Category: Physics

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

Quantum computers have the potential to solve complex problems in human health, drug discovery, and artificial intelligence millions of times faster than some of the world’s fastest supercomputers. A network of quantum computers could advance these discoveries even faster. But before that can happen, the computer industry will need a reliable way to string together billions of qubits—or quantum bits—with atomic precision.

Connecting qubits, however, has been challenging for the research community. Some methods form qubits by placing an entire silicon wafer in a rapid annealing oven at very high temperatures.

With these methods, qubits randomly form from defects (also known as color centers or quantum emitters) in silicon’s crystal lattice. And without knowing exactly where qubits are located in a material, a quantum computer of connected qubits will be difficult to realize.

But now, getting qubits to connect may soon be possible. A research team led by Lawrence Berkeley National Laboratory (Berkeley Lab) says that they are the first to use a femtosecond laser to create and “annihilate” qubits on demand, and with precision, by doping silicon with hydrogen.

The advance could enable quantum computers that use programmable optical qubits or “spin-photon qubits” to connect quantum nodes across a remote network. It could also advance a quantum internet that is not only more secure but could also transmit more data than current optical-fiber information technologies.

“To make a scalable quantum architecture or network, we need qubits that can reliably form on-demand, at desired locations, so that we know where the qubit is located in a material. And that’s why our approach is critical,” said Kaushalya Jhuria, a postdoctoral scholar in Berkeley Lab’s Accelerator Technology & Applied Physics (ATAP) Division. She is the first author on a new study that describes the technique in the journal Nature Communications.

“Because once we know where a specific qubit is sitting, we can determine how to connect this qubit with other components in the system and make a quantum network.”

“This could carve out a potential new pathway for industry to overcome challenges in qubit fabrication and quality control,” said principal investigator Thomas Schenkel, head of the Fusion Science & Ion Beam Technology Program in Berkeley Lab’s ATAP Division. His group will host the first cohort of students from the University of Hawaii in June where students will be immersed in color center/qubit science and technology.

Forming qubits in silicon with programmable control

The new method uses a gas environment to form programmable defects called “color centers” in silicon. These color centers are candidates for special telecommunications qubits or “spin photon qubits.” The method also uses an ultrafast femtosecond laser to anneal silicon with pinpoint precision where those qubits should precisely form. A femtosecond laser delivers very short pulses of energy within a quadrillionth of a second to a focused target the size of a speck of dust.

Spin photon qubits emit photons that can carry information encoded in electron spin across long distances—ideal properties to support a secure quantum network. Qubits are the smallest components of a quantum information system that encodes data in three different states: 1, 0, or a superposition that is everything between 1 and 0.

With help from Boubacar Kanté, a faculty scientist in Berkeley Lab’s Materials Sciences Division and professor of electrical engineering and computer sciences (EECS) at UC Berkeley, the team used a near-infrared detector to characterize the resulting color centers by probing their optical (photoluminescence) signals.

What they uncovered surprised them: a quantum emitter called the C_i center. Owing to its simple structure, stability at room temperature, and promising spin properties, the C_i center is an interesting spin photon qubit candidate that emits photons in the telecom band. “We knew from the literature that C_i can be formed in silicon, but we didn’t expect to actually make this new spin photon qubit candidate with our approach,” Jhuria said.

The researchers learned that processing silicon with a low femtosecond laser intensity in the presence of hydrogen helped to create the C_i color centers. Further experiments showed that increasing the laser intensity can increase the mobility of hydrogen, which passivates undesirable color centers without damaging the silicon lattice, Schenkel explained.

A theoretical analysis performed by Liang Tan, staff scientist in Berkeley Lab’s Molecular Foundry, shows that the brightness of the C_i color center is boosted by several orders of magnitude in the presence of hydrogen, confirming their observations from laboratory experiments.

“The femtosecond laser pulses can kick out hydrogen atoms or bring them back, allowing the programmable formation of desired optical qubits in precise locations,” Jhuria said.

The team plans to use the technique to integrate optical qubits in quantum devices such as reflective cavities and waveguides, and to discover new spin photon qubit candidates with properties optimized for selected applications.

“Now that we can reliably make color centers, we want to get different qubits to talk to each other—which is an embodiment of quantum entanglement—and see which ones perform the best. This is just the beginning,” said Jhuria.

“The ability to form qubits at programmable locations in a material like silicon that is available at scale is an exciting step towards practical quantum networking and computing,” said Cameron Geddes, Director of the ATAP Division.

by Theresa Duque, Lawrence Berkeley National Laboratory

by Nagoya University

A study led by Nagoya University in Japan revealed that a simple thermal reaction of gallium nitride (GaN) with metallic magnesium (Mg) results in the formation of a distinctive superlattice structure. This represents the first time researchers have identified the insertion of 2D metal layers into a bulk semiconductor.

By carefully observing the materials through various cutting-edge characterization techniques, the researchers uncovered new insights into the process of semiconductor doping and elastic strain engineering. They published their findings in the journal Nature.

GaN is an important wide bandgap semiconductor material that is poised to replace traditional silicon semiconductors in applications demanding higher power density and faster operating frequencies. These distinctive characteristics of GaN make it valuable in devices such as LEDs, laser diodes, and power electronics—including critical components in electric vehicles and fast chargers. The improved performance of GaN-based devices contributes to the realization of an energy-saving society and a carbon-neutral future.

In semiconductors, there are two essential and complementary types of electrical conductivity: p-type and n-type. The p-type semiconductor features primarily free carriers carrying positive charges, known as holes, whereas the n-type semiconductor conducts electricity through free electrons.

A semiconductor acquires p-type or n-type conductivity through a process called doping, which refers to the intentional introduction of specific impurities (known as dopants) into a pure semiconductor material to greatly alter its electrical and optical properties.

In the field of GaN semiconductors, Mg is the only known element to create p-type conductivity up to now. Despite 35 years since the first success of doping Mg into GaN, the full mechanisms of Mg doping in GaN, especially the solubility limit and segregation behavior of Mg, remain unclear. This uncertainty limits their optimization for optoelectronics and electronics.

To improve the conductivity of p-type GaN, Jia Wang, the first author of the study, and his colleagues conducted an experiment in which they patterned deposited metallic Mg thin films on GaN wafers and heated them up at a high temperature—a conventional process known as annealing.

Using state-of-the-art electron microscope imaging, the scientists observed the spontaneous formation of a superlattice featuring alternating layers of GaN and Mg. This is especially unusual since GaN and Mg are two types of materials with significant differences in their physical properties.

“Although GaN is a wide-bandgap semiconductor with mixed ionic and covalent bonding, and Mg is a metal featuring metallic bonding, these two dissimilar materials have the same crystal structure, and it is a strikingly natural coincidence that the lattice difference between hexagonal GaN and hexagonal Mg is negligibly small,” Wang said.

“We think that the perfect lattice match between GaN and Mg greatly reduces the energy needed to create the structure, playing a critical role in the spontaneous formation of such a superlattice.”

The researchers determined that this unique intercalation behavior, which they named interstitial intercalation, leads to compressive strain to the host material. Specifically, they found that the GaN being inserted with Mg layers sustains a high stress of more than 20 GPa, equivalent to 200,000 times atmospheric pressure, making it the highest compressive strain ever recorded in a thin-film material. This is much more than the compressive stresses commonly found in silicon films (in the range of 0.1 to 2 GPa).

Electronic thin films can undergo significant changes in electronic and magnetic properties because of this strain. The researchers found that the electrical conductivity in GaN via hole transport was significantly enhanced along the strained direction.

“Using such a simple and low-cost approach, we were able to enhance the transport of holes in GaN, which conducts more current,” Wang said. “This interesting finding in interactions between a semiconductor and a metal may provide new insights into semiconductor doping and improve the performance of GaN-based devices.”

Nagoya University and GaN
The fact that this study took place at Nagoya University is fitting, considering its reputation as the “cradle of GaN technology.” Hiroshi Amano, the corresponding author of the current study, and Isamu Akasaki from Nagoya University developed the first blue-light LEDs in the late 1980s, employing Mg-doped GaN. Their contributions, for which they received the Nobel Prize in Physics in 2014, have played an important role in creating a more energy-efficient society.

“The discovery of Mg-intercalated GaN superlattice structures and the identification of the novel mechanism of 2D-Mg doping offer a hard-earned opportunity to honor the pioneering achievements in the field of III-nitride semiconductor research,” said Wang. Having advanced the technology 10 years after the Nobel Prize, Wang referred to this timely finding as a “true gift of nature” that could potentially open new avenues and inspire more basic research in this field.

Among the authors of this research from Nagoya University were Jia Wang, Wentao Cai, Shun Lu, Emi Kano, Biplab Sarkar, Hirotaka Watanabe, Nobuyuki Ikarashi, Yoshio Honda, and Hiroshi Amano. In addition to Nagoya University, other contributing authors of this research include researchers from Meijo University and an optical group led by Professor Makoto Nakajima at Osaka University.

More information: Jia Wang et al, Observation of 2D-magnesium-intercalated gallium nitride superlattices, Nature (2024). DOI: 10.1038/s41586-024-07513-x

Journal information: Nature

Provided by Nagoya University

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 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 

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 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 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 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 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