QT/ Scientists discover new properties of magnetism that could change our computers
Quantum news biweekly vol.27, 9th May — 23d May
TL;DR
- A fundamental property of magnetism has been disclosed through new research. The discovery may be key to development of a new generation of powerful computers.
- Scientists have been relentlessly working on understanding the fundamental mechanisms at the base of high-temperature superconductivity with the ultimate goal to design and engineer new quantum materials superconducting close to room temperature.
- Researchers confirmed silicon nanoparticles are attracted to vortices in superfluid helium. Their simulations allowed them to visualize the process of vortex line recombination. The work may lead to improvements in quantum computing and optical spectroscopy.
- The famous double slit experiment shows that particles can travel on two paths at the same time — but only by looking at a lot of particles and analysing the results statistically. Now a two-path-interference experiment has been designed that only has to measure one specific particle to prove that it travelled on two paths.
- Computational detective work by physicists has confirmed cerium zirconium pyrochlore is a 3D quantum spin liquid, a solid material in which quantum entanglement and the geometric arrangement of atoms cause electrons to fluctuate between quantum magnetic states no matter how cold they become.
- Systems in which mechanical motion is controlled at the level of individual quanta are emerging as a promising quantum technology platform. New experimental work now establishes how quantum properties of such systems can be measured without destroying the quantum state — a key ingredient for tapping the full potential of mechanical quantum systems.
- A quantum system with only 51 charged atoms can take on more than two quadrillion different states. Calculating the system’s behavior is child’s play for a quantum simulator. But verifying the result is almost impossible, even with today’s supercomputers. A research team has now shown how these systems can be verified using equations formulated in the 18th century.
- A major hurdle for work at the forefront of fundamental physics is the inability to test cutting-edge theories in a laboratory setting. But a recent discovery opens the door for scientists to see ideas in action that were previously only understood in theory or represented in science fiction.
- A theoretical study shows that long-range entanglement can indeed survive at temperatures above absolute zero, if the correct conditions are met.
- A new materials database reveals more than 90,000 known ‘topological’ materials with persistent electronic properties.
- 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
Magnetic Bloch oscillations and domain wall dynamics in a near-Ising ferromagnetic chain
by Ursula B. Hansen, Olav F. Syljuåsen, Jens Jensen, Turi K. Schäffer, Christopher R. Andersen, Martin Boehm, Jose A. Rodriguez-Rivera, Niels B. Christensen, Kim Lefmann in Nature Communications
Our electronics can no longer shrink and are on the verge of overheating. But in a new discovery from the University of Copenhagen, researchers have uncovered a fundamental property of magnetism, which may become relevant for the development of a new generation of more powerful and less hot computers.
The ongoing miniaturization of components for computers which have electrons as their vehicles for information transfer has become challenged. Instead, it could be possible to use magnetism and thereby keep up the development of both cheaper and more powerful computers. This is one of the perspectives as scientists from the Niels Bohr Institute (NBI), University of Copenhagen.
“The function of a computer involves sending electric current through a microchip. While the amount is tiny, the current will not only transport information but also contribute to heating up the chip. When you have a huge number of components tightly packed, the heat becomes a problem. This is one of the reasons why we have reached the limit for how much you can shrink the components. A computer based on magnetism would avoid the problem of overheating,” says Professor Kim Lefmann, Condensed Matter Physics, NBI.
“Our discovery is not a direct recipe for making a computer based on magnetism. Rather we have disclosed a fundamental magnetic property which you need to control, if you want to design a such computer.”
To grasp the discovery, one needs to know that magnetic materials are not necessarily uniformly oriented. In other words, areas with magnetic north and south poles may exist side by side. These areas are termed domains, and the border between a north and south pole domain is the domain wall. While the domain wall is not a physical object it nevertheless has several particle-like properties. Thereby, it is an example of what physicists refer to as quasi-particles, meaning virtual phenomena which resemble particles.
“It is well established that one can move the position of the domain wall by applying a magnetic field. Initially, the wall will react similarly to a physical object which is subjected to gravity and accelerates until it impacts the surface below. However, other laws apply to the quantum world,” Kim Lefmann explains.
“At the quantum level, particles are not only objects they are also waves. This applies to a quasi-particle such as a domain wall as well. The wave properties imply that the acceleration is slowed down as the wall interacts with atoms in the surroundings. Soon, the acceleration will stop totally, and the position of the wall will start to oscillate.”
A similar phenomenon is seen for electrons. Here, it is known as Bloch oscillations named after the American-Swizz physicist and Nobel laureate Felix Bloch who discovered it in 1929. In 1996 Swiss theoretical physicists suggested that a parallel to Bloch oscillations could possibly exist in magnetism. Now — a little more than a quarter of a century later — Kim Lefmann and his colleagues managed to confirm this hypothesis. The research team has studied the movement of domain walls in the magnetic material CoCl2 ∙ 2D2O.
“We have known for a long time, that it would be possible to verify the hypothesis, but we also understood that it would require access to neutron sources. Uniquely, neutrons react to magnetic fields despite not being electrically charged. This makes them ideal for magnetic studies,” Kim Lefmann tells.
Neutron sources are large-scale scientific instruments. Worldwide, only some twenty facilities exist and competition for beam time is fierce.
“We have had beam time at NIST in USA, and ILL in France respectively. Fortunately, the conditions for magnetic research will improve greatly as the ESS (European Spallation Source, ed.) becomes operational in Lund, Sweden. Not just will our chances for beam time become better, since Denmark is a co-owner of the facility. The quality of the results will become roughly 100 times better, because the ESS will be an extremely powerful neutron source,” says Kim Lefmann.
To clarify, he emphasizes that even though quantum mechanics is involved, a computer based on magnetism would not be a type of quantum computer:
“In the future, quantum computers are expected to be able to tackle extremely complicated tasks. But even then, we will still need conventional computers for the more ordinary computing. This is where computers based on magnetism might become relevant alternatives as better than current computers.”
Enhanced charge density wave coherence in a light-quenched, high-temperature superconductor
by S. Wandel, F. Boschini, E. H. da Silva Neto, et al in Science
Scientists have been relentlessly working on understanding the fundamental mechanisms at the base of high-temperature superconductivity with the ultimate goal to design and engineer new quantum materials superconducting close to room temperature.
High temperature superconductivity is something of a holy grail for researchers studying quantum materials. Superconductors, which conduct electricity without dissipating energy, promise to revolutionize our energy and telecommunication power systems. However, superconductors typically work at extremely low temperatures, requiring elaborate freezers or expensive coolants. For this reason, scientist have been relentlessly working on understanding the fundamental mechanisms at the base of high-temperature superconductivity with the ultimate goal to design and engineer new quantum materials superconducting close to room temperature.
Fabio Boschini, Professor at the Institut national de la recherche scientifique (INRS), and North American scientists studied the dynamics of the superconductor yttrium barium copper oxide (YBCO), which offers superconductivity at higher-than-normal temperatures, via time-resolved resonant x-ray scattering at the Linac Coherent Light Source (LCLS) free-electron laser, SLAC (US). In this new study, researchers have been able to track how charge density waves in YBCO react to a sudden “quenching” of the superconductivity, induced by an intense laser pulse.
“We are learning that charge density waves — self-organized electrons behaving like ripples in water — and superconductivity are interacting at the nanoscale on ultrafast timescales. There is a very deep connection between superconductivity emergence and charge density waves,” says Fabio Boschini, co-investigator on this project and affiliate investigator at the Stewart Blusson Quantum Matter Institute (Blusson QMI).
“Up until a few years ago, researchers underestimated the importance of the dynamics inside these materials,” said Giacomo Coslovich, lead investigator and Staff Scientist at the SLAC National Accelerator Laboratory in California. “Until this collaboration came together, we really didn’t have the tools to assess the charge density wave dynamics in these materials. The opportunity to look at the evolution of charge order is only possible thanks to teams like ours sharing resources, and by the use of a free-electron laser to offer new insight into the dynamical properties of matter.”
Owing to a better picture of the dynamical interactions underlying high-temperature superconductors, the researchers are optimistic that they can work with theoretical physicists to develop a framework for a more nuanced understanding of how high-temperature superconductivity emerges.
The present work came about from a collaboration of researchers from several leading research centres and beamlines. “We began running our first experiments at the end of 2015 with the first characterization of the material at the Canadian Light Source, says Boschini. Over time, the project came to involve many Blusson QMI researchers, such as MengXing Na who I mentored and introduced to this work. She was integral to the data analysis.”
“This work is meaningful for a number of reasons, but it also really showcases the importance of forming long-lasting, meaningful collaborations and relationships,” said Na. “Some projects take a really long time, and it’s a credit to Giacomo’s leadership and perseverance that we got here.”
The project has linked at least three generations of scientists, following some as they progressed through their postdoctoral careers and into faculty positions. The researchers are excited to expand upon this work, by using light as an optical knob to control the on-off state of superconductivity.
Observing emergent hydrodynamics in a long-range quantum magnet
by M. K. Joshi, F. Kranzl, A. Schuckert, I. Lovas, C. Maier, R. Blatt, M. Knap, C. F. Roos in Science
At first glance, a system consisting of 51 ions may appear easily manageable. But even if these charged atoms are only changed back and forth between two states, the result is more than two quadrillion (1015) different orderings which the system can take on.
The behavior of such a system is almost impossible to calculate with conventional computers, especially since an excitation introduced to the system can propagate erratically. The excitation follows a statistical pattern referred to as a Lévy Flight. One characteristic of such movements is that, in addition to the smaller jumps which are to be expected, also significantly larger jumps take place. This phenomenon can also be observed in the flights of bees and in unusual fierce movements in the stock market.
While simulating the dynamics of a complex quantum system is a very tall order for even traditional super computers, the task is child’s play for quantum simulators. But how can the results of a quantum simulator be verified without the ability to perform the same calculations it can? Observation of quantum systems indicated that it might be possible to represent at least the long-term behavior of such systems with equations like the ones the Bernoulli brothers developed in the 18th century to describe the behavior of fluids.
In order to test this hypothesis, the authors used a quantum system which simulates the dynamics of quantum magnets. They were able to use it to prove that, after an initial phase dominated by quantum-mechanical effects, the system could actually be described with equations of the type familiar from fluid dynamics. Furthermore, they showed that the same Lévy Flight statistics which describe the search strategies used by bees also apply to fluid-dynamic processes in quantum systems.
The quantum simulator was built at the Institute for Quantum Optics and Quantum Information (IQOQI) of the Austrian Academy of Sciences at The University of Innsbruck Campus. “Our system effectively simulates a quantum magnet by representing the north and south poles of a molecular magnet using two energy levels of the ions,” says IQOQI Innsbruck scientist Manoj Joshi.
“Our greatest technical advance was the fact that we succeeded in individually addressing each one of the 51 ions individually,” observes Manoj Joshi. “As a result we were able to investigate the dynamics of any desired number of initial states, which was necessary in order to illustrate the emergence of the fluid dynamics.”
“While the number of qubits and the stability of the quantum states is currently very limited, there are questions for which we can already use the enormous computing power of quantum simulators today,” says Michael Knap, Professor for Collective Quantum Dynamics at the Technical University of Munich.
“In the near future, quantum simulators and quantum computers will be ideal platforms for researching the dynamics of complex quantum systems,” explains Michael Knap. “Now we know that after a certain point in time these systems follow the laws of classic fluid dynamics. Any strong deviations from that are an indication that the simulator isn’t working properly.”
Quantifying the presence of a neutron in the paths of an interferometer
by Hartmut Lemmel, Niels Geerits, Armin Danner, Holger F. Hofmann, Stephan Sponar in Physical Review Research
The double-slit experiment is the most famous and probably the most important experiment in quantum physics: individual particles are shot at a wall with two openings, behind which a detector measures where the particles arrive. This shows that the particles do not move along a very specific path, as is known from classical objects, but along several paths simultaneously: each individual particle passes through both the left and the right opening.
Normally, however, this can only be proven by carrying out the experiment over and over again and evaluating the results of many particle detections at the end. At TU Wien, it has now been possible to develop a new variant of such a two-way interference experiment that can correct this flaw: A single neutron is measured at a specific position — and due to the sophisticated measurement setup, this single measurement proofs already that the particle moved along two different paths at the same time. It is even possible to determine the ratio in which the neutron was distributed between the two paths. Thus, the phenomenon of quantum superposition can be proven without having to resort to statistical arguments.
“In the classical double-slit experiment, an interference pattern is created behind the double slit,” explains Stephan Sponar from the Atomic Institute at TU Wien. “The particles move as a wave through both openings at the same time, and the two partial waves then interfere with each other. In some places they reinforce each other, in other places they cancel each other out.”
The probability of measuring the particle behind the double slit at a very specific location depends on this interference pattern: where the quantum wave is amplified, the probability of measuring the particle is high. Where the quantum wave is cancelled out, the probability is low. Of course, this wave distribution cannot be seen by looking at a single particle. Only when the experiment is repeated many times does the wave pattern become increasingly recognisable point by point and particle by particle.
“So, the behaviour of individual particles is explained based on results that only become visible through the statistical investigation of many particles,” says Holger Hofmann from Hiroshima University, who developed the theory behind the experiment. “Of course, this is not entirely satisfactory. We have therefore considered how the phenomenon of two-way interference can be proven based on the detection of a single particle.”
This was made possible with the help of neutrons at the neutron source of ILL in Grenoble: The neutrons are sent onto a crystal that splits the quantum wave of the neutron into two partial waves, very similar to the classical double-slit experiment. The two partial neutron waves move along two different paths and are recombined again. They interfere and are then measured.
In addition, however, another property of the neutron is exploited: its spin — the angular momentum of the particle. It can be influenced by magnetic fields, the angular momentum of the neutron then points in a different direction. If the spin of the neutron is rotated on only one of the two paths, it is possible to determine afterwards which path it has taken. However, the interference pattern then also disappears, as a consequence of complementarity in quantum mechanics.
“We therefore rotate the spin of the neutron just a little,” explains Hartmut Lemmel, the first author of the current publication. “Then the interference pattern remains, because you can only obtain very little information about the path. In order to still obtain precise path information, this “weak” measurement is repeated many times in conventional experiments. However, one then obtains only a statistical statement about the whole ensemble of neutrons and can say little about each individual neutron.”
The situation is different if, after the two neutron partial waves have merged, another magnetic field is used to turn the spin back again. By trial and error, one determines the angle of rotation that is necessary to turn the spin of the superimposed state back into the original direction. The strength of this rotation is a measure of how strongly the neutron was present in each path. If it had taken only the path on which the spin has been rotated, the full angle of rotation would be necessary to rotate it back. If it had taken only the other path, no reverse rotation would be necessary at all. In the experiment carried out using a special asymmetric beam splitter, it was shown that the neutrons were present to one third in one path and to two thirds in the other.
Through detailed calculations, the team was able to show: Here, one does not merely detect an average value over the totality of all measured neutrons, but the statement applies to each individual neutron. It takes many neutrons to determine the optimal angle of rotation, but as soon as this is set, the path presence determined from it applies to every single neutron detected.
“Our measurement results support classical quantum theory,” says Stephan Sponar. “The novelty is that one does not have to resort to unsatisfactory statistical arguments: When measuring a single particle, our experiment shows that it must have taken two paths at the same time and quantifies the respective proportions unambiguously.”
This rules out alternative interpretations of quantum mechanics that attempt to explain the double-slit experiment with localised particles.
Parity measurement in the strong dispersive regime of circuit quantum acoustodynamics
by Uwe von Lüpke, Yu Yang, Marius Bild, Laurent Michaud, Matteo Fadel, Yiwen Chu in Nature Physics
Systems in which mechanical motion is controlled at the level of individual quanta are emerging as a promising quantum-technology platform. New experimental work now establishes how quantum properties of such systems can be measured without destroying the quantum state — a key ingredient for tapping the full potential of mechanical quantum systems.
When thinking about quantum mechanical systems, single photons and well-isolated ions and atoms may spring to mind, or electrons spreading through a crystal. More exotic in the context of quantum mechanics are genuinely mechanical quantum systems; that is, massive objects in which mechanical motion such as vibration is quantized. In a series of seminal experiments, quintessential quantum-mechanical features have been observed in mechanical systems, including energy quantization and entanglement. However, with a view to putting such systems to use in fundamental studies and technological applications, observing quantum properties is but a first step. The next one is to master the handling of mechanical quantum objects, so that their quantum states can be controlled, measured, and eventually exploited in device-like structures. The group of Yiwen Chu in the Laboratory of Solid State Physics at ETH Zurich has now made major progress in that direction. They report the extraction of information from a mechanical quantum system without destroying the precious quantum state. This advance paves the path to applications such as quantum error correction, and beyond.
The ETH physicists employ as their mechanical system a slab of high-quality sapphire, a little under half a millimetre thick. On its top sits a thin piezoelectrical transducer that can excite acoustic waves, which are reflected at the bottom and thus extend across a well-defined volume inside the slab. These excitations are the collective motion of a large number of atoms, yet they are quantized (in energy units known as phonons) and can be subjected, in principle at least, to quantum operations in very much the same ways as the quantum states of atoms, photons and electrons can be. Intriguingly, it is possible to interface the mechanical resonator with other quantum systems, and with superconducting qubits in particular. The latter are tiny electronic circuits in which electromagnetic energy states are quantized, and they are currently one of the leading platforms for building scalable quantum computers. The electromagnetic fields associated with the superconducting circuit enable the coupling of the qubit to the piezoelectrical transducer of the acoustic resonator, and thereby to its mechanical quantum states.
In such hybrid qubit-resonator devices, the best of two worlds can be combined. Specifically, the highly developed computational capabilities of superconducting qubits can be used in synchrony with the robustness and long lifetime of acoustical modes, which can serve as quantum memories or transducers. For such applications, however, merely coupling qubit and resonator states will be not enough. For example, a straightforward measurement of the quantum state in the resonator destroys it, making repeated measurements impossible. What is needed instead is the capability to extract information about the mechanical quantum state in a more gentle, well-controlled manner.
Demonstrating a protocol for such so-called quantum non-demolition measurements is what Chu’s doctoral students Uwe von Lüpke, Yu Yang and Marius Bild, working with Branco Weiss fellow Matteo Fadel and with support from semester project student Laurent Michaud, now achieved. In their experiments there is no direct energy exchange between the superconducting qubit and the acoustic resonator during the measurement. Instead, the properties of the qubit are made to depend on the number of phonons in the acoustic resonator, with no need to directly ‘touch’ the mechanical quantum state — think about a theremin, the musical instrument in which the pitch depends on the position of the musician’s hand without making physical contact with the instrument.
Creating a hybrid system in which the state of the resonator is reflected in the spectrum of the qubit is highly challenging. There are stringent demands on how long the quantum states can be sustained both in the qubit and in the resonator, before they fade away due to imperfections and perturbations from the outside. So the task for the team was to push the lifetimes of both the qubit and the resonator quantum states. And they succeeded, by making a series of improvements, including a careful choice of the type of superconducting qubit used and encapsulating the hybrid device in a superconducting aluminium cavity to ensure tight electromagnetic shielding.
Having successfully pushed their system into the desired operational regime (known as the ‘strong dispersive regime’), the team were able to gently extract the phonon-number distribution in their acoustic resonator after exciting it with different amplitudes. Moreover, they demonstrated a way to determine in one single measurement whether the number of phonons in the resonator is even or odd — a so-called parity measurement — without learning anything else about the distribution of phonons. Obtaining such very specific information, but no other, is crucial in a number of quantum-technological applications. For instance, a change in parity (a transition from an odd to an even number or vice versa) can signal that an error has affected the quantum state and that correcting is needed. Here it is essential, of course, that the to-be-corrected state is not destroyed.
Before an implementation of such error-correction schemes is possible, however, further refinement of the hybrid system is necessary, in particular to improve the fidelity of the operations. But quantum error correction is by far not the only use on the horizon. There is an abundance of exciting theoretical proposals in the scientific literature for quantum-information protocols as well as for fundamental studies that benefit from the fact that the acoustic quantum states reside in massive objects. These provide, for example, unique opportunities for exploring the scope of quantum mechanics in the limit of large systems and for harnessing the mechanical quantum systems as a sensor.
Visualization of quantized vortex reconnection enabled by laser ablation
by Yosuke Minowa, Shota Aoyagi, Sosuke Inui, Tomo Nakagawa, Gamu Asaka, Makoto Tsubota, Masaaki Ashida in Science Advances
Researchers have observed the vortices that form in superfluid helium by blasting silicon nanoparticles at them using laser ablation. The after observing the patterns of light scattering off the silicon nanoparticles Osaka Metropolitan University scientists performed a massive simulation of quantum vortex dynamics which confirmed that the observed nanoparticle swirls and loops were caused by quantum vortices. This work opens up new possibilities in optical research for other quantum properties of superfluid helium, such as the optical manipulation of quantized vortices due to the strong interaction between light and silicon nanoparticles.
The rules of quantum mechanics may seem very foreign; particles can act like waves and waves can act like particles. Weird quantum behavior is normally only found on a very small scale. However, when certain materials, like helium-4, are cooled to very low temperatures, the “waviness” of the particles has effects apparent even at the macroscopic scales.
“Supercooled” helium is an example of a Bose-Einstein condensation, where the waves representing atoms overlap until the whole fluid acts like a single massive particle. This process has no classical analogue and is a useful system for testing theories of quantum mechanics, because the transition to a superfluid in helium-4 occurs at relatively accessible temperatures. However, there is still a need to be able to visualize the motion of the superfluid.
Now, a researcher team has used silicon nanoparticles to help visualize the features of superfluid helium, similar to skipping stones across a river, to help visualize the flow of water. One of the special properties of superfluid helium is that any rotational motion can only occur in the form of quantized vortices. These are tiny, discrete whirlpools that each carry a fixed amount of angular momentum. The scientists used the nanoparticle “stones” to study the process of vortex reconnection, in which lines of vortices coalesce and exchange their parts. Because the light scatters off the nanoparticles, the vortex lines they are attracted to were clearly visible.
Osaka Metropolitan University researcher Makoto Tsubota led the team simulating the observed behavior of the silicon nanoparticles. “We performed the numerical simulation of quantized vortices fitted to the case of the experiments. The simulated vortices were the same as in observations! This agreement strongly supports that what we actually observed was the motion of quantized vortices,” exclaimed Professor Tsubota.
Moreover, Professor Tsubota noted, “A quantized vortex is a typical example of topological defects. Topological defects appear in various systems like superfluid helium, cold atoms, superconductors, liquid crystal, cosmology, etc. The present discovery will pave a novel way for investigating topological defects in these various systems.”
Sleuthing out exotic quantum spin liquidity in the pyrochlore magnet Ce2Zr2O7
by Anish Bhardwaj, Shu Zhang, Han Yan, Roderich Moessner, Andriy H. Nevidomskyy, Hitesh J. Changlani in npj Quantum Materials
Computational detective work by U.S. and German physicists has confirmed cerium zirconium pyrochlore is a 3D quantum spin liquid.
Despite the name, quantum spin liquids are solid materials in which quantum entanglement and the geometric arrangement of atoms frustrate the natural tendency of electrons to magnetically order themselves in relation to one another. The geometric frustration in a quantum spin liquid is so severe that electrons fluctuate between quantum magnetic states no matter how cold they become.
Theoretical physicists routinely work with quantum mechanical models that manifest quantum spin liquids, but finding convincing evidence that they exist in actual physical materials has been a decadeslong challenge. While a number of 2D or 3D materials have been proposed as possible quantum spin liquids, Rice University physicist Andriy Nevidomskyy said there’s no established consensus among physicists that any of them qualify. Nevidomskyy is hoping that will change based on the computational sleuthing he and colleagues from Rice, Florida State University and the Max Planck Institute for Physics of Complex Systems in Dresden, Germany.
“Based on all the evidence we have today, this work confirms that the single crystals of the cerium pyrochlore identified as candidate 3D quantum spin liquids in 2019 are indeed quantum spin liquids with fractionalized spin excitations,” he said.
The inherent property of electrons that leads to magnetism is spin. Each electron behaves like a tiny bar magnet with a north and south pole, and when measured, individual electron spins always point up or down. In most everyday materials, spins point up or down at random. But electrons are antisocial by nature, and this can cause them to arrange their spins in relation to their neighbors in some circumstances. In magnets, for example, spins are collectively arranged in the same direction, and in antiferromagnets they are arranged in an up-down, up-down pattern.
At very low temperatures, quantum effects become more prominent, and this causes electrons to arrange their spins collectively in most materials, even those where spins would point in random directions at room temperature. Quantum spin liquids are a counterexample where spins do not point in a definite direction — even up or down — no matter how cold the material becomes.
“A quantum spin liquid, by its very nature, is an example of a fractionalized state of matter,” said Nevidomskyy, associate professor of physics and astronomy and a member of both the Rice Quantum Initiative and the Rice Center for Quantum Materials (RCQM). “The individual excitations are not spin flips from up to down or vice versa. They’re these bizarre, delocalized objects that carry half of one spin degree of freedom. It’s like half of a spin.”
Nevidomskyy was part of the 2019 study led by Rice experimental physicist Pengcheng Dai that found the first evidence that cerium zirconium pyrochlore was a quantum spin liquid. The team’s samples were the first of their kind: Pyrochlores because of their 2-to-2-to-7 ratio of cerium, zirconium and oxygen, and single crystals because the atoms inside were arranged in a continuous, unbroken lattice. Inelastic neutron scattering experiments by Dai and colleagues revealed a quantum spin liquid hallmark, a continuum of spin excitations measured at temperatures as low as 35 millikelvin.
“You could argue that they found the suspect and charged him with the crime,” Nevidomskyy said. “Our job in this new study was to prove to the jury that the suspect is guilty.”
Nevidomskyy and colleagues built their case using state-of-the-art Monte Carlo methods, exact diagonalization as well as analytical tools to perform the spin dynamics calculations for an existing quantum mechanical model of cerium zirconium pyrochlore. The study was conceived by Nevidomskyy and Max Planck’s Roderich Moessner, and the Monte Carlo simulations were performed by Florida State’s Anish Bhardwaj and Hitesh Changlani with contributions from Rice’s Han Yan and Max Planck’s Shu Zhang.
“The framework for this theory was known, but the exact parameters, of which there are at least four, were not,” Nevidomskyy said. “In different compounds, these parameters could have different values. Our goal was to find those values for cerium pyrochlore and determine whether they describe a quantum spin liquid.
“It would be like a ballistics expert who is using Newton’s second law to calculate a bullet’s trajectory,” he said. “Newton’s law is known, but it only has predictive power if you supply the initial conditions like the bullet’s mass and initial velocity. Those initial conditions are analogous to these parameters. We had to reverse engineer, or sleuth out, ‘What are those initial conditions inside this cerium material?’ and, ‘Does that match the prediction of this quantum spin liquid?’”
To build a convincing case, the researchers tested the model against thermodynamic, neutron-scattering and magnetization results from previously published experimental studies of cerium zirconium pyrochlore.
“If you just have one piece of evidence, you might inadvertently find several models that still fit the description,” Nevidomskyy said. “We actually matched not one, but three different pieces of evidence. So, a single candidate had to match all three experiments.”
Some studies have implicated the same type of quantum magnetic fluctuations that arise in quantum spin liquids as a possible cause for unconventional superconductivity. But Nevidomskyy said the computational findings are primarily of fundamental interest to physicists.
“This satisfies our innate desire, as physicists, to find out how nature works,” he said. “There’s no application I know of that might benefit. It’s not immediately tied to quantum computing, although ideas exist for using fractionalized excitations as a platform for logical qubits.”
He said one particularly interesting point for physicists is the deep connection between quantum spin liquids and the experimental realization of magnetic monopoles, theoretical particles whose potential existence is still debated by cosmologists and high-energy physicists.
“When people talk about fractionalization, what they mean is the system behaves as if a physical particle, like an electron, splits into two halves that kind of wander around and then recombine somewhere later,” Nevidomskyy said. “And in pyrochlore magnets such as the one we studied, these wandering objects moreover behave like quantum magnetic monopoles.”
Magnetic monopoles can be visualized as isolated magnetic poles like either the upward or downward facing pole of a single electron.
“Of course, in classical physics one can never isolate just one end of a bar magnet,” he said. “The north and south monopoles always come in pairs. But in quantum physics, magnetic monopoles can hypothetically exist, and quantum theorists constructed these almost 100 years ago to explore fundamental questions about quantum mechanics.
“As far as we know, magnetic monopoles don’t exist in a raw form in our universe,” Nevidomskyy said. “But it turns out that a fancy version of monopoles does exist in these cerium pyrochlore quantum spin liquids. A single spin flip creates two fractionalized quasiparticles called spinons that behave like monopoles and wander around the crystal lattice.”
The study also found evidence that monopole-like spinons were created in an unusual way in cerium zirconium pyrochlore. Due to the tetrahedral arrangement of magnetic atoms in the pyrochlore, the study suggests they develop octupolar magnetic moments — spin-like magnetic quasiparticles with eight poles — at low temperatures. The research showed spinons in the material were produced from both these octupolar sources and more conventional, dipolar spin moments.
“Our modeling established the exact proportions of interactions of these two components with one another,” Nevidomskyy said. “It opens a new chapter in the theoretical understanding of not only the cerium pyrochlore materials but of octupolar quantum spin liquids in general.”
Acceleration-Induced Effects in Stimulated Light-Matter Interactions
by Barbara Šoda, Vivishek Sudhir, Achim Kempf in Physical Review Letters
A major hurdle for work at the forefront of fundamental physics is the inability to test cutting-edge theories in a laboratory setting. But a recent discovery opens the door for scientists to see ideas in action that were previously only understood in theory or represented in science fiction.
One such theory is on the Unruh effect. When astronauts in a spacecraft undergo super strong acceleration and see the light of stars stream by, then the Unruh effect is an additional warm glow on top of the streaming light.First predicted by Canadian physicist Bill Unruh, this effect is closely related to the glow from black holes predicted by Stephen Hawking. This is because black holes strongly accelerate everything towards them.
“Black holes are believed to be not entirely black,” says Barbara Šoda, a PhD student in physics at the University of Waterloo. “Instead, as Stephen Hawking discovered, black holes should emit radiation. This is because, while nothing else can escape a black hole, quantum fluctuations of radiation can.”
Similar to how the Hawking effect needs a black hole, the Unruh effect requires enormous accelerations to produce a significant glow. The Unruh effect was therefore thought to be so weak that it would be impossible to measure with the accelerations that can be achieved in experiments with current technology.
The research team found an innovative way to experiment on the Unruh effect through a novel use of high-intensity lasers. They discovered that shining a high-intensity laser on an accelerated particle can amplify the Unruh effect so much that it actually becomes measurable. In an unexpected twist, the team also discovered that by delicately balancing acceleration and deceleration, one should even be able to make accelerated matter transparent.
The ability to experiment on the Unruh effect as well as on the new phenomenon of acceleration-induced transparency provide a big boost for physicists, who have long been searching for ways to unify Einstein’s theory of general relativity with quantum mechanics.
“The theory of general relativity and the theory of quantum mechanics are currently still somewhat at odds, but there has to be a unifying theory that describes how things function in the universe,” says co-author Achim Kempf, a professor of applied mathematics and member of the Institute for Quantum Computing at Waterloo. “We’ve been looking for a way to unite these two big theories, and this work is helping to move us closer by opening up opportunities for testing new theories against experiments.”
The team is now setting out to conduct further laboratory experiments. They are also excited by the impacts of the research on some of the fundamental questions about physics and the nature of the universe.
“For over 40 years, experiments have been hindered by an inability to explore the interface of quantum mechanics and gravity,” says co-author Vivishek Sudhir, an assistant professor of mechanical engineering at the Massachusetts Institute of Technology and an affiliate of the Laser Interferometer Gravitational-Wave Observatory (LIGO). “We have here a viable option to explore this interface in a laboratory setting. If we can figure out some of these big questions, it could change everything.”
All topological bands of all nonmagnetic stoichiometric materials
by Maia G. Vergniory, Benjamin J. Wieder, Luis Elcoro, Stuart S. P. Parkin, Claudia Felser, B. Andrei Bernevig, Nicolas Regnault in Science
What will it take to make our electronics smarter, faster, and more resilient? One idea is to build them from materials that are topological.
Topology stems from a branch of mathematics that studies shapes that can be manipulated or deformed without losing certain core properties. A donut is a common example: If it were made of rubber, a donut could be twisted and squeezed into a completely new shape, such as a coffee mug, while retaining a key trait — namely, its center hole, which takes the form of the cup’s handle. The hole, in this case, is a topological trait, robust against certain deformations.
In recent years, scientists have applied concepts of topology to the discovery of materials with similarly robust electronic properties. In 2007, researchers predicted the first electronic topological insulators — materials in which electrons that behave in ways that are “topologically protected,” or persistent in the face of certain disruptions. Since then, scientists have searched for more topological materials with the aim of building better, more robust electronic devices. Until recently, only a handful of such materials were identified, and were therefore assumed to be a rarity.
Now researchers at MIT and elsewhere have discovered that, in fact, topological materials are everywhere, if you know how to look for them. In a paper, the team, led by Nicolas Regnault of Princeton University and the École Normale Supérieure Paris, reports harnessing the power of multiple supercomputers to map the electronic structure of more than 96,000 natural and synthetic crystalline materials. They applied sophisticated filters to determine whether and what kind of topological traits exist in each structure. Overall, they found that 90 percent of all known crystalline structures contain at least one topological property, and more than 50 percent of all naturally occurring materials exhibit some sort of topological behavior.
“We found there’s a ubiquity — topology is everywhere,” says Benjamin Wieder, the study’s co-lead, and a postdoc in MIT’s Department of Physics.
The team has compiled the newly identified materials into a new, freely accessible Topological Materials Database resembling a periodic table of topology. With this new library, scientists can quickly search materials of interest for any topological properties they might hold, and harness them to build ultra-low-power transistors, new magnetic memory storage, and other devices with robust electronic properties. The paper includes co-lead author Maia Vergniory of the Vergniory of the Donostia International Physics Center, Luis Elcoro of the University of Basque Country, Stuart Parkin and Claudia Felser of the Max Planck Institute, and Andrei Bernevig of Princeton University.
The new study was motivated by a desire to speed up the traditional search for topological materials.
“The way the original materials were found was through chemical intuition,” Wieder says. “That approach had a lot of early successes. But as we theoretically predicted more kinds of topological phases, it seemed intuition wasn’t getting us very far.”
Wieder and his colleagues instead utilized an efficient and systematic method to root out signs of topology, or robust electronic behavior, in all known crystalline structures, also known as inorganic solid-state materials.
For their study, the researchers looked to the Inorganic Crystal Structure Database, or ICSD, a repository into which researchers enter the atomic and chemical structures of crystalline materials that they have studied. The database includes materials found in nature, as well as those that have been synthesized and manipulated in the lab. The ICSD is currently the largest materials database in the world, containing over 193,000 crystals whose structures have been mapped and characterized.
The team downloaded the entire ICSD, and after performing some data cleaning to weed out structures with corrupted files or incomplete data, the researchers were left with just over 96,000 processable structures. For each of these structures, they performed a set of calculations based on fundamental knowledge of the relation between chemical constituents, to produce a map of the material’s electronic structure, also known as the electron band structure.
The team was able to efficiently carry out the complicated calculations for each structure using multiple supercomputers, which they then employed to perform a second set of operations, this time to screen for various known topological phases, or persistent electrical behavior in each crystal material.
“We’re looking for signatures in the electronic structure in which certain robust phenomena should occur in this material,” explains Wieder, whose previous work involved refining and expanding the screening technique, known as topological quantum chemistry.
From their high-throughput analysis, the team quickly discovered a surprisingly large number of materials that are naturally topological, without any experimental manipulation, as well as materials that can be manipulated, for instance with light or chemical doping, to exhibit some sort of robust electronic behavior. They also discovered a handful of materials that contained more than one topological state when exposed to certain conditions.
“Topological phases of matter in 3D solid-state materials have been proposed as venues for observing and manipulating exotic effects, including the interconversion of electrical current and electron spin, the tabletop simulation of exotic theories from high-energy physics, and even, under the right conditions, the storage and manipulation of quantum information,” Wieder notes.
Exponential Clustering of Bipartite Quantum Entanglement at Arbitrary Temperatures
by Tomotaka Kuwahara, Keiji Saito in Physical Review X
A theoretical study shows that long-range entanglement can indeed survive at temperatures above absolute zero, if the correct conditions are met.
Quantum computing has been earmarked as the next revolutionary step in computing. However current systems are only practically stable at temperatures close to absolute zero. A new theorem from a Japanese research collaboration provides an understanding of what types of long-range quantum entanglement survive at non-zero temperatures, revealing a fundamental aspect of macroscopic quantum phenomena and guiding the way towards further understanding of quantum systems and designing new room-temperature stable quantum devices.
When things get small, right down to the scale of one-thousandth the width of a human hair, the laws of classical physics get replaced by those of quantum physics. The quantum world is weird and wonderful, and there is much about it that scientists are yet to understand. Large-scale or “macroscopic” quantum effects play a key role in extraordinary phenomena such as superconductivity, which is a potential game-changer in future energy transport, as well for the continued development of quantum computers.
It is possible to observe and measure “quantumness” at this scale in particular systems with the help of long-range quantum entanglement. Quantum entanglement, which Albert Einstein once famously described as “spooky action at a distance,” occurs when a group of particles cannot be described independently from each other. This means that their properties are linked: if you can fully describe one particle, you will also know everything about the particles it is entangled with.
Long-range entanglement is central to quantum information theory, and its further understanding could lead to a breakthrough in quantum computing technologies. However, long-range quantum entanglement is stable at specific conditions, such as between three or more parties and at temperatures close to absolute zero (-273°C). What happens to two-party entangled systems at non-zero temperatures? To answer this question, researchers from the RIKEN Center for Advanced Intelligence Project, Tokyo, and Keio University, Yokohama, recently presented a theoretical study describing long-range entanglement at temperatures above absolute zero in bipartite systems.
“The purpose of our study was to identify a limitation on the structure of long-range entanglement at arbitrary non-zero temperatures,” explains RIKEN Hakubi Team Leader Tomotaka Kuwahara, one of the authors of the study, who performed the research while at the RIKEN Center for Advanced Intelligence Project. “We provide simple no-go theorems that show what kinds of long-range entanglement can survive at non-zero temperatures. At temperatures above absolute zero, particles in a material vibrate and move around due to thermal energy, which acts against quantum entanglement. At arbitrary non-zero temperatures, no long-range entanglement can persist between only two subsystems.”
The researchers’ findings are consistent with previous observations that long-range entanglement survives at a non-zero temperature only when more than three subsystems are involved. The results suggest this is a fundamental aspect of macroscopic quantum phenomena at room temperatures, and that quantum devices need to be engineered to have multipartite entangled states.
“This result has opened the door to a deeper understanding of quantum entanglement over large distances, so this is just the beginning.,” states Keio University’s Professor Keijo Saito, the co-author of the study. “We aim to deepen our understanding of the relationship between quantum entanglement and temperature in the future. This knowledge will spark and drive the development of future quantum devices that work at room temperatures, making them practical.”
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