Category: Physics

Quantum computers have the potential of outperforming classical computers in some optimization and data processing tasks. However, quantum systems are also more sensitive to noise and thus prone to errors, due to the known physical challenges associated with reliably manipulating qubits, their underlying units of information.

Engineers recently devised various methods to reduce the impact of these errors, which are known as quantum error mitigation (QEM) techniques. While some of these techniques achieved promising results, executing them on real quantum computers is often too expensive or unfeasible.

Researchers at IBM Quantum recently showed that simple and more accessible machine learning (ML) techniques could be used for QEM. Their paper, published in Nature Machine Intelligence, demonstrates that these techniques could achieve accuracies comparable to those of other QEM techniques, at a far lower cost.

“We started to think about how to depart from conventional physics-based QEM methods and whether ML techniques can help to reduce the cost of QEM so that more complex and larger-scale experiments could come within reach,” Haoran Liao, co-first author of the paper, told Phys.org.

“However, there seemed to be a fundamental paradox: How can classical ML learn what noise is doing in a quantum calculation running on a quantum computer that is doing something beyond classical computers? After all, quantum computers are of interest for their ability to run problems beyond the power of classical computers.”

As part of their study, the researchers carried out a series of tests, where they used state-of-the-art quantum computers and up to 100 qubits to complete different tasks. They focused on tasks that are impossible to complete via brute-force calculations performed on classical computers, but that could be tackled using more sophisticated computational methods running on powerful classical computers.

“This interesting ‘paradox’ is another motivation for us to think about whether careful designs can tailor ML models to find the complicated relationships between noisy and ideal outputs of a quantum computer to help quantum computations,” said Liao.

The primary objective of the recent study by Liao and his colleagues was to accelerate QEM using widely used ML techniques, demonstrating the potential of these techniques in real-world experiments. The team first started experimenting with a complex graph neural network (GNN), which they used to encode the entire structure of a quantum circuit and its properties, yet they found that this model performed not very well.

“It surprised us to find that a simpler model, random forest, worked very well across different types of circuits and noise models,” explained Liao.

“In our exploration of the ML techniques for QEM, we tried to shape and vary the noise to see how different techniques ML or conventional, perform in different scenarios, so we can better understand the capability of ML models in ‘learning’ the noise–we are not passive bystanders, but active agents.

“We not only tried to benchmark but also demystify the ‘black box’ nature of the ML models in the context of learning relationships between noisy and ideal outputs of a more powerful quantum computer. “

The experiments carried out by Liao and his colleagues demonstrate that ML could help to accelerate physics-based QEM. Remarkably, the model that they found to be most promising, known as random forest, is also fairly simple and could thus be easy to implement on a large scale.

“We assessed how much data is needed to train the ML models well to mitigate errors on a much larger set of testing data and clearly demonstrated a substantially lower overall overhead of the ML techniques for QEM without sacrificing accuracy,” said Liao.

The findings gathered by this team of researchers could have both theoretical and practical implications. In the future, they could help to enrich the present understanding of quantum errors, while also potentially improving the accessibility of QEM methods.

“Is the relationship between noisy and ideal outputs of a quantum computer fundamentally unlearnable? We didn’t know the answer,” said Liao.

“There are a lot of reasons why this would be impossible to determine. However, we showed this key relationship is in fact learnable by ML models in practice. We also showed that we are not passive bystanders, but we can shape the noise in the computation to improve the learnability further.”

Liao and his colleagues also successfully demonstrated that ML techniques for QEM are less costly, yet they can achieve accuracies comparable to those of alternative physics-based QEM techniques. Their experiments were the first to demonstrate the potential of machine learning algorithms for QEM at a utility scale.

“Even in the most conservative setting, ML-QEM demonstrates more than a 2-fold reduction in runtime overhead, addressing the primary bottleneck of error mitigation and a fundamentally challenging problem with a substantial leap in efficiency,” said Liao.

“Most conservatively, this translates into at least halving experiment durations—e.g., cutting an 80-hour experiment to just 40 hours—drastically reducing operational costs and doubling the regime of accessible experiments.”

After gathering these promising results, Liao and his colleagues plan to continue exploring the potential of AI algorithms for simplifying and upscaling QEM. Their work could inspire other research groups to conduct similar studies, potentially contributing to the advancement and future deployment of quantum computers.

“This is just the beginning, and we’re excited about the field of AI for quantum,” added Liao. “We would like to emphasize that at least in the context of QEM, ML is not to replace, but to facilitate physics-based methods, that they can build off each other.

“This opens the door for what’s possible with ML in quantum and is an invitation to the community that ML can perhaps accelerate and improve many other aspects of quantum computations.”

More information: Haoran Liao et al, Machine learning for practical quantum error mitigation, Nature Machine Intelligence (2024). DOI: 10.1038/s42256-024-00927-2.

Journal information: Nature Machine Intelligence 

© 2024 Science X Network

In the quest for ultra-precise timekeeping, scientists have turned to nuclear clocks. Unlike optical atomic clocks—which rely on electronic transitions—nuclear clocks utilize the energy transitions in the atom’s nucleus, which are less affected by outside forces, meaning this type of clock could potentially keep time more accurately than any previously existing technology.

However, building such a clock has posed major challenges—thorium-229, one of the isotopes used in nuclear clocks, is rare, radioactive, and extremely costly to acquire in the substantial quantities required for this purpose.

Reported in a study published in Nature, a team of researchers, led by JILA and NIST Fellow and University of Colorado Boulder Physics professor Jun Ye, in collaboration with Professor Eric Hudson’s team at UCLA’s Department of Physics and Astronomy, have found a way to make nuclear clocks a thousand times less radioactive and more cost-effective, thanks to a method creating thin films of thorium tetrafluoride (ThF₄).

The successful use of thin films marks a potential turning point in the development of nuclear clocks. Using thin-film technology in nuclear clocks is commensurate with semiconductors and photonic integrated circuits, suggesting that future nuclear clocks could be more accessible and scalable.

“A key advantage of nuclear clocks is their portability, and to fully unleash such an attractive potential, we need to make the systems more compact, less expensive, and more radiation-friendly to users,” said Ye.

The costs of nuclear clockmaking

JILA has been at the forefront of atomic and optical clock research for decades, with Ye’s laboratory making pioneering contributions to advancing the concept, design, and implementation of optical lattice clocks, which set new standards in precision timekeeping.

Physicists have been trying to observe the energy transition of thorium-229 for nearly 50 years. In September 2024, researchers in Ye’s laboratory reported the first high-resolution spectrum of the nuclear transition and determined the absolute frequency based on the JILA Sr optical lattice clock.

Their result was published as a cover article in Nature.

To build their nuclear clock setup, the team worked with radioactive thorium-229 crystals, collaborating with researchers at the University of Vienna.

“The growth of that crystal is an art in itself, and our collaborators in Vienna spent many years of effort to grow a nice single crystal for this measurement,” explains Chuankun Zhang, a graduate student at JILA and first author of both Nature studies.

Previous approaches using thorium-doped crystals required more radioactive material. As thorium-229 is often sourced from uranium via nuclear decay, this leads to additional radiation safety and cost considerations.

“Thorium-229 by weight is more expensive than some of the custom proteins I’ve worked with in the past,” adds JILA postdoctoral researcher Jake Higgins, also involved in this project, “so we had to make this work with as little material as possible.” The researchers collaborated closely with CU Boulder’s Environmental Health & Safety department to safely build and study their nuclear clock.

As the team worked to observe the nuclear transitions in thorium-doped crystals, they simultaneously pursued methods to make the clock safer and more cost-effective by developing thin film coatings to reduce the amount of radioactive thorium needed.

Vaporizing thorium

To produce the thin films, the researchers used a process called physical vapor deposition (PVD), which involved heating thorium fluoride in a chamber until it vaporized. The vaporized atoms then condensed on a substrate, forming a thin, even layer of thorium fluoride about 100 nanometers thick.

The researchers selected sapphire and magnesium fluoride as substrates because of their transparency to the ultraviolet light used to excite the nuclear transition.

“If we have a substrate very close by, the vaporized thorium fluoride molecules touch the substrate and stick to it, so you get a nice, even thin film,” Zhang says.

This method used just micrograms of thorium-229, making the product a thousand times less radioactive while producing a dense layer of active thorium nuclei. Working with the JILA Keck Metrology laboratory and JILA instrument maker Kim Hagen, the researchers reliably reproduced films that could be tested for potential nuclear transitions using a laser.

FInding energy transitions in thin film

However, the team faced a new challenge. Unlike in a crystal, where every thorium atom was situated in an ordered environment, the thin films produced variations in thorium environments, shifting their energy transitions and making them less consistent.

JILA graduate student Jack Doyle, who was also involved in this study, elaborates, “Wolfgang Pauli was rumored to have said that ‘God invented the bulk and the surface is of the devil,’ but he might as well have said this because the number of factors that are hard to learn about for a particular surface is immense.”

After preparing the films, JILA researchers sent them to Professor Eric Hudson at UCLA, who used a high-power laser with a much greater spectral width to test the nuclear transitions.

This broad-spectrum laser has all of its optical power concentrated in one spectral location instead of a frequency comb that has regularly spaced spectral lines over a larger spectral distance. This allowed the UCLA team to excite the thorium nuclei effectively, even though the observed linewidth is broader than previously seen in the previous study.

When the laser’s energy precisely matched the energy required for the transition, the nuclei emitted photons as they relaxed back to their original state. By detecting these emitted photons, the researchers could confirm successful nuclear excitations, verifying the thin film’s potential to serve as a frequency reference for nuclear clocks.

“We made the thin film, we characterized it, and it looked pretty good,” explains JILA graduate student Tian Ooi, who was also involved in this research. “It was cool to see that the nuclear decay signal was actually there.”

A new chapter of precision timekeeping

Based on their findings, the researchers are excited about the improvements in precision timekeeping to be gained by using thin films in nuclear clocks.

“The general advantage of using clocks in a solid state, as opposed to in a trapped-ion setting, is that the number of atoms is much, much larger,” Higgins elaborates. “There are orders and orders of magnitude more atoms than one could feasibly have in an ion trap, which helps with your clock stability.”

These thin films could additionally allow nuclear timekeeping to move beyond laboratory settings by making them compact and portable.

“Imagine something you can wear on your wrist,” Ooi says. “You can imagine being able to miniaturize everything to that level in the far, far future.”

While this level of portability is still a distant goal, it could revolutionize sectors that rely on precise timekeeping, from telecommunications to navigation.

“If we are lucky, it might even tell us about new physics,” Doyle adds.

More information: Eric Hudson, 229ThF4 thin films for solid-state nuclear clocks, Nature (2024). DOI: 10.1038/s41586-024-08256-5. www.nature.com/articles/s41586-024-08256-5

Journal information: Nature 

Provided by JILA 

Understanding and reliably measuring the geometric properties of quantum states can shed new light on the intricate underpinning of various physical phenomena. The quantum geometric tensor (QGT) is a mathematical object that provides a detailed description of how quantum states change in response to perturbations, thus offering insights about their underlying geometry.

While this mathematical object has been the focus of numerous theoretical studies, measuring it in experimental settings has proved more challenging. As a result, direct measurements of the QGT have so far been limited to artificial two-level systems.

Researchers at Massachusetts Institute of Technology, Seoul National University and other institutions recently devised a new approach to measure the QGT in crystalline solids. Their proposed method, introduced in a Nature Physics paper, relies on photoemission spectroscopy, a technique typically used to examine the electronic structure of materials.

“The work started as we were thinking about ways to probe the Berry curvature of electrons in solids,” Riccardo Comin, senior author of the paper, told Phys.org. “We originally devised an experiment based on the relationship between orbital angular momentum (probed by circular dichroic ARPES) and Berry curvature.”

The first experiment carried out by Comin and his colleagues was successful and it allowed them to compile the dataset that they used to conduct their recent study. This ultimately allowed them to develop their new approach for measuring QGT in solids, which they called “reconstruction of the full QGT.”

“The full scope of our method was developed thanks to work in Prof. Yang’s group, where the approach was broadened to include the reconstruction of the real part of the quantum geometric tensor (the quantum distance) from the energy dispersion of electronic bands,” said Comin.

“From there, we were able together to develop an approach that connects band theory with experimental data from ARPES, which is the key advancement of this paper.”

The approach devised by Comin, Prof. Yang and their colleagues is based on two independent but complementary approaches. Both of these approaches entail the analysis of data collected via angle-resolved photoemission spectroscopy (ARPES), as a means of retrieving both the real (i.e., quantum distance) and imaginary (i.e., Berry curvature) parts of the QGT.

“The method requires the use of spin- and polarization-resolved ARPES and relies on a minor set of approximations, which are outlined in the paper,” explained Comin.

“Notably, the method was conceived to be applicable to any generic material, regardless of its band structure details or symmetry properties. What makes our approach more powerful is that the QGT is resolved for each electron in reciprocal space.

“This is a significant step forward from existing methods which can mainly detect an integrated Berry curvature (a.k.a., the Chern number) via linear or nonlinear transport measurements.”

The recent study by Comin, Prof. Yang and their colleagues opens new possibilities for research focusing on the geometric properties of quantum states in solids. The new approach they developed could soon be used to study various crystalline systems, which could enrich the current understanding of their quantum geometric responses.

“The most important implication is that we now have a way to retrieve information about the electron wavefunction, and not just the electron energy levels (i.e., the electronic bands),” added Comin.

“This will make it possible to establish an even closer connection between experiments and theory. In our next studies, we plan to apply this method to a broad class of materials with nontrivial topology, to elucidate the detailed origin of quantum geometrical effects.”

More information: Mingu Kang et al, Measurements of the quantum geometric tensor in solids, Nature Physics (2024). DOI: 10.1038/s41567-024-02678-8.

Journal information: Nature Physics 

© 2024 Science X Network

Telling time in space is difficult, but it is absolutely critical for applications ranging from testing relativity to navigating down the road. Atomic clocks, such as those used on the Global Navigation Satellite System network, are accurate, but only up to a point.

Moving to even more precise navigation tools would require even more accurate clocks. There are several solutions at various stages of technical development, and one from Germany’s DLR, COMPASSO, plans to prove quantum optical clocks in space as a potential successor.

There are several problems with existing atomic clocks—one has to do with their accuracy, and one has to do with their size, weight, and power (SWaP) requirements. Current atomic clocks used in the GNSS are relatively compact, coming in at around .5 kg and 125 x 100 x 40 mm, but they lack accuracy. In the highly accurate clock world terminology, they have a “stability” of 10e-9 over 10,000 seconds. That sounds absurdly accurate, but it is not good enough for a more precise GNSS.

Alternatives, such as atomic lattice clocks, are more accurate, down to 10e-18 stability for 10,000. However, they can measure .5 x .5 x .5m and weigh hundreds of kilograms. Given satellite space and weight constraints, those are way too large to be adopted as a basis for satellite timekeeping.

To find a middle ground, ESA has developed a technology development roadmap focusing on improving clock stability while keeping it small enough to fit on a satellite. One such example of a technology on the roadmap is a cesium-based clock cooled by lasers and combined with a hydrogen-based maser, a microwave laser. NASA is not missing out on the fun either, with its work on a mercury ion clock that has already been orbitally tested for a year.

The work is published in the journal GPS Solutions.

COMPASSO hopes to surpass them all. Three key technologies enable the mission: two iodine frequency references, a “frequency comb,” and a “laser communication and ranging terminal.” Ideally, the mission will be launched to the ISS, where it will sit in space for two years, constantly keeping time. The accuracy of those measurements will be compared to alternatives over that time frame.

Lasers are the key to the whole system. The iodine frequency references display the very distinct absorption lines of molecular iodine, which can be used as a frequency reference for the frequency comb, a specialized laser whose output spectrum looks like it has comb teeth at specific frequencies. Those frequencies can be tuned to the frequency of the iodine reference, allowing for the correction of any drift in the comb.

The comb then provides a method for phase locking for a microwave oscillator, a key part of a standard atomic clock. Overall, this means that the stability of the iodine frequency reference is transferred to the frequency comb, which is then again transferred to the microwave oscillator and, therefore, the atomic clock. In COMPASSO’s case, the laser communication terminal is used to transmit frequency and timing information back to a ground station while it is active.

COMPASSO was initially begun in 2021, and a paper describing its details and some breadboarding prototypes were released this year. It will hop on a ride to the ISS in 2025 to start its mission to make the world a more accurately timed place—and maybe improve our navigation abilities as well.

More information: Frederik Kuschewski et al, COMPASSO mission and its iodine clock: outline of the clock design, GPS Solutions (2023). DOI: 10.1007/s10291-023-01551-0

Provided by Universe Today 

UNSW engineers have developed and built a special maser system that boosts microwave signals—such as those from deep space—but does not need to be super-cooled.

They say that diamonds are a girl’s best friend—but that might also soon be true for astronomers and astrophysicists following the new research. The team of quantum experts have developed a device known as a maser which uses a specially created purple diamond to amplify weak microwave signals, such as those which can come from deep space.

Most importantly, their maser works at room temperature, whereas previous such devices needed to be super-cooled, at great expense, down to about minus 269°C.

The amplified signals, originally emitted by pulsars, galaxies, or very distant spacecraft, could ultimately be crucial for expanding our understanding of the universe and fundamental physics.

The UNSW research team, led by Associate Professor Jarryd Pla, have published their findings in the journal Physical Review X, describing how a so-called spin system within the diamond can boost weak signals at room temperature.

“The microwaves enter the device and then the spins inside the diamond create copies of them, which in effect amplifies the microwave signals. Ideally, the microwave signals then come out much larger and with very little noise on top,” A/Prof. Pla says.

“Currently, electronic amplifiers are being used to detect signals from very distant spacecraft like Voyager 1 which is now more than 15 billion miles away from earth, but still sending out data.

“Those amplifiers are cryogenically cooled to reduce what is known as thermal noise, which is random electrical noise generated by the motion of electrons in the amplifier’s components. Otherwise, that noise would just overwhelm the signals being received.

“Our room temperature solid-state maser amplifier avoids all the complication and cost of having to cool everything down to extremely low temperatures and is also much more compact.”

In the paper, the researchers show their maser system can boost signals by a factor of up to 1,000.

Nitrogen vacancy center spins

The proof-of-concept maser developed by the UNSW team, which also includes Mr. Tom Day, lead author on the study, works by growing a diamond in a lab which contains imperfections known as nitrogen vacancy (NV) centers.

This NV is a deliberate defect where a nitrogen atom replaces a carbon atom next to an empty spot in the crystal structure, thereby creating a spin system.

When the spin system is placed inside a magnetic field and simultaneously exposed to a strong green laser beam, it is able to amplify incoming microwave signals..

As well as applications in space exploration, the room-temperature maser could also be hugely beneficial in defense applications, such as radar.

Radars work by sending out electromagnetic signals, which bounce off objects and return to the radar to provide information about the objects’ location, speed, and size. Being more easily able to detect and boost weak signals would therefore potentially be very useful.

Purple reigns

The research team acknowledge that further developments are needed to reduce noise in their maser system, but believe a commercial device could be operating within two to three years.

They are investigating the effect of increasing the concentration of NV-spins inside the diamond, as well as improving other components of the system, such as the resonator that the diamond sits in.

“In effect, we need to make the diamonds more purple,” says Mr. Day.

“The purple color is actually caused by red light emitted by the NV centers. Making darker samples means more NV centers, which ultimately produces higher levels of gain and lower levels of noise and makes the amplified signals clearer.

“However, if you grow these diamonds with very high densities of NV centers, then you can get unwanted defects, so that’s a materials engineering problem that we’re solving.”

A/Prof. Pla adds, “To be competitive against the current cryogenic amplifiers in terms of the low levels of noise then we know we need to boost the NV concentration in the diamond even further. And we think there is quite some way we can go, potentially up to an order of magnitude improvement there.

“We are also working with manufacturers from France and Japan to produce better resonators and we estimate that could also improve the noise by quite some margin.”

More information: Tom Day et al, Room-Temperature Solid-State Maser Amplifier, Physical Review X (2024). DOI: 10.1103/PhysRevX.14.041066

Journal information: Physical Review X 

Provided by University of New South Wales 

In-plane magnetic fields are responsible for inducing anomalous Hall effect in EuCd₂Sb₂ films, report researchers from the Institute of Science Tokyo. By studying how these fields change electronic structures, the team discovered a large in-plane anomalous Hall effect.

These findings, published in Physical Review Letters on December 3, 2024, pave the way for new strategies for controlling electronic transport under magnetic fields, potentially advancing applications in magnetic sensors.

The Hall effect is a fundamental phenomenon in material science. It occurs when a material carrying an electric current is exposed to a magnetic field, producing a voltage perpendicular to both the current and the magnetic field. This effect has been extensively studied in materials under out-of-plane magnetic fields. However, research on how in-plane magnetic fields induce this phenomenon has been very limited.

In recent years, in-plane magnetic fields have attracted growing interest due to their potential to unlock new material behaviors, particularly in materials with singular points in their electronic band structures, such as EuCd₂Sb₂.

Against this backdrop, a team of researchers from Institute of Science Tokyo and the RIKEN Center for Emergent Matter Science (CEMS), led by Associate Professor Masaki Uchida, explored how in-plane magnetic fields induce the anomalous Hall effect in EuCd₂Sb₂ films. Their study sheds light on how these fields induce a distinctive change in electronic band structures.

Uchida explains, “Our findings highlight a new way to manipulate the Hall effect in magnetic materials. This opens up exciting possibilities for future technologies that rely on precise magnetic field measurement, such as magnetic sensing.”

The team’s efforts revealed that in-plane magnetic fields lead to a significantly large anomalous Hall effect in EuCd₂Sb₂ thin films. This effect changes its sign with rotation of the in-plane magnetic field, exhibiting clear three-fold symmetry for rotation of the in-plane magnetic fields.

Furthermore, the study revealed that these effects are linked to an unusual out-of-plane shift of the singular points in electronic band structures. This shift corresponds to the manifestation of orbital magnetization, which is the rotational motion of an electron wave packet, formulated in modern terms as a quantum geometric tensor in solids.

This discovery deepens our understanding of how in-plane magnetic fields change the material’s internal structure.

The researchers also discovered that even small adjustments in the angle of the magnetic field could lead to significant variations in the in-plane anomalous Hall effect. This directional dependence further highlights the material’s versatility and its potential for use in technologies that require precise measurement of magnetic fields along specific directions.

Uchida concludes, “The present work not only heralds a breakthrough in experimentally studying orbital magnetization, but also stimulates materials development for future applications, revolutionizing the concept of the Hall effect ‘from out to in.'”

Overall, this study enhances our understanding of how in-plane magnetic fields influence the electronic properties of advanced materials, such as EuCd₂Sb₂, bringing us closer to developing materials with tailored magnetotransport properties for future technologies.

More information: Ayano Nakamura et al, In-Plane Anomalous Hall Effect Associated with Orbital Magnetization: Measurements of Low-Carrier Density Films of a Magnetic Weyl Semimetal, Physical Review Letters (2024). DOI: 10.1103/PhysRevLett.133.236602. On arXiv: DOI: 10.48550/arxiv.2405.16722

Journal information: Physical Review Letters  , arXiv 

Provided by Institute of Science Tokyo 

The future storage and processing of data stand to benefit greatly from tiny magnetic whirlpools known as skyrmions, which are robust against noise and may be useful in lower power consumption devices. The development of skyrmion-based technologies has received a boost from a simple and intuitive model for visualizing the complex motions of skyrmions developed by RIKEN researchers.

In addition to having mass and electric charge, electrons also possess spin—a form of intrinsic angular momentum.

In some crystalline materials, nearby electrons orient their spins relative to each other to form strange patterns. A skyrmion is one such example where the spin at the center points directly upward, while the surrounding spins gradually twist and point downwards to create a magnetic vortex.

As an electron moves through a lattice of skyrmions, its path bends due to the net (or “emergent”) magnetic field created by the skyrmions—a phenomenon known as the topological Hall effect. But this flow of electrons also causes the skyrmion lattice to move, resulting in an emergent electric field.

Given this complicated interplay, Max Birch and his colleagues at the RIKEN Center for Emergent Matter Science wanted to investigate what happens to the topological Hall effect when a skyrmion lattice moves.

The team chose the compound gadolinium palladium silicide (Gd₂PdSi₃) because its topological Hall effect is almost 100 times larger than those of similar materials. Their samples were also relatively small, making it easier to reach the high currents required to track skyrmion motion over micrometer length scales.

The researchers discovered that the skyrmion lattice motion totally canceled out the topological Hall effect. The paper is published in the journal Nature.

“It turns out that the topological Hall effect essentially scales with the velocity of the electrons relative to the skyrmions,” explains Birch. “Once the skyrmions reach the same velocity as the electrons, the topological Hall effect will vanish, since it is totally canceled out by the induced emergent electric field.”

An intuitive way of looking at this is that when the electrons and the skyrmions are moving at the same speed, their relative velocity is zero, and so there is no topological Hall effect.

Ironically, this demonstration on skyrmions, which are promising for forming the basis of future devices, means that they can be described by equations developed more than a century and a half ago. “We’ve shown that the topological effects can be expressed analogously to Maxwell’s equations of electromagnetism,” explains Birch.

This way of looking at skyrmion dynamics will be crucial for developing spintronic devices, the researchers believe.

More information: Max T. Birch et al, Dynamic transition and Galilean relativity of current-driven skyrmions, Nature (2024). DOI: 10.1038/s41586-024-07859-2

Journal information: Nature 

Provided by RIKEN 

Heating plasma to the ultra-high temperatures needed for fusion reactions requires more than turning the dial on a thermostat. Scientists consider multiple methods, one of which involves injecting electromagnetic waves into the plasma, the same process that heats food in microwave ovens. But when they produce one type of heating wave, they can sometimes simultaneously create another type of wave that does not heat the plasma, in effect wasting energy.

In response to the problem, scientists at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) have performed computer simulations confirming a technique that prevents the production of the unhelpful waves, known as slow modes, boosting the heat put into the plasma and increasing the efficiency of the fusion reactions.

“This is the first time scientists have used 2D computer simulations to explore how to reduce slow modes,” said Eun-Hwa Kim, a PPPL principal research physicist and lead author of the paper reporting the results in Physics of Plasmas. “The results could lead to more efficient plasma heating and possibly an easier path to fusion energy.”

The team, which included researchers from General Atomics who use the DIII-D tokamak fusion facility, determined that positioning a metal grate known as a Faraday screen at a slight five-degree slant with respect to the antenna producing the heating waves, also known as helicon waves, stops the production of the slow modes. Researchers want to avoid creating slow modes, because unlike helicon waves, they cannot penetrate the magnetic field lines confining the plasma to heat the core, where most fusion reactions occur. In addition, the slow modes are easily damped or snuffed out by the plasma itself. Therefore, any energy used to create slow modes is energy that is not used to heat the plasma and foster fusion reactions.

The researchers simulated the production of helicon waves and slow modes using the Petra-M computer code, a powerful and versatile program used to model electromagnetic waves in fusion devices and space plasmas. The simulations replicated conditions in the DIII-D tokamak, a doughnut-shaped plasma device operated by General Atomics for the DOE. The team performed a series of virtual experiments to test which of the following had the greatest effect on the production of slow modes—the antenna’s alignment, the Faraday screen’s alignment or the density of small particles known as electrons in front of the antenna.

The simulations confirmed suggestions made by previous researchers indicating that when the Faraday screen was aligned at an angle of five degrees or less from the orientation of the antenna, the screen—in effect—short-circuits the slow modes, making them fizzle out before they propagate into the plasma.

The suppression of slow modes depends greatly on how much the Faraday screen leans to the side.

“We found that when the screen’s orientation exceeds five degrees by only a little bit, the slow modes grow by a great deal,” said PPPL principal research physicist Masayuki Ono, one of the paper’s authors. “We were surprised by how sensitive the development of slow modes was to the screen alignment.”

Scientists could use this information to tweak the design of new fusion facilities to make their heating more powerful and efficient.

In the future, the scientists plan to increase their understanding of how to prevent slow modes by running computer simulations that consider more of the plasma’s properties and factor in more information about the antenna.

More information: E.-H. Kim et al, Full-wave simulations on helicon and parasitic excitation of slow waves near the edge plasma, Physics of Plasmas (2024). DOI: 10.1063/5.0222413

Journal information: Physics of Plasmas 

Provided by Princeton Plasma Physics Laboratory 

Figuring out certain aspects of a material’s electron structure can take a lot out of a computer—up to a million CPU hours, in fact. A team of Yale researchers, though, are using a type of artificial intelligence to make these calculations much faster and more accurately. Among other benefits, this makes it much easier to discover new materials. Their results are published in Nature Communications.

In the field of materials science, exploring the electronic structure of real materials is of particular interest, since it allows for better understanding of the physics of larger and more complex systems, such as moiré systems and defect states. Researchers typically will use a method known as density functional theory (DFT) to explore electronic structure, and for the most part it works fine.

“But the issue is that if you’re looking at excited state properties, like how materials behave when they interact with light or when they conduct electricity, then DFT really isn’t sufficient to understand the properties of the material,” said Prof. Diana Qiu, who led the study.

In that case, she said, researchers turn to higher-level theories that build on top of DFT. The problem for Qiu, though, is that her research focuses on problems that tend to be very computationally expensive. Sometimes, researchers will solve this by applying machine learning to their calculations. But it’s not much help in figuring out a material’s band structure, Qiu said, “which is what’s necessary to really understand the properties of the material.”

Qiu and her team decided to focus on the electrons’ wave function, which mathematically describes the quantum state of a particle. For the study, they used 2D materials, which are just a few atoms in thickness.

“We said ‘Okay, what is the wave function?'” said Qiu, assistant professor of mechanical engineering & materials science. “It’s this probability over space. We can always plot this as an image in space.”

They used a variational autoencoder, an AI image-processing tool, to create a dimensional representation of the wave function.

“This is done in what’s called an unsupervised way, so there’s no human guidance,” she said. “It takes this 100-gigabyte object and reduces it to 30 numbers, and those numbers represent the initial wave function. We then use that as the input for a second neural network that can then predict these more complicated excited state properties.”

Because the method avoids relying on intuition-based human guidance, it provides a more accurate representation. It’s also much more efficient and can be generalized for other applications.

“The representation that we can end up getting is also about 100 to 1,000 times smaller than what you need for feature selection,” she said. That’s important because the simpler the input, the better it can be used for other applications.

Qiu notes that conventional methods of calculating the band structure of something with three atoms could take between 100,000 and a million CPU hours. Their VAE-assisted method takes about an hour.

“The immediate practical application is that it gives us a way of really speeding up these complicated calculations, which means that now we can do these calculations for a wider range of materials, and that enables the discovery of new materials with properties that we care about.”

More information: Bowen Hou et al, Unsupervised representation learning of Kohn–Sham states and consequences for downstream predictions of many-body effects, Nature Communications (2024). DOI: 10.1038/s41467-024-53748-7

Journal information: Nature Communications 

Provided by Yale University 

Queen Mary University of London physicist Professor Chris White, along with his twin brother Professor Martin White from the University of Adelaide, have discovered a surprising connection between the Large Hadron Collider (LHC) and the future of quantum computing.

For decades, scientists have been striving to build quantum computers that leverage the bizarre laws of quantum mechanics to achieve far greater processing power than traditional computers. A recently identified property—amusingly called “magic”—is critical for building these machines, but its generation and enhancement remain a mystery.

For any given quantum system, magic is a measure that tells us how hard it is to calculate on a non-quantum computer. The higher the magic, the more we need quantum computers to describe the behavior. Studying the magic properties of quantum systems generates profound insights into the development and use of quantum computers.

This new research, published in Physical Review D, demonstrates for the first time that the LHC routinely produces “magic.” By studying the behavior of top quarks, the heaviest known fundamental particles, produced at the LHC, the researchers have predicted that “magic top quarks” will be made very often.

Interestingly, the amount of “magic” exhibited by these top quarks depends on how fast they are moving and their direction of travel, all of which can be measured by the ATLAS and CMS detectors that observe the results of the LHC proton collisions.

This discovery holds significant implications for understanding and potentially enhancing magic in other quantum systems. “While entanglement, where particles become linked, has been a major focus of quantum research,” explains Professor Chris White.

“Our work explores the concept of ‘magic’ in top quarks, which essentially measures how well-suited particles are for building powerful quantum computers.”

Professor Martin White adds, “The ATLAS experiment has already observed evidence of quantum entanglement. We have shown that the LHC can also observe more complex patterns of quantum behavior, at the highest energies yet attempted for these kinds of experiments.”

The potential benefits of quantum computers are vast, impacting fields like drug discovery and materials science. However, harnessing this power requires robust and controllable quantum states, and “magic” plays a critical role in achieving that control.

The White brothers’ research paves the way for a deeper understanding of the connection between quantum information theory and high-energy physics. “By studying ‘magic’ in top quark production,” Professor Chris White says, “we create a new bridge between these two exciting areas of physics.”

Furthermore, this research highlights the potential of the LHC as a unique platform for exploring the frontiers of quantum theory.

This discovery is not just about the heaviest particles in the universe; it’s about unlocking the potential of a revolutionary new computing paradigm.

More information: Chris D. White et al, Magic states of top quarks, Physical Review D (2024). DOI: 10.1103/PhysRevD.110.116016

Journal information: Physical Review D 

Provided by Queen Mary, University of London