Category: Physics

Scientists have discovered a new way that matter can exist—one that is different from the usual states of solid, liquid, gas or plasma—at the interface of two exotic materials made into a sandwich.

The new quantum state, called quantum liquid crystal, appears to follow its own rules and offers characteristics that could pave the way for advanced technological applications, the scientists said.

In an article published in the journal Science Advances, a Rutgers-led team of researchers described an experiment that focused on the interaction between a conducting material called the Weyl semimetal and an insulating magnetic material known as spin ice when both are subjected to an extremely high magnetic field. Both materials individually are known for their unique and complex properties.

“Although each material has been extensively studied, their interaction at this boundary has remained entirely unexplored,” said Tsung-Chi Wu, who earned his doctoral degree in June from the Rutgers graduate program in physics and astronomy and is the first author of the study. “We observed new quantum phases that emerge only when these two materials interact. This creates a new quantum topological state of matter at high magnetic fields, which was previously unknown.”

The team discovered that at the interface of these two materials, the electronic properties of the Weyl semimetal are influenced by the magnetic properties of the spin ice. This interaction leads to a very rare phenomenon called “electronic anisotropy” where the material conducts electricity differently in different directions. Within a circle of 360 degrees, the conductivity is lowest at six specific directions, they found. Surprisingly, when the magnetic field is increased, the electrons suddenly start flowing in two opposite directions.

This discovery is consistent with a characteristic seen in the quantum phenomenon known as rotational symmetry breaking and indicates the occurrence of a new quantum phase at high magnetic fields.

The findings are significant because they reveal new ways in which the properties of materials can be controlled and manipulated, Wu said. By understanding how electrons move in these special materials, scientists could potentially design new generations of ultra-sensitive quantum sensors of magnetic fields that work best in extreme conditions—such as in space or inside powerful machines.

Weyl semimetals are materials that allow electricity to flow in unusual ways with very high speed and zero energy loss because of special relativistic quasi-particles called Weyl fermions. Spin ice, on the other hand, are magnetic materials where the magnetic moments (tiny magnetic fields within the material) are arranged in a way that resembles the positions of hydrogen atoms in ice. When these two materials are combined, they create a heterostructure, composed of atomic layers of dissimilar materials.

Scientists have found that new states of matter appear under extreme conditions, including very low temperatures, high pressures or high magnetic fields, and behave in strange and fascinating ways. Experiments such as the Rutgers-led one may lead to a new, fundamental understanding of matter beyond the naturally occurring four states of matter, according to Wu.

“This is just the beginning,” Wu said. “There are multiple possibilities for exploring new quantum materials and their interactions when combined into a heterostructure. We hope our work will also inspire the physics community to explore these exciting new frontiers.”

Physicists led by Jak Chakhalian (left) and including Tsung-Chi Wu (right) and Michael Terilli (center) are studying new quantum phenomena that could pave the way for advanced technologies. Credit: Jeff Arban
The research was conducted using a combination of experimental techniques, led by the principal investigator for the project, Jak Chakhalian, the Claud Lovelace Endowed Professor of Experimental Physics in the Department of Physics and Astronomy and a co-author of the study. The work was theoretically supported by Jedediah Pixley, an associate professor in the Department of Physics and Astronomy, also a co-author of the study.

“The experiment-theory collaboration is what really makes the work possible,” Wu said. “It took us more than two years to understand the experimental results. The credit goes to the state-of-the-art theoretical modeling and calculations done by the Pixley group, particularly Jed Pixley and Yueqing Chang, a postdoctoral researcher. We are continuing our collaboration to push the frontier of the field as a Rutgers team.”

Most of the experiments were conducted at the National High Magnetic Field Laboratory (MagLab) in Tallahassee, Fla., which provided the unique conditions to study these materials at ultra-low temperatures and high magnetic fields.

“We had to initiate the collaboration and travel to the MagLab multiple times to perform these experiments, each time refining ideas and methods,” Wu said. “The ultra-low temperatures and high magnetic fields were crucial for observing these new phenomena.”

The research builds on previous Rutgers-led research published earlier this year by Chakhalian, Mikhail Kareev, Wu and other physicists. The report described how four years of continuous experimentation led to a novel method to design and build a unique, tiny, atoms-thick structure composed of a Weyl semimetal and spin ice. The quantum heterostructure was so difficult to create, the scientists developed a machine to make it: the Q-DiP, short for quantum phenomena discovery platform.

“In that paper, we described how we made the heterostructure,” said Chakhalian. “The new Science Advances paper is about what it can do.”

Dark matter (DM) is a type of matter estimated to account for 80% of the universe’s total mass, but it cannot be directly detected using conventional experimental techniques. As DM does not emit, reflect or absorb light, most previous dark matter searches were aimed at observing either its weak interactions with ordinary matter using highly sensitive detectors or other signatures linked to its presence or decay.

Researchers at Texas A&M University recently introduced a new approach that could enable the direct detection of this elusive type of matter, leveraging a process known as the DM internal pair production. Their proposed strategy, outlined in a paper published in Physical Review Letters, could open new possibilities for future DM searches focusing on a wide range of candidate particles.

“The particle nature of DM can be revealed when a DM particle scatters off a nucleus and produces a visible recoil signal,” the authors told Phys.org. “However, for light DM, transferring sufficient energy to a heavy nucleus is kinematically challenging, even if the DM is energetic. To overcome this limitation, we developed a framework where additional particles are produced in the final state, allowing the DM’s energy to be shared among them, while the nucleus remains largely at rest.”

The detection strategy proposed by Bhaskar Dutta, Aparajitha Karthikeyan, Mudit Rai, and Hyunyong Kim is predicted to enhance the detectability of light DM in scattering experiments. These are research efforts aimed at observing interactions between DM and ordinary matter that can leave detectable traces.

“We propose a novel dark matter–nucleus scattering process involving the emission of a muon pair, electron pair, or photons in short-baseline neutrino experiments, e.g., ongoing short-baseline neutrino facilities at Fermilab, upcoming DUNE, etc., where large dark matter fluxes are expected from the proton-target collision but hard to detect,” explained the authors.

“These energetic final states provide distinctive signatures to separate dark matter signals from neutrino backgrounds and offer new ways to probe the underlying dark matter models.”

The approach for detecting dark matter proposed by Dutta and his colleagues is rooted in theory predicting that energetic dark matter particles can collide with nuclei in dense materials, such as those employed by various large-scale DM experiments. These collisions can result in the exchange of a temporary quantum fluctuation of light, known as a virtual photon, which in turn prompts the formation of a lepton-antilepton pair.

The researchers propose a strategy for extracting energetic and visible signals associated with the formation of these pairs, which has so far proved difficult. This strategy could be employed as part of future DM searches, potentially contributing to its detection and shedding light on its origin and composition.

“So far, we have applied our newly developed mechanism in the context of short-baseline neutrino experiments,” added the authors. “Encouraged by these results, we plan to extend this approach to search for dark matter present in the galaxy or produced in astrophysical sources such as blazars.

“In such scenarios, the resulting energetic signals could be detectable in various dark matter direct and indirect detection experiments, as well as in large neutrino detectors such as DUNE, Hyper-Kamiokande, JUNO, IceCube, and KM3NeT.”

Exploring the BTZ black hole in (2+1)-dimensional gravity took me down a fascinating rabbit hole, connecting ideas I never expected—like black holes and topological phases in quantum matter! When I swapped the roles of space and time in the equations (it felt like turning my map upside down when I was lost in a new city), I discovered an interior version of the solution existing alongside the familiar exterior, each with its own thermofield double state.

What surprised me was how these states seem to communicate, even bridging regions where orientation flips—like walking through a door and suddenly left is right—reminding me of getting turned around on a mountain hike until I saw the landscape from a new perspective.

Digging deeper, I found that the weirdness of black holes with swapped space and time is connected to non-orientable spacetimes and topological invariants, revealing deep ties between gravity and the strange properties of quantum materials that emerge when you flip orientation.

In my recent research published in Physics Letters B, I explored the geometry of the BTZ black hole from a new angle by interchanging the spatial and temporal coordinates. The BTZ (Bañados-Teitelboim-Zanelli) black hole is a fundamental model in lower-dimensional gravity that helps us probe black hole physics, holographic dualities, and aspects of quantum gravity with relative mathematical simplicity.

My key insight was to examine what happens when the usual roles of space and time coordinates in the BTZ metric are swapped, giving rise to a richer geometric and quantum description of this black hole, and offering fresh perspectives on its interior structure, holographic states, and the topology of spacetime itself.

I began by deriving a new BTZ metric where space and time effectively exchange their characteristics. Usually, the BTZ metric clearly delineates time from space, particularly across the event horizon: outside the horizon, time flows as we expect, while spatial dimensions behave conventionally; inside the horizon, however, the temporal and spatial roles switch their roles, with time acting like a spatial coordinate. By explicitly interchanging these coordinates, I constructed a maximal extension of the black hole’s interior.

Remarkably, the line element describing this interior metric closely resembles that of the exterior solution, but with space and time swapped. This surprising symmetry suggested to me a sort of duality between the inside and outside regions of the black hole, providing an innovative way to analyze black hole interiors and their holographic duals. This new interior metric thus opens up novel avenues to probe black hole physics beyond the traditional horizon.

Building on this geometric insight, I proposed to associate two distinct thermofield double (TFD) states with the BTZ black hole. Typically, the TFD state is an entangled quantum state describing an eternal black hole holographically by coupling two copies of a conformal field theory (CFT). This state encapsulates the black hole’s exterior region, which connects two boundaries.

However, by considering the space-time interchange framework, I found that a complete quantum description requires two independent TFD states: one corresponding to the conventional exterior region, and the second encoding the interior region characterized by reversed spatial and temporal roles.

These two TFD states complement one another, collectively encoding the full bulk geometry. This richer dual-TFD structure expands the traditional holographic dictionary and may provide new insights into the quantum microstructure of black holes and the long-standing information puzzle.

Next, I analyzed the partition function that corresponds to the entire BTZ black hole geometry, now viewed as the combination of interior and exterior regions under the coordinate interchange. The partition function is fundamental in quantum statistical mechanics and quantum field theory because it encodes the full thermodynamic and spectral information of the system.

What I discovered is that the resulting partition function describes a non-orientable spacetime—a topology where one cannot consistently assign a global orientation across the manifold. This observation challenges the conventional assumption in gravitational physics that spacetimes are orientable, revealing a profound topological novelty.

Such non-orientable geometries might play an essential role in uncovering new quantum gravitational effects, especially within the enigmatic regime of black hole interiors.

I went further and constructed a thermofield double–like state that mediates between spacetime sectors with reversed space and time orientations. This state functions as a bridge between the two TFD states assigned to the exterior and interior regions, embodying a sort of temporal-spatial duality in the gravitational dual theory.

This construction points to deeper algebraic and geometric structures underlying holographic dualities—namely that black holes cannot be fully described by just one boundary state but instead by interconnected sectors distinguished by orientation reversals in time and space.

This insight underscores the important role of temporal-spatial dualities in gravitational physics and suggests new approaches for describing the quantum relationships among different regions of spacetime historically treated as separate.

Perhaps the most fascinating part of this work is the connection I found between the black hole’s partition function and topological invariants known from condensed matter physics—specifically those that classify many-body topological phases protected by orientation-reversing symmetries. In condensed matter, symmetry-protected topological (SPT) phases represent exotic quantum states robust against local perturbations, distinguished by global topological properties rather than local order parameters.

My findings reveal that the non-orientable spacetime geometry and its partition function link naturally to these topological invariants, suggesting that quantum states of black holes might be understood through the mathematical frameworks developed for topological quantum matter.

This interdisciplinary bridge opens a promising path for integrating ideas from quantum gravity, holography, and condensed matter physics, hinting that black hole interiors share striking similarities with symmetry-protected topological phases.

These insights enrich our conceptual tools for approaching quantum gravity, holography, black hole interiors, and the interplay of topology with quantum information. They also inspire future directions across quantum field theory, gravitational physics, and topological quantum matter—bringing us closer to a more cohesive understanding of the quantum nature of spacetime.

This story is part of Science X Dialog, where researchers can report findings from their published research articles. Visit this page for information about Science X Dialog and how to participate.

Neutrinos are fundamental particles characterized by no electric charge and very small masses, which are known to interact with other matter via the weak force or gravity. While these particles have been the focus of numerous research studies, the processes through which they acquire their masses have not yet been elucidated.

One hypothesis is that neutrino masses originate from interactions with ultralight dark matter, a type of dark matter theorized to be made up of particles or fields with extremely small masses below 10 electron volts (eV). Researchers at Shanghai Jiao Tong University and University of Salerno recently set out to test this hypothesis by comparing data collected by the Kamioka Liquid Scintillator Antineutrino Detector (KamLAND) experiment to theoretical predictions.

Their findings, published in a paper in Physical Review Letters, suggest that neutrino masses are not likely to have a dark origin.

“The motivation for this work came from one of the most pressing open questions in particle physics: the origin of neutrino masses,” Luca Visinelli, senior author of the paper, told Phys.org.

“Unlike other fermions, neutrinos are massless in the Standard Model, but experiments have clearly shown that they must possess a tiny mass, as demonstrated by the phenomenon of neutrino oscillations. Because neutrino masses are so small, their origin likely involves new physics beyond the Standard Model, potentially unrelated to the Higgs mechanism that gives mass to other known particles.”

In recent years, many physicists have been exploring the possibility that the masses of neutrinos arise from interaction with a so-called “dark sector,” or, in other words, with hypothetical fields and forces also connected to dark matter or dark energy. Visinelli and his colleagues Andrew Cheek and Hong-Yi Zhang drew from these earlier works to further test this hypothesis using a combination of theory and experimental observations.

“We developed a framework in which the smallness of neutrino masses results from their interaction with the dark sector, and then rigorously tested if such a connection can be detected using existing neutrino data, including short- and long-baseline neutrino oscillation experiments and solar neutrino measurements,” said Visinelli. “Our results suggest that such a dark-sector origin for neutrino masses is not supported by current data.”

Overall, the findings derived by the researchers suggest that it is highly unlikely that neutrino masses originate from interactions with a dark sector. On the other hand, it is far more likely for them to be explained by conventional particle physics phenomena. Their study could thus guide future efforts aimed at uncovering the origin of neutrino masses by narrowing down the processes that could explain their emergence.

“To test our hypothesis, we developed a theoretical model in which neutrinos acquire mass through interactions with new particles in a dark sector,” explained Visinelli. “Specifically, we considered the case where dark matter is composed of light bosonic fields. These fields behave as coherent waves oscillating in time, with the frequency determined by the mass of the boson.

“In this setup, we carefully analyzed how the frequency of these oscillations compares with the timescales relevant to neutrino experiments, for these oscillations can be either fast or slow depending on the dark matter mass.”

As part of their analyses, the researchers also considered the spatial variation of the dark matter field. In other words, they account for the fact that dark matter would have a characteristic oscillation length, which overlaps with the source and detector locations in neutrino experiments, as well as the movements of Earth in our galaxy.

“These considerations lead to a modification of the standard neutrino oscillation probabilities,” said Visinelli. “We show that such changes can be tested using data from current neutrino observatories, across a range of dark matter masses.

“Our study provides a testable framework connecting the origin of neutrino masses to interactions with a dark sector. This scenario can be constrained with a systematic analysis of current data from solar and baseline neutrinos, along with a careful modeling of the astrophysical setup.”

By comparing theoretical predictions with neutrino observations by the KamLAND experiment, the researchers showed that neutrino masses are more likely to originate from conventional particle physics processes or other new physics unrelated to dark matter. As more neutrino-related data becomes available, Visinelli and his colleagues plan to use them to revisit their framework and test new theoretical predictions.

“Upcoming results from the JUNO and DUNE experiments could allow us to search for subtle time variations in neutrino oscillation parameters, compared with what is expected by a vacuum mass solution,” added Visinelli.

“Beyond neutrinos, we’re also exploring the possibility of applying similar ideas to other quantum systems. For example, atomic and nuclear systems, like those studied in atomic clocks or precision magnetometers, can also be sensitive to oscillations induced by light bosonic dark matter.”

The ultrafast placement of an electron in a polar liquid generates collective molecular vibrations in a spherical nano-volume. The vibrations change the diameter of this sphere periodically for more than 100 picoseconds.

New results from ultrafast spectroscopy reveal how such oscillations in radial direction are distinguished from transversal excitations and how the two of them govern the electric behavior of the liquid. Tuning the concentration of generated electrons allows for adapting the electric properties of different liquids. The findings are published in the journal Physical Review Research.

Polar liquids such as water and alcohols consist of molecules with an electric dipole moment. Via their dipoles, the molecules exert electric forces on each other, by which coupled collective motions of large groups of molecules are rendered possible.

Collective motions have a direct impact on the electrical properties of the liquid, e.g., the microwave absorption, but have been explored to a limited degree only and are not understood at the molecular level.

Ionization of molecules in a liquid by a femtosecond light pulse generates electrons, which are transferred to a localized ground state within several hundreds of femtoseconds. The localized electron is embedded in molecular cloud of nanometer dimension, which encompasses thousands of molecules.

In the early phase of the localization process, collective coherent vibrations of the molecular cloud, so-called polarons, are excited, which manifest in a periodic modulation of optical absorption in the terahertz frequency range (1 THz = 1012 Hz = 1012 vibrations per second). The oscillation frequency is determined by the electron concentration in the liquid.

New results from the Max Born Institute, Berlin, Germany, demonstrate that polaron oscillations are connected with radial, that is, longitudinal motions of molecules in the cloud and that such oscillations are decoupled from the environment beyond the cloud.

The diameter of the cloud is determined by the screening length of the electric field from the central electron, a length of a few nanometers over which this field is present in the liquid.

Experiments in which electrons were generated by two light pulses separated in time, reveal a polaron frequency determined by the partial electron concentration created by each excitation pulse individually. After the second excitation, the polaron frequency remains unchanged, although the total electron concentration has doubled.

This surprising observation demonstrates a minor mutual coupling of different vibrating molecular clouds and an effective decoupling from their environment.

In contrast, the transversal excitations occurring after electron generation are additive on a macroscopic length scale. The related step-like change of THz absorption is determined by the total electron concentration in the liquid.

A second study done in collaboration with researchers in the U.S. and the United Kingdom and published in Physical Review A shows how to make the electrical properties of different liquids nearly identical.

Control of the electron concentration in different alcohols results in identical frequencies and line shapes of the polaron resonance. This “driven impostor” approach allows for faking the properties of a system by manipulating another one, a concept with potential for applications in optoelectronics and information processing.

A puzzling form of superconductivity that arises only under strong magnetic fields has been mapped and explained by a research team including Andriy Nevidomskyy, professor of physics and astronomy at Rice University. Their findings, published in Science, detail how uranium ditelluride (UTe2) develops a superconducting halo under strong magnetic fields.

Traditionally, scientists have regarded magnetic fields as detrimental to superconductors. Even moderate magnetic fields typically weaken superconductivity, while stronger ones can destroy it beyond a known critical threshold. However, UTe2 challenged these expectations when, in 2019, it was discovered to maintain superconductivity in critical fields hundreds of times stronger than those found in conventional materials.

“When I first saw the experimental data, I was stunned,” said Nevidomskyy, a member of the Rice Advanced Materials Institute and the Rice Center for Quantum Materials.

“The superconductivity was first suppressed by the magnetic field as expected but then reemerged in higher fields and only for what appeared to be a narrow field direction. There was no immediate explanation for this puzzling behavior.”

Superconducting resurrection in high fields
This phenomenon, initially identified by researchers at the University of Maryland (UMD) and the National Institute of Standards and Technology (NIST), has captivated physicists worldwide.

In UTe2, superconductivity vanished below 10 Tesla, a field strength that is already immense by conventional standards, but surprisingly reemerged at field strengths exceeding 40 Tesla.

This unexpected revival has been dubbed the Lazarus phase. Researchers determined that this phase critically depends on the angle of the applied magnetic field in relation to the crystal structure.

In collaboration with experimental colleagues at UMD and NIST, Nevidomskyy decided to map out the angular dependence of this high-field superconducting state. Their precise measurements revealed that the phase formed a toroidal, or donutlike, halo surrounding a specific crystalline axis.

“Our measurements revealed a three-dimensional superconducting halo that wraps around the hard b-axis of the crystal,” said Sylvia Lewin of NIST, a co-lead author on the study. “This was a surprising and beautiful result.”

Building theory to fit halo
To explain these findings, Nevidomskyy developed a theoretical model that accounted for the data without relying heavily on debated microscopic mechanisms. His approach employed an effective phenomenological framework with minimal assumptions about the underlying pairing forces that bind electrons into Cooper pairs.

The model successfully reproduced the nonmonotonic angular dependence observed in experiments, offering insights into how the orientation of the magnetic field influences superconductivity in UTe2.

Deeper understanding of interplay
The research team found that the theory, fitted with a few key parameters, aligned remarkably well with the experimental features, particularly the halo’s angular profile.

A key insight from the model is that Cooper pairs carry intrinsic angular momentum like a spinning top does in classical physics. The magnetic field interacts with this momentum, creating a directional dependence that matches the observed halo pattern.

This work lays the foundation for a deeper understanding of the interplay between magnetism and superconductivity in materials with strong crystal anisotropy like UTe2.

“One of the experimental observations is the sudden increase in the sample magnetization, what we call a metamagnetic transition,” said NIST’s Peter Czajka, co-lead author on the study.

“The high-field superconductivity only appears once the field magnitude has reached this value, itself highly angle-dependent.”

The exact origin of this metamagnetic transition and its effect on superconductivity is hotly debated by scientists, and Nevidomskyy said he hopes this theory would help elucidate it.

“While the nature of the pairing glue in this material remains to be understood, knowing that the Cooper pairs carry a magnetic moment is a key outcome of this study and should help guide future investigations,” he said.

As technology advances, photonic systems are gaining ground over traditional electronics, using light to transmit and process information more efficiently. One such optical system is laser beam scanning (LBS), where laser beams are rapidly steered to scan, sense, or display information.

This technology is used in applications ranging from barcode scanners at grocery stores to laser projectors in light shows. To process a wider range of signals or enable full-color output, these systems utilize multiplexers that merge the red, green, and blue (RGB) laser beams into a single beam.

Traditionally, this was achieved by directly modulating each laser, turning them on and off to control the output. However, this approach is relatively slow and energy intensive. A recent study by researchers at the TDK Corporation (Japan) reports the development of a faster and more energy-efficient RGB multiplexer based on thin-film lithium niobate (TFLN).

The work is published in Advanced Photonics Nexus.

Lithium niobate is a versatile material widely used in photonics due to its excellent electro-optic, nonlinear-optic, and acousto-optic properties. TFLN is widely used in infrared optical modulators and is increasingly valued for its ability to guide visible light. Unlike conventional systems, the TFLN-based approach uses electric fields to control how light propagates and combines, enabling higher modulation speeds and lower power consumption.

Describing the team’s motivation for the study, corresponding author Atsushi Shimura notes, “A TFLN-based RGB multiplexer is essential for LBS with lower power consumption and higher resolution; however, this had never been demonstrated, and the RGB multiplexer has been limited to glass-based photonic integrated circuit.”

The multiplexer, measuring just 2.3 millimeters in length, was created using a physical vapor deposition (“sputter”) technique to deposit the LN film, followed by etching to form waveguides that direct the laser light. With this approach, fabrication avoids the complex bonding process typically required with bulk lithium niobate, resulting in a scalable and cost-effective route to mass-producing compact light-based circuits.

The structure of the waveguides was carefully designed to guide light efficiently, with a trapezoidal cross-section that helped reduce signal loss. By adjusting the lengths of the combined sections, the researchers were able to fine-tune the performance for each color. When evaluated, the RGB combiner successfully combined red (638 nm), green (520 nm), and blue (473 nm) laser beams through carefully designed waveguides.

By adjusting the intensity of each beam, the researchers were able to generate mixed colors such as cyan (green + blue), magenta (red + blue), and yellow (red + green), and even white light, by combining all three primary colors. Such precise color control is essential for LBS-based displays.

While the results are encouraging, the study also highlights some important challenges that need to be addressed moving forward. A key issue is the lower crystal quality of sputter-deposited TFLN compared to bulk lithium niobate, which affects performance at shorter wavelengths.

For example, at 473 nm (blue light), the measured optical loss was between 7 and 10 dB, significantly higher than the simulated value of 3.1 dB. This loss was mainly caused by surface roughness in the waveguides, which scatters light and reduces overall efficiency.

“Optimizing fabrication processes to produce smoother surfaces is a key step toward realizing TFLN’s potential in visible-light photonics and applications,” Shimura remarks.

Despite these limitations, the results lay a foundation for developing scalable, faster, and more energy-efficient multiplexers for future visible-light LBS systems.

“This work demonstrates the feasibility of a passive RGB multiplexer as a first step toward developing active photonic integrated circuits,” Shimura notes.

The alchemist’s dream is to make gold from common metals, but can this be done? The physics needed to explain how to change one element into another is well understood and has been used for decades in accelerators and colliders, which smash sub-atomic particles together.

The most notable present-day example is the Large Hadron Collider at Cern, based in Geneva. But the costs of making gold this way are vast, and the quantities generated are minuscule.

For example, Cern’s Alice experiment estimated it produced only 29 picograms of gold while operating over four years. At that rate, it would take hundreds of times the lifetime of the universe to make a troy ounce of gold.

The Californian startup company Marathon Fusion has proposed a very different approach: to use the radioactivity from neutron particles in a nuclear fusion reactor to transform one form of mercury into another, called mercury-197.

This then decays into a stable form of gold: gold-197. This process of particle decay is where one subatomic particle spontaneously transforms into two or more lighter particles. The team from Marathon Fusion estimates that a fusion power plant could produce several tonnes of gold per gigawatt of thermal power in a single year of operation.

Bombarding the isotope mercury-198 with neutrons leads to the creation of the radioactive isotope mercury-197—which subsequently decays to the only stable isotope of gold.

The key is to have energetic enough neutrons to trigger the mercury decay sequence. If this could be made to work, then it is an interesting idea. But whether it could make a tidy profit is another matter.

To do this, a large neutron flux (a measure of the intensity of neutron radiation) is required. This can be generated using a standard fuel mix for fusion reactors, deuterium and tritium (both of which are forms of hydrogen), to create energy in the plasma of a fusion reactor.

Neutrons penetrate material easily and scatter off the nuclei (cores) in atoms, slowing down as they do so. Neutrons with energies above 6 million electron volts are required to transform mercury-198 into gold.

To come up with its estimates, Marathon Fusion has been using a fusion reactor’s “digital twin”—a computer model that simulates the physics of the fusion reaction and the resulting radioactive processes. A limitation of this type of work is that the digital twin needs to be validated against a real commercial fusion reactor—but none currently exist.

There are many challenges to overcome before scientists can realize a commercial fusion reactor. These include the creation of new materials for its construction, and understanding the science required both to operate the system to continuously extract power, and to develop AI systems that can help keep the plasma fusion reaction running.

Even some of the most advanced fusion experiments, such as the UK-based JET (Joint European Torus) project, could only generate relatively small amounts of energy. However, researchers in the UK have devised a new way to shrink the size of fusion reactors by changing the way the exhaust plasma is controlled. A prototype of this novel fusion reactor concept, called Spherical Tokomak for Energy Production (Step), aims to be ready by 2040.

Radioactive waste
On paper, it is possible to make gold from mercury in a fusion reactor. However, until commercial fusion reactors are realized, the assumptions used by Marathon Fusion in its digital twin studies will remain untested.

Furthermore, any gold produced at a fusion reactor would initially be radioactive, meaning it would be classified as radioactive waste—and thus need to be managed for quite some time after production.

As nuclear and particle physicists know well, it is very easy to forget to include important physical effects and critical details when creating a digital twin of an experiment. But while the processing of that waste into usable forms of pure gold would be a further challenge to address, it will not necessarily deter long-term investors.

For now, this remains an attractive proposition on paper—but we’re still some way off from kickstarting a new kind of Californian gold rush.

As the demand for more secure data transmission increases, conventional communication technologies are facing limitations imposed by classical physics, and are therefore approaching their limits in terms of security. Fortunately, quantum communication may help us overcome these restrictions.

Quantum communication harnesses the quantum nature of light by utilizing single photons as information carriers. This is a fundamentally different approach from conventional communication technologies and has the potential to lead to the development of secure, high-performance communication systems.

These future quantum technologies will require new single-photon emission sources. Recently, extremely thin two-dimensional semiconductors with a thickness of only a few atomic layers have shown great potential due to their excellent electrical and optical properties. Although increasing the efficiency of such single-photon generation is extremely important, the capacity of these materials and its strategy had not been thoroughly explored.

This inspired a team of researchers at Kyoto University to investigate what they predicted may be a functional single-photon emission source. They hypothesized that a semiconductor in single-layer tungsten diselenide, in which they introduced a single defect, would bind excitons—electron-hole pairs—to the defect and emit only a single photon. The study is published in the journal Science Advances.

Defect-localized exciton emission from monolayer WSe2 encapsulated by h-BNs. Credit: Science Advances (2025). DOI: 10.1126/sciadv.adr5562
To realize this idea, the team prepared a sample of monolayer tungsten diselenide, heating it to introduce a small number of defects and to artificially break the crystal symmetry, which resulted in two distinct luminescence peaks representing bright excitons and dark excitons.

The researchers then measured the luminescence and photon correlation at a temperature of about -265°C, applying an external magnetic field to control the emission, revealing that the emission intensity significantly increased even when they applied a relatively weak magnetic field.

Using photon correlation measurements, the team also observed that emitted light demonstrated photon antibunching, indicating that photons are emitted one by one. This suggests that, even under a magnetic field, it can function as a single-photon source, and that the magnetic field can enhance the efficiency of single-photon generation.

“This is significant because it shows that single-photon emissions can be generated and manipulated with an external magnetic field in a two-dimensional semiconductor, revealing it to be a promising platform for the development of secure, efficient, and compact quantum information devices,” says team leader Kazunari Matsuda.

Refraction—the bending of light as it passes through different media—has long been constrained by physical laws that prevent independent control over how light waves along different directions bend. Now, UCLA researchers have developed a new class of passive materials that can be structurally engineered to “program” refraction, enabling arbitrary control over the bending of light waves.

In a study published in Nature Communications, a team led by Dr. Aydogan Ozcan, the Chancellor’s Professor of Electrical & Computer Engineering at UCLA, has introduced a novel device called a refractive function generator (RFG) that can independently tailor the output direction of refracted light for each input direction. This device allows light to be steered, filtered, or redirected according to custom-designed rules—far beyond what standard materials or traditional metasurfaces can achieve.

Standard refraction, described by Snell’s law, links the input and output directions of light using fixed material properties. Even advanced metasurface designs only allow limited tunability of refraction.

The RFG, however, uses a very thin stack of passive transmissive layers—each structurally engineered through deep learning at a scale close to the diffraction limit of light—to define completely arbitrary refractive functions, effectively decoupling the input-output mappings of light refraction. The UCLA team demonstrated that these thin optical devices, spanning only a few tens of wavelengths in thickness, can perform sophisticated wave transformations such as permutation, filtered permutation and negative refraction.

To validate their approach, the researchers fabricated and experimentally tested RFGs using 3D printed materials and terahertz waves. These devices successfully bent light in precisely defined directions, successfully demonstrating arbitrary programming of refractive functions.

“This is a significant step forward in our ability to precisely control and engineer how light behaves,” said Dr. Ozcan. “By programming the refraction of light using structured 3D materials, we open up new design opportunities for optical computing, communications, and imaging systems.”

The study shows that these RFG devices can be designed using AI to be compact, efficient, and robust against fabrication imperfections and wavelength variations. The AI-based design framework also showcased further extensions, including wavelength and polarization multiplexing of RFGs, and unidirectional light routing using only passive, structured materials.

The authors of this work are Dr. Md Sadman Sakib Rahman, Tianyi Gan, Prof. Mona Jarrahi, and Prof. Aydogan Ozcan, all at the UCLA Samueli School of Engineering. This research was supported by the US ARO (Army Research Office).