QT/ Uncovering universal physics in the dynamics of a quantum system
Quantum news biweekly vol.52, 11th May — 25th May
TL;DR
- New experiments using one-dimensional gases of ultra-cold atoms reveal a universality in how quantum systems composed of many particles change over time following a large influx of energy that throws the system out of equilibrium.
- The connection between quantum physics and the theory of relativity is extremely hard to study. But now, scientists have set up a model system, which can help: Quantum particles can be tuned in such a way that the results can be translated into information about other systems, which are much harder to observe. This kind of ‘quantum simulator’ works very well and can lead to new insights about the nature of relativity and quantum physics.
- Researchers have identified novel van der Waals (vdW) magnets using cutting-edge tools in artificial intelligence (AI). In particular, the team identified transition metal halide vdW materials with large magnetic moments that are predicted to be chemically stable using semi-supervised learning. These two-dimensional (2D) vdW magnets have potential applications in data storage, spintronics, and even quantum computing.
- Physicists have discovered stacked pancakes of ‘liquid’ magnetism that may account for the strange electronic behavior of some layered helical magnets.
- Adapting a detector developed for space X-ray observation, researchers have successfully verify strong-field quantum electrodynamics with exotic atoms.
- In the study, a team of researchers describe what they believe to be the first measurement showing direct interaction between electrons spinning in a 2D material and photons coming from microwave radiation.
- Researchers have developed a new class of integrated photonic devices — ‘leaky-wave metasurfaces’ — that convert light initially confined in an optical waveguide to an arbitrary optical pattern in free space. These are the first to demonstrate simultaneous control of all four optical degrees of freedom. Because they’re so thin, transparent, and compatible with photonic integrated circuits, they can be used to improve optical displays, LIDAR, optical communications, and quantum optics.
- Quantum dots in semiconductors such as silicon or gallium arsenide have long been considered hot candidates for hosting quantum bits in future quantum processors. Scientists have now shown that bilayer graphene has even more to offer here than other materials. The double quantum dots they have created are characterized by a nearly perfect electron-hole-symmetry that allows a robust read-out mechanism — one of the necessary criteria for quantum computing.
- An international team of researchers has developed a comprehensive manual for engineering spin dynamics in nanomagnets — an important step toward advancing spintronic and quantum-information technologies.
- Large numbers can only be factorized with a great deal of computational effort. Physicists are now providing a blueprint for a new type of quantum computer to solve the factorization problem, which is a cornerstone of modern cryptography.
- And more!
Quantum Computing Market
According to the recent market research report ‘Quantum Computing Market with COVID-19 impact by Offering (Systems and Services), Deployment (On Premises and Cloud Based), Application, Technology, End-use Industry and Region — Global Forecast to 2026’, published by MarketsandMarkets, the Quantum Computing market is expected to grow from USD 472 million in 2021 to USD 1,765 million by 2026, at a CAGR of 30.2%. The early adoption of quantum computing in the banking and finance sector is expected to fuel the growth of the market globally. Other key factors contributing to the growth of the quantum computing market include rising investments by governments of different countries to carry out research and development activities related to quantum computing technology. Several companies are focusing on the adoption of QCaaS post-COVID-19. This, in turn, is expected to contribute to the growth of the quantum computing market. However, stability and error correction issues are expected to restrain the growth of the market.
According to ‘Quantum Computing Market Research Report: By Offering, Deployment Type, Application, Technology, Industry — Industry Share, Growth, Drivers, Trends and Demand Forecast to 2030’ report, the quantum computing market is projected to reach $64,988 million by 2030. Machine learning (ML) is expected to progress at the highest CAGR, during the forecast period, among all application categories, owing to the fact that quantum computing is being integrated in ML for improving the latter’s use case.
Latest Research
Observation of hydrodynamization and local prethermalization in 1D Bose gases
by Yuan Le, Yicheng Zhang, Sarang Gopalakrishnan, Marcos Rigol, David S. Weiss in Nature
New experiments using one-dimensional gases of ultra-cold atoms reveal a universality in how quantum systems composed of many particles change over time following a large influx of energy that throws the system out of equilibrium. A team of physicists at Penn State showed that these gases immediately respond, “evolving” with features that are common to all “many-body” quantum systems thrown out of equilibrium in this way.
“Many major advances in physics over the last century have concerned the behavior of quantum systems with many particles,” said David Weiss, Distinguished Professor of Physics at Penn State and one of the leaders of the research team. “Despite the staggering array of diverse ‘many-body’ phenomena, like superconductivity, superfluidity, and magnetism, it was found that their behavior near equilibrium is often similar enough that they can be sorted into a small set of universal classes. In contrast, the behavior of systems that are far from equilibrium has yielded to few such unifying descriptions.”
These quantum many-body systems are ensembles of particles, like atoms, that are free to move around relative to each other, Weiss explained. When they are some combination of dense and cold enough, which can vary depending on the context, quantum mechanics — the fundamental theory that describes the properties of nature at the atomic or subatomic scale — is required to describe their dynamics.
Dramatically out-of-equilibrium systems are routinely created in particle accelerators when pairs of heavy ions are collided at speeds near the speed-of-light. The collisions produce a plasma — composed of the subatomic particles “quarks” and “gluons” — that emerges very early in the collision and can be described by a hydrodynamic theory — similar to the classical theory used to describe air flow or other moving fluids — well before the plasma reaches local thermal equilibrium. But what happens in the astonishingly short time before hydrodynamic theory can be used?
“The physical process that occurs before hydrodynamics can be used has been called ‘hydrodynamization,” said Marcos Rigol, professor of physics at Penn State and another leader of the research team. “Many theories have been developed to try to understand hydrodynamization in these collisions, but the situation is quite complicated and it is not possible to actually observe it as it happens in the particle accelerator experiments. Using cold atoms, we can observe what is happening during hydrodynamization.”
The Penn State researchers took advantage of two special features of one-dimensional gases, which are trapped and cooled to near absolute zero by lasers, in order to understand the evolution of the system after it is thrown of out of equilibrium, but before hydrodynamics can be applied. The first feature is experimental. Interactions in the experiment can be suddenly turned off at any point following the influx of energy, so the evolution of the system can be directly observed and measured. Specifically, they observed the time-evolution of one-dimensional momentum distributions after the sudden quench in energy.
“Ultra-cold atoms in traps made from lasers allow for such exquisite control and measurement that they can really shed light on many-body physics,” said Weiss. “It is amazing that the same basic physics that characterize relativistic heavy ion collisions, some of the most energetic collisions ever made in a lab, also show up in the much less energetic collisions we make in our lab.”
The second feature is theoretical. A collection of particles that interact with each other in a complicated way can be described as a collection of “quasiparticles” whose mutual interactions are much simpler. Unlike in most systems, the quasiparticle description of one-dimensional gases is mathematically exact. It allows for a very clear description of why energy is rapidly redistributed across the system after it is thrown out of equilibrium.
“Known laws of physics, including conservation laws, in these one-dimensional gases imply that a hydrodynamic description will be accurate once this initial evolution plays out,” said Rigol. “The experiment shows that this occurs before local equilibrium is reached. The experiment and theory together therefore provide a model example of hydrodynamization. Since hydrodynamization happens so fast, the underlying understanding in terms of quasi-particles can be applied to any many-body quantum system to which a very large amount of energy is added.”
Experimental observation of curved light-cones in a quantum field simulator
by Mohammadamin Tajik, Marek Gluza, Nicolas Sebe, Philipp Schüttelkopf, Federica Cataldini, João Sabino, Frederik Møller, Si-Cong Ji, Sebastian Erne, Giacomo Guarnieri, Spyros Sotiriadis, Jens Eisert, Jörg Schmiedmayer in Proceedings of the National Academy of Sciences
The theory of relativity works well when you want to explain cosmic-scale phenomena — such as the gravitational waves created when black holes collide. Quantum theory works well when describing particle-scale phenomena — such as the behavior of individual electrons in an atom. But combining the two in a completely satisfactory way has yet to be achieved. The search for a “quantum theory of gravity” is considered one of the significant unsolved tasks of science.
This is partly because the mathematics in this field is highly complicated. At the same time, it is tough to perform suitable experiments: One would have to create situations in which phenomena of both the relativity theory play an important role, for example, a spacetime curved by heavy masses, and at the same time, quantum effects become visible, for example the dual particle and wave nature of light. At the TU Wien in Vienna, Austria, a new approach has now been developed for this purpose: A so-called “quantum simulator” is used to get to the bottom of such questions: Instead of directly investigating the system of interest (namely quantum particles in curved spacetime), one creates a “model system” from which one can then learn something about the system of actual interest by analogy. The researchers have now shown that this quantum simulator works excellently.
The basic idea behind the quantum simulator is simple: Many physical systems are similar. Even if they are entirely different kinds of particles or physical systems on different scales that, at first glance, have little to do with each other, these systems may obey the same laws and equations at a deeper level. This means one can learn something about a particular system by studying another.
“We take a quantum system that we know we can control and adjust very well in experiments,” says Prof. Jörg Schmiedmayer of the Atomic Institute at TU Wien. “In our case, these are ultracold atomic clouds held and manipulated by an atom chip with electromagnetic fields.”
Suppose you properly adjust these atomic clouds so that their properties can be translated into another quantum system. In that case, you can learn something about the other system from the measurement of the atomic cloud model system — much like you can learn something about the oscillation of a pendulum from the oscillation of a mass attached to a metal spring: They are two different physical systems, but one can be translated into the other.
“We have now been able to show that we can produce effects in this way that can be used to resemble the curvature of spacetime,” says Mohammadamin Tajik of the Vienna Center for Quantum Science and Technology (VCQ) — TU Wien, first author of the current paper. In the vacuum, light propagates along a so-called “light cone.” The speed of light is constant; at equal times, the light travels the same distance in each direction. However, if the light is influenced by heavy masses, such as the sun’s gravitation, these light cones are bent. The light’s paths are no longer perfectly straight in curved spacetimes. This is called “gravitational lens effect.” The same can now be shown in atomic clouds. Instead of the speed of light, one examines the speed of sound.
“Now we have a system in which there is an effect that corresponds to spacetime curvature or gravitational lensing, but at the same time, it is a quantum system that you can describe with quantum field theories,” says Mohammadamin Tajik. “With this, we have a completely new tool to study the connection between relativity and quantum theory.”
The experiments show that the shape of light cones, lensing effects, reflections, and other phenomena can be demonstrated in these atomic clouds precisely as expected in relativistic cosmic systems. This is not only interesting for generating new data for basic theoretical research — solid-state physics and the search for new materials also encounter questions that have a similar structure and can therefore be answered by such experiments.
“We now want to control these atomic clouds better to determine even more far-reaching data. For example, interactions between the particles can still be changed in a very targeted way,” explains Jörg Schmiedmayer. In this way, the quantum simulator can recreate physical situations that are so complicated that they cannot be calculated even with supercomputers.
The quantum simulator thus becomes a new, additional source of information for quantum research — in addition to theoretical calculations, computer simulations, and direct experiments. When studying the atomic clouds, the research team hopes to come across new phenomena that may have been entirely unknown up to now, which also take place on a cosmic, relativistic scale — but without a look at tiny particles, they might never have been discovered.
Artificial Intelligence Guided Studies of van der Waals Magnets
by Trevor David Rhone, Romakanta Bhattarai, Haralambos Gavras, Bethany Lusch, Misha Salim, Marios Mattheakis, Daniel T. Larson, Yoshiharu Krockenberger, Efthimios Kaxiras in Advanced Theory and Simulations
A team of researchers led by Rensselaer Polytechnic Institute’s Trevor David Rhone, assistant professor in the Department of Physics, Applied Physics, and Astronomy, has identified novel van der Waals (vdW) magnets using cutting-edge tools in artificial intelligence (AI). In particular, the team identified transition metal halide vdW materials with large magnetic moments that are predicted to be chemically stable using semi-supervised learning. These two-dimensional (2D) vdW magnets have potential applications in data storage, spintronics, and even quantum computing.
Rhone specializes in harnessing materials informatics to discover new materials with unexpected properties that advance science and technology. Materials informatics is an emerging field of study at the intersection of AI and materials science.
2D materials, which can be as thin as a single atom, were only discovered in 2004 and have been the subject of great scientific curiosity because of their unexpected properties. 2D magnets are significant because their long-range magnetic ordering persists when they are thinned down to one or a few layers. This is due to magnetic anisotropy. The interplay with this magnetic anisotropy and low dimensionality could give rise to exotic spin degrees of freedom, such as spin textures that can be used in the development of quantum computing architectures. 2D magnets also span the full range of electronic properties and can be used in high-performance and energy-efficient devices.
Rhone and team combined high-throughput density functional theory (DFT) calculations, to determine the vdW materials’ properties, with AI to implement a form of machine learning called semi-supervised learning. Semi-supervised learning uses a combination of labeled and unlabeled data to identify patterns in data and make predictions. Semi-supervised learning mitigates a major challenge in machine learning — the scarcity of labeled data.
“Using AI saves time and money,” said Rhone. “The typical materials discovery process requires expensive simulations on a supercomputer that can take months. Lab experiments can take even longer and can be more expensive. An AI approach has the potential to speed up the materials discovery process.”
Using an initial subset of 700 DFT calculations on a supercomputer, an AI model was trained that could predict the properties of many thousands of materials candidates in milliseconds on a laptop. The team then identified promising candidate vdW materials with large magnetic moments and low formation energy. Low formation energy is an indicator of chemical stability, which is an important requirement for synthesizing the material in a laboratory and subsequent industrial applications.
“Our framework can easily be applied to explore materials with different crystal structures, as well,” said Rhone. “Mixed crystal structure prototypes, such as a data set of both transition metal halides and transition metal trichalcogenides, can also be explored with this framework.”
“Dr. Rhone’s application of AI to the field of materials science continues to produce exciting results,” said Curt Breneman, dean of Rensselaer’s School of Science. “He has not only accelerated our understanding of 2D materials that have novel properties, but his findings and methods are likely to contribute to new quantum computing technologies.”
Anisotropic Melting of Frustrated Ising Antiferromagnets
by Matthew W. Butcher, Makariy A. Tanatar, Andriy H. Nevidomskyy in Physical Review Letters
Physicists have discovered “stacked pancakes of liquid magnetism” that may account for the strange electronic behavior of some layered helical magnets.
The materials in the study are magnetic at cold temperatures and become nonmagnetic as they thaw. Experimental physicist Makariy Tanatar of Ames National Laboratory at Iowa State University noticed perplexing electronic behavior in layered helimagnetic crystals and brought the mystery to the attention of Rice theoretical physicist Andriy Nevidomskyy, who worked with Tanatar and former Rice graduate student Matthew Butcher to create a computational model that simulated the quantum states of atoms and electrons in the layered materials.
Magnetic materials undergo a “thawing” transition as they warm up and become nonmagnetic. The researchers ran thousands of Monte Carlo computer simulations of this transition in helimagnets and observed how the magnetic dipoles of atoms inside the material arranged themselves during the thaw. Their results were published in a recent study in Physical Review Letters. At a submicroscopic level, the materials under study are composed of thousands of 2D crystals stacked one atop another like pages in a notebook. In each crystal sheet, atoms are arrayed in lattices, and the physicists modeled quantum interactions both within and between sheets.
“We’re used to thinking that if you take a solid, like a block of ice, and you heat it up, eventually it will become a liquid, and at a higher temperature, it will evaporate and become a gas,” said Nevidomskyy, an associate professor of physics and astronomy and member of the Rice Quantum Initiative. “A similar analogy can be made with magnetic materials, except that nothing evaporates in a true sense of the word.
“The crystal is still intact,” he said. “But if you look at the arrangement of the little magnetic dipoles — which are like compass needles — they start out in a correlated arrangement, meaning that if you know which way one of them is pointing, you can determine which way any of them points, regardless how far away it is in the lattice. That is the magnetic state — the solid in our analogy. As you heat up, the dipoles eventually will become completely independent, or random, with respect to one another. That’s known as a paramagnet, and it is analogous to a gas.”
Nevidomskyy said physicists typically think of materials either having magnetic order or lacking it.
“A better analogy from the classical viewpoint would be a block of dry ice,” he said. “It kind of forgets about the liquid phase and goes straight from ice into gas. That’s what magnetic transitions are usually like in the textbooks. We are taught that you start with something correlated, let’s say a ferromagnet, and at some point the order parameter disappears, and you end up with a paramagnet.”
Tanatar, a research scientist at Ames’ Superconductivity and Magnetism Low-Temperature Laboratory, had found signs that the transition from magnetic order to disorder in helical magnets was marked by a transitory phase in which electronic properties, like resistance, differed by direction. For instance, they might differ if they were measured horizontally, from side to side, as opposed to vertically from top to bottom. This directional behavior, which physicists call anisotropy, is a hallmark of many quantum materials like high-temperature superconductors.
“These layered materials don’t look the same in the vertical and horizontal directions,” said Nevidomskyy. “That’s the anisotropy. Makariy’s intuition was that anisotropy was affecting how magnetism melts in the material, and our modeling demonstrated that to be true and showed why it happens.”
The model showed that the material passes through an intermediate phase as it transitions from magnetic order to disorder. In that phase, dipole interactions are much stronger within sheets than between them. Moreover, the correlations between the dipoles resembled those of a liquid, rather than a solid. The result is “flattened puddles of magnetic liquids that are stacked up like pancakes,” Nevidomskyy said. In each puddlelike pancake, dipoles point roughly in the same direction, but that sense of direction varies between neighboring pancakes.
“It’s a bunch of atoms all with their dipoles pointing in the same direction,” Nevidomskyy said. “But then, if you go up one layer, all of them are pointing in a different random direction.”
The atomic arrangement in the material “frustrates” the dipoles and keeps them from aligning in a uniform direction throughout the material. Instead, the dipoles in the layers shift, rotating slightly in response to changes in neighboring pancakes.
“Frustrations make it difficult for the arrows, these magnetic dipoles, to decide where they want to point, at one angle or another,” Nevidomskyy said. “And to relieve that frustration, they tend to rotate and shift in each layer.”
Tanatar said, “The idea is that you have two competing magnetic phases. They are fighting each other, and as a result you have a transition temperature for these phases that is lower than it would be without competition. And in this competition scenario, the phenomena that lead to magnetic order are different from the phenomena when you don’t have this competition.”
Tanatar and Nevidomskyy said that while there’s no immediate application for the discovery, it may nevertheless offer hints about the still-unexplained physics of other anisotropic materials like high-temperature superconductors. Despite the name, high-temperature superconductivity occurs at very cold temperatures. One theory suggests that materials may become superconductors when they are cooled in the vicinity of a quantum critical point, a temperature sufficient to suppress long-range magnetic order and give rise to effects brought about by strong quantum fluctuations. For example, several magnetic “parent” materials have been shown to harbor superconductivity close to a quantum critical point where magnetism disappears.
“Once you suppress the main effect, the long-range magnetic ordering, you may give way to weaker effects like superconductivity,” Tanatar said. “This is one of the leading theories of unconventional superconductivity. In our study, we show that you can do the same thing in a different way, with frustration or competing interactions.”
Proof-of-Principle Experiment for Testing Strong-Field Quantum Electrodynamics with Exotic Atoms: High Precision X-Ray Spectroscopy of Muonic Neon
by T. Okumura, T. Azuma, D. A. Bennett, I. Chiu, W. B. Doriese, et al in Physical Review Letters
An international collaboration of researchers, including the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) has succeeded in a proof-of-principle experiment to verify strong-field quantum electrodynamics with exotic atoms, by performing high-precision measurements of the energy spectrum of muonic characteristic X-rays emitted from muonic atoms using a state-of-the-art X-ray detector, reports a new study.
The group’s results are a significant step toward verifying fundamental physical laws under strong electric fields, which humankind has not yet been able to create artificially. The highly efficient and accurate X-ray energy determination method using state-of-the-art quantum technology demonstrated in this research is expected to be applied to various research fields, such as non-destructive elemental analysis methods using muonic atoms.
It has always been a dream of scientists to discover physical laws. They have been found or proposed to explain observed phenomena that cannot be understood by existing theories. In many cases, the discovery of new physics requires the development of new experimental techniques and improved accuracy of measurements. The most precisely tested theory of physical laws is Quantum ElectroDynamics (QED), which describes the microscopic interactions between charged particles and light. Scientists are constantly pushing the limits of how far QED accurately describes our physical reality.
By taking full advantage of the excellent energy resolution of the TES detector, the energy of the muonic characteristic X-rays was determined with an absolute uncertainty of less than 1/10,000, and contributions from vacuum polarization in strong-field quantum electrodynamics were successfully verified with a high precision of 5.8 %. The TES detector was originally developed for space X-ray observation. Takahashi’s current project at Kavli IPMU has been to carry out unprecedented cross-disciplinary research using this detector. His team includes Kavli IPMU Project Assistant Professor Shin’ichiro Takeda, Project Researcher Miho Katsuragawa, and at-the-tune graduate student Kairi Mine, who took part in the muon experiments.
The collaboration’s demonstration of the experimental technique using muonic atoms is expected to lead to a great leap forward in the study of QED verification under strong electric fields. The effects of QED are more pronounced in environments with strong electric fields, but theoretical calculations become more difficult in this case. Therefore, a strong electric field environment is very important for QED verification. For many years, experiments using highly charged ions (HCIs), which are atoms stripped of multiple electrons, have been conducted as an approach to realize a strong electric field environment. The electric field felt by the bound electrons in HCIs becomes stronger as the atomic number becomes larger, and the shielding effect is suppressed by the stripping of many electrons. HCI research using large accelerators is still vigorously pursued. However, even for HCIs with large atomic numbers, the effect of the finite size of the nucleus cannot be ignored. It has been pointed out that this effect is not precisely known, and thus the accuracy of QED verification, which compares experimental results with theory, is greatly compromised.
To verify QED under strong electric fields in a different way than with HCIs, international research groups have focused on “exotic atoms,” in which a negatively charged particle is bound to the nucleus instead of the electron. Among the variety of exotic atoms, muonic atoms are composed of negative muons (elementary particles about 200 times heavier than electrons) and nuclei. Negative muons can nowadays be extracted as beams from large accelerators. Muonic atoms are characterized by the extremely close proximity of the negative muon to the nucleus, with the orbital radius of a bound muon being approximately 1/200th that of a bound electron. As a result, the electric field felt by the muon is about 40,000 times stronger than the electric field felt by a bound electron of the same quantum level in an HCI, resulting in a huge QED effect. In addition, by using negative muons, which occupy high angular momentum quantum levels with small overlap with the nucleus, it is possible to conduct experiments in which the effect of the finite size of the nucleus is largely suppressed. By precisely measuring the energy of muonic characteristic X-rays emitted when muonic atoms are deexcited from a specific level to lower levels, QED can be verified under a strong electric field .
Thus, muonic atoms are a promising experimental target for strong-field QED verification. However, there are several problems to overcome. The largest is that a number of muonic atoms must be prepared in an isolated environment. The presence of atoms or molecules in the vicinity of the muonic atoms may cause rapid electron transfer and changes the energy of the muonic characteristic X-rays. The solution is to use dilute gas targets with a small number density (low pressure), but the number of produced muonic atoms and the resulting intensity of muonic characteristic X-rays are reduced. The international research group conducted experiments at the Japan Proton Accelerator Research Complex (J-PARC) in Tokai-mura, Ibaraki, where the world’s most intense low-velocity muon beam is available. In order to determine the energy with sufficient accuracy even with low-intensity muonic characteristic X-rays, the experiment was conducted with a superconducting transition edge sensor (TES) microcalorimeter, which is a highly efficient and high-resolution X-ray detector.
Using rare gas neon (10Ne) atoms as the target, they have achieved an energy resolution that is one order of magnitude higher than that of conventional semiconductor detectors (FWHM [11]: 5.2 eV) under dilute conditions of 0.1 atm and successfully measured the muonic characteristic X-rays. The peaks shown are mainly due to the overlap of muonic characteristic X-rays from six different transitions, and the energy of the muonic characteristic X-rays was determined to a high accuracy of 0.002% by analyzing contributions from each of them.
They repeated the measurements while changing the pressure of the neon gas target and confirmed that the energy of the muonic X-rays is constant within experimental error regardless of the pressure of the neon gas target. Thus, it can be concluded that the muonic neon atoms were in an isolated environment. They compared the latest theoretical calculations with the experimental results and confirmed that they agreed within the experimental error. We succeeded in verifying the effect of vacuum polarization under a strong electric field with an extremely high accuracy of 5.8%. This is comparable to the accuracy of strong-field QED using the multiply charged uranium ion U91+, which is the most accurate observation to date.
Dirac revivals drive a resonance response in twisted bilayer graphene
by Erin Morissette, Jiang-Xiazi Lin, Dihao Sun, Liangji Zhang, Song Liu, Daniel Rhodes, Kenji Watanabe, Takashi Taniguchi, James Hone, Johannes Pollanen, Mathias S. Scheurer, Michael Lilly, Andrew Mounce, J. I. A. Li in Nature Physics
For two decades, physicists have tried to directly manipulate the spin of electrons in 2D materials like graphene. Doing so could spark key advances in the burgeoning world of 2D electronics, a field where super-fast, small and flexible electronic devices carry out computations based on quantum mechanics.
Standing in the way is that the typical way in which scientists measure the spin of electrons — an essential behavior that gives everything in the physical universe its structure — usually doesn’t work in 2D materials. This makes it incredibly difficult to fully understand the materials and propel forward technological advances based on them. But a team of scientists led by Brown University researchers believe they now have a way around this longstanding challenge.
In the study, the team — which also include scientists from the Center for Integrated Nanotechnologies at Sandia National Laboratories, and the University of Innsbruck — describe what they believe to be the first measurement showing direct interaction between electrons spinning in a 2D material and photons coming from microwave radiation. Called a coupling, the absorption of microwave photons by electrons establishes a novel experimental technique for directly studying the properties of how electrons spin in these 2D quantum materials — one that could serve as a foundation for developing computational and communicational technologies based on those materials, according to the researchers.
“Spin structure is the most important part of a quantum phenomenon, but we’ve never really had a direct probe for it in these 2D materials,” said Jia Li, an assistant professor of physics at Brown and senior author of the research. “That challenge has prevented us from theoretically studying spin in these fascinating material for the last two decades. We can now use this method to study a lot of different systems that we could not study before.”
The researchers made the measurements on a relatively new 2D material called “magic-angle” twisted bilayer graphene. This graphene-based material is created when two sheets of ultrathin layers of carbon are stacked and twisted to just the right angle, converting the new double-layered structure into a superconductor that allows electricity to flow without resistance or energy waste. Just discovered in 2018, the researchers focused on the material because of the potential and mystery surrounding it.
“A lot of the major questions that were posed in 2018 have still yet to be answered,” said Erin Morissette, a graduate student in Li’s lab at Brown who led the work.
Physicists usually use nuclear magnetic resonance or NMR to measure the spin of electrons. They do this by exciting the nuclear magnetic properties in a sample material using microwave radiation and then reading the different signatures this radiation causes to measure spin.
The challenge with 2D materials is that the magnetic signature of electrons in response to the microwave excitation is too small to detect. The research team decided to improvise. Instead of directly detecting the magnetization of the electrons, they measured subtle changes in electronic resistance, which were caused by the changes in magnetization from the radiation using a device fabricated at the Institute for Molecular and Nanoscale Innovation at Brown. These small variations in the flow of the electronic currents allowed the researchers to use the device to detect that the electrons were absorbing the photos from the microwave radiation.
The researchers were able to observe novel information from the experiments. The team noticed, for instance, that interactions between the photons and electrons made electrons in certain sections of the system behave as they would in an anti-ferromagnetic system — meaning the magnetism of some atoms was canceled out by a set of magnetic atoms that are aligned in a reverse direction.
The new method for studying spin in 2D materials and the current findings won’t be applicable to technology today, but the research team sees potential applications the method could lead to in the future. They plan to continue to apply their method to twisted bilayer graphene but also expand it to other 2D material.
“It’s a really diverse toolset that we can use to access an important part of the electronic order in these strongly correlated systems and in general to understand how electrons can behave in 2D materials,” Morissette said.
Leaky-wave metasurfaces for integrated photonics
by Heqing Huang, Adam C. Overvig, Yuan Xu, Stephanie C. Malek, Cheng-Chia Tsai, Andrea Alù, Nanfang Yu in Nature Nanotechnology
Researchers at Columbia Engineering have developed a new class of integrated photonic devices — “leaky-wave metasurfaces” — that can convert light initially confined in an optical waveguide to an arbitrary optical pattern in free space. These devices are the first to demonstrate simultaneous control of all four optical degrees of freedom, namely, amplitude, phase, polarization ellipticity, and polarization orientation — a world record. Because the devices are so thin, transparent, and compatible with photonic integrated circuits (PICs), they can be used to improve optical displays, LIDAR (Light Detection and Ranging), optical communications, and quantum optics.
“We are excited to find an elegant solution for interfacing free-space optics and integrated photonics — these two platforms have traditionally been studied by investigators from different subfields of optics and have led to commercial products addressing completely different needs,” said Nanfang Yu, associate professor of applied physics and applied mathematics who is a leader in research on nanophotonic devices. “Our work points to new ways to create hybrid systems that utilize the best of both worlds — free-space optics for shaping the wavefront of light and integrated photonics for optical data processing — to address many emerging applications such as quantum optics, optogenetics, sensor networks, inter-chip communications, and holographic displays.”
The key challenge of interfacing PICs and free-space optics is to transform a simple waveguide mode confined within a waveguide — athin ridge defined on a chip — into a broad free-space wave with a complex wavefront, and vice versa. Yu’s team tackled this challenge by building on their invention last fall of “nonlocal metasurfaces” and extended the devices’ functionality from controlling free-space light waves to controlling guided waves.
Specifically, they expanded the input waveguide mode by using a waveguide taper into a slab waveguide mode — a sheet of light propagating along the chip. “We realized that the slab waveguide mode can be decomposed into two orthogonal standing waves — waves reminiscent of those produced by plucking a string,” said Heqing Huang, a PhD student in Yu’s lab and co-first author of the study.
“Therefore, we designed a ‘leaky-wave metasurface’ composed of two sets of rectangular apertures that have a subwavelength offset from each other to independently control these two standing waves. The result is that each standing wave is converted into a surface emission with independent amplitude and polarization; together, the two surface emission components merge into a single free-space wave with completely controllable amplitude, phase, and polarization at each point over its wavefront.”
Yu’s team experimentally demonstrated multiple leaky-wave metasurfaces that can convert a waveguide mode propagating along a waveguide with a cross-section on the order of one wavelength into free-space emission with a designer wavefront over an area about 300 times the wavelength at the telecom wavelength of 1.55 microns. These include: A leaky-wave metalens that produces a focal spot in free space. Such a device will be ideal for forming a low-loss, high-capacity free-space optical link between PIC chips; it will also be useful for an integrated optogenetic probe that produces focused beams to optically stimulate neurons located far away from the probe.
Aleaky-wave optical-lattice generator that can produce hundreds of focal spots forming a Kagome lattice pattern in free space. In general, the leaky-wave metasurface can produce complex aperiodic and three-dimensional optical lattices to trap cold atoms and molecules. This capability will enable researchers to study exotic quantum optical phenomena or conduct quantum simulations hitherto not easily attainable with other platforms, and enable them to substantially reduce the complexity, volume, and cost of atomic-array-based quantum devices. For example, the leaky-wave metasurface could be directly integrated into the vacuum chamber to simplify the optical system, making portable quantum optics applications, such as atomic clocks, a possibility.
A leaky-wave vortex-beam generator that produces a beam with a corkscrew-shaped wavefront. This could lead to a free-space optical link between buildings that relies on PICs to process information carried by light, while also using light waves with shaped wavefronts for high-capacity intercommunication. A leaky-wave hologram that can displace four distinct images simultaneously: two at the device plane (at two orthogonal polarization states) and another two at a distance in the free space (also at two orthogonal polarization states). This function could be used to make lighter, more comfortable augmented reality goggles and more realistic holographic 3D displays.
Particle–hole symmetry protects spin-valley blockade in graphene quantum dots
by L. Banszerus, S. Möller, K. Hecker, E. Icking, K. Watanabe, T. Taniguchi, F. Hassler, C. Volk, C. Stampfer in Nature
Quantum dots in semiconductors such as silicon or gallium arsenide have long been considered hot candidates for hosting quantum bits in future quantum processors. Scientists at Forschungszentrum Jülich and RWTH Aachen University have now shown that bilayer graphene has even more to offer here than other materials. The double quantum dots they have created are characterized by a nearly perfect electron-hole-symmetry that allows a robust read-out mechanism — one of the necessary criteria for quantum computing.
The development of robust semiconductor spin qubits could help the realization of large-scale quantum computers in the future. However, current quantum dot based qubit systems are still in their infancy. In 2022, researchers at QuTech in the Netherlands were able to create 6 silicon-based spin qubits for the first time. With graphene, there is still a long way to go. The material, which was first isolated in 2004, is highly attractive to many scientists. But the realization of the first quantum bit has yet to come.
“Bilayer graphene is a unique semiconductor,” explains Prof. Christoph Stampfer of Forschungszentrum Jülich and RWTH Aachen University. “It shares several properties with single-layer graphene and also has some other special features. This makes it very interesting for quantum technologies.”
One of these features is that it has a bandgap that can be tuned by an external electric field from zero to about 120 milli-electronvolt. The band gap can be used to confine charge carriers in individual areas, so-called quantum dots. Depending on the applied voltage, these can trap a single electron or its counterpart, a hole — basically a missing electron in the solid-state structure. The possibility of using the same gate structure to trap both electrons and holes is a feature that has no counter part in conventional semiconductors.
“Bilayer graphene is still a fairly new material. So far, mainly experiments that have already been realized with other semiconductors have been carried out with it. Our current experiment now goes really beyond this for the first time,” Christoph Stampfer says. He and his colleagues have created a so-called double quantum dot: two opposing quantum dots, each housing an electron and a hole whose spin properties mirror each other almost perfectly.
“This symmetry has two remarkable consequences: it is almost perfectly preserved even when electrons and holes are spatially separated in different quantum dots,” Stampfer said. This mechanism can be used to couple qubits to other qubits over a longer distance. And what’s more, “the symmetry results in a very robust blockade mechanism which could be used to read out the spin state of the dot with high fidelity.”
“This goes beyond what can be done in conventional semiconductors or any other two-dimensional electron systems,” says Prof. Fabian Hassler of the JARA Institute for Quantum Information at Forschungszentrum Jülich and RWTH Aachen University, co-author of the study. “The near-perfect symmetry and strong selection rules are very attractive not only for operating qubits, but also for realizing single-particle terahertz detectors. In addition, it lends itself to coupling quantum dots of bilayer graphene with superconductors, two systems in which electron-hole symmetry plays an important role. These hybrid systems could be used to create efficient sources of entangled particle pairs or artificial topological systems, bringing us one step closer to realizing topological quantum computers.”
Controlling Selection Rules for Magnon Scattering in Nanomagnets by Spatial Symmetry Breaking
by Arezoo Etesamirad, Julia Kharlan, Rodolfo Rodriguez, Igor Barsukov, Roman Verba in Physical Review Applied
An international team of researchers at the University of California, Riverside, and the Institute of Magnetism in Kyiv, Ukraine, has developed a comprehensive manual for engineering spin dynamics in nanomagnets — an important step toward advancing spintronic and quantum-information technologies.
Despite their small size, nanomagnets — found in most spintronic applications — reveal rich dynamics of spin excitations, or “magnons,” the quantum-mechanical units of spin fluctuations. Due to its nanoscale confinement, a nanomagnet can be considered to be a zero-dimensional system with a discrete magnon spectrum, similar to the spectrum of an atom.
“The magnons interact with each other, thus constituting nonlinear spin dynamics,” said Igor Barsukov, an assistant professor of physics and astronomy at UC Riverside and a corresponding author on the study. “Nonlinear spin dynamics is a major challenge and a major opportunity for improving the performance of spintronic technologies such as spin-torque memory, oscillators, and neuromorphic computing.”
Barsukov explained that the interaction of magnons follows a set of rules — the selection rules. The researchers have now postulated these rules in terms of symmetries of magnetization configurations and magnon profiles. The new work continues the efforts to tame nanomagnets for next-generation computation technologies. In a previous publication, the team demonstrated experimentally that symmetries can be used for engineering magnon interactions.
“We recognized the opportunity, but also noticed that much work needed to be done to understand and formulate the selection rules,” Barsukov said.
According to the researchers, a comprehensive set of rules reveals the mechanisms behind the magnon interaction.
“It can be seen as a guide for spintronics labs for debugging and designing nanomagnet devices,” said Arezoo Etesamirad, the first author of the paper who worked in the Barsukov lab and recently graduated with a doctoral degree in physics. “It lays the foundation for developing an experimental toolset for tunable magnetic neurons, switchable oscillators, energy-efficient memory, and quantum-magnonic and other next-generation nanomagnetic applications.”
Submerged single-photon LiDAR imaging sensor used for real-time 3D scene reconstruction in scattering underwater environments
by Aurora Maccarone, Kristofer Drummond, Aongus McCarthy, Ulrich K. Steinlehner, Julian Tachella, Diego Aguirre Garcia, Agata Pawlikowska, Robert A. Lamb, Robert K. Henderson, Stephen McLaughlin, Yoann Altmann, Gerald S. Buller in Optics Express
For the first time, researchers have demonstrated a prototype lidar system that uses quantum detection technology to acquire 3D images while submerged underwater. The high sensitivity of this system could allow it to capture detailed information even in extremely low-light conditions found underwater.
“This technology could be useful for a wide range of applications,” said research team member Aurora Maccarone, a Royal Academy of Engineering research fellow from Heriot-Watt University in the United Kingdom. “For example, it could be used to inspect underwater installations, such as underwater wind farm cables and the submerged structure of the turbines. Underwater lidar can also be used for monitoring or surveying submerged archaeology sites and for security and defense applications.”
Obtaining 3D images through ocean water can be challenging because it is light-limited, and any particles in the water will scatter light and distort the image. However, single-photon detection, which is a quantum-based technique, allows very high penetration and works even in low-light conditions.
Researchers from Heriot-Watt University and the University of Edinburgh describe experiments in which an entire single-photon lidar system was submerged in a large water tank. The new demonstrations bring the technology closer to practical applications compared to the research team’s earlier experiments with underwater single-photon detection, which were performed in carefully controlled laboratory conditions with the optical setup placed outside the water tank and data analysis performed offline. They also implemented new hardware and software developments that allow the 3D images acquired by the system to be reconstructed in real time.
“This work aims to make quantum detection technologies available for underwater applications, which means that we will be able to image the scene of interest in very low light conditions,” said Maccarone. “This will impact the use of offshore cable and energy installations, which are used by everyone. This technology could also allow monitoring without the presence of humans, which would mean less pollution and a less invasive presence in the marine environment.”
Lidar systems create images by measuring how long it takes laser light to be reflected from objects in the scene and travel back to the system’s receiver, known as the “time of flight.” In the new work, the researchers sought to develop a way to acquire 3D images of targets that are obscured by turbid water and thus not visible to conventional lidar imaging systems. They designed a lidar system that uses a green pulsed laser source to illuminate the scene of interest. The reflected pulsed illumination is detected by an array of single-photon detectors, which allows ultrafast low light detection and greatly reduces measurement time in photon-starved environments such as highly attenuating water.
“By taking time-of-flight measurements with picosecond timing resolution, we can routinely resolve millimeter details of the targets in the scene,” said Maccarone. “Our approach also allows us to distinguish the photons reflected by the target from those reflected by particles in the water, making it particularly suitable to performing 3D imaging in highly turbid waters where optical scattering can ruin image contrast and resolution.”
The fact that this approach requires thousands of single-photon detectors, all producing many hundreds of events per second, makes it extremely challenging to retrieve and process the data necessary to reconstruct the 3D image in a short time, especially for real-time applications. To solve this problem, the researchers developed algorithms specifically for imaging in highly scattering conditions and applied them in conjunction with widely available graphics processing unit (GPU) hardware. The new technique builds on some important technological advances.
“Heriot-Watt University has a long track record in single-photon detection techniques and image processing of single-photon data, which allowed us to demonstrate advanced single?photon imaging in extremely challenging conditions,” said Maccarone. “The University of Edinburgh has achieved fundamental advances in the design and fabrication of single-photon avalanche diode detector arrays, which allowed us to build compact and robust imaging systems based on quantum detection technologies.”
After optimizing the optical setup on a laboratory optical bench, the researchers connected the lidar system to a GPU to achieve real-time processing of the data while also implementing a number of image processing approaches for three-dimensional imaging. Once the system was working properly, they moved it to a tank that was 4 meters long, 3 meters wide, and 2 meters deep. With the system submerged in the water, the researchers added a scattering agent in a controlled manner to make the water more turbid. Experiments at three different turbidity levels demonstrated successful imaging in controlled highly scattering scenarios at distances of 3 meters.
“Single-photon technologies are rapidly developing, and we have demonstrated very promising results in underwater environments,” said Maccarone. “The approach and image processing algorithms could also be used in a wider range of scenarios for improved vision in free space such as in fog, smoke or other obscurants.”
Scalable set of reversible parity gates for integer factorization
by Martin Lanthaler, Benjamin E. Niehoff, Wolfgang Lechner in Communications Physics
Today’s computers are based on microprocessors that execute so-called gates. A gate can, for example, be an AND operation, i.e. an operation that adds two bits. These gates, and thus computers, are irreversible. That is, algorithms cannot simply run backwards. “If you take the multiplication 2*2=4, you cannot simply run this operation in reverse, because 4 could be 2*2, but likewise 1*4 or 4*1,” explains Wolfgang Lechner, professor of theoretical physics at the University of Innsbruck. If this were possible, however, it would be feasible to factorize large numbers, i.e. divide them into their factors, which is an important pillar of cryptography.
Martin Lanthaler, Ben Niehoff and Wolfgang Lechner from the Department of Theoretical Physics at the University of Innsbruck and the quantum spin-off ParityQC have now developed exactly this inversion of algorithms with the help of quantum computers. The starting point is a classical logic circuit, which multiplies two numbers. If two integers are entered as the input value, the circuit returns their product. Such a circuit is built from irreversible operations.
“However, the logic of the circuit can be encoded within ground states of a quantum system,” explains Martin Lanthaler from Wolfgang Lechner’s team. “Thus, both multiplication and factorization can be understood as ground-state problems and solved using quantum optimization methods.”
“The core of our work is the encoding of the basic building blocks of the multiplier circuit, specifically AND gates, half and full adders with the parity architecture as the ground state problem on an ensemble of interacting spins,” says Martin Lanthaler.
The coding allows the entire circuit to be built from repeating subsystems that can be arranged on a two-dimensional grid. By stringing several of these subsystems together, larger problem instances can be realized. Instead of the classical brute force method, where all possible factors are tested, quantum methods can speed up the search process: To find the ground state, and thus solve an optimization problem, it is not necessary to search the whole energy landscape, but deeper valleys can be reached by “tunneling.”
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