Category: Physics

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 

Quantum technologies are radically transforming our understanding of the universe. One emerging technology is macroscopic mechanical oscillators, devices that are vital in quartz watches, mobile phones, and lasers used in telecommunications. In the quantum realm, macroscopic oscillators could enable ultra-sensitive sensors and components for quantum computing, opening new possibilities for innovation in various industries.

Controlling mechanical oscillators at the quantum level is essential for developing future technologies in quantum computing and ultra-precise sensing. But controlling them collectively is challenging, as it requires near-perfect units, i.e., identical.

Most research in quantum optomechanics has centered on single oscillators, demonstrating quantum phenomena like ground-state cooling and quantum squeezing. But this hasn’t been the case for collective quantum behavior, where many oscillators act as one. Although these collective dynamics are key to creating more powerful quantum systems, they demand exceptionally precise control over multiple oscillators with nearly identical properties.

Scientists led by Tobias Kippenberg at EPFL have now achieved the long-sought goal: They successfully prepared six mechanical oscillators in a collective state, observed their quantum behavior, and measured phenomena that only emerge when oscillators act as a group. The research, published in Science, marks a significant step forward for quantum technologies, opening the door to large-scale quantum systems.

“This is enabled by the extremely low disorder among the mechanical frequencies in a superconducting platform, reaching levels as low as 0.1%,” says Mahdi Chegnizadeh, the first author of the study. “This precision allowed the oscillators to enter a collective state, where they behave as a unified system rather than independent components.”

To enable the observation of quantum effects, the scientists used sideband cooling, a technique that reduces the energy of oscillators to their quantum ground state—the lowest possible energy allowed by quantum mechanics.

Sideband cooling works by shining a laser at an oscillator, with the laser’s light tuned slightly below the oscillator‘s natural frequency. The light’s energy interacts with the vibrating system in a way that subtracts energy from it. This process is crucial for observing delicate quantum effects, as it reduces thermal vibrations and brings the system near stillness.

By increasing the coupling between the microwave cavity and the oscillators, the system transitions from individual to collective dynamics.

“More interestingly, by preparing the collective mode in its quantum ground state, we observed quantum sideband asymmetry, which is the hallmark of quantum collective motion. Typically, quantum motion is confined to a single object, but here it spanned the entire system of oscillators,” says Marco Scigliuzzo, a co-author of the study.

The researchers also observed enhanced cooling rates and the emergence of “dark” mechanical modes, i.e., modes that did not interact with the system’s cavity and retained higher energy.

The findings provide experimental confirmation of theories about collective quantum behavior in mechanical systems and open new possibilities for exploring quantum states. They also have major implications for the future of quantum technologies, as the ability to control collective quantum motion in mechanical systems could lead to advances in quantum sensing and generation of multi-partite entanglement.

All devices were fabricated in the Center of MicroNanoTechnology (CMi) at EPFL.

More information: Mahdi Chegnizadeh et al, Quantum collective motion of macroscopic mechanical oscillators, Science (2024). DOI: 10.1126/science.adr8187. www.science.org/doi/10.1126/science.adr8187

Journal information: Science 

Provided by Ecole Polytechnique Federale de Lausanne 

The magnetic moment of the muon is an important precision parameter for putting the Standard Model of particle physics to the test. After years of work, the research group led by Professor Hartmut Wittig of the PRISMA+ Cluster of Excellence at Johannes Gutenberg University Mainz (JGU) has calculated this quantity using the so-called lattice quantum chromodynamics method (lattice QCD method).

Their result agrees with the latest experimental measurements, in contrast to earlier theoretical calculations.

After the experimental measurements had been pushed to ever higher precision in recent years, attention had increasingly turned to the theoretical prediction and the central question of whether it deviates significantly from the experimental results and thus provides evidence for the existence of new physics beyond the Standard Model.

The anomalous magnetic moment is an intrinsic property of elementary particles such as the electron or its heavier brother, the muon. Calculating this quantity with sufficient accuracy within the framework of the Standard Model is an enormous challenge.

With the only exception of gravity, all fundamental interactions contribute to the anomalous magnetic moment. In particular, the contributions of the strong interaction, which describes the forces between the basic building blocks of protons and neutrons, the quarks, cause great difficulties for physicists.

The main source of uncertainty in the theoretical calculation of the anomalous magnetic moment of the muon is the contribution of the so-called hadronic vacuum polarization (HVP). Traditionally, this contribution has been determined using experimental data—this is called the “data-driven” method.

In fact, over many years, this technique provided a significant deviation from the experimental measured value and thus also one of the most promising indications of the existence of new physics.

Result of the PRISMA+ Cluster of Excellence

Wittig’s group has now published a new result for the HVP contribution as a preprint in the open access archive arXiv, which was obtained using the complementary method of lattice QCD.

“Our work confirms earlier evidence suggesting a clear divergence between the data-driven method and lattice QCD calculations,” says Wittig. “At the same time, we have to conclude from our result that the Standard Model has once again been confirmed, because our result agrees with the experimental measurement.”

In 2020, the “Muon g-2 Theory Initiative”—an international group of 130 physicists with strong participation from Mainz—published a reference value for the theoretical prediction of the anomalous magnetic moment of the muon within the framework of the Standard Model, which is based on the data-driven method.

This actually showed a clear deviation from the new direct measurements of this quantity, which have been carried out at Fermilab near Chicago since 2021.

However, since the publication of new results from the CMD-3 experiment in Novosibirsk in February 2023, this reference value has come into question, as the Standard Model prediction varies greatly depending on which data set is used.

In order to overcome the disadvantages of the data-driven method, Wittig’s group has focused on calculations using the lattice QCD method, which allows the contributions of the strong interaction to be calculated numerically using supercomputers. The advantage of such an approach is that, unlike the value published in 2020, it provides results that do not require experimental data.

Agreement with the experimental mean value

Wittig’s group focused on calculating the contribution of the HVP, which provides the largest contribution of the strong interaction to the anomalous magnetic moment of the muon. In their recent work, the team has found a new value for the muon’s anomalous magnetic moment that is consistent with the current experimental mean and far from the 2020 theoretical estimate.

“After years of work on reducing the uncertainties of our calculations and overcoming the computational challenges associated with performing such lattice QCD calculations, we have obtained the HVP contribution with an overall accuracy of just below 1% and a good balance between statistical and systematic uncertainties,” says Wittig. “This allows us to reassess the validity of the Standard Model.”

Even if the new result once again confirms the Standard Model, there are still many puzzles. Where the difference between the lattice QCD and the data-driven method comes from and how the result of the CMD-3 experiment should be evaluated is not yet fully understood.

“We still have a long way to go to achieve our long-term goal of reducing the total error to around 0.2%. No matter how you look at it, we can’t get around the fact that there are discrepancies in the anomalous magnetic moment of the muon that need to be explained. There is still a lot for us to understand,” concludes Wittig.

More information: Dalibor Djukanovic et al, The hadronic vacuum polarization contribution to the muon g-2 at long distances, arXiv (2024). DOI: 10.48550/arxiv.2411.07969

Journal information: arXiv