Quantum Gases Won’t Take the Heat

The quantum world blatantly defies intuitions that we’ve developed while living among relatively large things, like cars, pennies and dust motes. In the quantum world, tiny particles can maintain a special connection over any distance, pass through barriers and simultaneously travel down multiple paths.

A less widely known quantum behavior is dynamical localization, a phenomenon in which a quantum object stays at the same temperature despite a steady supply of energy—bucking the assumption that a cold object will always steal heat from a warmer object.

This assumption is one of the cornerstones of thermodynamics—the study of how heat moves around. The fact that dynamical localization defies this principle means that something unusual is happening in the quantum world—and that dynamical localization may be an excellent probe of where the quantum domain ends and traditional physics begins. Understanding how quantum systems maintain, or fail to maintain, quantum behavior is essential not only to our understanding of the universe but also to the practical development of quantum technologies.

“At some point, the quantum description of the world has to changeover to the classical description that we see, and it's believed that the way this happens is through interactions,” says UMD postdoctoral researcher Colin Rylands of the Joint Quantum Institute.UCSB labEquipment at the University of California, Santa Barbra for creating and manipulating quantum gases. It is being used to investigate the dynamical localization of interacting atoms, which is related to new work by JQI researchers. (Credit: Tony Mastres, UCSB)

Until now, dynamical localization has only been observed for single quantum objects, which has prevented it from contributing to attempts to pin down where the changeover occurs. To explore this issue, Rylands, together with Prof. Victor Galitski and other colleagues, investigated mathematical models to see if dynamical localization can still arise when many quantum particles interact. To reveal the physics, they had to craft models to account for various temperatures, interaction strengths and lengths of times. The team’s results, published in Physical Review Letters, suggest that dynamical localization can occur even when strong interactions are part of the picture.

“This result is an example of where a single quantum particle behaves completely differently from a classical particle, and then even with the addition of strong interactions the behavior still resembles that of the quantum particle rather than the classical,” says Rylands, who is the first author of the article.

A Quantum Merry-Go-Round

The result extends dynamical localization beyond its single-particle origins, into the regime of many interacting particles. But in order to visualize the effect, it’s still useful to start with a single particle. Often, that single particle is discussed in terms of a rotor, which you can picture as a playground merry-go-round (or anything else that spins in a circle). The energy of a rotor (and its temperature) is directly related to how fast it is spinning. And a rotor with a steady supply of energy—one that is given a regular “kick”—is a convenient way of visualizing the differences in the flow of energy in quantum and classical physics.

For example, imagine Hercules tirelessly swiping at a merry-go-round. Most of his swipes will speed it up, but occasionally a swipe will land poorly and slow it down. Under these (imaginary) conditions, a normal merry-go-round would spin faster and faster, building up more and more energy until vibrations finally shake the whole thing apart. This represents how a normal rotor, in theory, can heat up forever without hitting an energy limit.

In the quantum world, things go down differently. For a quantum merry-go-round each swipe doesn’t simply increase or decrease the speed. Instead, each swipe produces a quantum superposition over different speeds, representing the chance of finding the rotor spinning at different rates. It’s not until you make a measurement that a particular speed emerges from the quantum superposition caused by the preceding kicks.

Previous research, both theoretical and experimental, has shown that at first a quantum rotor doesn’t behave very differently from a normal rotor because of this distinction—on average a quantum merry-go-round will also have more energy after experiencing more kicks. But once a quantum rotor has been kicked enough, its speed tends to plateau. After a certain point, the persistent effort of our quantum Hercules fails to increase the quantum merry-go-round’s energy (on average).

This behavior is conceptually similar to another thermodynamics-defying quantum phenomenon called Anderson localization. Philip Anderson, one of the founders of condensed-matter physics, earned a Noble Prize for the discovery of the phenomenon. He and his colleagues explained how a quantum particle, like an electron, could become trapped despite many apparent opportunities to move. They explained that imperfections in the arrangement of atoms in a solid can lead to quantum interference among the paths available to a quantum particle, changing the likelihood of it taking each path. In Anderson localization, the chance of being on any path becomes almost zero, leaving the particle trapped in place.

Dynamical localization looks a lot like Anderson localization but instead of getting trapped at a particular position, a particle’s energy gets stuck. As a quantum object, a rotor’s energy and thus speed are restricted to a set of quantized values. These values form an abstract grid or lattice similar to the locations of atoms in a solid and can produce an interference among energy states similar to the interference among paths in physical space. The probabilities of the different possible energies, instead of the possible paths of a particle, interfere, and the energy and speed get stuck near a single value, despite ongoing kicks.

Exploring a New Quantum Playground

While Anderson localization provided researchers with a perspective to understand a single kicked quantum rotor, it left some ambiguity about what happens to many interacting rotors that can toss energy back and forth. A common expectation was that the extra interactions would allow normal heating by disrupting the quantum balance that limits the increase of energy.

Galitski and colleagues identified a one-dimensional system where they thought the expectation may not hold true. They chose an interacting one-dimensional Bose gas as their playground. In a Bose gas, particles zipping back and forth down a line play the part of the rotors spinning in place. The gas atoms follow the same basic principles as kicked rotors but are more practical to work with in a lab. In labs, lasers can be used to contain the gas and also to cool the atoms in the gas down to a low temperature, which is essential to ensuring a strong quantum behavior.

Once the team selected this playground, they explored mathematical models of the many interacting gas atoms. Exploring the gas at a variety of temperatures, interaction strengths and number of kicks required the team to switch between several different mathematical techniques to get a full picture. In the end their results combined to suggest that when a gas with strong interactions starts near zero temperature it can experience dynamical localization. The team named this phenomenon “many-body dynamical localization.”

"These results have important implications and fundamentally demonstrate our incomplete understanding of these systems," says Robert Konik, a coauthor of the paper and physicist at Brookhaven National Lab. "They also contain the seed of possible applications because systems that do not accept energy should be less sensitive to quantum decoherence effects and so might be useful for making quantum computers."

Experimental Support

Of course, a theoretical explanation is only half the puzzle; experimental confirmation is essential to knowing if a theory is on solid ground. Fortunately, an experiment on the opposite coast of the U.S. has been pursuing the same topic. Conversations with Galitski inspired David Weld, an associate physics professor at the University of California, Santa Barbra, to use his team’s experimental expertise to probe many-body dynamical localization.

“Usually it's not easy to convince an experimentalist to do an experiment based on theory,” says Galitski. “This case was kind of serendipitous, that David already had almost everything ready to go.”

Weld’s team is using a quantum gas of lithium atoms that is confined by lasers to create an experiment similar to the theoretical model Galitski’s team developed. (The main difference is that in the experiment the atoms move in three dimensions instead of just one.)

In the experiment, Weld and his team kick the atoms hundreds of times using laser pulses and repeatedly observe their fate. For different runs of the experiment they tuned the interaction strength of the atoms to different values.

“It's nice because we can go to a noninteracting regime quite perfectly, and that's something that it's pretty easy to calculate the behavior of,” says Weld. “And then we can continuously turn up the interaction and move into a regime that's more like what Victor and his coworkers are talking about in this latest paper. And we do observe localization, even in the presence of the strongest interactions that we can add to the system. That's been a surprise to me.”

Their preliminary results confirm the prediction that many-body dynamical localization can occur even when strong interactions are part of the picture. This opens new opportunities for researchers to try to pin down the boundary between the quantum and classical world.

“It's nice to be able to show something that people didn't expect and also for it to be experimentally relevant,” says Rylands.

Story by Bailey Bedford

In addition to Rylands, Galitski and Konik, former JQI graduate student Efim Rozenbaum, who is now a consultant with Boston Consulting Group, was also a co-author of the paper.

Research Contact: Colin Rylands This email address is being protected from spambots. You need JavaScript enabled to view it.
 
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Peeking into a World of Spin-3/2 Materials

Researchers have been pushing the frontiers of the quantum world for over a century. And time after time, spin has been a rich source of new physics.

Spin, like mass and electrical charge, is an intrinsic property of quantum particles. It is central to understanding how quantum objects will respond to a magnetic field, and it divides all quantum objects into two types. The half-integer ones, like the spin-1/2 electron, refuse to share the same quantum state, whereas the integer ones, like the spin-1 photon, don’t have a problem cozying up together. So, spin is essential when delving into virtually any topic governed by quantum mechanics, from the Higgs Boson to superconductors.  fignewsIn a material, the momentum and energy of an electron are tied together by a “dispersion relation” (pictured above). This relationship influences the electrons’ behavior, sometimes making them behave as particles with different quantum properties. (Credit: Igor Boettcher/University of Maryland)

Yet after almost a century of playing a central role in quantum research, questions about spin remain. For example, why do all the elementary particles that we know about only have spin values of 0, 1/2, or 1? And what new behaviors might exist for particles with spin values greater than 1?

The first question may remain a cosmic mystery, but there are opportunities to explore the second. Inside of a material, a particle’s surroundings can cause it to behave like it has a new spin value. In the past couple years, researchers have discovered materials in which electrons behave like their spin has been bumped up, from 1/2 to 3/2. UMD postdoctoral researcher Igor Boettcher of the Joint Quantum Institute explored the new behaviors these spins might produce in a recent paper featured on the cover of Physical Review Letters.

Instead of looking at a particular material, Boettcher focused on the math that describes interactions between spin-3/2 electrons at low temperatures. These electrons can interact in more ways than their mundane spin-1/2 counterparts, which unlocks new phases—or collective behaviors—that researchers can look for in experiments. Boettcher sifted through the possible phases, searching for the ones that are likely to be stable at low temperatures. He looked at which phases tie up the least energy in the interactions, since as the temperature drops a material becomes most stable in the form containing the least energy (like steam condensing into liquid water and eventually freezing into ice).

He found three promising phases to hunt for in experiments. Which of these phases, if any, arise in a particular material will depend on its unique properties. Still, Boettcher’s predictions provide researchers with signals to keep an eye out for during experiments. If one of the phases forms, he predicts that common measurement techniques will reveal a signature shift in the electrical properties.

Boettcher’s work is an early step in the exploration of spin-3/2 materials. He hopes that one day the field might be comparable to that of graphene, with researchers constantly racing to explore new physics, produce better quality materials, and identify new transport properties.

“I really hope that this will develop into a big field, which will require both experimentalists and the theorists to do their part so that we can really learn something about the spin-3/2 particles and how they interact.” says Boettcher. “This is really just the beginning right now, because these materials just popped up.”

Story by Bailey Bedford

 
Research Contact: Igor Boettcher  This email address is being protected from spambots. You need JavaScript enabled to view it.
 
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New Protocol Helps Classify Topological Matter

Topological materials have captured the interest of many scientists and may provide the basis for a new era in materials development. On April 10, 2020 in the journal Science Advances, physicists working with Andreas Elben, Jinlong Yu, Peter Zoller and Benoit Vermersch, including Associate Professor Mohammad Hafezi and former Joint Quantum Institute (JQI) postdoctoral researcher Guanyu Zhu (currently a research staff member at IBM T. J. Watson Research Center), presented a new method for identifying and characterizing topological invariants on various experimental platforms, testing their protocol in a quantum simulator made of neutral atoms.

Quantum simulators are an emerging tool for preparing and investigating complex quantum states. They can be realized in a variety of different physical systems—such as ultracold atoms in optical lattices, Rydberg atoms, trapped ions or superconducting quantum bits—and they promise to enhancemany body topological invariants from randomized measurements finalTopological phases of matter are a particularly fascinating class of quantum states. (Credit: Harald Ritsch/IQOQI Innsbruck) the study of exotic states of matter.

In particular, this new breed of simulator may be able to prepare topological states of matter, which researchers find particularly fascinating. In 2016, David Thouless, Duncan Haldane and Michael Kosterlitz were awarded the Nobel Prize in Physics for their theoretical discoveries about topological states. Scientists now know that these states of matter are characterized by nonlocal quantum correlations, making them particularly robust against local distortions that inevitably occur in experiments.

But it’s often hard to know if a material sample in the lab is in a topological phase. "Identifying and characterizing such topological phases in experiments is a great challenge," say Vermersch, Yu and Elben from the Center for Quantum Physics at the University of Innsbruck and the Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences. "Topological phases cannot be identified by local measurements because of their special properties. We are therefore developing new measurement protocols that will enable experimental physicists to characterize these states in the laboratory".

In recent years this identification has been achieved for systems without any interactions. However, for interacting systems, which in the future could also be used as topological quantum computers, this has not been possible so far.

In the new work, the research team proposed and experimentally tested protocols that might enable other experimenters to measure topological invariants. These mathematical expressions distinguish different topological phases, making it possible to classify interacting topological states in a wide variety of systems.

"The idea of our method is to first prepare such a topological state in a quantum simulator,” explains Elben. “Now so-called random measurements are performed, and topological invariants are extracted from statistical correlations of these random measurements.”

The key to the method is that although the topological invariants are highly complex, non-local correlation functions, they can still be extracted from statistical correlations of simple and local random measurements. “The many-body invariants characterizing different types of topological orders are path-integrals in topological quantum field theory, corresponding to various types of space-time manifolds, such as the real-projective plane,” says Zhu. “It is kind of a miracle that we eventually realized that these highly abstract quantities in theory can actually be measured in relatively simple experiments.”

And as some members of the research group have recently shown, such random measurements are possible in experiments today. "Our protocols for measuring the topological invariants can therefore be directly applied in the existing experimental platforms," says Vermersch.

In addition to Elben, Yu, Zoller, Vermersch, Zhu and Hafezi, the co-authors included Frank Pollmann from the Technical University of Munich. The research was financially supported by the European Research Council and the EU flagship for quantum technologies, as well as the Army Research Office MURI program and the NSF Physics Frontier Center at JQI.

This story was originally published by the Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences in Innsbruck. It was adapted with permission by the JQI: https://jqi.umd.edu/news/new-protocol-helps-classify-topological-matter

Research Contact: Mohammad Hafezi, This email address is being protected from spambots. You need JavaScript enabled to view it.
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Donuts, Donut Holes and Topological Superconductors

Topology—the mathematical study of shapes that describes how a donut differs from a donut hole—has turned out to be remarkably relevant to understanding our physical world. For decades, it’s captured the hearts and minds of physicists, who have spent that time uncovering just how deep the connection between topology and physics runs. Among many other things, they’ve unearthed a prediction, born of topology, for a new particle with promising applications to quantum computing.topology shapes1a color scheme 6E. Edwards, IQUIST

In this episode of the Joint Quantum Institute's Relatively Certain podcast, Dina Genkina sits down with Associate Professor Jay Sau and Professor Johnpierre Paglione, the director of the Quantum Materials Center. They take a trip back to the 1980s, when the story of topology in physics began, and arrive at a recent discovery by Paglione and his collaborators of a (possible) topological superconductor.

This episode of Relatively Certain was produced by Dina Genkina, Chris Cesare, and Emily Edwards. It features music by Dave Depper, Frequency Decree, Chad Crouch and Scott Holmes.

Relatively Certain is a production of the Joint Quantum Institute, a research partnership between the University of Maryland and the National Institute of Standards and Technology, and you can find it on iTunesGoogle PlaySoundcloud or Spotify.

  

Charting a Course Toward Quantum Simulations of Nuclear Physics

In nuclear physics, like much of science, detailed theories alone aren’t always enough to unlock solid predictions. There are often too many pieces, interacting in complex ways, for researchers to follow the logic of a theory through to its end. It’s one reason there are still so many mysteries in nature, including how the universe’s basic building blocks coalesce and form stars and galaxies. The same is true in high-energy experiments, in which particles like protons smash together at incredible speeds to create extreme conditions similar to those just after the Big Bang.for jqi article v.1.1 gallary copyTrapped ion quantum simulators may soon offer new means to explore the properties of matter emerging from complex interactions among quarks, gluons and the other fundamental building blocks of nature. (Credit: A. Shaw and Z. Davoudi/University of Maryland)

Fortunately, scientists can often wield simulations to cut through the intricacies. A simulation represents the important aspects of one system—such as a plane, a town’s traffic flow or an atom—as part of another, more accessible system (like a computer program or a scale model). Researchers have used their creativity to make simulations cheaper, quicker or easier to work with than the formidable subjects they investigate—like proton collisions or black holes.

Simulations go beyond a matter of convenience; they are essential for tackling cases that are both too difficult to directly observe in experiments and too complex for scientists to tease out every logical conclusion from basic principles. Diverse research breakthroughs—from modeling the complex interactions of the molecules behind life to predicting the experimental signatures that ultimately allowed the identification of the Higgs boson—have resulted from the ingenious use of simulations.

But conventional simulations only get you so far. In many cases, a simulation requires so many computations that the best computers ever built can’t make meaningful progress—not even if you are willing to wait your entire life.

Now, quantum simulators (which exploit quantum effects like superposition and entanglement) promise to bring their power to bear on many problems that have refused to yield to simulations built atop classical computers—including problems in nuclear physics. But to run any simulation, quantum or otherwise, scientists must first determine how to faithfully represent their system of interest in their simulator. They must create a map between the two.

Assistant professor Zohreh Davoudi, a computational nuclear physicist, is collaborating with researchers at the Joint Quantum Institute to explore how quantum simulations might aid nuclear physicists. They are working to create some of the first maps between the theories that describe the underpinnings of nuclear physics and the early quantum simulators and quantum computers being put together in labs.

“It seems like we are at the verge of going into the next phase of computing that takes advantage of quantum mechanics,” says Davoudi. “And if nuclear scientists don't get into this field now—if we don't start to move our problems into such quantum hardware, we might not be able to catch up later because quantum computing is evolving very fast.”

Davoudi and several colleagues, including Chris Monroe and Mohammad Hafezi, designed their approach to making maps with an eye toward compatibility with the quantum technologies on the horizon. In a new paper published April 8, 2020 in the journal Physical Review Research, they describe their new method and how it creates new simulation opportunities for researchers to explore.

“It is not yet clear exactly where quantum computers will be usefully applied,” says Monroe, who is also a professor of physics at UMD and co-founder of the quantum computing startup IonQ. “One strategy is to deploy them on problems that are based in quantum physics. There are many approaches in electronic structure and nuclear physics that are so taxing to normal computers that quantum computers may be a way forward.”

Patterns and Control

As a first target, the team set their sights on lattice gauge theories. Gauge theories describe a wide variety of physics, including the intricate dance of quarks and gluons—the fundamental particles in nuclear physics. Lattice versions of gauge theories simplify calculations by restricting all the particles and their interactions to an orderly grid, like pieces on a chessboard.

Even with this simplification, modern computers can still choke when simulating dense clumps of matter or when tracking how matter changes over time. The team believes that quantum computers might overcome these limitations and eventually simulate more challenging types of gauge theories—such as quantum chromodynamics, which describes the strong interactions that bind quarks and gluons into protons and neutrons and hold them together as atomic nuclei.

Davoudi and her colleagues chose trapped atomic ions—the specialty of Monroe—as the physical system for performing their simulation. In these systems, ions, which are electrically charged atoms, hover, each trapped by a surrounding electric or magnetic field. Scientists can design these fields to arrange the ions in various patterns that can be used to store and transfer information. For this proposal, the team focused on ions organized into a straight line.

Researchers use lasers to control each ion and its interactions with neighbors—an essential ability when creating a useful simulation. The ions are much more accessible than the smaller particles that intrigue Davoudi. Nuclear physicists can only dream of achieving the same level of control over the interactions at the hearts of atoms.

“Take a problem at the femtometer scale and expand it to micron scale—that dramatically increases our level of control,” says Hafezi, who is also an associate professor in the Department of Electrical and Computer Engineering and the Department of Physics at UMD. “Imagine you were supposed to dissect an ant. Now the ant is stretched to the distance between Boston and Los Angeles.”

While designing their map-making method, the team looked at what can be done with off-the-shelf lasers. They realized that current technology allows ion trappers to set up lasers in a new, efficient way that allows for simultaneous control of three different spin interactions for each ion.

“Trapped-ion systems come with a toolbox to simulate these problems,” says Hafezi. “Their amazing feature is that sometimes you can go back and design more tools and add it to the box.”

With this opportunity in mind, the researchers developed a procedure for producing maps with two desirable features. First, the maps maximize how faithfully the ion-trap simulation matches a desired lattice gauge theory. Second, they minimize the errors that occur during the simulation.

In the paper, the researchers describe how this approach might allow a one-dimensional string of ions to simulate a few simple lattice gauge theories, not only in one dimension but also higher dimensions. With this approach, the behavior of ion spins can be tailored and mapped to a variety of phenomena that can be described by lattice gauge theories, such as the generation of matter and antimatter out of a vacuum.

“As a nuclear theorist, I am excited to work further with theorists and experimentalists with expertise in atomic, molecular, and optical physics and in ion-trap technology to solve more complex problems,” says Davoudi. “I explained the uniqueness of my problem and my system, and they explained the features and capabilities of their system, then we brainstormed ideas on how we can do this mapping.”

Monroe points out that “this is exactly what is needed for the future of quantum computing. This ‘co-design’ of devices tailored for specific applications is what makes the field fresh and exciting.”

Analog vs. Digital

The simulations proposed by Davoudi and her colleagues are examples of analog simulations, since they directly represent elements and interactions in one system with those of another system. Generally, analog simulators must be designed for a particular problem or set of problems. This makes them less versatile than digital simulators, which have an established set of discrete building blocks that can be put together to simulate nearly anything given enough time and resources.

The versatility of digital simulations has been world-altering, but a well-designed analog system is often less complex than its digital counterpart. Carefully designed quantum analog simulations might deliver results for certain problems before quantum computers can reliably perform digital simulations. This is similar to just using a wind tunnel instead of programming a computer to model the way the wind buffets everything from a goose to an experimental fighter plane.

Monroe’s team, in collaboration with coauthor Guido Pagano, a former JQI postdoctoral researcher who is now an assistant professor at Rice University, is working to implement the new analog approach within the next couple of years. The completed system should be able to simulate a variety of lattice gauge theories.

The authors say that this research is only the beginning of a longer road. Since lattice gauge theories are described in mathematically similar ways to other quantum systems, the researchers are optimistic that their proposal will find uses beyond nuclear physics, such as in condensed matter physics and materials science. Davoudi is also working to develop digital quantum simulation proposals with Monroe and Norbert Linke, another JQI Fellow. She hopes that the two projects will reveal the advantages and disadvantages of each approach and provide insight into how researchers can tackle nuclear physics problems with the full might of quantum computing.

“We want to eventually simulate theories of a more complex nature and in particular quantum chromodynamics that is responsible for the strong force in nature,” says Davoudi. “But that might require thinking even more outside the box.”

Original story by Bailey Bedford

In addition to Davoudi, Hafezi and Monroe, co-authors of the paper include former JQI postdoctoral researcher and current assistant professor at Rice University Guido Pagano; JQI graduate student Alireza Seif, and UMD Physics graduate student Andrew Shaw.

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