Category: Physics

by Tejasri Gururaj , Phys.org

A new study in Physical Review Letters (PRL) introduces the concept of pseudomagic quantum states, which appear to have high stabilizerness (or complexity) and can move us closer to achieving quantum supremacy.

Quantum supremacy or quantum advantage is the ability of quantum computers to simulate or run computations that classical computers can’t (due to their limited computational abilities).

Achieving universal quantum computation is the ability of quantum computers to be able to perform any arbitrary quantum computation, and quantum supremacy is at the heart of this.

The new PRL study explores nonstabilizer states or magic states. These are quantum states that allow quantum computations that cannot be efficiently simulated on classical computers. This complexity is what gives quantum computers their potential power.

Phys.org spoke to co-authors of the paper Andi Gu, a Ph.D. student at Harvard University, and Dr. Lorenzo Leone, a postdoctoral researcher at Freie Universität, Berlin.

“The starting point to understand our research is that quantum computation is more powerful than classical computation. In quantum computing, the term nonstabilizerness or magic refers to a measure of the non-classical resources possessed by a quantum state,” explained Gu.

Stabilizer vs. nonstabilizer quantum states

Every quantum system can be represented as a quantum state, a mathematical equation containing all the information about the system.

A stabilizer state is a type of quantum state that can be efficiently simulated (or executed) on a classical computer.

“These states—along with a restricted set of quantum operations called stabilizer operations—form a classically simulable framework. However, stabilizer states and operations alone are not sufficient for achieving universal quantum computation,” explained Dr. Leone.

To perform computations that are truly quantum and beyond classical capabilities, nonstabilizer states are required. These states can enable quantum computers to perform tasks infeasible for classical computers. However, one of the main challenges is constructing these magic states.

Nonstabilizer states are inherently challenging to construct as they require more complex quantum operations.

“In this context, nonstabilizerness is best viewed as a resource because it is essential for achieving quantum advantage. The more nonstabilizerness a quantum state possesses, the more powerful it is as a resource for quantum computation,” explained Gu.

Pseudomagic states

The researchers found a way around this challenge by introducing the concept of pseudomagic quantum states.

Pseudomagic quantum states appear to have the properties of nonstabilizer states (complexity and non-classical operations) but are computationally indistinguishable from random quantum states, at least to an observer with limited computational resources.

In simple terms, this means that pseudomagic quantum states appear like magic states but are far less complex to construct. Especially to someone with a not-so-powerful computer, pseudomagic quantum states are indistinguishable from random quantum states.

“This indistinguishability arises from the fact that efficiently distinguishing between pseudomagic states and truly magical states would require an exponential amount of computational resources, making it infeasible for any realistic observer,” said Dr. Leone.

Gu added, “Just as pseudorandom number generators produce sequences that appear random to computationally limited classical observers, pseudomagic states are engineered to appear highly nonstabilizer to computationally bounded quantum observers.”

Laying down the foundations

Over the course of six theorems, the researchers laid out the theoretical foundation for pseudomagic states as well as their implications for quantum computing applications.

They constructed the pseudomagic states in a way that the gap between their actual and apparent nonstabilizerness was tunable.

“This means that we can create states that may seem to be powerful resources for quantum computation, even though they are not as resource-intensive as they appear,” explained Dr. Leone.

The core of this framework revolved around the concept of stabilizer entropy. This is a measure of the nonstabilizerness (or complexity) of a quantum system.

What is unique about the stabilizer entropy is that, unlike other measurements of nonstabilizerness, it is computationally less draining.

Implications for quantum computing applications

The researchers focused on three areas where pseudomagic states could have implications, beginning with quantum cryptography.

According to the study, pseudomagic states introduce a new protocol for quantum cryptography based on EFI (or Efficiently preparable, statistically Far, but computationally Indistinguishable) pairs.

These pairs can improve the security of data communication and can be constructed using pseudomagic states.

The researchers also show that pseudomagic states can provide new insights into quantum chaos and scrambling, which are important for understanding the behavior of complex quantum systems and the spread of quantum information.

“By demonstrating that the apparent magic of a quantum state can differ from its actual magic, our work highlights the need to consider the limitations of realistic, computationally limited observers when studying quantum systems and their applications,” explained Gu.

Finally, they also demonstrate that pseudomagic states can be used to build more efficient fault-tolerant quantum computers using a process called magic state distillation.

Magic state distillation is essentially a purification process that improves the fidelity of the magic states, making them more suitable for use in quantum algorithms and error-correction schemes.

The researchers wish to explore the relationship between pseudomagic states and concepts in quantum information theory in the future. Additionally, they want to explore the experimental realization of pseudomagic states with existing and near-term quantum devices.

“This could lead to the development of practical applications that harness the unique properties of these states,” concluded Dr. Leone.

by Tejasri Gururaj

by Rachel Kremen, Princeton Plasma Physics Laboratory

The furious exhaust heat generated by a fusing plasma in a commercial-scale reactor may not be as damaging to the vessel’s innards as once thought, according to researchers at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL), Oak Ridge National Laboratory and the ITER Organization (ITER).

“This discovery fundamentally changes how we think about the way heat and particles travel between two critically important regions at the edge of a plasma during fusion,” said PPPL Managing Principal Research Physicist Choongseok Chang, who led the team of researchers behind the discovery. A new paper detailing their work was recently published in the journal Nuclear Fusion, following previous publications on the subject.

To achieve fusion, temperatures inside a tokamak—the doughnut-shaped device that holds the plasma—must soar higher than 150 million degrees Celsius. That’s 10 times hotter than the center of the sun. Containing something that hot is challenging, even though the plasma is largely held away from the inner surfaces using magnetic fields. Those fields keep most of the plasma confined in a central region known as the core, forming a doughnut-shaped ring.

Some particles and heat escape the confined plasma, however, and strike the material facing the plasma. New findings by PPPL researchers suggest that particles escaping the core plasma inside a tokamak collide with a larger area of the tokamak than once thought, greatly reducing the risk of damage.

Past research based on physics and experimental data from present-day tokamaks suggested exhaust heat would focus on a very narrow band along a part of the tokamak wall known as the divertor plates. Dedicated to removing exhaust heat and particles from the burning plasma, the divertor is critical to a tokamak’s performance.

“If all of this heat hits this narrow area, then this part of the divertor plate will be damaged very quickly,” said Chang, who works in the PPPL Theory Department. “It could mean frequent stretches of downtime. Even if you are just replacing this part of the machine, it’s not going to be quick.”

The problem hasn’t stopped the operation of existing tokamaks which are not as powerful as those that will be needed for a commercial-scale fusion reactor. However, for the last few decades, there has been significant concern that a commercial-scale device would create plasmas so dense and hot that the divertor plates might be damaged. One proposed plan involved adding impurities to the edge of the plasma to radiate away the energy of the escaping plasma, reducing the intensity of the heat hitting the divertor material, but Chang said this plan was still challenging.

Simulating the escape route

Chang decided to study how the particles were escaping and where the particles would land on such a device as ITER, the multinational fusion facility under assembly in France. To do so, his group created a plasma simulation using a computer code known as X-Point Included Gyrokinetic Code (XGC). This code is one of several developed and maintained by PPPL that are used for fusion plasma research.

The simulation showed how plasma particles traveled across the magnetic field surface, which was intended to be the boundary separating the confined plasma from the unconfined plasma, including the plasma in the divertor region. This magnetic field surface—generated by external magnets—is called the last confinement surface.

A couple of decades ago, Chang and his co-workers found that charged particles known as ions were crossing this barrier and hitting the divertor plates. They later discovered these escaping ions were causing the heat load to be focused on a very narrow area of the divertor plates.

A few years ago, Chang and his co-workers found that the plasma turbulence can allow negatively charged particles called electrons to cross the last confinement surface and widen the heat load by 10 times on the divertor plates in ITER. However, the simulation still assumed the last confinement surface was undisturbed by the plasma turbulence.

“In the new paper, we show that the last confinement surface is strongly disturbed by the plasma turbulence during fusion, even when there are no disturbances caused by external coils or abrupt plasma instabilities,” Chang said. “A good last confinement surface does not exist due to the crazy, turbulent magnetic surface disturbance called homoclinic tangles.”

In fact, Chang said the simulation showed that electrons connect the edge of the main plasma to the divertor plasmas. The path of the electrons as they follow the path of these homoclinic tangles widens the heat strike zone 30% more than the previous width estimate based on turbulence alone.

He explained, “This means it is even less likely that the divertor surface will be damaged by the exhaust heat when combined with the radiative cooling of the electrons by impurity injection in the divertor plasma. The research also shows that the turbulent homoclinic tangles can reduce the likelihood of abrupt instabilities at the edge of the plasma, as they weaken their driving force.”

“The last confinement surface in a tokamak should not be trusted,” Chang said. “But ironically, it may raise fusion performance by lowering the chance for divertor surface damage in steady-state operation and eliminating the transient burst of plasma energy to divertor surface from the abrupt edge plasma instabilities, which are two among the most performance-limiting concerns in future commercial tokamak reactors.”

by Rachel Kremen

by Howard Hughes Medical Institute

A team led by researchers at HHMI’s Janelia Research Campus has adapted a class of techniques employed in astronomy to unblur images of far-away galaxies for use in the life sciences, providing biologists with a faster and cheaper way to get clearer and sharper microscopy images. The findings are published in the journal Optica.

Astronomers figured out long ago how to make the images their telescopes capture of far-away galaxies clearer and sharper. By using techniques that measure how light is distorted by the atmosphere, they can apply corrections to cancel out aberrations.

Microscopists have been adapting these methods to generate clearer images of thick biological samples, which also bend light and create distortions. But these techniques—a class of methods called adaptive optics—are complex, expensive, and slow, making them out of reach for many labs.

Now, in hopes of making adaptive optics more widely available to biologists, a team led by researchers at HHMI’s Janelia Research Campus has turned their attention to a class of techniques called phase diversity that’s been widely used in astronomy but is new to the life sciences.

These phase diversity methods add additional images with known aberrations to a blurry image with an unknown aberration, providing enough additional information to unblur the original image. Unlike many other adaptive optics techniques, phase diversity doesn’t require any major changes to an imaging system, making it a potentially attractive route for microscopy.

To implement the new method, the team first adapted the astronomy algorithm for use in microscopy and validated it with simulations. Next, they built a microscope with a deformable mirror, whose reflective surface can be changed, and two additional lenses—minor modifications to an existing microscope that create the known aberration. They also improved the software used to carry out the phase diversity correction.

As a test of their new method, the team demonstrated that they could calibrate the microscope’s deformable mirror 100 times faster than with competing methods. Next, they showed that the new method could sense and correct randomly generated aberrations, providing clearer images of fluorescent beads and fixed cells.

The next step is to test the method on real-world samples, including living cells and tissues, and extend its use to more complex microscopes. The team also hopes to make the method more automated and easier to use. They hope the new method, which is faster and cheaper to implement than current techniques, could one day make adaptive optics accessible to more labs, helping biologists see more clearly when peering deep inside tissues.

More information: Courtney Johnson et al, Phase-diversity-based wavefront sensing for fluorescence microscopy, Optica (2024). DOI: 10.1364/OPTICA.518559

Provided by Howard Hughes Medical Institute 

Researchers have used inkjet printing to create a compact multispectral version of a light field camera. The camera, which fits in the palm of the hand, could be useful for many applications including autonomous driving, classification of recycled materials and remote sensing.

3D spectral information can be useful for classifying objects and materials; however, capturing 3D spatial and spectral information from a scene typically requires multiple devices or time-intensive scanning processes. The new light field camera solves this challenge by simultaneously acquiring 3D information and spectral data in a single snapshot.

“To our knowledge, this is the most advanced and integrated version of a multispectral light field camera,” said research team leader Uli Lemmer from the Karlsruhe Institute of Technology in Germany. “We combined it with new AI methods for reconstructing the depth and spectral properties of the scene to create an advanced sensor system for acquiring 3D information.”

In the journal Optics Express, the researchers report that the new camera and image reconstruction methods can be used to distinguish objects in a scene based on their spectral characteristics. Using inkjet printing to make the camera’s key optical components allows it to be easily customized or manufactured in large volumes.

“Reconstructed 3D data from camera images are finding widespread use in virtual and augmented reality, autonomous cars, robotics, smart home devices, remote sensing and other applications,” said Michael Heizmann, a member of the research team.

“This new technology could, for example, allow robots to better interact with humans or improve the accuracy of classifying and separating materials in recycling. It could also be potentially used to classify healthy and diseased tissues.”

Adding color with inkjet printing

Light field cameras, also called plenoptic cameras, are specialized imaging devices that capture the direction and intensity of light rays. After image acquisition, computational processing is used to reconstruct 3D image information from the acquired data. These cameras typically use microlens arrays that are aligned with the pixels of a high-resolution camera chip.

To create a multispectral light field camera, the researchers used inkjet printing to deposit a single droplet of material to form each individual lens on one side of ultrathin microscope slides and then printed fully aligned color filter arrays on the opposite side of the microscope slides. The resulting optical component was integrated directly onto a CMOS camera chip. The inkjet printing method allowed precise alignment between the optical components, significantly reducing the manufacturing complexity and enhancing efficiency.

Because this setup produces spectral and depth information that are interwoven in the camera image, the researchers developed methods to separate each component. They found that an approach based on deep learning worked best for extracting the desired information directly from the acquired measurements.

Spectral-based object detection

“Tackling the challenge of creating a multispectral light field camera was only possible by combining recent advances from manufacturing, system design and AI-based image reconstruction,” said Qiaoshuang Zhang, first author of the paper. “This work pushes the boundaries of inkjet printing—a versatile method with high precision and industrial scalability—for manufacturing photonic components.”

The researchers tested the camera by recording a test scene that contained multicolor 3D objects at different distances. The image reconstruction algorithm was trained and tested on many synthetic and real multispectral images. The results demonstrate that the prototype camera can simultaneously acquire 3D spatial and spectral information and that different objects can be imaged and distinguished by their different spectral composition and depth information within one single snapshot.

Now that they have completed this first proof-of-concept, the researchers are exploring various applications where a light field camera with an ability to acquire multispectral information could be useful.

More information: Qiaoshuang Zhang et al, Compact multispectral light field camera based on an inkjet-printed microlens array and color filter array, Optics Express (2024). DOI: 10.1364/OE.521646 

Qiaoshuang Zhang

Waves pick up information from their environment through which they propagate. A theory of information carried by waves has now been developed at TU Wien—with astonishing results that can be utilized for technical applications.

Ultrasound is used to analyze the body, radar systems to study airspace or seismic waves to study the interior of our planet. Many areas of research are dealing with waves that are deflected, scattered or reflected by their surroundings. As a result, these waves carry a certain amount of information about their environment, and this information must then be extracted as comprehensively and precisely as possible.

Searching for the best way to do this has been the subject of research around the world for many years. TU Wien has now succeeded in describing the information carried by a wave about its environment with mathematical precision. This has made it possible to show how waves pick up information about an object and then transport it to a measuring device.

This can now be used to generate customized waves to extract the maximum amount of information from the environment—for more precise imaging processes, for example. This theory was confirmed with microwave experiments. The results were published in the journal Nature Physics.

Where exactly is the information located?

“The basic idea is quite simple: you send a wave at an object and the part of the wave that is scattered back from the object is measured by a detector,” says Prof Stefan Rotter from the Institute of Theoretical Physics at TU Wien.

“The data can then be used to learn something about the object—for example, its precise position, speed or size.” This information about the environment that this wave carries with it is known as “Fisher information.”

However, it is often not possible to capture the entire wave. Usually, only part of the wave reaches the detector. This raises the question: Where exactly is this information actually located in the wave? Are there parts of the wave that can be safely ignored? Would a different waveform perhaps provide more information to the detector?

“To get to the bottom of these questions, we took a closer look at the mathematical properties of this Fisher information and came up with some astonishing results,” says Rotter.

“The information fulfills a so-called continuity equation—the information in the wave is preserved as it moves through space, according to laws which are very similar laws to the conservation of energy, for example.”

A comprehensible path of information

Using the newly developed formalism, the research team has now been able to calculate exactly at which point in space the wave actually carries how much information about the object. It turns out that the information about different properties of the object (such as position, speed and size) can be hidden in completely different parts of the wave.

As the theoretical calculations show, the information content of the wave depends precisely on how strongly the wave is influenced by certain properties of the object under investigation.

“For example, if we want to measure whether an object is a little further to the left or a little further to the right, then the Fisher information is carried precisely by the part of the wave that comes into contact with the right and left edges of the object,” says Jakob Hüpfl, the doctoral student who played a key role in the study.

“This information then spreads out, and the more of this information reaches the detector, the more precisely the position of the object can be read from it.”

Microwave experiments confirm the theory

In Ulrich Kuhl’s group at the University of Cote d’Azur in Nice, experiments were carried out by Felix Russo as part of his master’s thesis: A disordered environment was created in a microwave chamber using randomly positioned Teflon objects. Between these objects, a metallic rectangle was placed whose position was to be determined.

Microwaves were sent through the system and then picked up by a detector. The question now was: How well can the position of the metal rectangle be deduced from the waves caught in the detector in such a complicated physical situation and how does the information flow from the rectangle to the detector?

By precisely measuring the microwave field, it was possible to show exactly how the information about the horizontal and vertical position of the rectangle spreads: it emanates from the respective edges of the rectangle and then moves along with the wave—without any information being lost, just as predicted by the newly developed theory.

Possible applications in many areas

“This new mathematical description of Fisher information has the potential to improve the quality of a variety of imaging methods,” says Rotter. If it is possible to quantify where the desired information is located and how it propagates, then it also becomes possible, for example, to position the detector more appropriately or to calculate customized waves that transport the maximum amount of information to the detector.

“We tested our theory with microwaves, but it is equally valid for a wide variety of waves with different wavelengths,” emphasizes Rotter. “We provide simple formulas that can be used to improve microscopy methods as well as quantum sensors.”

by Vienna University of Technology

by QuTech

Researchers at QuTech have found a way to make Majorana particles in a two-dimensional plane. This was achieved by creating devices that exploit the combined material properties of superconductors and semiconductors. The inherent flexibility of this new 2D platform should allow one to perform experiments with Majoranas that were previously inaccessible. The results are published in Nature.

Quantum computers operate fundamentally differently from classical computers. While classical computers use bits as the basic unit of information, which can be either 0 or 1, quantum computers use qubits, which can exist in a state of 0, 1, or both simultaneously.

This principle of superposition, combined with new quantum algorithms could allow quantum computers to solve certain problems much more efficiently than classical computers. However, the qubits that store this quantum information are inherently more fragile than classical bits.

Majorana qubits are based on states of matter that are topologically protected. This means that small local disturbances cannot destroy the state of the qubit. This robustness to external influences makes Majorana qubits highly desirable for quantum computing, since quantum information encoded in these states would remain stable for significantly longer times.

Producing a full Majorana qubit requires several steps. The first of these is the ability to reliably engineer Majoranas and to demonstrate that they indeed possess the special properties that make them promising candidates for qubits.

Previously, researchers at QuTech—a collaboration between the TU Delft and TNO—have used a one-dimensional nanowire to demonstrate a new approach to studying Majoranas by creating a Kitaev chain. In this approach, a chain of semiconductor quantum dots are connected via superconductors to produce Majoranas.

The extension of this result to two dimensions has several important implications. First author Bas ten Haaf explains, “By implementing the Kitaev-chain in two dimensions, we show that the underlying physics is universal and platform independent.”

His colleague and co-first author Qingzheng Wang adds, “Given the long-standing challenges with reproducibility in the Majorana research, our results are really encouraging.”

The ability to create Kitaev chains in two-dimensional systems opens up several avenues for future Majorana research. Principal investigator Srijit Goswami explains, “I believe we are now in a position where we can do interesting physics with Majoranas in order to probe their fundamental properties. For example, we can increase the number of sites in the Kitaev chain and systematically study the protection of Majorana particles.

“In the longer term, the flexibility and scalability of the 2D platform should allow us to think about concrete strategies to create networks of Majoranas and integrate them with auxiliary elements needed for control and readout of a Majorana qubit.”

More information: Srijit Goswami, A two-site Kitaev chain in a two-dimensional electron gas, Nature (2024). DOI: 10.1038/s41586-024-07434-9. www.nature.com/articles/s41586-024-07434-9

Journal information: Nature 

by CERN

When you receive a present on your birthday, you might be the kind of person who tears off the wrapping paper immediately to see what’s inside the box. Or maybe you like to examine the box, guessing the contents from its shape, size, weight or the sound it makes when you shake it.

When physicists at the Large Hadron Collider (LHC) analyze their datasets in search of new physics phenomena such as new particles, they usually take one of two different approaches. They either perform a direct search for a specific new kind of particle, equivalent to tearing off the wrapping paper immediately, or use an indirect strategy based on quantum mechanics and its subtle wonders, similar to shaking the box and guessing what’s inside.

At the annual LHCP conference that took place in Boston, the CMS collaboration reported how it used the second approach to look for new physics in a rare decay of a particle called B⁰ meson.

The physics process that drives the decay of a particle into lighter ones can be influenced by new, unknown particles, which might be too heavy to be produced at the LHC. This influence could change the decay process in ways that can be measured and compared to predictions of the Standard Model of particle physics.

In the same way as shaking the box containing your birthday present could give you a clue about what’s inside, any deviation from the Standard Model predictions could give physicists a hint of new physics.

The decay of the B⁰ meson, which is made up of a bottom quark and a down quark, into a K*⁰ meson (containing a strange quark and a down quark) and two muons is particularly suited to this approach. This is because it occurs via a rare penguin transition that is highly sensitive to possible contributions from new heavy particles.

In its new study, the CMS team used all the data collected by its detector between 2016 and 2018, during the second run of the LHC, to “shake” this B⁰ decay “box.” This box offers many ways to look for new physics. One is to weigh the box, i.e. measure the rate at which the decay occurs. Another is to take two twin boxes—for example, one corresponding to the decay into two muons and the other to the decay into two electrons—and check if they weigh the same.

In their new study, the CMS researchers looked at the shape of the box, i.e. they examined how the particles produced in the decay share the energy of the parent B⁰ meson and measured at what angles they fly away from each other. They then determined a set of parameters using these energies and angles, and compared the results with two sets of predictions from the Standard Model.

For most parameters, the results are in line with these two sets of Standard Model predictions. However, for two parameters, known as P₅‘ and P₂, and for specific energies of the two muons, the results are in tension with the two available predictions. Overall, the results are in agreement with the previous results from the ATLAS, LHCb and Belle experiments, while improving upon their level of precision.

Unfortunately, there is a charming, “naughty” kind of penguin that’s crashing the birthday party: a charm quark that participates in the rare penguin transition. This complicates the Standard Model predictions and makes it difficult to draw a conclusion. To advance, researchers need better predictions, more data and improved analysis techniques.

More information: CMS Physics Analysis Summary: cms-results.web.cern.ch/cms-re … PH-21-002/index.html

Provided by CERN 

by Waseda University

Magnetic monopoles are elementary particles with isolated magnetic charges in three dimensions. In other words, they behave as isolated north or south poles of a magnet. Magnetic monopoles have attracted continuous research interest since physicist Paul Dirac’s first proposal in 1931. However, real magnetic monopoles have not yet been observed and their existence remains an open question. On the other hand, scientists have discovered quasi-particles that mathematically behave as magnetic monopoles in condensed matter systems, resulting in interesting phenomena.

Recently, researchers discovered that a material called manganese germanide (MnGe) has a unique periodic structure, formed by special magnetic configurations called hedgehogs and antihedgehogs, which is called a magnetic hedgehog lattice.

In these special configurations, the magnetic moments point radially outward (hedgehog) or inward (antihedgehog), resembling the spines of a hedgehog. These hedgehogs and antihedgehogs act like magnetic monopoles and antimonopoles, serving as sources or sinks of emergent magnetic fields.

MnGe exhibits what is known as a triple-Q hedgehog lattice. However, recent experiments have shown that the substitution of Ge with Si (MnSi_1-xGe_x) transforms the arrangement into the quadruple-Q hedgehog lattice (4Q-HL).

This new arrangement, also found in the perovskite ferrite SrFeO₃, provides a promising avenue for studying and controlling the properties of hedgehog lattices. Moreover, these magnetic monopoles can also induce electric fields through moving following Maxwell’s laws of electromagnetism. To understand the resulting new physical phenomena, it is essential to study the inherent excitations of hedgehog lattices.

In a recent study, Professor Masahito Mochizuki and Ph.D. course student Rintaro Eto, both from the Department of Applied Physics at Waseda University, theoretically studied the collective excitation modes of 4Q-HLs in MnSi_1-xGe_x and SrFeO₃. Their study was published in the journal Physical Review Letters on 31 May 2024.

“Our research clarified the unknown dynamical nature of emergent magnetic monopoles in magnetic materials for the first time. This can inspire future experiments on hedgehog-hosting materials with applications in electronic devices and for bridging particle physics and condensed-matter physics,” says Mochizuki.

Utilizing the three-dimensional Kondo-lattice model, the researchers reproduced the two distinct 4Q-HLs found in MnSi_1-xGe_x and SrFeO₃ and analyzed their dynamical properties. They discovered that the 4Q-HLs have collective excitation modes associated with the oscillation of Dirac strings.

A Dirac string is a theoretical concept in quantum mechanics which describes a string that connects a magnetic monopole and a magnetic antimonopole, in this case, a hedgehog and an antihedgehog.

The researchers found that the number of these excitation modes depends on the number and configuration of Dirac strings, offering a way to experimentally determine the spatial configuration of hedgehogs and antihedgehogs and their unique topology in real magnets such as MnSi_1-xGe_x and SrFeO₃.

This finding offers insights into the dynamics of hedgehog lattices in other magnets as well. Moreover, the finding enables us to switch on and off the excitation modes through controlling the presence or absence of the Dirac strings with external magnetic field.

Explaining the significance of their results, Eto said, “The collective spin excitation modes revealed in the study are elementary excitations that directly reflect the presence (or absence) of emergent magnetic monopoles. Thus, our findings will be a fundamental guideline for studying more detailed dynamical nature of emergent monopoles in magnetic materials in the future.

“Moreover, they might become the building blocks of novel field-switchable spintronic devices such as nano-sized power generators, light-voltage converters, and light/microwave filters based on emergent electromagnetism.”

These discoveries have the potential to open new research avenues in fundamental physics and lead to the development of new technologies involving emergent magnetic monopoles in magnets.

More information: Rintaro Eto et al, Theory of Collective Excitations in the Quadruple- Q Magnetic Hedgehog Lattices, Physical Review Letters (2024). DOI: 10.1103/PhysRevLett.132.226705. On arXiv: DOI: 10.48550/arxiv.2403.01765

Journal information: Physical Review Letters  , arXiv 

Provided by Waseda University 

by FLEET

A team of Monash researchers have uncovered for the first time the full effects of interactions between exciton-polaritons and their associated dark excitonic reservoir. The study, “Polaronic polariton quasiparticles in a dark excitonic medium,” was published in Physical Review B.

Exciton-polaritons (polaritons, for short) are hybrid mixtures of light and matter that inherit the best properties of both. They form in semiconductors sandwiched between two mirrors, through which a laser is shone.

“Polaritons capture useful properties of both matter and light,” says lead author Kenneth Choo. “Their ‘matter’ part means we can manipulate them through their interactions. Meanwhile, the ‘light’ part makes the polariton almost massless, resulting in the formation of a superfluid condensate that flows effortlessly.

“Put them together, and a new kind of ‘liquid light’ that we can guide and control is formed, potentially the basis for the next generation of computing that uses light to build circuits,” says Kenneth, who is a Ph.D. candidate at Monash.

But with every yang there is a yin, and so it is with the polariton’s dark counterpart—a dark exciton.

These dark excitons are uncoupled to light yet still able to interact with the polaritons.

“Whenever the polaritons are formed in experiments, they tend to be accompanied by a substantial reservoir of these dark excitons which form together with them. The fact that they are dark, however, means it has been difficult to see their true effect,” says corresponding author Prof Meera Parish.

A famous technique dresses up for the occasion

The Monash researchers used a framework known as Fermi liquid theory to explain the effect of the dark reservoir on the bright polaritons. The essential idea is to map the (many, many) possible interactions onto a new ‘quasiparticle’—one with different masses, energies and lifetimes to the original. This allows the many-particle system to be treated as if it were a single particle.

One such quasiparticle is the polaron, which can be thought of as a ‘zone of influence’ around the original particle.

By using this technique and including all possible two-particle interactions, they found that the bright polariton becomes ‘dressed’ by excitations of the dark medium, forming a new quasiparticle called the polaron polariton.

Physically, this shows up as a reduction in the strength of the light-matter coupling as the dark exciton density is increased, an effect which is commonly seen in experiment.

In contrast, the strength of the interactions between the dark reservoir and the polaritons are increased, especially when the spins of the reservoir and polariton are different. This effect can be so strong that an additional quasiparticle is generated—the biexciton polariton—which is associated with the bound state of two opposite-spin excitons.

This phenomenon, known as a Feshbach resonance, was first observed in ultracold atomic gases, and offers the promise of being able to tune the interaction strength to whatever value is desired. It would then be possible to create reservoir ‘hills’ and ‘valleys’ that guide the flow of the polariton condensate, and then change them around at the flick of a (laser) switch.

What’s next?

This study opens up new opportunities in the field of reservoir engineering. Already, there has been experimental progress in using this dark reservoir to trap and focus the bright polaritons, and the study shows how the reservoir can be used to fundamentally change the polaritons themselves.

In addition, this study provides a new way to look at old results—by accounting for the hitherto unknown influence of the reservoir, a more accurate determination of interaction strengths can be obtained.

More information: Kenneth Choo et al, Polaronic polariton quasiparticles in a dark excitonic medium, Physical Review B (2024). DOI: 10.1103/PhysRevB.109.195432. On arXiv: DOI: 10.48550/arxiv.2312.00985

Journal information: Physical Review B  , arXiv 

Provided by FLEET

by Tejasri Gururaj , Phys.org

A new study published in Nature Physics introduces a theory of electron-phonon coupling that is affected by the quantum geometry of the electronic wavefunctions.

The movement of electrons in a lattice and their interactions with the lattice vibrations (or phonons) play a pivotal role in phenomena like superconductivity (resistance-free conductivity).

Electron-phonon coupling (EPC) is the interaction between free electrons and phonons, which are quasiparticles representing the vibrations of a crystal lattice. EPC leads to the formation of Cooper pairs (pairs of electrons), responsible for superconductivity in certain materials.

The new study explores the realm of quantum geometry in materials and how these can contribute to the strength of EPC.

Phys.org spoke to the first author of the study, Dr. Jiabin Yu, Moore Postdoctoral Fellow at Princeton University.

Speaking of the motivation behind the study, Dr. Yu said, “My motivation is to go beyond the common wisdom and find out how the geometric and topological properties of wavefunctions affect interactions in quantum materials. In this work, we focus on EPC, one of the most important interactions in quantum materials.”

Electronic wavefunctions and EPC

A quantum state is described by a wavefunction, a mathematical equation holding all the information about the state. An electronic wavefunction is basically a way to measure the probability of where the electron is located in the lattice (arrangement of atoms in a material).

“In condensed matter physics, people have long used energies to study the behavior of materials. In the last several decades, a paradigm shift led us to understand that the geometric and topological properties of wavefunctions are crucial in understanding and classifying realistic quantum materials,” explained Dr. Yu.

In the context of EPC, the interaction between the two depends on the location of the electron within the crystal lattice. This means that the electronic wavefunction, to some extent, governs which electrons can couple with phonons and impact the conductivity properties of that material.

The researchers in this study wanted to explore the effect of quantum geometry on the EPC in materials.

Quantum geometry

A wavefunction, as mentioned before, describes the state of a quantum particle or system.

These wavefunctions are not always static, and their shape, structure, and distribution can evolve over space and time, just like how a wave in the ocean changes. But unlike waves in the ocean, quantum mechanical wavefunctions follow the laws of quantum mechanics.

Quantum geometry explores this variation of spatial and temporal characteristics of wavefunctions.

“The geometric properties of single-particle wavefunctions are called band geometry or quantum geometry,” explained Dr. Yu.

In condensed matter physics, the band structure of materials describes the energy levels available to electrons in a crystal lattice. Think of them as steps of a ladder, with the energy increasing the higher you go.

Quantum geometry influences the band structure by affecting the spatial extent and shape of electron wavefunctions within the lattice. In simple terms, the distribution of electrons affects the energy structure or layout for electrons in a crystal lattice.

The energy levels in a lattice are crucial as they determine important properties like conductivity. Additionally, the band structure will vary from material to material.

Gaussian approximation and hopping

The researchers built their model by using Gaussian approximation. This method simplifies complex interactions (such as those between electrons and phonons) by approximating the distribution of variables like energies as Gaussian (or normal) distributions.

This makes it easier to handle mathematically and draw conclusions about the influence of quantum geometry on EPC.

“The Gaussian approximation is essentially a way to relate the real-space electron hopping to the momentum-space quantum geometry,” said Dr. Yu.

Electron hopping is a phenomenon in crystal lattices where electrons move from one site to another. For hopping to occur effectively, the wavefunctions of electrons at neighboring sites must overlap, allowing electrons to tunnel through the potential barriers between sites.

The researchers found that the overlapping was affected by the quantum geometry of the electronic wavefunction, thus affecting hopping.

“The EPC often comes from the change of the hopping with respect to the lattice vibrations. So naturally, the EPC should be enhanced by strong quantum geometry,” explained Dr. Yu.

They quantified this by measuring the EPC constant, which tells the strength of the coupling or interaction, using the Gaussian approximation.

To test their theory, they applied it to two materials, graphene and magnesium diboride (MgB₂).

Superconductors and applications

The researchers chose to test their theory on graphene and MgB₂ because both materials have superconducting properties driven by EPC.

They found that for both materials, EPC was strongly influenced by geometric contributions. Specifically, the geometric contributions were measured to be 50% and 90% for graphene and MgB₂, respectively.

They also found the existence of a lower bound or limit for the contributions due to quantum geometry. In simple words, there is a minimum contribution towards the EPC constant due to quantum geometry, and the rest of the contribution is from the energy of the electrons.

Their work suggests that increasing superconducting critical temperature, which is the temperature below which superconductivity is observed, can be done by enhancing EPC.

Certain superconductors like MgB₂ are phonon-mediated, meaning that EPC directly affects their superconducting properties. According to the research, strong quantum geometry implies strong EPC, opening a new route to search for relatively high-temperature superconductors.

“Even if EPC cannot mediate superconductivity alone, it can help cancel part of the repulsive interaction and help generate superconductivity,” added Dr. Yu.

Future work

The theory developed by the researchers has only been tested for certain materials, which means it is not universal. Dr. Yu believes that the next step is to generalize this theory to make it applicable to all materials.

This is especially important for developing and understanding different quantum materials (like topological insulators) that could be affected by quantum geometry.

“Quantum geometry is ubiquitous in quantum materials. Researchers know it should affect many quantum phenomena, but often lack theories that clearly capture this effect. Our work is one step towards such a general theory, but we are still far away from fully understanding it,” concluded Dr. Yu.

More information: Jiabin Yu et al, Non-trivial quantum geometry and the strength of electron–phonon coupling, Nature Physics (2024). DOI: 10.1038/s41567-024-02486-0.

Journal information: Nature Physics