Category: Physics

In the current issue of Nature Photonics, Prof. Dr. Oliver G. Schmidt, Dr. Libo Ma and partners present a strategy for observing and manipulating the optical Berry phase in Möbius ring microcavities. In their research paper, they discuss how an optical Berry phase can be generated and measured in dielectric Möbius rings. Furthermore, they present the first experimental proof of the existence of a variable Berry phase for linearly or elliptically polarized resonant light.

A Möbius strip is a fascinating object. You can easily create a Möbius strip when twisting the two ends of a strip of paper by 180 degrees and connecting them together. Upon closer inspection, you realize that this ribbon has only one surface that cannot be distinguished between inside and outside or below and above. Because of this special topological property, the Möbius strip has become an object of countless mathematical discourses, artistic representations and practical applications, for example, in paintings by M.C. Escher, as a wedding ring, or as a drive belt to wear both sides of the belt equally.

Optical ring resonators

Closed bands or rings also play an important role in optics and optoelectronics. Until now, however, they have not consisted of Möbius strips and they are not made of paper, but are made of optical materials, for example, silicon and silicon dioxide or polymers. These “normal” rings are also not centimeters in size, but micrometers. If light with a certain wavelength propagates in a micro ring, constructive interference causes optical resonances to occurs. This principle can be exemplified by a guitar string, which produces different tones at different lengths—the shorter the string, the shorter the wavelength, and the higher the tone.

An optical resonance or constructive interference occurs exactly when the circumference of the ring is a multiple of the wavelength of the light. In these cases the light resonates in the ring and the ring is called an optical ring resonator. In contrast, the light is strongly attenuated and destructive interference occurs when the circumference of the ring is an odd multiple of half the wavelength of the light. Thus, an optical ring resonator enhances light of certain wavelengths and strongly attenuates light of other wavelengths that do not “fit” in the ring. In technological terms, the ring resonator acts as an optical filter that, integrated on a photonic chip, can selectively “sort” and process light. Optical ring resonators are central elements of optical signal processing in today’s data communication networks.

How polarized light circulates in the Möbius strip

Besides the wavelength, polarization is an essential property of light. Light can be polarized in various ways, for example linearly or circularly. If light propagates in an optical ring resonator, the polarization of the light does not change and remains the same at every point in the ring.

The situation changes fundamentally if the optical ring resonator is replaced by a Möbius strip or better, a Möbius ring. To better understand this case, it helps to consider the detail of the geometry of the Möbius ring. The cross-section of a Möbius ring is typically a slender rectangle in which two edges are much longer than their two adjacent edges, such as in a thin strip of paper.

Let us now assume that linearly polarized light circulates in the Möbius ring. Because the polarization prefers to align itself in the direction of the long cross-sectional side of the Möbius ring, the polarization continuously rotates up to 180 degrees while passing completely around the Möbius ring. This is a huge difference to a “normal” ring resonator, in which the polarization of the light is always maintained.

And that’s not all. The twisting of the polarization causes a change in the phase of the light wave, so that the optical resonances no longer occur at full wavelength multiples that fit into the ring, but at odd multiples of half the wavelength. Part of the research group had already predicted this effect theoretically in 2013. This prediction, in turn, is based on work by physicist Michael Berry, who introduced the eponymous “Berry phase” in 1983, describing the change in the phase of light whose polarization changes as it propagates.

First experimental evidence

In the current article published in the journal Nature Photonics, the Berry phase of light circulating in a Möbius ring is experimentally revealed for the first time. For this purpose, two rings with the same diameter were made. The first is a “normal” ring and the second is a Möbius ring. And as predicted, the optical resonances in the Möbius ring appear at different wavelengths compared to the “normal” ring.

The experimental results, however, go much beyond previous predictions. For example, the linear polarization not only rotates, but also becomes increasingly elliptical. The resonances do not occur exactly at odd multiples of half the wavelength, but quite generally at non-integer multiples. To find out the reason for this deviation, Möbius rings with decreasing strip width were made. This study revealed that the degree of ellipticity in the polarization and the deviation of the resonance wavelength compared to the “normal” ring became progressively weaker as the Möbius strip became narrower and narrower.

This can be easily understood because the special topological properties of the Möbius ring merge into the properties of a “normal” ring when the width of the band decreases to that of its thickness. However, this also means that the Berry phase in Möbius rings can be easily controlled by simply changing the design of the band.

In addition to the fascinating new fundamental properties of optical Möbius rings, new technological applications are also opening up. The tunable optical Berry phase in Möbius rings could serve for all-optical data processing of classical bits as well as qubits and support quantum logic gates in quantum computation and simulation.

More information: Jiawei Wang et al, Experimental observation of Berry phases in optical Möbius-strip microcavities, Nature Photonics (2022). DOI: 10.1038/s41566-022-01107-7

Journal information: Nature Photonics 

Provided by Chemnitz University of Technology

A team of physicists, biologists and engineers at Cornell University, in the U.S., has discovered some of the factors that lead to more or less spray when cutting onions and found a couple of ways to reduce the amount of eye irritation. The group has published a paper describing their study on the arXiv preprint server.

Prior research has shown that eye irritation when cutting onions is caused by the release of syn-propanethial-S-oxide into the air along with other juices in the onion. For this new study, the team in New York wanted to know what factors led to more or less of the juices being spewed into the air during slicing.

To find out, the research team outfitted a special guillotine that could be fitted with different types of blades. They also coated onion chunks with paint to allow for better viewing of the cutting process. They used the guillotine to cut samples, each of which was recorded. Trials varied knife size, sharpness and cutting speed. They even used an electron microscope to accurately measure the knives before use.

The videos revealed that the differences in the amount of spray released, and thus the amount of eye irritation, were due to the sharpness of the knife and the speed at which it cut the onion. The sharper the knife, and slower the cut, the less spray. This was because duller knives tended to push down on the onion, forcing its layers to bend inward—as the cut ensued, the layers sprang back, forcing juice out into the air.

They also noted that as the juice droplets were flung into the air, they tended to fragment into smaller drops, which allowed them to persist longer. Faster cutting also resulted in more juice generation, and thus more mist to irritate the eyes.

They conclude that onion cutters use the sharpest knife they can find and cut their onions slowly.

The ZEUS laser facility at the University of Michigan has roughly doubled the peak power of any other laser in the U.S. with its first official experiment at 2 petawatts (2 quadrillion watts).

At more than 100 times the global electricity power output, this huge power lasts only for the brief duration of its laser pulse—just 25 quintillionths of a second long.

“This milestone marks the beginning of experiments that move into unexplored territory for American high field science,” said Karl Krushelnick, director of the Gérard Mourou Center for Ultrafast Optical Science, which houses ZEUS.

Research at ZEUS will have applications in medicine, national security, materials science and astrophysics, in addition to plasma science and quantum physics. ZEUS is a user facility—meaning that research teams from all over the country and internationally can submit experiment proposals that go through an independent selection process.

“One of the great things about ZEUS is it’s not just one big laser hammer, but you can split the light into multiple beams,” said Franklin Dollar, professor of physics and astronomy at the University of California, Irvine, whose team is running the first user experiment at 2 petawatts.

“Having a national resource like this, which awards time to users whose experimental concepts are most promising for advancing scientific priorities, is really bringing high-intensity laser science back to the U.S.”

Dollar’s team and the ZEUS team aim to produce electron beams with energies equivalent to those made in particle accelerators that are hundreds of meters in length. This would be 5–10 times higher energy than any electron beams previously produced at the ZEUS facility.

“We aim to reach higher electron energies using two separate laser beams—one to form a guiding channel and the other to accelerate electrons through it,” said Anatoly Maksimchuk, U-M research scientist in electrical and computer engineering, who leads the development of the experimental areas.

They hope to do this in part with a redesigned target. They lengthened the cell that holds the gas that the laser pulse rams into, helium in this experiment. This interaction produces plasma, ripping electrons off the atoms so that the gas becomes a soup of free electrons and positively charged ions. Those electrons get accelerated behind the laser pulse-like wakesurfers close behind a speedboat—a phenomenon called wakefield acceleration.

Light moves slower through plasma, enabling the electrons to catch up to it. In a less dense, longer target, the electrons spend more time accelerating before they catch up to the laser pulse, enabling them to hit higher top speeds.

This demonstration of ZEUS’s power paves the way for the signature experiment, expected later this year, when the accelerated electrons will collide with laser pulses coming the opposite way. In the moving frame of the electrons, the 3-petawatt laser pulse will seem to be a million times more powerful—a zettawatt-scale pulse. This gives ZEUS its full name of “Zettawatt Equivalent Ultrashort laser pulse System.”

“The fundamental research done at the NSF ZEUS facility has many possible applications, including better imaging methods for soft tissues and advancing the technology used to treat cancer and other diseases,” said Vyacheslav Lukin, program director in the NSF Division of Physics, which oversees the ZEUS project.

“Scientists using the unique capabilities of ZEUS will expand the frontiers of human knowledge in new ways and provide new opportunities for American innovation and economic growth.”

The ZEUS facility fits in a space similar in size to a school gymnasium. At one corner of the room, a laser produces the initial infrared pulse. Optical devices called diffraction gratings stretch it out in time so that when the pump lasers dump power into the pulse, it doesn’t get so intense that it starts tearing the air apart. At its biggest, the pulse is 12 inches across and a few feet long.

After four rounds of pump lasers adding energy, the pulse enters the vacuum chambers. Another set of gratings flattens it to a 12-inch disk that is just 8 microns thick—about 10 times thinner than a piece of printer paper. Even at 12 inches across, its intensity could turn the air into plasma, but then it is focused down to 0.8 microns wide to deliver maximum intensity to the experiments.

“As a midscale-sized facility, we can operate more nimbly than large-scale facilities like particle accelerators or the National Ignition Facility,” said John Nees, U-M research scientist in electrical and computer engineering, who leads the ZEUS laser construction. “This openness attracts new ideas from a broader community of scientists.”

The road to 2 petawatts has been slow and careful. Just getting the pieces they need to assemble the system has been harder than expected. The biggest challenge is a sapphire crystal, infused with titanium atoms. Almost 7 inches in diameter, it is the critical component of the final amplifier of the system, which brings the laser pulse to full power.

“The crystal that we’re going to get in the summer will get us to 3 petawatts, and it took four and a half years to manufacture,” said Franko Bayer, project manager for ZEUS. “The size of the titanium sapphire crystal we have, there are only a few in the world.”

In the meantime, jumping from the 300 terawatt power of the previous HERCULES laser to just 1 petawatt on ZEUS resulted in worrying darkening of the gratings. First, they had to determine the cause: Were they permanently damaged or just darkened by carbon deposits from the powerful beam tearing up molecules floating in the imperfect vacuum chamber?

When it turned out to be carbon deposits, Nees and the laser team had to figure out how many laser shots could run safely between cleanings. If the gratings became too dark, they could distort the laser pulses in a way that damages optics further along the path.

Finally, the ZEUS team has already spent a total of 15 months running user experiments since the grand opening in October 2023 because there is still plenty of science that could be done with a 1 petawatt laser.

So far, it has welcomed 11 separate experiments with a total of 58 experimenters from 22 institutions, including international researchers. Over the next year—between user experiments—the ZEUS team will continue upgrading the system toward its full power.

My telescope, set up for astrophotography in my light-polluted San Diego backyard, was pointed at a galaxy unfathomably far from Earth. My wife, Cristina, walked up just as the first space photo streamed to my tablet. It sparkled on the screen in front of us.

“That’s the Pinwheel galaxy,” I said. The name is derived from its shape—albeit this pinwheel contains about a trillion stars.

The light from the Pinwheel traveled for 25 million years across the universe—about 150 quintillion miles—to get to my telescope.

My wife wondered: “Doesn’t light get tired during such a long journey?”

Her curiosity triggered a thought-provoking conversation about light. Ultimately, why doesn’t light wear out and lose energy over time?

Let’s talk about light

I am an astrophysicist, and one of the first things I learned in my studies is how light often behaves in ways that defy our intuitions.

Light is electromagnetic radiation: basically, an electric wave and a magnetic wave coupled together and traveling through space-time. It has no mass. That point is critical because the mass of an object, whether a speck of dust or a spaceship, limits the top speed it can travel through space.

But because light is massless, it’s able to reach the maximum speed limit in a vacuum—about 186,000 miles (300,000 kilometers) per second, or almost 6 trillion miles per year (9.6 trillion kilometers). Nothing traveling through space is faster. To put that into perspective: In the time it takes you to blink your eyes, a particle of light travels around the circumference of Earth more than twice.

As incredibly fast as that is, space is incredibly spread out. Light from the sun, which is 93 million miles (about 150 million kilometers) from Earth, takes just over eight minutes to reach us. In other words, the sunlight you see is eight minutes old.

Alpha Centauri, the nearest star to us after the sun, is 26 trillion miles away (about 41 trillion kilometers). So by the time you see it in the night sky, its light is just over four years old. Or, as astronomers say, it’s four light years away.

With those enormous distances in mind, consider Cristina’s question: How can light travel across the universe and not slowly lose energy?

Actually, some light does lose energy. This happens when it bounces off something, such as interstellar dust, and is scattered about.

But most light just goes and goes, without colliding with anything. This is almost always the case because space is mostly empty—nothingness. So there’s nothing in the way.

When light travels unimpeded, it loses no energy. It can maintain that 186,000-mile-per-second speed forever.

It’s about time
Here’s another concept: Picture yourself as an astronaut on board the International Space Station. You’re orbiting at 17,000 miles (about 27,000 kilometers) per hour. Compared with someone on Earth, your wristwatch will tick 0.01 seconds slower over one year.

That’s an example of time dilation—time moving at different speeds under different conditions. If you’re moving really fast, or close to a large gravitational field, your clock will tick more slowly than someone moving slower than you, or who is farther from a large gravitational field. To say it succinctly, time is relative.

Now consider that light is inextricably connected to time. Picture sitting on a photon, a fundamental particle of light; here, you’d experience maximum time dilation. Everyone on Earth would clock you at the speed of light, but from your reference frame, time would completely stop.

That’s because the “clocks” measuring time are in two different places going vastly different speeds: the photon moving at the speed of light, and the comparatively slowpoke speed of Earth going around the sun.

What’s more, when you’re traveling at or close to the speed of light, the distance between where you are and where you’re going gets shorter. That is, space itself becomes more compact in the direction of motion—so the faster you can go, the shorter your journey has to be. In other words, for the photon, space gets squished.

Which brings us back to my picture of the Pinwheel galaxy. From the photon’s perspective, a star within the galaxy emitted it, and then a single pixel in my backyard camera absorbed it, at exactly the same time. Because space is squished, to the photon the journey was infinitely fast and infinitely short, a tiny fraction of a second.

But from our perspective on Earth, the photon left the galaxy 25 million years ago and traveled 25 million light years across space until it landed on my tablet in my backyard.

And there, on a cool spring night, its stunning image inspired a delightful conversation between a nerdy scientist and his curious wife.

Researchers at the National Synchrotron Light Source II (NSLS-II), a U.S. Department of Energy (DOE) Office of Science user facility at DOE’s Brookhaven National Laboratory, depend on the facility’s bright, stable electron beam to carry out groundbreaking experiments. Behind the scenes, a dedicated team of physicists, engineers, designers, and technicians in the facility’s accelerator complex are not only maintaining this system for reliable operation but also looking into ways to improve performance and unlock new areas of synchrotron science for the light source’s research community.

In an inventive new design that has been years in the making, the team has unveiled a proof-of-principle prototype for a new “complex bend” lattice design. This unique magnet array has sparked discussion about some intriguing possibilities for the future of NSLS-II’s accelerator, and the design is lighting the way for necessary next steps.

Building on a legacy of accelerator innovation

Synchrotrons like NSLS-II use powerful magnets to steer and focus an ultrabright beam of electrons as the beam circulates around a circular track called a storage ring. This beam produces intense X-rays that are directed to experimental stations known as beamlines, where they are used for a wide range of scientific investigations.

For decades, light sources around the world have relied on the Chasman-Green lattice, a complex arrangement of magnets also known as the double bend achromat lattice. Developed in the 1970s by Brookhaven Lab scientists Renate Chasman and George Kenneth Green, this design has become a foundational element of modern synchrotron light sources.

The NSLS-II storage ring is currently comprised of 30 double-bend achromat cells that support the construction of nearly 60 beamlines. At this time, 29 beamlines are operational and several more are in various stages of development.

In 2018, a group of scientists in NSLS-II’s Accelerator Division, led by Division Director Timur Shaftan, began exploring a new design for an upgraded lattice that showed promise in achieving very strong localized focusing and minimized beam emittance. Emittance is the measurement of how much the electrons in a beam will spread, which is important to light source facilities. The tighter the beam is, the brighter it will be.

For this reason, accelerator physicists are on the lookout for ways to minimize emittance as much as they can. This quest helped motivate a novel “complex bend” lattice, which requires closely spaced, high-strength, combined-function magnets. These magnets are placed along a curved trajectory, known as a “Halbach” cylinder arrangement, which requires the use of compact permanent magnet quadrupoles (PMQs). These magnets would serve a combined function, meaning they would operate not only as a quadrupole but also as a dipole component if needed.

Permanent magnets, in contrast to the widely used electromagnets that are currently in use at NSLS-II, would bring several benefits, particularly in their size and efficiency, but they also present some interesting challenges, namely the design and fabrication of these unique magnet modules.

Evolving prototypes

To test this revolutionary design, the accelerator team began developing prototypes at multiple scales. The initial prototype—a small, low-energy version—paved the way for a full-scale, high-energy model designed in collaboration with the Lab’s Collider-Accelerator Department. The next step is to install two full-scale prototypes in the NSLS-II accelerator tunnel to demonstrate their operational feasibility, a major milestone that calls for careful planning and cautious optimism.

“We are planning for success,” said Bernard Kosciuk, a mechanical engineer in the Accelerator Division’s Instrumentation Group. “Before installation, we need to take rigorous magnetic measurements in the lab that will instill a high level of confidence in the prototype’s performance. We want to address any uncertainty to ensure we can commission the prototype.”

These prototypes are a critical step toward developing an accelerator that uses the complex bend design, an approach that could deliver major benefits to NSLS-II. These permanent magnets will not require utility infrastructure or maintenance, and they will likely reduce power consumption and cost by an estimated 80%, from 1.7 megawatts to 0.3.

The low emittance would increase brightness by an estimated factor of 10–100, depending on the photon energy, which would make experiments at the beamlines more efficient. The space saved by the compact size would leave ample room for insertion devices and other accelerator equipment, allowing for new possibilities at future beamlines.

The six-year journey from concept to full-scale prototype has already built up momentum fairly quickly. Guimei Wang, leader of the accelerator coordination group at NSLS-II, led a team that successfully demonstrated the first complex bend prototype using the NSLS-II linac beam, a 200 megaelectron volt test setup, and a scaled-down version of a 3 gigaelectron volt machine. This first proof of concept would operate at 15 times less energy than a full-scale version.

“There were some challenges with this novel design,” recalled Wang. “Strong focusing made it difficult to match optics perfectly and avoid beam blowout. The compact size also made it difficult to incorporate diagnostic equipment. We were able to find innovative ways around some of these issues, including using machine learning to tune the beam, a flexible vacuum chamber connection, and a micrometer-resolution imaging system.

“The initial prototype ended up being a successful step in the project, though. The design maintained high gradient, comparable to a full-scale magnet, while significantly reducing the magnet size.”

This initial work led to a three-year Laboratory Directed Research and Development proposal to scale up the prototype to a single girder assembly with combined-function PMQs and a vacuum chamber. Vacuum systems are integral to accelerators like this, but the radically different magnet design would place significant challenges on the vacuum chamber.

“The incredibly small aperture and straight magnetic elements required precision machining to accommodate a smooth beam radius while also allowing sufficient exit slot geometry,” explained Robert Todd, leader of the Vacuum Group at NSLS-II.

“This pushed the limits of machining and welding to create a thin-walled chamber, two meters in length, within tolerance and capable of withstanding vacuum forces. A careful design and machining approach led to a successful vendor collaboration and fabrication.”

“We had to come up with creative solutions because nothing like this had really been done before,” said Sushil Sharma, a senior advisor in the Accelerator Division and the lead on this project. “To keep our facility competitive with the rest of the world, we have to do a lot of research and development ourselves.”

There was another aspect of this prototype that needed to be realized. The low-energy version used simple PMQs that were able to focus and defocus, but bending was produced by displacing these magnets perpendicular to the beam. This new prototype needed to be designed to realize the multifunctional aspect of the magnets and allow bending to be produced by the PMQ itself at a larger size with optimized permanent magnet wedges, which required significant changes to their magnetization angles.

These changes, which were crucial to moving forward, were made possible through a collaboration with Brookhaven’s Collider-Accelerator Department. Stephen Brooks, an accelerator physicist within the department, was instrumental in developing the two-dimensional geometry for the PMQs.

Carefully closing in on the design of the future

An important aspect of the PMQs is their field quality, especially in combined function magnets. If a magnet is designed to have a dipole and quadrupole field, it should not show any other higher-order field. This was measured very precisely with a tiny copper coil that was created for the task in a collaboration with DOE’s Fermi National Accelerator Laboratory and ended up being a significant project in and of itself. Three of these coils were made and used to test the performance of this new magnetic measurement system. Passing those tests would lead to the biggest challenge yet.

“This prototype met the specifications we set, but to be installed in the storage ring, we need to validate its scalability, cost-effective production, and operational reliability,” said Sharma. “We’re currently working on the proposal to create two fully functional prototypes that fit these refined specifications and install them in the NSLS-II storage ring.”

These aren’t the only explorations that are happening. A team of scientists from NSLS-II’s Accelerator Division also participated in the recent “1,000 Scientist AI Jam Session” hosted by DOE in cooperation with OpenAI and Anthropic. At this event, teams of scientists and engineers from nine DOE national laboratories brought some of their toughest challenges to test advanced AI models and see how they could help with their research in the future.

“Our goal was to test the ability of AI tools to help us solve some of the problems related to the PMQ design in relation to the NSLS-II upgrade,” recalled Patrick N’Gotta, a magnet physicist in NSLS-II’s Accelerator Division who participated in the event.

“We investigated the mathematical model of the magnet, ideas for field correction, ways to improve our correction algorithm code, and the possibility of predicting a correction based on previous correction data. Overall, we’re convinced that AI is an interesting tool that can help our work and save us time if provided with the proper input and guidance.”

As designs are being tested and data is being analyzed from each study, NSLS-II staff and members of the greater light source community have been putting their heads together in a number of discussions and workshops.

In the last four years, the subject of an accelerator upgrade has been a topic of immense interest for scientists and engineers across the entire facility, with the complex bend being the core element for an upgraded machine. There is currently another large, exploratory science workshop with a focus on the accelerator upgrade being organized in early fall of 2025.

“Complex bend methodology conceived at NSLS-II is planned to become the main building block for a future facility upgrade, taking advantage of high-gradient, high-quality fields produced by complex permanent magnets for the low-emittance ring lattice design,” said Shaftan. “It will be really exciting to see the hard work that led up to where we are now become the accelerators of the future.”

Transistors are the fundamental building blocks behind today’s electronic revolution, powering everything from smartphones to powerful servers by controlling the flow of electrical currents. But imagine a parallel world, where we could apply the same level of control and sophistication—not to electricity, but to heat.

This is precisely the frontier being explored through quantum thermal transistors, devices designed to replicate electronic transistor functionality at the quantum scale, but for heat.

The rapidly growing field of quantum thermodynamics has been making impressive strides, exploring how heat and energy behave when quantum mechanical effects dominate. Innovations such as quantum thermal diodes, capable of directing heat flow in a specific direction, and quantum thermal transistors, which amplify heat flows similarly to how electronic transistors amplify electric signals, are groundbreaking examples of this progress.

These devices promise revolutionary advances in managing heat at nanoscale, critical for developing next-generation quantum and nanoscale technologies.

Despite this progress, the quantum thermal transistor lacked a comprehensive, practical model akin to the widely used Ebers-Moll model in electronics, which simplified complex transistor behaviors into understandable, manageable forms. Such models were instrumental in the rapid advancement and widespread adoption of electronic transistors, serving as fundamental tools for engineers and designers.

Addressing this crucial gap, our team at Monash University’s Advanced Computing and Simulation Laboratory (AχL), Australia, has developed a novel equivalent model for quantum thermal transistors.

This innovative model, recently published in APL Quantum, leverages a unique quantum analogy to the Ebers-Moll electronic transistor model.

From Ebers-Moll model to quantum thermal transistors
We focused on a quantum thermal transistor consisting of two quantum two-level systems (qubits) interacting with a three-level system (qutrit), which collectively mimic the behavior of a traditional electronic transistor, but with heat instead of electric current.

Our research demonstrates that this quantum thermal transistor behavior can be effectively captured and explained using a simplified, yet powerful equivalent model composed of two quantum thermal diodes connected in a configuration analogous to the classical Ebers-Moll model.

This not only makes quantum thermal transistor technology more accessible and intuitive but also provides critical insights into optimizing their operation, such as determining ideal coupling strengths for maximum thermal amplification.

This development represents a significant step forward, laying a foundational framework that parallels the success of classical transistor models in electronics. By enabling clearer visualization, simulation, and design of quantum thermal circuits, this model opens the door to transformative advancements in thermotronic technologies with applications in thermal management.

Ultimately, translating electronic principles into thermal counterparts at the quantum scale represents a frontier of technological innovation, promising a new era where thermal energy is precisely managed much like electrical current in electronics, shaping the future of sustainable, efficient, and powerful quantum technologies.

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.

The orbital angular momentum of electrons has long been considered a minor physical phenomenon, suppressed in most crystals and largely overlooked. Scientists at Forschungszentrum Jülich have now discovered that in certain materials it is not only preserved but can even be actively controlled. This is due to a property of the crystal structure called chirality, which also influences many other processes in nature.

The discovery has the potential to lead to a new class of electronic components capable of transmitting information with exceptional robustness and energy efficiency.

From electronics to spintronics, and now to orbitronics: In classical electronics, it is primarily the charge of the electron that counts. In modern approaches such as quantum computing and spintronics, the focus has shifted to the electron’s spin.

Now, another property is entering the spotlight: orbital angular momentum (OAM). In simple terms, OAM describes how the electron moves within an atom—not in a classical orbit, but as a quantum mechanical distribution within an orbital.

“For decades, spin was considered the key parameter for new quantum-based technologies. But orbital angular momentum also has great potential as an information carrier—and is significantly more robust,” explains Dr. Christian Tusche from the Peter Grünberg Institute (PGI-6) at Forschungszentrum Jülich. The physicist is one of the lead authors of the study published in Advanced Materials.

The orbital angular momentum is one of the fundamental quantum numbers of the electron, similar to spin, which describes the apparent rotation of the electron. However, OAM is rarely observable in crystals. It is usually suppressed by the symmetrical electric and magnetic fields in the crystal lattice—an effect known as “quenching.”

In so-called chiral materials such as the cobalt silicide (CoSi) studied, this is different, as the team led by Dr. Tusche, together with partners in Taiwan, Japan, Italy, the U.S., and Germany, has now been able to show. The word “chiral” comes from the ancient Greek “cheir” for hand.

“These crystal structures lack mirror symmetry and are either left- or right-handed—just like the human hand. You can turn them around and they remain mirror images of each other,” explains Dr. Tusche. Chirality occurs frequently in nature. Sugar molecules, amino acids, and DNA all exhibit chiral structures.

Using high-resolution momentum microscopy and circularly polarized light, the researchers were able to resolve the orbital angular momentum in the chiral semiconductor for the first time—both inside the crystal and on its surface.

For the measurements, they used the NanoESCA momentum microscope operated by Forschungszentrum Jülich at the Elettra synchrotron in Trieste, Italy. They discovered that the handedness of the crystal—left- or right-handed—predictably affects the orbital angular momentum of the electrons.

Textures of the orbital angular momentum with mirror-image Fermi arcs that depend on the handedness of the crystal. Credit: Advanced Materials (2025). DOI: 10.1002/adma.202418040
New link between crystal structure and electron
“Our results show that the structure of the crystal directly influences the angular momentum of the electrons—an effect that we were able to measure directly. This opens up a whole new door for materials research and information processing,” emphasizes experimental physicist Dr. Ying-Jiun Chen.

Dr. Dongwook Go, theoretical physicist at the Peter Grünberg Institute (PGI-1) in Jülich, adds, “The discovery is particularly important for the emerging field of orbitronics, which uses orbital angular momentum as an information carrier for the next generation of quantum technology.”

A characteristic feature of the resulting orbital angular momentum texture are differently formed Fermi arcs: open, arc-shaped structures that become visible in so-called momentum space representations, as generated by momentum microscopy.

This opens up new perspectives for applications. In the future, information could be transmitted and stored not just via the charge or spin of electrons, but also through the direction and orientation of their orbital angular momentum. This so-called orbitronics—electronics based on orbital properties—could thus provide the foundation for a new class of electronic devices.

Applications
The development of this future technology is part of the EIC Pathfinder project OBELIX, in which Prof. Yuriy Mokrousov from the University of Mainz is also involved. The theoretical physicist is also group leader at the Peter Grünberg Institute (PGI-1) in Jülich and contributed fundamental theoretical models to the recent discovery.

Prof. Claus Michael Schneider also sees great promise. “For instance, it seems conceivable to use orbital angular momentum as an information carrier. Or one might employ circularly polarized light to selectively influence a crystal’s chirality, enabling a light-controlled, non-mechanical switch as an alternative to the transistor.

“Furthermore, coupling between orbital angular momentum and spin could allow integration into existing spintronics concepts—for example, in hybrid quantum devices,” says the director of the Peter Grünberg Institute for Electronic Properties (PGI-6) at Forschungszentrum Jülich.

In conventional heat-assisted magnetic recording (HAMR), a laser is used to locally heat the recording medium to facilitate data writing. However, the thermal energy applied is largely dissipated within the medium and does not contribute directly to the recording efficiency. Moreover, this high-temperature process consumes substantial energy and raises concerns regarding the magnetic and physical degradation of the medium, especially under repeated use.

The research team focused on the temperature gradient generated within the recording medium during laser irradiation. They developed a novel structure by inserting an antiferromagnetic manganese-platinum (MnPt) layer beneath the iron-platinum (FePt) recording layer. This structure achieved approximately 35% improvement in recording efficiency compared to conventional HAMR.

This enhancement stems from spin currents generated by the temperature gradient, which induce spin torque that assists magnetic switching—effectively augmenting the conventional thermal assist effect. Furthermore, the study demonstrated that spin torque can be applied to hard disk drives (HDDs), paving the way for a new class of recording technologies.

Building on these results, the team aims to apply the technology to FePt nanogranular media and advance TST-HAMR as a practical recording method for future HDDs. This could lead to higher-capacity and more energy-efficient HDDs, contributing to the advancement of next-generation storage technologies.

by National Institute for Materials Science

edited by Gaby Clark, reviewed by Robert Egan

A recent study has mathematically clarified how the presence of crystals and gas bubbles in magma affects the propagation of seismic P-waves. The researchers derived a new equation that characterizes the travel of these waves through magma, revealing how the relative proportions of crystals and bubbles influence wave velocity and waveform properties.

The ratio of crystals to bubbles in subterranean magma reservoirs is crucial for forecasting volcanic eruptions. Due to the inaccessibility of direct observations, scientists analyze seismic P-waves recorded at the surface to infer these internal characteristics.

Previous studies have predominantly focused on the influence of gas bubbles, with limited consideration given to crystal content. Moreover, conventional models have primarily addressed variations in wave velocity and amplitude decay, without capturing detailed waveform transformations.

In the study, published in Physics of Fluids, the researchers developed a new equation by integrating two distinct mathematical models of magma flow. The results show that P-wave velocity decreases as the proportion of bubbles increases relative to crystals, with bubbles exerting a more significant influence than crystals.

Conversely, attenuation effects were found to be more strongly affected by crystals. The analysis further revealed that waveform characteristics depend on frequency and bubble content, with discernible differences emerging between the two underlying models.

The new equation enables the time-dependent calculation of P-waveforms based on the bubble and crystal content in magma. Looking ahead, the research team intends to integrate this model with machine learning techniques to estimate the internal composition of magma from observed P-waveforms, with the goal of enhancing the accuracy of volcanic eruption prediction systems.

by University of Tsukuba

edited by Lisa Lock, reviewed by Robert Egan

Magnets and superconductors go together like oil and water—or so scientists have thought. But a new finding by MIT physicists is challenging this century-old assumption.

In a paper appearing in the journal Nature, the physicists report that they have discovered a “chiral superconductor”—a material that conducts electricity without resistance, and also, paradoxically, is intrinsically magnetic. What’s more, they observed this exotic superconductivity in a surprisingly ordinary material: graphite, the primary material in pencil lead.

Graphite is made from many layers of graphene—atomically thin, lattice-like sheets of carbon atoms—that are stacked together and can easily flake off when pressure is applied, as when pressing down to write on a piece of paper. A single flake of graphite can contain several million sheets of graphene, which are normally stacked such that every other layer aligns. But every so often, graphite contains tiny pockets where graphene is stacked in a different pattern, resembling a staircase of offset layers.

The MIT team has found that when four or five sheets of graphene are stacked in this “rhombohedral” configuration, the resulting structure can exhibit exceptional electronic properties that are not seen in graphite as a whole.

In their new study, the physicists isolated microscopic flakes of rhombohedral graphene from graphite, and subjected the flakes to a battery of electrical tests. They found that when the flakes are cooled to 300 millikelvins (about -273 degrees Celsius), the material turns into a superconductor, meaning that any electrical current passing through the material can flow through without resistance.

They also found that when they swept an external magnetic field up and down, the flakes could be switched between two different superconducting states, just like a magnet. This suggests that the superconductor has some internal, intrinsic magnetism. Such switching behavior is absent in other superconductors.

“The general lore is that superconductors do not like magnetic fields,” says Long Ju, assistant professor of physics at MIT. “But we believe this is the first observation of a superconductor that behaves as a magnet with such direct and simple evidence. And that’s quite a bizarre thing because it is against people’s general impression of superconductivity and magnetism.”

Ju is senior author of the study, which includes MIT co-authors Tonghang Han, Zhengguang Lu, Zach Hadjri, Lihan Shi, Zhenghan Wu, Wei Xu, Yuxuan Yao, Jixiang Yang, Junseok Seo, Shenyong Ye, Muyang Zhou, and Liang Fu, along with collaborators from Florida State University, the University of Basel in Switzerland, and the National Institute for Materials Science in Japan.

Graphene twist
In everyday conductive materials, electrons flow through in a chaotic scramble, whizzing by each other, and pinging off the material’s atomic latticework. Each time an electron scatters off an atom, it has—in essence—met some resistance, and loses some energy as a result, normally in the form of heat. In contrast, when certain materials are cooled to ultracold temperatures, they can become superconducting, meaning that the material can allow electrons to pair up, in what physicists term “Cooper pairs.”

Rather than scattering away, these electron pairs glide through a material without resistance. With a superconductor, then, no energy is lost in translation.

Since superconductivity was first observed in 1911, physicists have shown many times that zero electrical resistance is a hallmark of a superconductor. Another defining property was first observed in 1933, when the physicist Walther Meissner discovered that a superconductor will expel an external magnetic field. This “Meissner effect” is due in part to a superconductor’s electron pairs, which collectively act to push away any magnetic field.

Physicists have assumed that all superconducting materials should exhibit both zero electrical resistance, and a natural magnetic repulsion. Indeed, these two properties are what could enable magnetic levitation (Maglev) trains, whereby a superconducting rail repels and therefore levitates a magnetized car.

Ju and his colleagues had no reason to question this assumption as they carried out their experiments at MIT. In the last few years, the team has been exploring the electrical properties of pentalayer rhombohedral graphene. The researchers have observed surprising properties in the five-layer, staircase-like graphene structure; most recently, that it enables electrons to split into fractions of themselves. This phenomenon occurs when the pentalayer structure is placed atop a sheet of hexagonal boron nitride (a material similar to graphene), and slightly offset by a specific angle, or twist.

Curious as to how electron fractions might change with changing conditions, the researchers followed up their initial discovery with similar tests, this time by misaligning the graphene and hexagonal boron nitride structures. To their surprise, they found that when they misaligned the two materials and sent an electrical current through, at temperatures less than 300 millikelvins, they measured zero resistance. It seemed that the phenomenon of electron fractions disappeared, and what emerged instead was superconductivity.

The researchers went a step further to see how this new superconducting state would respond to an external magnetic field. They applied a magnet to the material, along with a voltage, and measured the electrical current coming out of the material. As they dialed the magnetic field from negative to positive (similar to a north and south polarity) and back again, they observed that the material maintained its superconducting, zero-resistance state, except in two instances, once at either magnetic polarity.

In these instances, the resistance briefly spiked, before switching back to zero, and returning to a superconducting state.

“If this were a conventional superconductor, it would just remain at zero resistance, until the magnetic field reaches a critical point, where superconductivity would be killed,” says Zach Hadjri, a first-year student in the group. “Instead, this material seems to switch between two superconducting states, like a magnet that starts out pointing upward, and can flip downwards when you apply a magnetic field. So it looks like this is a superconductor that also acts like a magnet, which doesn’t make any sense.”

‘One of a kind’
As counterintuitive as the discovery may seem, the team observed the same phenomenon in six similar samples. They suspect that the unique configuration of rhombohedral graphene is the key. The material has a very simple arrangement of carbon atoms. When cooled to ultracold temperatures, the thermal fluctuation is minimized, allowing any electrons flowing through the material to slow down, sense each other, and interact.

Such quantum interactions can lead electrons to pair up and superconduct. These interactions can also encourage electrons to coordinate. Namely, electrons can collectively occupy one of two opposite momentum states, or “valleys.” When all electrons are in one valley, they effectively spin in one direction, versus the opposite direction. In conventional superconductors, electrons can occupy either valley, and any pair of electrons is typically made from electrons of opposite valleys that cancel each other out. The pair overall then has zero momentum, and does not spin.

In the team’s material structure, however, they suspect that all electrons interact such that they share the same valley, or momentum state. When electrons then pair up, the superconducting pair overall has a “non-zero” momentum, and spinning, that—along with many other pairs—can amount to an internal, superconducting magnetism.

“You can think of the two electrons in a pair spinning clockwise, or counterclockwise, which corresponds to a magnet pointing up, or down,” Tonghang Han, a fifth-year student in the group, explains. “So we think this is the first observation of a superconductor that behaves as a magnet due to the electrons’ orbital motion, which is known as a chiral superconductor. It’s one of a kind. It is also a candidate for a topological superconductor, which could enable robust quantum computation.”

“Everything we’ve discovered in this material has been completely out of the blue,” says Zhengguang Lu, a former postdoc in the group and now an assistant professor at Florida State University. “But because this is a simple system, we think we have a good chance of understanding what is going on, and could demonstrate some very profound and deep physics principles.”

“It is truly remarkable that such an exotic chiral superconductor emerges from such simple ingredients,” adds Liang Fu, professor of physics at MIT. “Superconductivity in rhombodedral graphene will surely have a lot to offer.”