Category: Physics

by Kate Blackwood, Cornell University

Picture a nanoscale dance floor full of independently moving particles. When things really start to heat up—or, in this case, cool down—particles partner off, but on opposite sides of the space, “dancing” in synch as if telepathically.

In the ultra-pure isotope helium-three (³He), this dance starts at a very specific, very low temperature, when it converts into the superfluid phase (where its superfluid component has no viscosity and thus flows without friction) through a mechanism called pairing. Pairs of particles form over huge atomic distances in three dimensions.

“It’s something like a dance in space,” said Jeevak Parpia, professor of physics in the College of Arts and Sciences (A&S). “The effect of this pairing, called a ‘fluctuation,’ is to scatter other non-paired partners and disrupt the overall transport of momentum.”

These superfluid fluctuation effects were predicted almost 50 years ago, but no one had the instrumentation to see it; now, enabled by a custom thermometer that is accurate at super-low temperatures and sensitive enough to capture this subtle effect, Cornell researchers have observed the phenomenon in experiments—possibly gaining new insight for quantum computing and the physics of the early universe.

“Observation of Suppressed Viscosity in the Normal State of ³He due to Superfluid Fluctuations” was published Sept. 20 in Nature Communications. Parpia led the study, and research was primarily conducted by postdoctoral researcher Yefan Tian and doctoral student Rakin Baten. Eric Smith, Ph.D. ’72, was an essential team member and Erich Mueller, professor of physics (A&S), provided theoretical support.

To observe the minute changes of superfluid fluctuations at ultralow temperatures, the researchers used a tiny thermometer, 1.25 mm in diameter and 1.25 mm long, a homemade device they started to build during the COVID pandemic and are still refining.

“The low noise is essential,” Parpia said. “After all, we are looking for a small effect and if the temperature is ‘blurry’ or noisy then this small upturn [the sign of a superfluid fluctuation] is going to buried in the noise.”

As the only “quantum fluid,” helium is unique, Parpia said. All other elements, when cooled down, undergo phase transitions from liquid to solid; but while helium does change from a gas to the liquid state, the atoms don’t solidify unless a great deal of pressure is applied. This is because each atom’s mass is so small that the motion of the atoms is bigger than the separation of atoms. Even near absolute zero, helium atom components called quasiparticles (also known as excitations) are moving quickly and colliding with each other.

“Fluctuations are signaling that a change is coming, just like a wind gust can signal a storm,” Parpia said. “They occur just above the actual superfluid transition and disrupt the transfer of information. That’s because the quasiparticles pair up and have a very short lifetime, less than a millionth of a second even a few micro-degrees above the superfluid transition.”

A similar pairing mechanism also occurs in superconductors, which conduct charge (electricity) without resistance.

“Once a current is established in a superconductor, for example in a loop, it would flow forever,” Parpia said. “Superfluids are superconductors on steroids. The atoms, not just the electrons, flow without resistance. But unlike electronic superconductors where disorder is almost ubiquitous—it’s very hard to make a superconductor without defects, or ‘dirt,’ if you will—helium-three is ultrapure. So it is the best model system to study some exotic properties.”

Excitations in helium-three may be useful as a platform for quantum computation, Mueller said. A strategy known as “topological quantum computation” relies on the fact that pairs of excitations in certain exotic superconductors, such as those seen in helium-three, are theorized to act as quantum bits (qubits).

“While it has been challenging to find (or create) superconducting devices with the right types of excitations, there are predictions that helium-three could work. The first step is showing that helium-three has these ‘topological’ excitations,” he said. “Characterizing the superfluid fluctuations is an important step towards investigating this possibility.”

There are also suggestions that phase transitions in helium-three are ideal model systems that emulate the physics of the early universe, Parpia said, when energy first started to be differentiated into different forms and different fundamental forces emerged.

“Because the physics of helium is one of extreme purity and ultra-low temperatures, paradoxically that’s what makes it a good model for this ultra-high energy inflationary ‘epoch’ in the early universe,” he said. “How neat would it be if we were able to understand some aspect of the early universe in the lab!”

More information: Rakin N. Baten et al, Observation of suppressed viscosity in the normal state of ³He due to superfluid fluctuations, Nature Communications (2023). DOI: 10.1038/s41467-023-41422-3

Journal information: Nature Communications 

Provided by Cornell University 

by Karyn Hede, Pacific Northwest National Laboratory

Imagine trying to tune a radio to a single station but instead encountering static noise and interfering signals from your own equipment. That is the challenge facing research teams searching for evidence of extremely rare events that could help understand the origin and nature of matter in the universe. It turns out that when you are trying to tune into some of the universe’s weakest signals, it helps to make your instruments very quiet.

Around the world more than a dozen teams are listening for the pops and electronic sizzle that might mean they have finally tuned into the right channel. These scientists and engineers have gone to extraordinary lengths to shield their experiments from false signals created by cosmic radiation.

Most such experiments are found in very inaccessible places—such as a mile underground in a nickel mine in Sudbury, Ontario, Canada, or in an abandoned gold mine in Lead, South Dakota—to shield them from naturally radioactive elements on Earth. However, one such source of fake signals comes from natural radioactivity in the very electronics that are designed to record potential signals.

Radioactive contaminants, even at concentrations as tiny as one part-per-billion, can mimic the elusive signals that scientists are seeking. Now, a research team at the Department of Energy’s Pacific Northwest National Laboratory, working with Q-Flex Inc., a small business partner in California, has produced electronic cables with ultra-pure materials.

These cables are specially designed and manufactured to have such extremely low levels of the radioactive contaminants that they will not interfere with highly sensitive neutrino and dark matter experiments.

The scientists report in the journal EPJ Techniques and Instrumentation that the cables have applications not only in physics experiments, but they may also be useful to reduce the effect of ionizing radiation interfering with future quantum computers.

“We have pioneered a technique to produce electronic cabling that is a hundred times lower than current commercially available options,” said PNNL principal investigator Richard Saldanha. “This manufacturing approach and product has broad application across any field that is sensitive to the presence of even very low levels of radioactive contaminants.”

An ultra-quiet choreographed ballet

Small amounts of naturally occurring radioactive elements are found everywhere: in rocks, dirt and dust floating in the air. The amount of radiation that they emit is so low that they do not pose any health hazards, but it’s still enough to cause problems for next-generation neutrino and dark matter detectors.

“We typically need to get a million or sometimes a billion times cleaner than the contamination levels you would find in just a little speck of dirt or dust,” said PNNL chemist Isaac Arnquist, who co-authored the research article and led the measurement team.

For these experiments, Saldanha, Arnquist, and colleagues Maria Laura di Vacri, Nicole Rocco and Tyler Schlieder evaluated the amount of uranium, thorium and potassium at each step of the dozen or so processing steps that ultimately produce a detector cable. The team then developed special cleaning and fabrication techniques to reduce the contamination down to insignificant levels. Working in an ultraclean, dust and contaminant-free laboratory, the researchers meticulously plan out their every move.

“I almost think of us as performance athletes because everything, every movement we make, is extremely thought out. It’s almost like we’re choreographed dancers,” said Arnquist. “When we handle a detector sample material, there’s no wasted extraneous motion or interaction with the sample because that interaction could impart some contamination that limits how well we can measure the materials.”

After several years of work and hundreds of measurements, the resulting cables are now so free of contaminants that they will not impact the operation of next-generation dark matter and neutrino experiments such as DAMIC-M, OSCURA, and nEXO. The research team points out in their study that low-radioactivity cables can increase the sensitivity of the experiments and even allow more flexibility in detector design.

Getting closer to the ‘a-ha’ moment

So, exactly what are the researchers looking for in these experiments? In the case of both dark matter and neutrinoless double beta decay, they are hoping to record extremely rare events that could solve two key mysteries of the universe. Both of these mysteries pose fundamental questions about why the universe looks the way it does.

The galaxies that fill our universe would not have formed without the existence of dark matter. Dark matter makes up around 85% of the matter of the universe, and yet, we have never observed dark matter directly, only its gravitational imprint on the universe. Perhaps more intriguing, the question of why there is matter in the universe at all may hinge on a unique property of the smallest known particles of matter—the neutrino.

Unlike all other fundamental particles, neutrinos could possibly interact as both matter and anti-matter. If true, this could result in an extremely rare nuclear decay called neutrinoless double beta decay. Scientists are building large experiments consisting of many tons of sensitive material with the hope of finding the first evidence of neutrinoless double beta decay within the next decade.

“Every step we take to eliminate interfering radioactivity gets us closer to finding evidence for dark matter or neutrinoless double beta decay,” said Saldanha.

“These flexible cables have many conductive pathways, which are needed to read out complicated signals,” added Arnquist. “When, say, dark matter interacts with the detector or a neutrinoless double beta decay occurs, it’s going to create an event that needs to be accurately recorded—read out—to make the discovery. We need to put a complex electronic part that is extremely clean of radioactive elements into the heart of the detector.”

“Next generation searches for neutrinoless double beta decay will be among the lowest radioactivity experiments ever constructed,” said David Moore, a Yale University physicist and PNNL collaborator.

“These detectors use such pure materials that even a small amount of normal cabling materials would overwhelm the radioactivity from the entire rest of the detector, so developing ultra-low-background cables to read out such detectors is a major challenge. This recent work from PNNL and Q-Flex is key to enabling these detectors and will reduce the cabling background to a small fraction of what was possible with previous technologies.”

Planning is already underway to upgrade the highly sensitive DAMIC-M dark matter experiment and the new ultra-pure cables are one of the key improvements scheduled for installation in the detector.

“One component that we can’t avoid in our detector are the cables that transmit the signals, which must be of very low radioactivity,” said Alvaro E Chavarria, a physicist at the University of Washington and a collaborator on the DAMIC-M project.

“Prior to this recent PNNL development, the best solution was microcoax cables, which carry too few signals and would have required a significant redesign of our detector. This development is super exciting, since it enables the use of the industry-standard flex-circuit technology for low-background applications.”

Recent research findings by PNNL scientists and other collaborators indicate that the performance of some quantum computing devices can be affected by the presence of trace radioactive contaminants. While radioactivity is not currently what limits the capabilities of existing quantum computers, it is possible that quantum devices of the future might need low-radioactivity cables to enhance their performance.

“We see the potential for these cables to find applications in a wide range of sensitive radiation detectors and perhaps other applications such as quantum computing,” Saldanha said.

More information: Isaac J. Arnquist et al, Ultra-low radioactivity flexible printed cables, EPJ Techniques and Instrumentation (2023). DOI: 10.1140/epjti/s40485-023-00104-6

Provided by Pacific Northwest National Laboratory 

by Matt Shipman, North Carolina State University

Researchers have demonstrated a material for next generation dynamic windows, which would allow building occupants to switch their windows between three modes: transparent, or “normal” windows; windows that block infrared light, helping to keep a building cool; and tinted windows that control glare while maintaining the view.

Dynamic windows based on electrochromism—meaning their opacity changes in response to electric stimulus—are not a new concept. But, to this point, most dynamic windows were either clear or dark.

“Our work demonstrates that there are more options available,” says Veronica Augustyn, co-corresponding author of a paper on the work and the Jake and Jennifer Hooks Distinguished Scholar in Materials Science and Engineering at North Carolina State University. “Specifically, we’ve shown that you can allow light to pass through the windows while still helping to keep buildings cooler and thus more energy efficient.”

The paper, “Dual-Band Electrochromism in Hydrous Tungsten Oxide,” is published in the journal ACS Photonics.

The key to more dynamic window materials is water.

Specifically, the researchers found that when water is bound within the crystalline structure of a tungsten oxide—forming tungsten oxide hydrate—the material exhibits a previously unknown behavior.

Tungsten oxides have long been used in dynamic windows. That’s because tungsten oxide is normally transparent. But when you apply an electrical signal, and inject lithium ions and electrons into the material, the material becomes dark and blocks light.

Researchers have now shown that you can effectively tune the wavelengths of light that are blocked when you inject lithium ions and electrons into a related material called tungsten oxide hydrate. When lithium ions and electrons are injected into the hydrate material, it first transitions into a “heat blocking” phase, allowing visible wavelengths of light to pass through, but blocking infrared light. If more lithium ions and electrons are injected, the material then transitions into a dark phase, blocking both visible and infrared wavelengths of light.

“The presence of water in the crystalline structure makes the structure less dense, so the structure is more resistant to deformation when lithium ions and electrons are injected into the material,” says Jenelle Fortunato, first author of the paper and a postdoctoral fellow at NC State.

“Our hypothesis is that, because the tungsten oxide hydrate can accommodate more lithium ions than regular tungsten oxide before deforming, you get two modes. There’s a ‘cool’ mode—when injection of lithium ions and electrons affects the optical properties, but structural change hasn’t occurred yet—which absorbs infrared light. And then, after the structural change occurs, there’s a ‘dark’ mode that blocks both visible and infrared light.”

“The discovery of dual-band (infrared and visible) light control in a single material that’s already well-known to the smart windows community may accelerate development of commercial products with enhanced features,” says Delia Milliron, co-corresponding author of the paper and the Ernest Cockrell, Sr. Chair #1 in Engineering at the University of Texas at Austin.

“More broadly speaking, the unforeseen role of structural water in producing distinctive electrochemical properties may inspire the research community beyond smart window developers, leading to innovation in energy storage and conversion materials.”

More information: Jenelle Fortunato et al, Dual-Band Electrochromism in Hydrous Tungsten Oxide, ACS Photonics (2023). DOI: 10.1021/acsphotonics.3c00921

Journal information: ACS Photonics 

Provided by North Carolina State University 

by Ingrid Fadelli , Phys.org

Quantum entanglement is a physical process through which pairs of particles become connected and remain so even when separated by vast distances. This fascinating phenomenon has been the focus of numerous research studies, due to its mysterious nature and promising real-world applications.

Ben Kain, a researcher at College of the Holy Cross, recently introduced a simulation-based model that outlines the possible connection between entangled particles and wormholes, hypothetical connections between distant regions in space-time. His model, presented in Physical Review Letters, is a concrete framework that could be used to test and study recent theories introduced by physicists Juan Maldacena and Leonard Susskind.

“In 2019 I studied something called Dirac stars,” Kain told Phys.org. “Fermions, which are described by the Dirac equation, when coupled to general relativity have star-like solutions in which the fermions can hold their configuration through their gravitational interaction. As a side note, traditional descriptions of stars, which of course are filled with fermions, do not fully account for general relativity.”

With the help of two undergraduate students at College of the Holy Cross, Kain previously wrote code that would allow him to simulate Dirac stars. A few years ago, other researchers discovered that when these Dirac systems include an electric charge, they could contain wormholes.

Wormholes are solutions to Einstein’s field equations of gravity, which can be visualized as tunnels with two ends located in distant places and/or at different points in time. Recent papers hinting that Dirac stars with electric charges have wormhole solutions assumed that the wormholes were traversable, meaning that particles could travel from one side to the other.

“I thought it would be very interesting if I could simulate this wormhole and confirm if the wormhole was traversable,” Kain said. “The Dirac system I focused on makes use of two fermions (i.e., two particles that obey the Pauli exclusion principle). My simulations require the system to be spherically symmetric, as this makes it easier to solve. To be spherically symmetric, the total angular momentum of the system must be zero. This ends up requiring the two fermions to be in a state called the ‘singlet state,’ which entangles the particles. “

A decade or so ago, physicists Maldacena and Susskind introduced the idea that entangled particles are connected by wormholes. This is a bold and radical conjecture, as it offers a gravity-related explanation (i.e., wormholes) for a quantum mechanical phenomenon (i.e., entanglement).

“Entanglement requires faster-than-light communication, although this faster-than-light communication cannot be exploited by humans to send messages to one another faster than light,” Kain explained. “Maldacena and Susskind suggested that this faster-than-light communication might occur through a wormhole. They further suggested that the wormhole must be non-traversable (i.e., humans cannot travel through it) to be consistent with humans not being able to exploit the system for sending messages faster than light.”

In his recent paper, Kain introduced a new model that could help to explore Maldacena and Susskind’s hypothesis. This model is based on the simulation of two entangled fermions connected by a wormhole.

When running this simulation, Kain found that in this scenario, black holes quickly form, covering both ends of the wormhole. These black holes ultimately make the wormhole non-traversable, meaning that nothing can pass through it and reach the other end.

“Since the model describes two entangled fermions connected by a non-traversable wormhole, it is a concrete model for studying Maldacena and Susskind’s conjecture,” Kain said. “They named their conjecture ER = EPR. ER stands for an Einstein-Rosen bridge, which was the first name for a wormhole. EPR stands for Einstein-Podolsky-Rosen, who were the first people to study entangled particles. The model I studied is thus a concrete example of ER = EPR.”

This recent paper introduces a new model to explore the possible connection between quantum entanglement and wormholes. Kain hopes that by examining his model further, researchers will be able to determine whether Maldacena and Susskind’s hypothesis is correct, while also determining how a wormhole could facilitate faster-than-light communication, which is a key requirement of entanglement.

“One idea I have for future works is to extend the simulations to allow matter to travel into one side of the wormhole, and hence into a black hole, and to travel across the wormhole,” Kain added. “I am interested in how this might affect the system.”

More information: Ben Kain, Probing the Connection between Entangled Particles and Wormholes in General Relativity, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.131.101001

Journal information: Physical Review Letters 

by Light Publishing Center, Changchun Institute of Optics, Fine Mechanics And Physics, CAS

Over the past decade, there has been remarkable advancement in state-of-the-art technologies, leading to a profound alteration in the way individuals interact and exchange information, resulting in the emergence of a “hyper-connected community.” Nonetheless, this interconnected environment has led to a gradual decline in the security of information, encompassing vital aspects like privacy and secrecy, which people highly value.

Consequently, it becomes imperative to prioritize the safeguarding of sensitive data through an encryption framework. As a response, optical encryption techniques have seen extensive progress and deployment, offering solutions for preserving the confidentiality of information.

In a paper published in Light: Science & Applications, a team of scientists, led by Professor Cheolmin Park from Nanopolymers Laboratory, Department of Materials Science and Engineering, Yonsei University, Korea, Professor Seung Soon Jang from Computational NanoBio Technology Lab, School of Materials Science and Engineering, Georgia Institute of Technology, US, and their research team have developed three-dimensional (3-D) volumetric optical encryption with moldable and printable dual-light-emitting fluorescent–phosphorescent materials.

They presented fluorescent perovskite nanocrystals embedded in metal–organic frameworks (MOFs) designed for phosphorescent host–guest interactions. When guest molecules for room temperature phosphorescence were securely fixed within each periodic cavity of a molecular framework, the host–guest pairs were rigidified with fixed molecular separation, owing to the crystalline structure of the MOFs, resulting in stable room temperature phosphorescence without additional processes.

As such, their dual-light-emitting materials efficiently conceal actual information with transient phosphorescence behind fake information written with fluorescence under UV exposure. Moreover, a 3-D cubic skeleton is developed with the dual-light-emitting powder dispersed in 3-D-printable polymer filaments for volumetric dual-pattern encryption.

This method and technique will open new avenues for developing room-temperature multi-light-emitting materials and a strategy for multidimensional information encryption with enhanced capacity and security.

Summarizing their research, the scientists say, “We design a fluorescent-phosphorescent dual-light-emitting materials for optical information encryption. Utilizing dual-light-emitting materials that exhibit distinct and individual fluorescence and phosphorescence emissions, 2-D pochoir pattern encryption effectively conceals real information through transient phosphorescence, while fake information is presented through fluorescence.”

“Furthermore, a 3-D cubic skeleton is developed with the dual-light-emitting powder dispersed in 3-D-printable polymer filaments for volumetric dual-pattern encryption. Achieving viewing-angle-dependent encryption was successfully demonstrated using cube-type skeletons constructed from polymer composite filaments, employing both a standard smartphone camera and software.”

“Since our polymer composites with dual-light-emitting materials is readily moldable with conventional 3-D printers, a high-security and cost-effective optical encoding/decoding system can be achieved,” they added.

“The presented technique can be used to develop solid-state multiwavelength light-emitting materials suitable for a variety of wearable, patchable, and stretchable sensors and displays. This breakthrough could open a new venue for a high-security information encryption technique suitable for personal biodata protection technologies, which are rapidly progressing owing to human–machine interface research,” the scientists added.

More information: Jin Woo Oh et al, Dual-light emitting 3D encryption with printable fluorescent-phosphorescent metal-organic frameworks, Light: Science & Applications (2023). DOI: 10.1038/s41377-023-01274-4

Journal information: Light: Science & Applications 

Provided by Light Publishing Center, Changchun Institute of Optics, Fine Mechanics And Physics, CAS

by Intelligent Computing

A quantum machine can drastically speed up certain kinds of computation, but only if two or more quantum bits in the machine are entangled—that is, capable of displaying related behavior despite being separated.

Seeking a way for users of cloud-based quantum computing services to detect qubit entanglement, Jiheon Seong and Joonwoo Bae of the Korea Advanced Institute of science and Technology developed and tested an entanglement witness circuit. It works to certify entanglement even when the cloud-based service allows only limited control of the machine. Their research was published in Intelligent Computing.

Researchers want to build circuits that generate entanglement among qubits. However, until they use a circuit, they do not know whether it is an entanglement-generating circuit or not. A costly procedure called quantum tomography can be performed, or the researcher can use an entanglement witness. The entanglement witness is a mathematical function relating two specific qubits and their states. The value of its output signals whether the states of the qubits are entangled or separable.

Unfortunately, it is not always possible to use an entanglement witness without direct access to the quantum machine. In a lab setting, and in the IBMQ cloud-based quantum computing service, a researcher can choose which of the machine’s hardware qubits to allocate to a circuit. In the IonQ cloud-based quantum computing service, the user does not have this level of control, and thus cannot be sure of getting the appropriate values for computing the output of the entanglement witness function.

A researcher’s only input to the IBMQ and IonQ cloud services is a quantum circuit. To address this limitation, Seong and Bae designed special entanglement witness circuits that use the entanglement witness strategy for certifying the presence of entangled qubits. Researchers can use these circuits to detect entanglement using only the measurement statistics output by the service.

They do not need to be able to control qubit allocation. Entanglement witness circuits enable researchers using cloud-based computing to satisfy an “essential requirement” in the process of seeking quantum advantages.

Moreover, the new entanglement witness circuits are built on a recently developed framework called EW 2.0, which is twice as efficient at detecting entanglement.

Seong and Bae describe entanglement detection for two- and three-qubit entanglement-generating circuits, outline two schemes for constructing entanglement witness circuits for entanglement-generating circuits and share results of experiments using the IBMQ and IonQ cloud-based quantum computing services.

More information: Jiheon Seong et al, Detecting Entanglement-Generating Circuits in Cloud-Based Quantum Computing, Intelligent Computing (2023). DOI: 10.34133/icomputing.0051

Provided by Intelligent Computing

by Ecole Polytechnique Federale de Lausanne

Glass, despite its apparent transparency and rigidity, is a complex and intriguing material. When a liquid is cooled to form a glass, its dynamics slows down significantly, resulting in its unique properties.

This process, known as “glass transition,” has puzzled scientists for decades. But one of its intriguing aspects is the emergence of “dynamical heterogeneities,” where the dynamics become increasingly correlated and intermittent as the liquid cools down and approaches the glass transition temperature.

In a new study, researchers propose a new theoretical framework to explain these dynamical heterogeneities in glass-forming liquids. The idea is that relaxation in these liquids occurs through local rearrangements, which influence each other via elastic interactions. By investigating the interplay between local rearrangements, elastic interactions, and thermal fluctuations, the researchers have formulated a comprehensive theory for the collective dynamics of these complex systems.

The study is a collaboration between Professor Matthieu Wyart at EPFL and his colleagues at Max Planck Institute in Dresden, the ENS, the Université Grenoble Alpes, and the Center for Systems Biology Dresden. It is now published in Physical Review X.

The team developed a “scaling theory” that explains the growth of the dynamical correlation length observed in glass-forming liquids. This correlation length is linked to “thermal avalanches,” which are rare events induced by thermal fluctuations, which then trigger a subsequent burst of faster dynamics.

The study’s theoretical framework also provides insights into the Stoke-Einstein breakdown, a phenomenon where the viscosity of the liquid becomes uncoupled from the diffusion of its particles.

To validate their theoretical predictions, the researchers conducted extensive numerical simulations in various conditions. These simulations supported the accuracy of their scaling theory and its ability to describe the observed dynamics in glass-forming liquids.

The study not only deepens our understanding of glass dynamics but also suggests a new handle to tackle the properties of some other complex systems where the dynamics is intermittent and jerky- features known to occur in a range of situations, from the brain’s activity or the sliding between frictional objects.

“Our work connects the growth of the dynamical correlation length in liquids to avalanche-type relaxations, well studied, for example, in the context of disordered magnets, granular materials, and earthquakes,” says Matthieu Wyart. “As such, this approach builds unexpected bridges between other fields. Our description of how avalanches are affected by exogeneous fluctuations, including thermal ones, may thus be of more general interest.”

More information: Ali Tahaei et al, Scaling Description of Dynamical Heterogeneity and Avalanches of Relaxation in Glass-Forming Liquids, Physical Review X (2023). DOI: 10.1103/PhysRevX.13.031034

Journal information: Physical Review X 

Provided by Ecole Polytechnique Federale de Lausanne 

by University of Science and Technology of China

A research team led by Prof. Wang Qun from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS) has made significant progress in the theoretical study of vector meson spin physics, specifically regarding the intriguing behavior of ϕ mesons generated during collisions between gold nuclei.

Their results, published in Physical Review Letters, titled “Spin Alignment of Vector Mesons in Heavy-Ion Collisions,” represent a significant milestone that challenges conventional theoretical models.

Vector fields are an effective representation of strong interactions between exotic quarks. In the hadronization phase of relativistic heavy-ion collisions, where chiral symmetry is spontaneously broken, the strongly interacting matter can be described by quarks and by the SU(3) pseudo-Goldstone boson field surrounding the quarks.

The vector field is determined by the gradient degree of the Goldstone boson field, where the vector field coupled to strange quarks and antistrange quarks is called the ϕ-vector field.

In 2019, the research group led by Wang suggested that the influence of the ambient vector field felt by the strange quarks and anti-strange quarks and leads to a significant deviation of the spin alignment of the ϕ meson by 1/3.

In the latest work, researchers derived the relativistic spin Boltzmann equation for vector mesons from the Kadanoff-Baym equation, thus establishing a link between the spin alignment of ϕ mesons and the spin polarization of their component exotic quarks, and anti-exotic quarks, during hadronization.

In this research work, they selected the transverse rise and longitudinal fall of the vector field as two parameters of the model, taking into account the asymmetry of the quark gluon plasma in the transverse direction (perpendicular to the direction of the beam) and the longitudinal direction (along the direction of the beam).

The values of the two parameters for different collision energies are determined by the spin alignment in the normal direction of the reaction surface as well as in the direction of the collision parameters, which can well explain the dependence of the spin alignment on the transverse momentum of the ϕ meson.

This study could further promote the development of high-energy nuclear spin physics and offer a new frontier direction in heavy-ion collision physics.

More information: Xin-Li Sheng et al, Spin Alignment of Vector Mesons in Heavy-Ion Collisions, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.131.042304

Journal information: Physical Review Letters 

by University of Science and Technology of China

Researchers from the University of Science and Technology of China(USTC) of the Chinese Academy of Sciences (CAS) have developed an ultra-cold atom quantum simulator to study the relationship between the non-equilibrium thermalization process and quantum criticality in lattice gauge field theories. The research was led by Pan Jianwei and Yuan Zhensheng, in collaboration with Zhai Hui from Tsinghua University and Yao Zhiyuan from Lanzhou University.

Their findings reveal that multi-body systems possessing gauge symmetry tend to thermalize to an equilibrium state more easily when situated in a quantum phase transition critical region. The results were published in Physical Review Letters.

Gauge theory and statistical mechanics are two foundational theories of physics. From the Maxwell’s equations of classical electromagnetism to quantum electrodynamics and the Standard Model, which describe the interactions of fundamental particles, all adhere to specific gauge symmetries. On the other hand, statistical mechanics connects the microscopic states of large ensembles of particles (such as atoms and molecules) to their macroscopic statistical behaviors, based on the principle of maximum entropy proposed by Boltzmann and others. It elucidates, for instance, how the energy distribution of microscopic particles affects macroscopic quantities like pressure, volume, or temperature.

So, does a quantum many-body system described by gauge theory thermalize to a thermodynamic equilibrium when it’s far from equilibrium? Answering this question would advance our understanding of gauge theory, statistical mechanics, and their interrelation. While theoretical physicists have proposed various models to analyze this issue, it remains experimentally challenging to construct a physical system that is both described by gauge theory and that can be artificially manipulated and observed during its thermalization process.

The emergence of ultracold atomic quantum simulators has provided an ideal experimental platform for studying gauge theories and statistical physics concurrently. In 2020, a research team from USTC developed an ultracold atomic optical lattice quantum simulator with 71 lattice points. This marked the first experimental simulation of the quantum phase transition process in the U(1) lattice gauge theory, specifically the Schwinger Model.

In 2022, the team simulated the thermalization dynamics of transitioning from a non-equilibrium to an equilibrium state in lattice gauge field theories. For the first time experimentally, they verified the “loss” of initial state information due to quantum many-body thermalization under gauge symmetry constraints.

Collaborators on this project, Zhai Hui and Yao Zhiyuan, have pointed out through theoretical research that there exists a correlation between quantum thermalization and quantum phase transitions in such lattice gauge models. Starting from the antiferromagnetic Neel state, they predicted that the system can achieve full thermalization only in the vicinity of the quantum phase transition point.

Observing the relationship between quantum thermalization and quantum phase transitions in lattice gauge theories poses new challenges to previous experimental capabilities: the challenge lies in how to control and detect many-body quantum states in situ with single lattice point precision and distinguishable atomic numbers.

On the foundation of their ultracold atomic quantum simulator, the team has combined techniques including quantum gas microscopy, spin-dependent superlattices, and programmable optical potentials. This amalgamation has paved the way for the development of atomic operations and detection techniques with single-site precision and distinguishable particle numbers.

Leveraging these advancements, the researchers were able to prepare and probe multi-atomic quantum states with any atomic configuration. Moreover, they tracked the dynamical evolution of many-body quantum states under the constraints of gauge symmetry.

In their study, the team experimentally prepared initial states with specific atomic configurations. They utilized the method of adiabatic evolution to investigate the quantum phase transition process under gauge symmetry constraints. For the first time in experimental conditions, they accurately pinpointed the phase transition point through finite-size scaling theory.

In addition, they explored the annealing dynamics of the same initial configuration when far from equilibrium. Their work unveiled a pattern wherein many-body systems with gauge symmetry, when near the quantum phase transition critical point, tend to thermally stabilize into an equilibrium state.

The journal Physics highlighted their achievements in an article titled “Watching a Quantum System Thermalize.”

More information: Han-Yi Wang et al, Interrelated Thermalization and Quantum Criticality in a Lattice Gauge Simulator, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.131.050401

Charles Day, Watching a Quantum System Thermalize, Physics (2023). DOI: 10.1103/Physics.16.s115

Journal information: Physical Review Letters 

Provided by University of Science and Technology of China

by Ingrid Fadelli , Phys.org

Black holes are regions in space characterized by extremely strong gravity, which prevents all matter and electromagnetic waves from escaping it. These fascinating cosmic bodies have been the focus of countless research studies, yet their intricate physical nuances are yet to be fully uncovered.

Researchers at University of California–Santa Barbara, University of Warsaw and University of Cambridge recently carried out a theoretical study focusing on a class of black holes known as extremal Kerr black holes, which are uncharged stationary black holes with a coinciding inner and outer horizon. Their paper, published in Physical Review Letters, shows that these black holes’ unique characteristics could make them ideal “amplifiers” of new, unknown physics.

“This research has its origin in a previous project started during my visit to UC Santa Barbara,” Maciej Kolanowski, one of the researchers who carried out the study, told Phys.org. “I started discussing very cold (so called, extremal) black holes with Gary Horowitz (UCSB) and Jorge Santos (at Cambridge). Soon we realized that in fact, generic extremal black holes look very different than it was previously believed.”

In their previous paper, Kolanowski, Horowitz and Santos showed that in the presence of a cosmological constant extremal black holes are affected by infinite tidal forces. This means that if living beings were to fall into the black hole, they would be crushed by gravity before they moved even remotely close to the black hole’s center. Yet the team showed that if the cosmological constant is zero, as it is assumed to be in many astrophysical scenarios, this effect vanishes.

“The spark for the current paper arose at UC Santa Barbara’s weekly Gravity Lunch,” Grant Remmen explained. “Chatting with Horowitz after a talk on his work on black hole horizon singularities, I asked whether other effects could give rise to such phenomena. My previous work on effective field theories (EFTs), particularly development of physics models with quantum corrections, gave me an idea. Talking with Horowitz, I wondered whether the higher-derivative terms in a gravitational EFT (i.e., quantum corrections to the Einstein equations) could themselves lead to singularities on the horizons of extreme black holes.”

After Remmen shared his idea with Horowitz, they started collaboration with Kolanowski and Santos, aimed at testing this idea via a series of calculations. In their calculations, the researchers considered Einstein gravity coupled to its leading quantum corrections.

“The Einstein equations are linear in the Riemann tensor, a mathematical object describing the curvature of spacetime,” Remmen explained. “In three space dimensions, the leading corrections to Einstein are terms that are cubic (third power) and quartic (fourth power) in the curvature. Because curvature is a measure of derivatives of the spacetime geometry, such terms are called ‘higher-derivative terms.’ We calculated the effect of these higher-derivative terms on rapidly spinning black holes.”

Extremal black holes rotate at a maximum possible rate corresponding to the horizon moving at the speed of light. The researchers’ calculations showed that the higher-derivative EFT corrections of extremal black holes make their horizons singular, with infinite tidal forces. This is in stark contrast with typical black holes, which have finite tidal forces that only become infinite at the center of the black hole.

“Surprisingly, EFT corrections make the singularity jump all the way from the center of the black hole out to the horizon, where you wouldn’t expect it to be,” Remmen said. “The value of the coefficient in front of a given EFT term—the ‘dial settings’ in the laws of physics—are dictated by the couplings and types of particle that are present at high energies and short distances. In this sense, EFT coefficients are sensitive to new physics.”

Kolanowski, Horowitz, Remmen and Santos also found that the strength of the divergence in tides at the horizon of extremal black holes, and the possible occurrence of tidal singularity, heavily depends on the EFT coefficients. The results of their calculations thus suggest that the spacetime geometry near the horizon of these black holes is sensitive to new physics at higher energies.

“Interestingly, this unexpected singularity is present for the values of these EFT coefficients generated by the Standard Model of particle physics,” Remmen said.

“Our results are surprising, since they imply that the low-energy description of physics can break down in a situation where you wouldn’t expect that to happen. In physics, there’s usually a sense of ‘decoupling’ between different distance scales. For example, you don’t need to know the details of water molecules to describe waves using hydrodynamics. Yet for rapidly spinning black holes, that’s precisely what happens: the low-energy EFT breaks down at the horizon.”

Overall, the calculations carried out by this team of researchers hint at the promise of extremal Kerr black holes for probing new physical phenomena. While the horizon of these black holes can be very large, it was not expected to have an infinitely large curvature (i.e., infinite tidal forces) in the EFT. Their results show that it does.

“In future work, we are interested in exploring whether the singularities can be resolved by ultraviolet physics,” Remmen added. “A pressing question is whether the sensitivity of the horizon to new physics persists all the way to the Planck scale, or whether the horizon ‘smooths out’ at the short-distance scale associated with the EFT. We are also looking for other situations in which short distance effects might show up unexpected at large distances.”

More information: Gary T. Horowitz et al, Extremal Kerr Black Holes as Amplifiers of New Physics, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.131.091402

Journal information: Physical Review Letters 

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