Research News - UMD Physics

Tug-of-War Unlocks Menagerie of Quantum Phases of Matter

Details: Published: Tuesday, January 25 2022 02:46

Phases are integral to how we define our world. We navigate through the phases of our lives, from child to teenager to adult, chaperoned along the way by our changing traits and behaviors. Nature, too, undergoes phase changes. Lakes can freeze for the winter, thaw in the spring and lose water to evaporation in the dog days of summer. It’s useful to capture and study the differences that accompany these dramatic shifts.

In physics, phases of matter play a key role, and there are more phases than just the familiar solid, liquid and gas. Physicists have built a modest taxonomy of the different phases that matter can inhabit, and they’ve explored the alchemy of how one phase can be converted into another. Now, scientists are discovering new ways to conjure up uniquely quantum phases that may be foundational to quantum computers and other quantum tech of the future.

“There's a whole world here,” says Associate Professor Maissam Barkeshli, a Fellow of the Joint Quantum Institute and a member of the Condensed Matter Theory Center. “There’s a whole zoo of phases that we could study by having competing processes in random quantum circuits.”In new numerical experiments, quantum particles (black dots), which travel upward through time, are subject to random quantum processes (blue, green and yellow blocks). Depending on the likelihood of the different kinds of processes, the quantum particles ultimately end up in different entanglement phases. This figure shows five examples of randomly chosen processes acting on a small number of particles. (Credit: A. Lavasani/JQI)

Often when physicists study phases of matter they examine how a solid slab of metal or a cloud of gas changes as it gets hotter or colder. Sometimes the changes are routine—we’ve all boiled water to cook pasta and frozen it to chill our drinks. Other times the transformations are astonishing, like when certain metals get cold enough to become superconductors(link is external) or a gas heats up and breaks apart into a glowing plasma soup(link is external).

However, changing the temperature is only one way to transmute matter into different phases. Scientists also blast samples with strong electric or magnetic fields or place them in special chambers and dial up the pressure. In these experiments, researchers are hunting for a stark transition in a material’s behavior or a change in the way its atoms are organized.

In a new paper published recently in the journal Physical Review Letters(link is external), Barkeshli and two colleagues continued this tradition of exploring how materials respond to their environment. But instead of looking for changes in conductivity or molecular structure, they focused on changes in a uniquely quantum property: entanglement, or the degree to which quantum particles give up their individuality and become correlated with each other. The amount of entanglement and the distinct way that it spreads out among a group of particles defines different entanglement phases.

In all the entanglement phases studied in the new paper, the particles are fixed in place. They don’t move around and form new links, like what happens when ice melts into water. Instead, transitioning from phase to phase requires a metamorphosis in the way that the particles are entangled with each other—a change that’s invisible if you only pay attention to the local behavior of the particles and their links. To reveal this change, the researchers used a quantity called the topological entanglement entropy, which captures, in a single number, the amount of entanglement present in a collection of particles. Different entanglement phases have different amounts of entanglement entropy, so calculating this number picks out which entanglement phase the particles are in.

The researchers used UMD’s supercomputers to conduct numerical experiments and study the entanglement phases of a grid of quantum particles. They studied which entanglement phase the particles end up in when subjected to a tug-of-war between three competing quantum processes. One process performs a quantum measurement on an individual particle, forcing it to choose between one of two states and removing some entanglement from the grid. Another process, which the researchers were the first to include, is also a quantum measurement, but instead of measuring a single particle it measures four neighboring particles at a time. This, too, removes some entanglement, but it can also spread entanglement in a controlled way. The final process twists and spins the particles around, like what happens when a magnet influences a compass needle. This tends to inject more entanglement into the grid.

On their own, each of the three processes will pull the particles into three different entanglement phases. After many applications of the process that twists the particles around, entanglement will be spread far and wide—all the particles will end up entangled with each other. The single particle measurements have the opposite effect: They remove entanglement and halt its spread. The four-particle measurements, which spread entanglement in a controlled way, lead to an in-between phase.

The researchers began their numerical experiments by preparing all the particles in the same way. Then, they randomly selected both a process and which cluster of particles it was applied to. After many rounds of random applications, they ceased their prodding and calculated the topological entanglement entropy. Over many runs, the researchers also varied the likelihood of selecting the different processes, tuning how often each of the processes gets applied relative to the others. By performing these experiments many times, the researchers constructed a phase diagram—basically a map of how much entanglement is left after many rounds of random quantum nudges.

The results add to an emerging body of work that studies the effects of applying random quantum processes—including a paper published in Nature Physics earlier this year(link is external) by the same team—but the inclusion of the four-particle measurements in the new result produced a richer picture. In addition to some expected features, like three distinct entanglement phases corresponding to the three processes, the researchers found a couple of surprises.

In particular, they found that entanglement spread widely throughout the system using only the two quantum measurement processes, even though neither process would produce that phase on its own. They may have even spotted a stable phase perched between the phase created by the single-particle measurements alone and the phase created by the four-particle measurements alone, an unlikely phenomenon akin to balancing something on the edge of a knife.

But besides creating the phase diagram itself, the authors say that their technique supplies a new way to prepare phases that are already well known. For instance, the phase created by the four-particle measurements is key to quantum error correcting codes and topological quantum computation. One way of preparing this phase would require making the four-particle measurements, interpreting the results of those measurements, and feeding that information back into the quantum computer by performing additional highly controlled quantum procedures. To prepare the same phase with the new technique, the same four-particle measurements still must be made, but they can be done in a random fashion, with other quantum processes interspersed, and there is no need to interpret the results of the measurements—a potential boon for researchers looking to build quantum devices.

“It is a kind of shortcut in the sense that it's a way of realizing something interesting without needing as much control as you thought you needed,” Barkeshli says.

The authors note that the new work also contributes to the growing study of non-equilibrium phases of quantum matter, which includes exotic discoveries like time crystals and many-body localization. These contrast with equilibrium phases of matter in which systems exchange heat with their environment and ultimately share the same temperature, settling down into stable configurations. The key difference between equilibrium and non-equilibrium phases is the continual nudges that the application of random processes provides.

"Our work shows that the peculiar nature of measurements in quantum mechanics could be leveraged into realizing exotic non-equilibrium phases of matter,” says Ali Lavasani, a graduate student in the UMD Department of Physics and the first author of the new paper. “Moreover, this technique might also lead to novel non-equilibrium phases of matter which do not have any counterpart in equilibrium settings, just like driven systems give rise to time crystals that are forbidden in equilibrium systems.”

Original story by Chris Cesare: https://jqi.umd.edu/news/tug-war-unlocks-menagerie-quantum-phases-matter

In addition to Barkeshli and Lavasani, the paper had one additional author: Yahya Alavirad, a former graduate student in physics at the University of Maryland who is now a postdoctoral scholar in physics at the University of California San Diego.

Reference Publication: "Topological Order and Criticality in (2+1)D Monitored Random Quantum Circuits," Ali Lavasani, Yahya Alavirad, Maissam Barkeshli, Phys. Rev. Lett., 127, 235701 (2021)

Research Contacts: Maissam Barkeshli This email address is being protected from spambots. You need JavaScript enabled to view it.; Ali Lavasani This email address is being protected from spambots. You need JavaScript enabled to view it.

Enhancing Simulations of Curved Space with Qubits

Details: Published: Tuesday, January 18 2022 02:46

One of the mind-bending ideas that physicists and mathematicians have come up with is that space itself—not just objects in space—can be curved. When space curves (as happens dramatically near a black hole), sizes and directions defy normal intuition. Something as straightforward as defining a straight line requires careful consideration.

Understanding curved spaces is important to expanding our knowledge of the universe, but it is fiendishly difficult to study curved spaces in a lab setting (even using simulations). A previous collaboration between researchers at JQI explored using labyrinthine circuits made of superconducting resonators to simulate the physics of certain curved spaces (see the previous story for additional background information and motivation of this line of research). In particular, the team looked at hyperbolic lattices that represent spaces—called negatively curved spaces(link is external)—that have more space than can fit in our everyday “flat” space. Our three-dimensional world doesn’t even have enough space for a two-dimensional negatively curved space.

Now, in a paper published in the journal Physical Review Letters(link is external) on Jan. 3, 2022, the same collaboration between the groups of Alicia Kollár and Alexey Gorshkov expands the potential applications of the technique to include simulating more intricate physics. They’ve laid a theoretical framework for adding qubits—the basic building blocks of quantum computers—to serve as matter in a curved space made of a circuit full of flowing microwaves. Specifically, they considered the addition of qubits that change between two quantum states when they absorb or release a microwave photon—an individual quantum particle of the microwaves that course through the circuit.(Left image) Microwave photons that create an interaction between pairs of qubits (black dots on the edge) in a hyperbolic space are most likely to travel along the shortest path (dotted line). In both images, the darker colors show where photons are more likely to be found. (Right image) A quantum state formed by a qubit (grey dot containing parallel black lines) and an attached microwave photon that can be found at one of the intersections of the grid representing a curved space. (Credit: Przemyslaw Bienias/JQI)

“This is a new frontier in tabletop experiments studying effects of curvature on physical phenomena,” says first author Przemyslaw Bienias, a former Joint Quantum Institute (JQI) assistant research scientist who is now working for Amazon Web Services as a Quantum Research Scientist. “Here we have a system where this curvature is huge and it's very exciting to see how it influences the physics.”

For researchers to use these simulations they need a detailed understanding of how the simulations represent a curved space and even more importantly under what situations the simulation fails. In particular, the edges that must exist on the physical circuits used in the simulations must be carefully considered since scientists are often interested in an edgeless, infinite curved space. This is especially important for hyperbolic lattices because they have nearly the same number of sites on the edge of the lattice as inside. So the team identified situations where the circuits should reflect the reality of an infinite curved space despite the circuit’s edge and situations where future researchers will have to interpret results carefully.

The team found that certain properties, like how likely a qubit is to release a photon, shouldn’t be dramatically impacted by the circuit’s edge. But other aspects of the physics, like the proportion of states that photons occupy at a given shared total energy, will be strongly influenced by the edge.

With proper care, this type of simulation will provide a peek into how negatively curved spaces are a foundation for an entirely new world of physics.

“In this paper, we asked the question, ‘What happens when you add qubits to the photons living on those hyperbolic lattices?’” Bienias says. “We are asking, ‘What type of physics emerges there and what type of interactions are possible?’”

The researchers first looked at how the microwaves and a single qubit in the circuit can combine. The team predicts that the size of special quantum states in which a photon is attached to a particular qubit—a bound state—will be limited by the curved space in a way that doesn’t happen in flat space. The right-side image above shows such a state with the darker coloring showing where the photon is most likely to be found around the qubit represented by the grey dot.

They then investigated what happens when there are multiple qubits added to a circuit full of microwaves. The photons traveling between qubits serve as intermediaries and allow the qubits to interact. The team’s analysis suggests that the photons that are causing qubits to interact tend to travel along the shortest path between the two points in the circuit—corresponding to the shortest distance in the simulated curved space. One of these paths through the curved space is shown in the left-side image above. This result matches physicists’ current expectations of such a space and is a promising sign that the simulations will reveal useful results in more complex situations.

Additionally, the researchers predict that the curvature will limit the range of the interactions between qubits similar to the way it limits the size of the individual bound states. Simulations using this setup could allow scientists to explore the behaviors of many particles interacting in a curved space, which is impractical to study using brute numerical calculation.

These results build upon the previous research and provide additional tools for exploring new physics using superconducting circuits to simulate curved space. The inclusion of interactions explored in this paper could aid in using the simulations to investigate the topic called AdS/CFT correspondence that combines theories of quantum gravity and quantum field theories.

“Hyperbolic connectivity is immensely useful in classical computation, underlying, for example, some of the most efficient classical error correcting codes in use today,” Kollár says. “We now know that adding qubits to a hyperbolic resonator lattice will endow the qubits’ interactions with hyperbolic structure, rather than the native flat curvature of the lab. This opens the door to allow us to carry out direct experiments to examine the effect of hyperbolic connectivity on quantum bits and quantum information.”

Original story by Bailey Bedford: https://jqi.umd.edu/news/enhancing-simulations-curved-space-qubits

In addition to Kollár, Gorshkov and Bienias, other co-authors of the paper were Ron Belyansky, a JQI physics graduate student, and Igor Boettcher, a former JQI postdoctoral researcher and current assistant professor at the University of Alberta.

Reference Publication

"Circuit Quantum Electrodynamics in Hyperbolic Space: From Photon Bound States to Frustrated Spin Models," Przemyslaw Bienias, Igor Boettcher, Ron Belyansky, Alicia J. Kollar, Alexey V. Gorshkov, Phys. Rev. Lett., 128, 013601 (2022)

In a Smooth Move, Ions Ditch Disorder and Keep Their Memories

Details: Published: Monday, December 20 2021 10:46

A Persian adage, notably wielded by Abe Lincoln(link is external) and the band OK Go(link is external), expresses the ephemeral nature of the world: “This, too, shall pass.”

Physicists have their own version of this rule. It says that wiggles and wrinkles—really any small disturbances—tend to get ironed out over time. For instance, a couple drops of blue food coloring mixed into some cake batter will impart a blue tint to the whole batch; fresh water from a river funneled into the salty ocean will spread out and make a slightly less salty ocean; and a gush of cold wind entering your room will mingle with the air inside and reach a single, cooler temperature. The basic idea is that, given enough time, everything will reach equilibrium, regardless of where it started.

There are a few notable exceptions to this equanimous rule. In the quantum world of atoms and electrons, particles confined in a container made of electric and magnetic fields—akin to a bowl confining cake batter—can get stuck in place if the container isn’t smooth. When this “bowl” is rough, disorderly, and random, the particles can’t make up their minds about which way to go and instead stay put. Oddly, even when a bunch of these localized particles are allowed to influence each other, they can manage to stay localized, not exchanging energy and avoiding equilibrium. This effect, known as many-body localization (MBL), imparts particles with a kind of memory of where they started.

Now, scientists have found a new way to create disturbances that do not fade away. Instead of relying on disorder to freeze things in place, they tipped the quantum particles’ container to one side—a trick that is easier to conjure in the lab. A collaboration between the experimental group of College Park Professor Christopher Monroe and the theoretical group of Alexey Gorshkov, who is also a Fellow of the Joint Quantum Institute, and the Joint Center for Quantum Information and Computer Science and a physicist at the National Institute of Standards and Technology, has used trapped ions to implement this new technique, confirming that it prevents their quantum particles from reaching equilibrium. The team also measured the slowed spread of information with the new tipping technique for the first time. They published their results(link is external) recently in the journal Nature.

“One advantage of this method of many-body localization is that we don't need that disorder,” says Fangli Liu, former graduate student in physics at the University of Maryland (now a research scientist at QuEra Computing) and lead theorist on the work. “In the original system the disorder is realized in a random form. But with this method, each time you do a measurement you will have exactly the same result. It gives us the possibility to more efficiently use this many-body localization to do something interesting.”Researchers have demonstrated a new way for atomic ions to host disturbances that do not fade away. (credit: E. Edwards/JQI)

Instead of color (as in the dough example) or temperature (in the case of air in your room), the disturbance in the JQI experiment was in the ions’ spins—their little internal magnets that can point up or down (or a bit of both at the same time, as in a quantum superposition). These ion spins sit in a container shaped not like a bowl but instead like a single row of an egg carton, with each ion residing in a different dimple of the container. Normally, after some time all spins would point in the same direction uniformly, with no memory of whether each spin pointed up or down to begin with.

By controlling the ions individually, the scientists can prepare one spin that points up while the rest point down. With an egg carton container that’s flat (like it’s sitting on a table), the single spin disturbance can hop between ions, chatting with neighbors and ultimately causing all the ions to agree on a uniform configuration. In traditional many-body localization, where randomness and disorder rule the Normally, ion spins that start out pointing in opposite directions will interact and reach an equilibrium, with no trace of where they started. But when the tilt in their container is large enough, they keep pointing in their original direction, creating a many-body localized state that remembers its initial configuration. (Credit: Adapted from article by the authors/JQI)day, the egg-carton dimples become offset up or down from each other in a random way, paralyzing each spin in its spot.

Instead of adding disorder, the team tilted the egg carton, offsetting each dimple a little higher than its neighbor to the left in a smooth, consistent way. This caused the spins to get localized as well, but for a very different reason. Quantum particles have wave-like properties, and once they start rolling down in the direction of a tilt, they can get reflected by the edges of the egg carton dimples. So instead of rolling downhill forever, they roll down and bounce back up over and over again, which confines them to their small region of the container.

For a single particle, this pinning mechanism has been known since the 1930s. But whether it would persist in the face of interactions between many particles and halt equilibration has only recently been explored. Indeed, the idea that tilting the egg carton would result in a breakdown of equilibration was only proposed in 2019.

The JQI team confirmed this in their experiment. Using tightly focused lasers, they adjusted each ion individually and prepared them in a highly disturbed state, with spins pointing in alternating directions. At the same time, they had extra lasers shining on all the ions together, allowing them to talk to each other even while far apart. If the tilt was high enough, the team found, the ions’ spins remained in their original configuration for an extended period, refusing to succumb to equilibrium.

In addition to a conceptual leap, creating MBL without disorder may come with certain practical advantages. First, it is experimentally easier to implement a smooth tilt (in fact, a small tilt was present in the JQI experiment whether they wanted it or not). Second, it makes measurements much more straightforward. And third, this method is immune to an accidental break down of MBL. In regular disorder-based MBL, the random offsets of the dimples need to be large. If they aren’t, localization can break down in some spots and infect the whole batch. With a smooth tilt, there’s no such risk.

This opens the possibility of using many-body localization to create a robust memory. MBL might help maintain quantum information in future quantum computers or help preserve curiosities like time crystals or topological phases.

In the past year, two other experiments realizing this method were reported. The team of H. Wang in Hangzhou, China set it up using superconducting qubits(link is external), and Monika Aidelburger’s team in Munich, Germany made it happen(link is external) with ultracold atoms.

“There's a lot of shared themes between our three papers,” says William Morong, a postdoctoral researcher at JQI and lead author on the work, “and I would say all of them together give a more complete picture of the phenomenon then each individually.”

The JQI group was the only one, however, to demonstrate another key property of many-body localization: the slow spread of entanglement between their ions. The team used a technique adapted from nuclear magnetic resonance imaging to measure the crawling pace with which entanglement spread across their atoms, a hallmark of MBL.

“I think that our work shows the exciting progress that has been made in modern quantum simulation platforms,” Morong says, “We are reaching the point where we have enough control over collections of quantum particles in these platforms that we can read a theoretical paper describing some interesting effect that emerges in a specific system, program in the forces that we need to create this effect for ourselves, and measure subtle signatures in the quantum entanglement between the particles that are only revealed when you can observe each particle individually. "

Original story by Dina Genkina: https://jqi.umd.edu/news/smooth-move-ions-ditch-disorder-and-keep-their-memories

In addition to Liu, Morong, Monroe and Gorshkov, authors on the paper included former graduate student in physics Patrick Becker (now a physicist at Booz Allen Hamilton), graduate student in physics Kate Collins, postdoctoral researcher at JQI Lei Feng, former graduate student in physics (now a postdoctoral fellow at Indiana University in Bloomington) Antonis Kyprianidis, former postdoctoral fellow at JQI (now assistant professor of physics at Rice University) Guido Pagano, and undergraduate researcher Tianyu You.

Reference Publication

"Observation of {Stark} many-body localization without disorder," W. Morong, F. Liu, P. Becker, K. S. Collins, L. Feng, A. Kyprianidis, G. Pagano, T. You, A.V. Gorshkov, C. Monroe, Nature, 599, 393–398 (2021)

Research Contact

William Morong: This email address is being protected from spambots. You need JavaScript enabled to view it.

(link sends e-mail)

Graphene’s Magic Act Relies on a Small Twist

Details: Published: Monday, October 18 2021 10:46

Carbon is not the shiniest element, nor the most reactive, nor the rarest. But it is one of the most versatile.

Carbon is the backbone of life on earth and the fossil fuels that have resulted from the demise of ancient life. Carbon is the essential ingredient for turning iron into steel, which underlies technologies from medieval swords to skyscrapers and submarines. And strong, lightweight carbon fibers are used in cars, planes and windmills. Even just carbon on its own is extraordinarily adaptable: It is the only ingredient in (among other things) diamonds, buckyballs(link is external) and graphite(link is external) (the stuff used to make pencil lead).

This last form, graphite, is at first glance the most mundane, but thin sheets of it host a wealth of uncommon physics. Research into individual atom-thick sheets of graphite—called graphene(link is external)—took off after 2004 when scientists developed a reliable way to produce it (using everyday adhesive tape to repeatedly peel layers apart). In 2010 early experiments demonstrating the quantum richness of graphene earned two researchers the Nobel Prize in physics.

In recent years, graphene has kept on giving. Researchers have discovered that stacking layers of graphene two or three at a time (called, respectively, bilayer graphene or trilayer graphene) and twisting the layers relative to each other opens fertile new territory for scientists to explore. Research into these stacked sheets of graphene is like the Wild West, complete with the lure of striking gold and the uncertainty of uncharted territory.

Researchers at JQI and the Condensed Matter Theory Center (CMTC) at the University of Maryland, including JQI Fellows Sankar Das Sarma and Jay Sau and others, are busy creating the theoretical physics foundation that will be a map of this new landscape. And there is a lot to map; the phenomena in graphene range from the familiar like magnetism to more exotic things like strange metallicity(link is external), different versions of the quantum Hall effect, and the Pomeranchuk effect—each of which involve electrons coordinating to produce unique behaviors. One of the most promising veins for scientific treasure is the appearance of superconductivity (lossless electrical flow) in stacked graphene.

“Here is a system where almost every interesting quantum phase of matter that theorists ever could imagine shows up in a single system as the twist angle, carrier density, and temperature are tuned in a single sample in a single experiment,” says Das Sarma, who is also the Director of the CMTC. “Sounds like magic or science fantasy, except it is happening every day in at least ten laboratories in the world.”

The richness and diversity of the electrical behaviors in graphene stacks has inspired a stampede of research. The 2021 American Physical Society March Meeting included 13 sessions addressing the topics of graphene or twisted bilayers, and Das Sarma hosted a day long virtual conference in June for researchers to discuss twisted graphene and the related research inspired by the topic. The topic of stacked graphene is extensively represented in scientific journals, and the online arXiv preprint server has over 2,000 articles posted about “bilayer graphene”—nearly 1,000 since 2018.

Perhaps surprisingly, graphene’s wealth of quantum research opportunities is tied to its physical simplicity.In a sheet of graphene, a carbon atom sits at the corner of each hexagon. (Credit: Paul Chaikin with modifications by Bailey Bedford)

Graphene is a repeating honeycomb sheet with a carbon atom residing at every corner. The carbon atoms hold strongly to one another, making imperfections in the pattern uncommon. Each carbon atom contributes an electron that can freely move between atoms, and electrical currents are very good at traveling through the resulting sheets. Additionally, graphene is lightweight, has a tensile strength(link is external) that is more than 300 times greater than that of steel and is unusually good at absorbing light. These features make it convenient to work with, and it is also easy to obtain.

Graphene’s pure, consistent structure is an excellent embodiment of the physics ideal of a two-dimensional solid material. This makes it the perfect playground for understanding how quantum physics plays out in the material without the researchers having to worry about complications from the additional mess that occurs in most materials. There are then a variety of new properties that are unlocked by stacking layers of graphene on top of each other. Each layer can be rotated (by what scientists call a “twist angle”) or shifted relative to the hexagonal pattern of its neighbors.

Graphene’s structural and electrical properties make it easy to change the quantum landscape that electrons experience in an experiment, giving researchers several options for how to customize, or tune, graphene’s electrical properties. Combining these basic building blocks has already resulted in a wealth of different results, and they aren’t done experimenting.

A ‘Magical’ Flourish

In the quantum world of electrons in graphene, the way that layers sit atop one another is important. When adjacent sheets in a bilayer are twisted with respect to each other, some atoms in the top sheet end up almost right above their corresponding neighbor while in other places atoms end up far away (on an atomic scale) from any atom in the other sheet. These differences form giant, repeating patterns similar to the distribution of atoms in the single sheet but over a much longer scale, as shown in the image at the top of the story and in the interactive visual bellow.

Every change of the angle also changes the scale of the larger pattern that forms the quantum landscape through which the electrons travel. The quantum environments formed by various repeating patterns (or a lack of any organization) are one of the main reasons that electrons behave differently in various materials; in particular, a material’s quantum environment dictates the interactions electrons experience. So each miniscule twist of a graphene layer opens a whole new world of electrical possibilities.

“This twist is really a new tuning knob that was absent before the discovery of these 2D materials,” says Fengcheng Wu, who has worked on graphene research with Das Sarma as a JQI and CMTC postdoc and now collaborates with him as a professor at Wuhan University in China. “In physics, we don't have too many tuning knobs. We have temperature, pressure, magnetic field, and electric field. Now we have a new tuning knob which is a big thing. And this twist angle also provides new opportunities to study physics.”

Researchers have discovered that at a special, small twist angle (about 1.1 degrees)—whimsically named the “magic angle”—the environment is just right to create strong interactions that radically change its properties. When that precise angle is reached, the electrons tend to cluster around certain areas of the graphene, and new electrical behaviors suddenly appear as if summoned with a dramatic magician’s flourish. Magic angle graphene behaves as a poorly-conducting insulator in some circumstances and in other cases goes to the opposite extreme of being a superconductor—a material that transports electricity without any loss of energy.

The discovery of magic-angle graphene and that it has certain quantum behaviors similar to a high-temperature superconductor was the Physics World 2018 Breakthrough of the Year. Superconductors have many valuable potential uses, like revolutionizing energy infrastructure and making efficient maglev trains. Finding a convenient, room-temperature superconductor has been a holy grail for scientists.

The discovery of a promising new form of superconductivity and a plethora of other electrical oddities, all with a convenient new knob to play with, are significant developments, but the most exciting thing for physicists is all the new questions that the discoveries have raised. Das Sarma has investigated many aspects of layered graphene, resulting in more than 15 papers on the topic since 2019; he says two of the questions that most interest him are how graphene becomes superconducting and how it becomes magnetic.

“Various graphene multilayers are turning out to be a richer playground for physics than any other known condensed matter or atomic collective system—the occurrence of superconductivity, magnetism, correlated insulator, strange metal here is coupled with an underlying nontrivial topology, providing an interplay among interaction, band structure, and topology which is unique and unprecedented,” says Das Sarma. “The subject should remain in the forefront of research for a long time."

Strange Bedfellows

Scientists have known about superconductivity and magnetism for a long time, but graphene isn’t where they expected to find them. Finding both individually was a surprise, but scientists have also found the two phenomena occurring simultaneously in some experiments.

Superconductivity and magnetism are usually antagonists, so their presence together in a graphene stack suggests there is something unusual happening. Researchers, like Das Sarma, hope that uncovering which interactions lead to these phenomena in graphene will give them a deeper understanding of the underlying physics and maybe allow them to discover more materials with exotic and useful properties.

A hint at the treasure possibly waiting to be discovered are measurements of twisted bilayer graphene’s electrical properties, which resemble behaviors seen in certain high-temperature superconductors. This suggests that graphene might be crucial to solving the mysteries surrounding high-temperature superconductivity.

The current clues point to the peculiarities of electron interactions being the key to understanding the topic. Superconductivity requires electrons to pair up, so the interactions that drive the pairing in graphene stacks are naturally of interest.

In an article published in Physical Review B(link is external), Das Sarma, Wu and Euyheon Hwang, who was formerly a JQI research scientist and is now a professor at Sungkyunkwan University in South Korea, proposed that what binds pairs of electrons in twisted bilayer graphene may be surprisingly mundane. They think the pairing mechanism may be the same as that in the most well understood superconductors. But they also think that the conventional origin may result in unconventional pairs.

Fengcheng Wu (Credit: Fengcheng Wu)Their analysis suggests that it is not just the interactions that electrons have with each other that are enhanced at the magic angle but also the electron’s interactions with vibrations of the carbon atoms. The vibrations, called phonons, are the quantum mechanical version of sound and other vibrations in materials.

In the best understood superconductors, it is phonons that bind electrons into pairs. In these superconductors, the partnered electrons are required to have opposite values of their spin—a quantum property related to how quantum particles orient themselves in a magnetic field. But the team’s theory suggests that in graphene this traditional pairing mechanism can not only pair electrons with opposite spins but also pair electrons with the same spin. Their description of the pairing method provides a possible explanation to help understand superconductivity in twisted bilayer graphene and graphene-based materials more generally.

“Unconventional superconductivity is highly sought after in physics, as it is exotic on its own and may also find applications in topological quantum computing,” says Wu. “Our theory provides a conventional mechanism towards unconventional superconductivity.”

More recently, Das Sarma, Sau, Wu and Yang-Zhi Chou, who is a JQI and CMTC post-doctoral researcher, collaborated to develop a tool to help scientists understand a variety of graphene stacks. A paper on this research(link is external) was recently accepted in Physical Review Letters. They made a theoretical framework to explore the way that electrons behave on a hexagonal grid. They were inspired by experiments on magic-angle twisted trilayer graphene. Twisted trilayer graphene has the middle layer twisted relative to the top and bottom layers, like a cheese sandwich with the slice twisted so that the corners stick out. This graphene sandwich has attracted attention because it hosts superconductivity at a higher temperature than the two-stack version.

The team’s theoretical model provides a description of the electrons’ behavior in a particular quantum world. Using it on the case of twisted trilayer graphene, they showed that the uncommon pairing of electrons with the same spin could dominate the electrons behavior and be the source of twisted trilayer graphene’s superconductivity.

This new tool provides a starting place for investigating other graphene experiments. And the way the identified pairing mechanism influences the electrons may be significant in future discussions of the role of magnetism in graphene experiments.

Magnetism in stacked graphene is its own mysterious magic trick. Magnetism isn’t found in graphite or single layers of graphene but somehow appears when the stacks align. It’s especially notable because superconductivity and magnetism normally can’t coexist in a material the way they appear to in graphene stacks.

“This unconventional superconducting state in twisted trilayer graphene can resist a large magnetic field, a property that is rarely seen in other known superconducting materials,” says Chou.

In another article in Physical Review B(link is external), Das Sarma and Wu tackled the conundrum of the simultaneous presence of both superconductivity and magnetism in twisted double bilayer graphene—a system like bilayer graphene but where the twist is between two pairs of aligned graphene sheets (for a total of four sheets). This construction with additional layers has attracted attention because it creates a quantum environment that is more sensitive than a basic bilayer to an electric field applied through the stack, giving researchers a greater ability to tweak the superconductivity and magnetism and observe them in different quantum situations.

In the paper, the team provides an explanation for the source of magnetism and how an applied electric field could produce the observed change to a stack’s magnetic behavior. They believe the magnetism arises in a completely different way than it does in more common magnets, like iron-based refrigerator magnets. In an iron magnet, the individual iron atoms each have their own small magnetic field. But the team believes that in graphene the carbon atoms aren’t becoming magnetic. Instead, they think the magnetism comes from electrons that are freely moving throughout the sheet.

Their theory suggests that double bilayer graphene becomes magnetic because of how the electrons push each other apart better in the particular quantum environment. This additional push could lead to the electrons coordinating their individual magnetic fields to make a larger field.

The coordination of electron spins might also be relevant to the pairing of electrons and the formation of potential superconductivity. Spin can be imagined as an arrow that wants to line up with any surrounding magnetic field. Superconductivity normally fails when the magnetism is strong enough that it tears apart the two opposite facing spins. But both spins being aligned in the pairs would explain the two phenomena peacefully coexisting in graphene experiments.

Around the Next Twist in the River

While these theories serve as a guide for researchers pushing forward into the uncharted territory of graphene research, they are far from being a definitive map. At the conference Das Sarma organized in June, a researcher presented new observations(link is external) of superconductivity in three stacked graphene sheets without any twist(link is external). These stacks offset so that none of the layers are right on top of each other; each hexagon has some of its carbon atoms placed at the center of the other layers’ hexagons. The experiment(link is external) revealed two distinct areas of superconductivity, one of which is disturbed by magnetism and the other not. This suggests that the twist may not be the magical ingredient that produces all of the exotic phenomena, but it also raises new questions, offers a route for identifying which electronic behaviors are created or enhanced by the “magic” twist, and provides a new opportunity to investigate the fundamental sources of the underlying physics.Yang-Zhi Chou (Credit: Yang-Zhi Chou)

Inspired by this work and previous observations of magnetism in the same collaboration of Das Sarma, Sau, Wu and Chou mathematically explored the way phonon coupling of electrons might be playing out in these twist-less stacks. The team’s analysis suggest that phonon pairing is the likely driver of both types of superconductivity, with one occurring with matching spins and one with opposite spins. This work(link is external), led by Chou, was recently accepted in Physical Review Letters and has been chosen as a PRL Editors' Suggestion.

These results represent only a fraction of work on graphene experiments at JQI and the CMTC, and many other researchers have tackled additional aspects of this rich topic. But there remains much to discover and understand before the topic of layered graphene is charted and tamed territory. These early discoveries hint that as researchers dig deeper, they may uncover new veins of research representing a wealth of opportunities to understand new physics and maybe even develop new technologies.

“Applications are hard to predict, but the extreme tunability of these systems showing so many different phases and phenomena makes it likely that there could be applications,” Das Sarma says. “At this stage, it is very exciting fundamental research.”

Original story by Bailey Bedford: https://jqi.umd.edu/news/graphenes-magic-act-relies-on-small-twist

Novel Design May Boost Efficiency of On-Chip Frequency Combs

Details: Published: Tuesday, October 12 2021 10:46

On the cover of the Pink Floyd album Dark Side of the Moon, a prism splits a ray of light into all the colors of the rainbow. This multicolored medley, which owes its emergence to the fact that light travels as a wave, is almost always hiding in plain sight; a prism simply reveals that it was there. For instance, sunlight is a mixture of many different colors of light, each bobbing up and down with their own characteristic frequency. But taken together the colors merge into a uniform yellowish glow.

A prism, or something like it, can also undo this splitting, mixing a rainbow back into a single beam. Back in the late 1970s, scientists figured out how to generate many colors of light, evenly spaced in frequency, and mix them together—a creation that became known as a frequency comb because of the spiky way the frequencies lined up like the teeth on a comb. They also overlapped the crests of the different frequencies in one spot, making the colors come together to form short pulses of light rather than one continuous beam.

As frequency comb technology developed, scientists realized that they could enable new laboratory developments(link is external), such as ultra-precise optical atomic clocks, and by 2005 frequency combs had earned two scientists a share of the Nobel Prize(link is external) in physics. These days, frequency combs are finding uses in modern technology, by helping self-driving cars to “see” and allowing optical fibers to transmit many channels worth of information at once, among others.

Now, a collaboration of researchers at the University of Maryland (UMD) has proposed a way to make chip-sized frequency combs ten times more efficient by harnessing the power of topology—a field of abstract math that underlies some of the most peculiar behaviors of modern materials. The team, led by Mohammad Hafezi and Kartik Srinivasan, as well as Yanne Chembo, an associate professor of electrical and computer engineering at UMD and a member of the Institute for Research in Electronics and Applied Physics, published their result(link is external) recently in the journal Nature Physics.

“Topology has emerged as a new design principle in optics in the past decade,” says Hafezi, “and it has led to many intriguing new phenomena, some with no electronic counterpart. It would be fascinating if one also finds an application of these ideas.”

Small chips that can generate a frequency comb have been around for almost fifteen years. They are produced with the help of micro-ring resonators—circles of material that sit atop a chip and guide light around in a loop. These circles are usually made of a silicon compound that is 10 to 100 microns in diameter and printed directly on a circuit board.

Light can be sent into the micro-ring from an adjacent piece of silicon compound, deposited in a straight line nearby. If the frequency of light matches one of the natural frequencies of the resonator, the light will go around and around thousands of times—or resonate—building up the light intensity in the ring before leaking back out into the straight-line trace.

Circling around thousands of times gives the light many chances to interact with the silicon (or other compound) it’s traveling through. This interaction causes other colors of light to pop up, distinct from the color sent into the resonator. Some of those colors will also resonate, going around and around the circle and building up power. These resonant colors are at evenly spaced frequencies—they correspond to wavelengths of light that are an integer fraction of the ring circumference, folding neatly into the circle and forcing the frequencies to form the teeth of a comb. At precisely the right input power and color, the crests of all the colors overlap automatically, making a stable comb. The evenly spaced colors that make up the comb come together to form a single, narrow pulse of light circulating around the ring.

“If you tune the power and the frequency of the light going into the resonator to be just right, magically at the output you get these pulses of light,” says Sunil Mittal, a postdoctoral researcher at the Joint Quantum Institute (JQI) and the lead author of the paper.

On-chip frequency combs allow for compact appRendering of a light-guiding lattice of micro-rings that researchers predict will create a highly efficient frequency comb. (Credit: S. Mittal/JQI)lications. For example, light detection and ranging (LIDAR) allows self-driving cars to detect what’s around them by bouncing short pulses of light produced by a frequency comb off its surroundings. When the pulse comes back to the car, it’s compared against another frequency comb to get an accurate map of the surroundings. In telecommunications, combs can be used to transmit more information in one optical fiber by writing different data onto each of the comb teeth using a technique called wavelength-division multiplexing (WDM).

But chip-scale frequency combs also have their limitations. In one micro-ring, the fraction of power that can be converted from the input into a comb at the output—the mode efficiency—is fundamentally limited to only 5%.

Mittal, Hafezi, and their collaborators have previously pioneered a micro-ring array with built-in topological protection, and used it to supply single photons on demand and generate made-to-order entangled photons. They wondered if a similar setup—a square lattice of micro-ring resonators with extra “link” rings—could also be adapted to improve frequency comb technology.

In this setting, the micro-rings along the outer edge of the lattice become distinct from all the rings in the middle. Light sent into the lattice spends most of its time along this outer edge and, due to the nature of the topological constraints, it doesn’t scatter into the center. The researchers call this outer circle of micro-rings a super-ring.

The team hoped to find magic conditions that would form a frequency comb in the pulses circulating around the super-ring. But this is tricky: Each of the rings in the lattice can have its own pulse of light circling round and round. To get one big pulse of light going around the super-ring, the pulses within each micro-ring would have to work together, syncing up to form an overall pulse going around the entire boundary.

Mittal and his collaborators didn’t know at what frequency or power this would happen, or if it would work at all. To figure it out, Mittal wrote computer code to simulate how light would traverse the 12 by 12 ring lattice. To the team’s surprise, not only did they find parameters that made the micro-ring pulses sync up into a super-ring pulse, but they also found that the efficiency was a factor of ten higher than possible for a single ring comb.

With “magic” input color and power, a lattice of micro-rings produces a single pulse of light circulating around the super-ring outer edge. This pulse is made up of equally spaced frequencies forming a highly efficient comb. (Credit: S. Mittal/JQI)

This improvement owes everything to the cooperation between micro-rings. The simulation showed that the comb’s teeth were spaced in accordance with the size of individual micro-rings, or wavelengths that fold neatly around the small circle. But if you zoomed in on any of the individual teeth, you’d see that they were really subdivided into smaller, more finely spaced sub-teeth, corresponding to the size of the super-ring. Simply put, the incoming light was coupled with a few percent efficiency into each of these extra sub-teeth, allowing the aggregate efficiency to top 50%.

The team is working on an experimental demonstration of this topological frequency comb. Using simulations, they were able to single out silicon nitride as a promising material for the micro-rings, as well as figure out what frequency and power of light to send in. They believe constructing their superefficient frequency comb should be within reach of current state-of-the art experimental techniques.

If such a comb is built, it may become important to the future development of several key technologies. The higher efficiency could benefit applications like LIDAR in self-driving cars or compact optical clocks. Additionally, the presence of finely spaced sub-teeth around each individual tooth could, for example, also help add more information channels in a WDM transmitter.

And the team hopes this is just the beginning. “There could be many applications which we don't even know yet,” says Mittal. “We hope that there'll be much more applications and more people will be interested in this approach.”

Original story by Dina Genkina: https://jqi.umd.edu/news/novel-design-may-boost-efficiency-on-chip-frequency-combs

In addition to Mittal, Chembo, Hafezi (who is also a professor of physics and of electrical and computer engineering at UMD, as well as a member of the Quantum Technology Center and the The Institute for Research in Electronics and Applied Physics), and Srinivasan (who is also a Fellow of the National Institute of Standards and Technology), the team included Assistant Research Scientist Gregory Moille.

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Novel Design May Boost Efficiency of On-Chip Frequency Combs

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