some kids can also jump at a rate that is an integer (1X, 2X, 3X…) multiple of the other kids, which will create a kind of interference pattern of jumps. The kids will go the highest at the time when the most kids are simultaneously jumping. Similarly, if the team wants to accumulate more and more momentum, they need to apply the grating (kick) at a particular time with respect to the ion trap oscillations.

At the other extreme, the kids can all jump at random times and their bounces will cancel each other leading to reduced or even zero jumping amplitude over all. You could imagine that perfect cancellation could take place at special combinations of jumps as well. Likewise, the team can even gain back the original spin superposition, with the motion entirely removed. If they flash on the grating at the precise moment the motion packets overlap, this kick neatly disentangles the spin from the motion.

This exquisite control over the spin-motion entanglement by adjusting the delay between kicks is equivalent to an interferometer using pairs of spin dependent kicks. Interferometers have numerous applications, ranging from radio astronomy to cellular imaging. When the same type of ultrafast interferometer applied to multiple atoms, their internal spin states can become entangled, which is a fundamental building block of a quantum computer.

This article was written by S. Kelley and E. Edwards. The animation was created by S. Kelley. Permissions for re-use: Contact JQI

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  • Rajibul Islam awarded Distinguished Dissertation Award
  • May 10, 2013

Rajibul Islam was recently awarded UMDs Distinguished Dissertation Award for his thesis work on quantum magnetism with ions in Chris Monroe's Trapped Ion Quantum Information group. According to the graduate school's website, "The Distinguished Dissertation Award recognizes original work that makes an unusually significant contribution to the discipline. Both methodological and substantive quality will be judged.  Awards will be given each year in four broad disciplinary areas: 1) Mathematics, Physical Sciences, and Engineering; 2) Social Sciences; 3) Humanities and Fine Arts; and 4) Biological and Life Sciences." Islam received the award in area (1). This year 625 students across all disciplines received PhDs from UMD.

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  • Turning on Frustration
  • 16 atomic ions simulate a quantum antiferromagnet
  • May 2, 2013

Frequency Sidebar:

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  • Quantum Dot Commands Light
  • A solid state ultrafast logic gate on a photon
  • March 31, 2013

The research described here was supported in part by the PFC @ JQI.

This news item was written by E. Edwards

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  • The Future of Ion Traps
  • Technology will continue to be a leader in the development of quantum computing architectures
  • March 7, 2013

Recently Science Magazine invited JQI fellow Chris Monroe and Duke Professor Jungsang Kim to speculate on ion trap technology as a scalable option for quantum information processing. The article is highlighted on the cover of this week’s (March 8, 2013) issue, which is dedicated to quantum information. The cover portrays a photograph of a surface trap that was fabricated by Sandia National Labs and used to trap ions at JQI and Duke, among other laboratories.

"Scaling the Ion Trap Quantum Processor," C. Monroe and J. Kim, Science, March 8, 2013

This news item was written by E. Edwards

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  • An Ideal Material
  • Solving a mystery leads to the discovery of a true topological insulator
  • January 29, 2013

☨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:

 *Experimental papers:

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  • The First Controllable Atom SQUID
  • From PML at NIST
  • November 28, 2012

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  • First Observation of the Hall Effect in a Bose-Einstein Condensate
  • June 19, 2012

* See reference publication.

** See, for example,

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Brief Reports

Solitons — solitary waves that behave more like discrete particles than waves — occur in diverse physical systems, from water in a canal to light waves in optical-fiber telecommunications. They can also exist in Bose-Einstein... Read more

A new paper from scientists at the National Institute of Standards and Technology (NIST) and the University of Maryland suggests that intentionally creating a traffic jam out of a ring of several thousand... Read more

These days,... Read more

In a paper published as the cover story in Nature on August 4, 2016, PFC-funded researchers introduced the first... Read more

Physical systems that retain no memory of their initial conditions are said to have thermalized, since it is often—but not always—an exchange of heat and energy with another system that causes the memory loss. The opposite case is localization, where information about the initial arrangement... Read more

Pumps have been around for millennia, but recently physicists have sought to build a different kind of pump—one that could use the rules of quantum mechanics to pump atoms or charges in a quantized way. PFC-supported researchers have created the first pump based on the geometry of quantum... Read more

Rydberg atoms are a popular choice for quantum device proposals because they interact strongly with each other and their individual and collective behavior is easy to manipulate. While experimenting with rubidium, one of the most popular Rydberg systems, PFC-supported researchers discovered an... Read more

In the fractional quantum Hall (FQH) effect, the collective action of electrons in a material form particle-like “quasiparticles” that can appear to possess fractional charge, such as 1/3. In quantum Hall systems, these quasiparticles can become trapped around specially tailored defects, forming... Read more

Ultracold atomic systems can be used to model condensed-matter physics, providing precise control of system variables often not achievable in real materials. This involves inducing charge-neutral particles to behave as if they were charged particles in a magnetic field. To this end, PFC-... Read more

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

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

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

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

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

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

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 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

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

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

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

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

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