QT/ New quantum device generates single photons and encodes information
Quantum news biweekly vol.58, 22nd August — 5th September
TL;DR
- A new approach to quantum light emitters generates a stream of circularly polarized single photons, or particles of light, that may be useful for a range of quantum information and communication applications. A team stacked two different, atomically thin materials to realize this chiral quantum light source.
- Using a trapped-ion quantum computer, the research team witnessed the interference pattern of a single atom caused by a ‘conical intersection’. Conical intersections are known throughout chemistry and are vital to rapid photo-chemical processes such as light harvesting in human vision or photosynthesis.
- The measurement values determined in sufficiently precise measurements of physical systems will vary based on the relation between the past and the future of a system determined by its interactions with the meter. This finding may explain why quantum experiments often produce paradoxical results that can contradict our common-sense idea of physical reality.
- Researchers have designed a new type of quantum computer that uses fermionic atoms to simulate complex physical systems. The processor uses programmable neutral atom arrays and is capable of simulating fermionic models in a hardware-efficient manner using fermionic gates. The team demonstrated how the new quantum processor can efficiently simulate fermionic models from quantum chemistry and particle physics.
- Scientists have implemented a quantum-based method to observe a quantum effect in the way light-absorbing molecules interact with incoming photons. Known as a conical intersection, the effect puts limitations on the paths molecules can take to change between different configurations. The observation method makes use of a quantum simulator, developed from research in quantum computing, and offers an example of how advances in quantum computing are being used to investigate fundamental science.
- New research shows that electrical stimuli passed between neighboring electrodes can also affect non-neighboring electrodes. Known as non-locality, this discovery is a crucial milestone toward creating brain-like computers with minimal energy requirements.
- In 1956, theoretical physicist David Pines predicted that electrons in a solid can do something strange. While they normally have a mass and an electric charge, Pines asserted that they can combine to form a composite particle that is massless, neutral, and does not interact with light. He called this particle a ‘demon.’ Now, researchers have finally found Pines’ demon 67 years after it was predicted.
- New interference radar functions improve the distance resolution between objects using radar waves. The results may have important ramifications in military, construction, archaeology, mineralogy and many other domains of radar applications. It addresses a nine decades-old problem that requires scientists and engineers to sacrifice detail and resolution for observation distance — underwater, underground, and in the air.
- Researchers used magnetic imaging to obtain the first direct visualization of how electrons flow in a special type of insulator, and by doing so they discovered that the transport current moves through the interior of the material, rather than at the edges, as scientists had long assumed.
- Theoreticians report that they found no evidence of any universal topological signatures after performing the first ab initio investigation of high harmonic generation from topological insulators.
- 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
Proximity-induced chiral quantum light generation in strain-engineered WSe2/NiPS3 heterostructures
by Xiangzhi Li, Andrew C. Jones, Junho Choi, Huan Zhao, Vigneshwaran Chandrasekaran, Michael T. Pettes, Andrei Piryatinski, Märta A. Tschudin, Patrick Reiser, David A. Broadway, Patrick Maletinsky, Nikolai Sinitsyn, Scott A. Crooker, Han Htoon in Nature Materials
A new approach to quantum light emitters generates a stream of circularly polarized single photons, or particles of light, that may be useful for a range of quantum information and communication applications. A Los Alamos National Laboratory team stacked two different, atomically thin materials to realize this chiral quantum light source.
“Our research shows that it is possible for a monolayer semiconductor to emit circularly polarized light without the help of an external magnetic field,” said Han Htoon, scientist at Los Alamos National Laboratory. “This effect has only been achieved before with high magnetic fields created by bulky superconducting magnets, by coupling quantum emitters to very complex nanoscale photonics structures or by injecting spin-polarized carriers into quantum emitters. Our proximity-effect approach has the advantage of low-cost fabrication and reliability.”
The polarization state is a means of encoding the photon, so this achievement is an important step in the direction of quantum cryptography or quantum communication.
“With a source to generate a stream of single photons and also introduce polarization, we have essentially combined two devices in one,” Htoon said.
As described, the research team worked at the Center for Integrated Nanotechnologies to stack a single-molecule-thick layer of tungsten diselenide semiconductor onto a thicker layer of nickel phosphorus trisulfide magnetic semiconductor. Xiangzhi Li, postdoctoral research associate, used atomic force microscopy to create a series of nanometer-scale indentations on the thin stack of materials. The indentations are approximately 400 nanometers in diameter, so over 200 of such indents can easily be fit across the width of a human hair.
The indentations created by the atomic microscopy tool proved useful for two effects when a laser was focused on the stack of materials. First, the indentation forms a well, or depression, in the potential energy landscape. Electrons of the tungsten diselenide monolayer fall into the depression. That stimulates the emission of a stream of single photons from the well.
The nanoindentation also disrupts the typical magnetic properties of the underlying nickel phosphorus trisulfide crystal, creating a local magnetic moment pointing up out of the materials. That magnetic moment circularly polarizes the photons being emitted. To provide experimental confirmation of this mechanism, the team first performed high magnetic field optical spectroscopy experiments in collaboration with National High Magnetic Field Laboratory’s Pulsed Field Facility at Los Alamos. The team then measured the minute magnetic field of the local magnetic moments in collaboration with the University of Basel in Switzerland. The experiments proved that the team had successfully demonstrated a novel approach to control the polarization state of a single photon stream.
The team is currently exploring ways to modulate the degree of circular polarization of the single photons with the application of electrical or microwave stimuli. That capability would offer a way to encode quantum information into the photon stream. Further coupling of the photon stream into waveguides — microscopic conduits of light — would provide the photonic circuits that allow the propagation of photons in one direction. Such circuits would be the fundamental building blocks of an ultra-secure quantum internet.
Direct observation of geometric-phase interference in dynamics around a conical intersection
by C. H. Valahu, V. C. Olaya-Agudelo, R. J. MacDonell, T. Navickas, A. D. Rao, M. J. Millican, J. B. Pérez-Sánchez, J. Yuen-Zhou, M. J. Biercuk, C. Hempel, T. R. Tan, I. Kassal in Nature Chemistry
Scientists at the University of Sydney have, for the first time, used a quantum computer to engineer and directly observe a process critical in chemical reactions by slowing it down by a factor of 100 billion times.
Joint lead researcher and PhD student, Vanessa Olaya Agudelo, said: “It is by understanding these basic processes inside and between molecules that we can open up a new world of possibilities in materials science, drug design, or solar energy harvesting.
“It could also help improve other processes that rely on molecules interacting with light, such as how smog is created or how the ozone layer is damaged.”
Specifically, the research team witnessed the interference pattern of a single atom caused by a common geometric structure in chemistry called a ‘conical intersection’. Conical intersections are known throughout chemistry and are vital to rapid photo-chemical processes such as light harvesting in human vision or photosynthesis.
Chemists have tried to directly observe such geometric processes in chemical dynamics since the 1950s, but it is not feasible to observe them directly given the extremely rapid timescales involved. To get around this problem, quantum researchers in the School of Physics and the School of Chemistry created an experiment using a trapped-ion quantum computer in a completely new way. This allowed them to design and map this very complicated problem onto a relatively small quantum device — and then slow the process down by a factor of 100 billion.
“In nature, the whole process is over within femtoseconds,” said Ms Olaya Agudelo from the School of Chemistry. “That’s a billionth of a millionth — or one quadrillionth — of a second.”
“Using our quantum computer, we built a system that allowed us to slow down the chemical dynamics from femtoseconds to milliseconds. This allowed us to make meaningful observations and measurements. “This has never been done before.”
Joint lead author Dr Christophe Valahu from the School of Physics said: “Until now, we have been unable to directly observe the dynamics of ‘geometric phase’; it happens too fast to probe experimentally. “Using quantum technologies, we have addressed this problem.”
Dr Valahu said it is akin to simulating the air patterns around a plane wing in a wind tunnel: “Our experiment wasn’t a digital approximation of the process — this was a direct analogue observation of the quantum dynamics unfolding at a speed we could observe,” he said.
In photo-chemical reactions such as photosynthesis, by which plants get their energy from the Sun, molecules transfer energy at lightning speed, forming areas of exchange known as conical intersections. This study slowed down the dynamics in the quantum computer and revealed the tell-tale hallmarks predicted — but never before seen — associated with conical intersections in photochemistry.
Dependence of measurement outcomes on the dynamics of quantum coherent interactions between the system and the meter
by Tomonori Matsushita, Holger F. Hofmann in Physical Review Research
Whenever the precision of a measurement approaches the uncertainty limit defined by quantum mechanics, the outcomes of the measurement depend on the dynamics of the interactions with the meter used to determine a physical property of the system. This finding may explain why quantum experiments often produce conflicting results and may contradict basic assumptions regarding physical reality.
Two quantum physicists from Hiroshima University recently analyzed the dynamics of a measurement interaction, where the value of a physical property is identified with a quantitative change in the meter state. This is a difficult problem, because quantum theory does not identify the value of a physical property unless the system is in a so-called “eigenstate” of that physical property, a very small set of special quantum states for which the physical property has a fixed value. The researchers solved this fundamental problem by combining information about the past of the system with information about its future in a description of the dynamics of the system during the measurement interaction, demonstrating that the observable values of a physical system depend on the dynamics of the measurement interaction by which they are observed.
“There is much disagreement about the interpretation of quantum mechanics because different experimental results cannot be reconciled with the same physical reality,” said Holger Hofmann, professor in the Graduate School of Advanced Science and Engineering at Hiroshima University in Hiroshima, Japan.
“In this paper, we investigate how quantum superpositions in the dynamics of the measurement interaction shape the observable reality of a system seen in the response of a meter. This is a major step towards explaining the meaning of “superposition” in quantum mechanics,” said Hofmann.
In quantum mechanics, a superposition describes a situation in which two possible realities seem to co-exist, even though they can be distinguished clearly when an appropriate measurement is performed. The analysis of the team’s study suggests that superpositions describe different kinds of reality when different measurements are performed. The reality of an object depends on the object’s interactions with its surroundings.
“Our results show that the physical reality of an object cannot be separated from the context of all its interactions with the environment, past, present and future, providing strong evidence against the widespread belief that our world can be reduced to a mere configuration of material building blocks,” said Hofmann.
According to quantum theory, the meter shift that represents the value of the physical property observed in a measurement depends on the dynamics of the system caused by the fluctuations of the back-action by which the meter disturbs the state of the system. Quantum superpositions between the different possible system dynamics shape the meter response and assign specific values to it.
The authors further explained that the fluctuations in the system dynamics depend on the strength of the measurement interaction. In the limit of weak interactions, the fluctuations of the system dynamics are negligibly small and the meter shift can be determined from the Hamilton-Jacobi equation, a classical differential equation expressing the relation between a physical property and the dynamics associated with it.
When the measurement interaction is stronger, complicated quantum interference effects between different system dynamics are observed. Fully resolved measurements require a complete randomization of the system dynamics. This corresponds to a superposition of all possible system dynamics, where quantum interference effects select only those components of the quantum process that correspond to the eigenvalues of the physical property. Eigenvalues are the values that textbook quantum mechanics assigns to measurement outcomes — precise photon numbers, spin up or spin down, and so forth. As the new results show, these values are a result of the complete randomization of the dynamics. Different values need to be considered when the system dynamics is not completely randomized by the measurement.
Interestingly, this observation provides a new perspective on the use of measurement outcomes in descriptions of reality. It is common to assume that localized particles or integer spin values are measurement independent elements of reality, but these research results suggest that these values are only created by quantum interferences in sufficiently strong measurements. Our understanding of the meaning of experimental data may be in need of a fundamental revision.
Hofmann and his team look forward to further clarifying the contradictory results observed in many quantum experiments. “Context-dependent realities can explain a wide range of seemingly paradoxical quantum effects. We are now working on better explanations of these phenomena. Ultimately, the goal is to develop a more intuitive understanding of the fundamental concepts of quantum mechanics that avoids the misunderstandings caused by a naive belief in the reality of microscopic objects,” said Hofmann.
Fermionic quantum processing with programmable neutral atom arrays
by D. González-Cuadra, D. Bluvstein, M. Kalinowski, R. Kaubruegger, N. Maskara, P. Naldesi, T. V. Zache, A. M. Kaufman, M. D. Lukin, H. Pichler, B. Vermersch, Jun Ye, P. Zoller in Proceedings of the National Academy of Sciences
Researchers from Austria and USA have designed a new type of quantum computer that uses fermionic atoms to simulate complex physical systems. The processor uses programmable neutral atom arrays and is capable of simulating fermionic models in a hardware-efficient manner using fermionic gates. The team led by Peter Zoller demonstrated how the new quantum processor can efficiently simulate fermionic models from quantum chemistry and particle physics.
Fermionic atoms are atoms that obey the Pauli exclusion principle, which means that no two of them can occupy the same quantum state simultaneously. This makes them ideal for simulating systems where fermionic statistics play a crucial role, such as molecules, superconductors and quark-gluon plasmas. “In qubit-based quantum computers extra resources need to be dedicated to simulate these properties, usually in the form of additional qubits or longer quantum circuits,” explains Daniel Gonzalez Cuadra from the research group led by Peter Zoller at the Institute for Quantum Optics and Quantum Information (IQOQI) of the Austrian Academy of Sciences (ÖAW) and the Department of Theoretical Physics at the University of Innsbruck, Austria.
A fermionic quantum processor is composed of a fermionic register and a set of fermionic quantum gates. “The register consists on a set of fermionic modes, which can be either empty or occupied by a single fermion, and these two states form the local unit of quantum information,” says Daniel Gonzalez Cuadra. “The state of the system we want to simulate, such as a molecule composed of many electrons, will be in general a superposition of many occupation patterns, which can be directly encoded into this register.” This information is then processed using a fermionic quantum circuit, designed to simulate for example the time evolution of a molecule. Any such circuit can be decomposed into a sequence of just two types of fermionic gates, a tunneling and an interaction gate.
The researchers propose to trap fermionic atoms in an array of optical tweezers, which are highly focused laser beams that can hold and move atoms with high precision. “The required set of fermionic quantum gates can be natively implemented in this platform: tunneling gates can be obtained by controlling the tunneling of an atom between two optical tweezers, while interaction gates are implemented by first exciting the atoms to Rydberg states, carrying a strong dipole moment,” says Gonzalez Cuadra.
Fermionic quantum processing is particularly useful to simulate the properties of systems composed of many interacting fermions, such as electrons in a molecule or in a material, or quarks inside a proton, and has therefore applications in many fields, ranging from quantum chemistry to particle physics. The researchers demonstrate how their fermionic quantum processor can efficiently simulate fermionic models from quantum chemistry and lattice gauge theory, which are two important fields of physics that are hard to solve with classical computers.
“By using fermions to encode and process quantum information, some properties of the simulated system are intrinsically guaranteed at the hardware level, which would require additional resources in a standard qubit-based quantum computer,” says Daniel Gonzalez Cuadra. “I am very excited about the future of the field, and I would like to keep contributing to it by identifying the most promising applications for fermionic quantum processing, and by designing tailored algorithms that can run in near-term devices.”
Quantum simulation of conical intersections using trapped ions
by Jacob Whitlow, Zhubing Jia, Ye Wang, Chao Fang, Jungsang Kim, Kenneth R. Brown in Nature Chemistry
Researchers at Duke University have implemented a quantum-based method to observe a quantum effect in the way light-absorbing molecules interact with incoming photons. Known as a conical intersection, the effect puts limitations on the paths molecules can take to change between different configurations.
The observation method makes use of a quantum simulator, developed from research in quantum computing, and addresses a long-standing, fundamental question in chemistry critical to processes such as photosynthesis, vision and photocatalysis. It is also an example of how advances in quantum computing are being used to investigate fundamental science.
“As soon as quantum chemists ran into these conical intersection phenomena, the mathematical theory said that there were certain molecular arrangements that could not be reached from one to the other,” said Kenneth Brown, the Michael J. Fitzpatrick Distinguished Professor of Engineering at Duke. “That constraint, called a geometric phase, isn’t impossible to measure, but nobody has been able to do it. Using a quantum simulator gave us a way to see it in its natural quantum existence.”
Conical intersections can be visualized as a mountain peak touching the tip of its reflection coming from above and govern the motion of electrons between energy states. The bottom half of the conical intersection represents the energy states and physical locations of an unexcited molecule in its ground state. The top half represents the same molecule but with its electrons excited, having absorbed energy from an incoming light particle. The molecule can’t stay in the top state — its electrons are out of position relative to their host atoms. To return to the more favorable lower energy state, the molecule’s atoms begin rearranging themselves to meet the electrons. The point where the two mountains meet — the conical intersection — represents an inflection point. The atoms can either fail to get to the other side by readjusting to their original state, dumping excess energy in the molecules around them in the process, or they can successfully make the switch.
Because the atoms and electrons are moving so fast, however, they exhibit quantum effects. Rather than being in any one shape — at any one place on the mountain — at any given time, the molecule is actually in many shapes at once. One could think of all these possible locations as being represented by a blanket wrapped around a portion of the mountainous landscape. But due to a mathematical quirk in the system that emerges from the underlying mathematics, called a geometric phase, certain molecular transformations can’t happen. The blanket can’t wrap entirely around the mountain.
“If a molecule has two different paths to take to get to the same final shape, and those paths happen to surround a conical intersection, then the molecule wouldn’t be able to take that shape,” said Jacob Whitlow, a doctoral student working in Brown’s laboratory. “It’s an effect that’s hard to gain intuition for, because geometric phase is weird even from a quantum mechanical standpoint.”
Measuring this quantum effect has always been challenging because it is both short-lived, on the order of femtoseconds, and small, on the scale of atoms. And any disruption to the system will prevent its measurement. While many smaller pieces of the larger conical intersection phenomenon have been studied and measured, the geometric phase has always eluded researchers.
“If conical intersections exist — which they do — then the geometric phase has to exist,” said Brown, who also holds appointments in Duke physics and chemistry. “But what does it mean to say something exists that you can’t measure?”
In the paper, Whitlow and coworkers used a five-ion quantum computer built by the group of Jungsang Kim, the Schiciano Family Distinguished Professor of Electrical and Computer Engineering at Duke. The quantum computer uses lasers to manipulate charged atoms in a vacuum, providing a high level of control. Whitlow and Zhubing Jia, a PhD student in Brown’s laboratory, also expanded the capability of the system by developing ways to physically nudge the floating ions within their electromagnetic traps.
Based on how the ions are moved and the quantum state that they’re placed in, they can fundamentally exhibit the exact same quantum mechanisms as the motion of atoms around a conical intersection. And because the quantum dynamics of the trapped ions are about a billion times slower than those of a molecule, the researchers were able to make direct measurements of the geometric phase in action.
The results look something like a two-dimensional crescent moon. As depicted in the conical intersection graph, certain configurations on one side of the cone fail to reach the other side of the cone even though there is no energy barrier. The experiment, Brown says, is an elegant example of how even today’s rudimentary quantum computers can model and reveal the inner quantum workings of complex quantum systems.
“The beauty of trapped ions is that they get rid of the complicated environment and make the system clean enough to make these measurements,” said Brown.
Spatial Interactions in Hydrogenated Perovskite Nickelate Synaptic Networks
by Ravindra Singh Bisht, Jaeseoung Park, et al in Nano Letters
We often believe computers are more efficient than humans. After all, computers can complete a complex math equation in a moment and can also recall the name of that one actor we keep forgetting. However, human brains can process complicated layers of information quickly, accurately, and with almost no energy input: recognizing a face after only seeing it once or instantly knowing the difference between a mountain and the ocean. These simple human tasks require enormous processing and energy input from computers, and even then, with varying degrees of accuracy.
Creating brain-like computers with minimal energy requirements would revolutionize nearly every aspect of modern life. Funded by the Department of Energy, Quantum Materials for Energy Efficient Neuromorphic Computing (Q-MEEN-C) — a nationwide consortium led by the University of California San Diego — has been at the forefront of this research.
UC San Diego Assistant Professor of Physics Alex Frañó is co-director of Q-MEEN-C and thinks of the center’s work in phases. In the first phase, he worked closely with President Emeritus of University of California and Professor of Physics Robert Dynes, as well as Rutgers Professor of Engineering Shriram Ramanathan. Together, their teams were successful in finding ways to create or mimic the properties of a single brain element (such as a neuron or synapse) in a quantum material. Now, in phase two, new research from Q-MEEN-C shows that electrical stimuli passed between neighboring electrodes can also affect non-neighboring electrodes. Known as non-locality, this discovery is a crucial milestone in the journey toward new types of devices that mimic brain functions known as neuromorphic computing.
“In the brain it’s understood that these non-local interactions are nominal — they happen frequently and with minimal exertion,” stated Frañó, one of the paper’s co-authors. “It’s a crucial part of how the brain operates, but similar behaviors replicated in synthetic materials are scarce.”
Like many research projects now bearing fruit, the idea to test whether non-locality in quantum materials was possible came about during the pandemic. Physical lab spaces were shuttered, so the team ran calculations on arrays that contained multiple devices to mimic the multiple neurons and synapses in the brain. In running these tests, they found that non-locality was theoretically possible.
When labs reopened, they refined this idea further and enlisted UC San Diego Jacobs School of Engineering Associate Professor Duygu Kuzum, whose work in electrical and computer engineering helped them turn a simulation into an actual device. This involved taking a thin film of nickelate — a “quantum material” ceramic that displays rich electronic properties — inserting hydrogen ions, and then placing a metal conductor on top. A wire is attached to the metal so that an electrical signal can be sent to the nickelate. The signal causes the gel-like hydrogen atoms to move into a certain configuration and when the signal is removed, the new configuration remains.
“This is essentially what a memory looks like,” stated Frañó. “The device remembers that you perturbed the material. Now you can fine tune where those ions go to create pathways that are more conductive and easier for electricity to flow through.”
Traditionally, creating networks that transport sufficient electricity to power something like a laptop requires complicated circuits with continuous connection points, which is both inefficient and expensive. The design concept from Q-MEEN-C is much simpler because the non-local behavior in the experiment means all the wires in a circuit do not have to be connected to each other. Think of a spider web, where movement in one part can be felt across the entire web. This is analogous to how the brain learns: not in a linear fashion, but in complex layers. Each piece of learning creates connections in multiple areas of the brain, allowing us to differentiate not just trees from dogs, but an oak tree from a palm tree or a golden retriever from a poodle.
To date, these pattern recognition tasks that the brain executes so beautifully, can only be simulated through computer software. AI programs like ChatGPT and Bard use complex algorithms to mimic brain-based activities like thinking and writing. And they do it really well. But without correspondingly advanced hardware to support it, at some point software will reach its limit.
Frañó is eager for a hardware revolution to parallel the one currently happening with software, and showing that it’s possible to reproduce non-local behavior in a synthetic material inches scientists one step closer. The next step will involve creating more complex arrays with more electrodes in more elaborate configurations.
“This is a very important step forward in our attempts to understand and simulate brain functions,” said Dynes, who is also a co-author. “Showing a system that has non-local interactions leads us further in the direction toward how our brains think. Our brains are, of course, much more complicated than this but a physical system that is capable of learning must be highly interactive and this is a necessary first step. We can now think of longer range coherence in space and time”
“It’s widely understood that in order for this technology to really explode, we need to find ways to improve the hardware — a physical machine that can perform the task in conjunction with the software,” Frañó stated. “The next phase will be one in which we create efficient machines whose physical properties are the ones that are doing the learning. That will give us a new paradigm in the world of artificial intelligence.”
Pines’ demon observed as a 3D acoustic plasmon in Sr2RuO4
by Ali A. Husain, Edwin W. Huang, Matteo Mitrano, Melinda S. Rak, Samantha I. Rubeck, Xuefei Guo, Hongbin Yang, Chanchal Sow, Yoshiteru Maeno, Bruno Uchoa, Tai C. Chiang, Philip E. Batson, Philip W. Phillips, Peter Abbamonte in Nature
In 1956, theoretical physicist David Pines predicted that electrons in a solid can do something strange. While they normally have a mass and an electric charge, Pines asserted that they can combine to form a composite particle that is massless, neutral, and does not interact with light. He called this particle a “demon.” Since then, it has been speculated to play an important role in the behaviors of a wide variety of metals. Unfortunately, the same properties that make it interesting have allowed it to elude detection since its prediction.
Now, a team of researchers led by Peter Abbamonte, a professor of physics at the University of Illinois Urbana-Champaign, have finally found Pines’ demon 67 years after it was predicted. As the researchers report, they used a nonstandard experimental technique that directly excites a material’s electronic modes, allowing them to see the demon’s signature in the metal strontium ruthenate.
“Demons have been theoretically conjectured for a long time, but experimentalists never studied them,” Abbamonte said. “In fact, we weren’t even looking for it. But it turned out we were doing exactly the right thing, and we found it.”
One of the most important discoveries of condensed matter physics is that electrons lose their individuality in solids. Electric interactions make the electrons combine to form collective units. With enough energy, the electrons can even form composite particles called plasmons with a new charge and mass determined by the underlying electric interactions. However, the mass is usually so large that plasmons cannot form with the energies available at room temperature.
Pines found an exception. If a solid has electrons in more than one energy band, as many metals do, he argued that their respective plasmons can combine in an out-of-phase pattern to form a new plasmon that is massless and neutral: a demon. Since demons are massless, they can form with any energy, so they may exist at all temperatures. This has led to speculation that they have important effects on the behavior of multi-band metals.
Demons’ neutrality means that they do not leave a signature in standard condensed matter experiments. “The vast majority of experiments are done with light and measure optical properties, but being electrically neutral means that demons don’t interact with light,” Abbamonte said. “A completely different kind of experiment was needed.”
Abbamonte recalls that he and his collaborators were studying strontium ruthenate for an unrelated reason — the metal is similar to high-temperature superconductors without being one. Hoping to find clues to why the phenomenon occurs in other systems, they were conducting the first survey of the metal’s electronic properties.
The research group of Yoshi Maeno, a professor of physics at Kyoto University, synthesized high-quality samples of the metal which Abbamonte and former graduate student Ali Husain examined with momentum-resolved electron energy-loss spectroscopy. A nonstandard technique, it uses energy from electrons shot into the metal to directly observe the metal’s features, including plasmons that form. As the researchers were looking through the data, though, they found something unusual: an electronic mode with no mass.
Husain, now a research scientist at Quantinuum, recalled, “At first, we had no idea what it was. Demons are not in the mainstream. The possibility came up early on, and we basically laughed it off. But, as we started ruling things out, we started to suspect that we had really found the demon.”
Edwin Huang, a Moore Postdoctoral Scholar at UIUC and condensed matter theorist, was eventually asked to calculate the features of strontium ruthenate’s electronic structure. “Pines’ prediction of demons necessitates rather specific conditions, and it was not clear to anyone whether strontium ruthenate should have a demon at all,” he said. “We had to perform a microscopic calculation to clarify what was going on. When we did this, we found a particle consisting of two electron bands oscillating out-of-phase with nearly equal magnitude, just like Pines described.”
According to Abbamonte, it was no accident that his group discovered the demon “serendipitously.” He emphasized that he and his group were using a technique that is not widely employed on a substance that has not been well studied. That they found something unexpected and significant is a consequence of simply trying something different, he believes.
“It speaks to the importance of just measuring stuff,” he said. “Most big discoveries are not planned. You go look somewhere new and see what’s there.”
Super Interferometric Range Resolution
by John C. Howell, Andrew N. Jordan, Barbara Šoda, Achim Kempf in Physical Review Letters
New interference radar functions employed by a team of researchers from Chapman University and other institutions improve the distance resolution between objects using radar waves. The results may have important ramifications in military, construction, archaeology, mineralogy and many other domains of radar applications.
This first proof-of-principle experiment opens a new area of research with many possible applications that can be disruptive to the multi-billion dollar radar industry. There are many new avenues to pursue both in theory and experiment.
The discovery addresses a nine decades-old problem that requires scientists and engineers to sacrifice detail and resolution for observation distance — underwater, underground, and in the air. The previous bound limited the distance estimated between objects to be one quarter of the wavelength of radio waves; this technology improves the distance resolution between objects using radar waves.
“We believe this work will open a host of new applications as well as improve existing technologies,” says John Howell, the lead author of the article. “The possibility of efficient humanitarian demining or performing high-resolution, non-invasive medical sensing is very motivating,” Howell adds.
Howell and a team of researchers from the Institute for Quantum Studies at Chapman University, the Hebrew University of Jerusalem, the University of Rochester, the Perimeter Institute and the University of Waterloo have demonstrated range resolution more than 100 times better than the long-believed limit. This result breaks the trade-off between resolution and wavelength, allowing operators to use long wavelengths and now have high spatial resolution.
By employing functions with both steep and zero-time gradients, the researchers showed that it was possible to measure extremely small changes in the waveform to precisely predict the distance between two objects while still being robust to absorption losses. To an archaeologist this creates the ability to distinguish between a coin deep underground from a pottery shard.
The breakthrough idea relies on the superposition of specially-crafted waveforms. When a radio wave reflects from two different surfaces, the reflected radio waves add to form a new radio wave. The research team uses purpose-designed pulses to generate a new kind of superposed pulse. The composite wave has unique sub-wavelength features that can be used to predict the distance between the objects.
“In radio engineering, interference is a dirty word and thought of as a deleterious effect. Here, we turn this attitude on its head, and use wave interference effects to break the long-standing bound on radar ranging by orders of magnitude,” says Andrew Jordan, director of Quantum Studies at Chapman University. “In remote radar sensing, only a small amount of the electromagnetic radiation is returned to the detector. The tailored waveforms that we designed have the important property of being self-referencing, so properties of the target can be distinguished from loss of signal.”
Howell adds, “We are now working to demonstrate that it is possible to not only measure the distance between two objects, but many objects or perform detailed characterization of surfaces.”
Direct visualization of electronic transport in a quantum anomalous Hall insulator
by G. M. Ferguson, Run Xiao, Anthony R. Richardella, David Low, Nitin Samarth, Katja C. Nowack in Nature Materials
Cornell researchers used magnetic imaging to obtain the first direct visualization of how electrons flow in a special type of insulator, and by doing so they discovered that the transport current moves through the interior of the material, rather than at the edges, as scientists had long assumed.
The finding provides new insights into the electron behavior in so-called quantum anomalous Hall insulators and should help settle a decades-long debate about how current flows in more general quantum Hall insulators. These insights will inform the development of topological materials for next-generation quantum devices. The lead author is Matt Ferguson, Ph.D.’22, currently a postdoctoral researcher at the Max Planck Institute for Chemical Physics of Solids in Germany.
The project, led by Katja Nowack, assistant professor of physics in the College of Arts and Sciences and the paper’s senior author, has its origins in what’s known as the quantum Hall effect. First discovered in 1980, this effect results when a magnetic field is applied to a specific material to trigger an unusual phenomena: The interior of the bulk sample becomes an insulator while an electrical current moves in a single direction along the outer edge. The resistances are quantized, or restricted, to a value defined by the fundamental universal constant and drop to zero.
A quantum anomalous Hall insulator, first discovered in 2013, achieves the same effect by using a material that is magnetized. Quantization still occurs and longitudinal resistance vanishes, and the electrons speed along the edge without dissipating energy, somewhat like a superconductor. At least that is the popular conception.
“The picture where the current flows along the edges can really nicely explain how you get that quantization. But it turns out, it’s not the only picture that can explain quantization,” Nowack said. “This edge picture has really been the dominant one since the spectacular rise of topological insulators starting in the early 2000s. The intricacies of the local voltages and local currents have largely been forgotten. In reality, these can be much more complicated than the edge picture suggests.”
Only a handful of materials are known to be quantum anomalous Hall insulators. For their new work, Nowack’s group focused on chromium-doped bismuth antimony telluride — the same compound in which the quantum anomalous Hall effect was first observed a decade ago.
The sample was grown by collaborators led by physics professor Nitin Samarth at Pennsylvania State University. To scan the material, Nowack and Ferguson used their lab’s superconducting quantum interference device, or SQUID, an extremely sensitive magnetic field sensor that can operate at low temperatures to detect dauntingly tiny magnetic fields. The SQUID effectively images the current flows — which are what generate the magnetic field — and the images are combined to reconstruct the current density.
“The currents that we are studying are really, really small, so it’s a difficult measurement,” Nowack said. “And we needed to go below one Kelvin in temperature to get a good quantization in the sample. I’m proud that we pulled that off.”
When the researchers noticed the electrons flowing in the bulk of the material, not at the boundary edges, they began to dig through old studies. They found that in the years following the original discovery of the quantum Hall effect in 1980, there was much debate about where the flow occurred — a controversy unknown to most younger materials scientists, Nowack said.
“I hope the newer generation working on topological materials takes note of this work and reopens the debate. It’s clear that we don’t even understand some very fundamental aspects of what happens in topological materials,” she said. “If we don’t understand how the current flows, what do we actually understand about these materials?”
Answering those questions might also be relevant for building more complicated devices, such as hybrid technologies that couple a superconductor to a quantum anomalous Hall insulator to produce even more exotic states of matter.
“I’m curious to explore if what we observe holds true across different material systems. It might be possible that in some materials, the current flows, yet differently,” Nowack said. “For me this highlights the beauty of topological materials — their behavior in an electrical measurement are dictated by very general principles, independent of microscopic details. Nevertheless, it’s crucial to understand what happens at the microscopic scale, both for our fundamental understanding and applications. This interplay of general principles and the finer nuances makes studying topological materials so captivating and fascinating.”
Are There Universal Signatures of Topological Phases in High-Harmonic Generation? Probably Not..
by Ofer Neufeld, Nicolas Tancogne-Dejean, Hannes Hübener, Umberto De Giovannini, Angel Rubio in Physical Review X
Topology plays an enormous role in modern condensed matter physics and beyond. It describes how solid materials can combine two very different and somewhat contradictory properties — for example, topological insulators are materials whose bulk acts as an insulator, but whose surfaces and edges can conduct electricity nonetheless. In the last few decades, the concept of topology has changed the way scientists think of electronic structure and material properties altogether. Moreover, it has paved the way towards technological applications that incorporate topological materials in electronics.
At the same time, topology is quite tricky to measure, often requiring combinations of multiple experimental techniques such as photoemission and transport measurements. A method known as high harmonic spectroscopy has recently emerged as a key technique to observe the topology of a material. In this approach a material is irradiated by intense laser light. The interactions between electrons in the material and the laser result in the emission of a broadband optical spectrum — which contains clues about the topological phase of the solid. With the help of theoretical calculations, those clues can be extracted in order to measure the material topology.
However, theoreticians at the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg, Germany, now report that they found no evidence of any universal topological signatures after performing the first ab initio investigation of high harmonic generation from topological insulators. Focusing on a quantum spin Hall insulator in a monolayer of Bismuth atoms, and a quantum anomalous Hall insulator in a single monolayer of Na3Bi, the researchers questioned the underlying assumptions of topological high harmonic spectroscopy: That topological information is imprinted on the emitted spectra and can be subsequently extracted.
“We specifically set out to avoid common approximations and simplified models,” explains lead author Ofer Neufeld. “In this vast and thorough analysis, we could not identify any universal topological signatures, hinting that it is unlikely such signatures exist. Even if at first glance some features seemed to strongly correlate with a topological property, whenever we dug in to their origin it was never topological.”
Instead, the non-topological aspects of the system dominated its response, suggesting that topology may play a more minor role than previously thought.
“For instance, a solid can react differently to laser light that is left or right elliptically polarized,” Nicolas Tancogne-Dejean, the paper’s second author, explains. “Initially it might seem that that typical response originates in the topology. However, on closer examination this effect turns out to stem from the crystal structure, rather than the topological structure.”
The team’s findings raise important questions about the potential use of topology for applications in highly nonlinear optics. On a more positive note, the MPSD theoreticians stress that they do not rule out the existence of topological signatures in high harmonic generation altogether. However, they argue that other non-topological aspects of the material usually dominate the resulting spectra, such as the band structure, lattice symmetry and the chemical nature of the participating orbitals.
“We hope that our study will not only provide a ‘cautionary tale’ to warn others of potentially misleading topological fingerprints, but more importantly, that it will motivate the community to come up with more complex and robust ideas for how to measure topology through nonlinear optics,” Neufeld concludes.
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