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

Electromagnetic noise poses a major problem for communications, prompting wireless carriers to invest heavily in technologies to overcome it. But for a team of scientists exploring the atomic realm, measuring tiny fluctuations in noise could hold the key to discovery.

“Noise is usually thought of as a nuisance, but physicists can learn many things by studying noise,” said Nathalie de Leon, an associate professor of electrical and computer engineering at Princeton University. “By measuring the noise in a material, they can learn its composition, its temperature, how electrons flow and interact with one another, and how spins order to form magnets. It is generally difficult to measure anything about how the noise changes in space or time.”

Using specially designed diamonds, a team of researchers at Princeton and the University of Wisconsin-Madison have developed a technique to measure noise in a material by studying correlations, and they can use this information to learn the spatial structure and time-varying nature of the noise. This technique, which relies on tracking tiny fluctuations in magnetic fields, represents a stark improvement over previous methods that averaged many separate measurements.

De Leon is a leader in the fabrication and use of highly controlled diamond structures called nitrogen-vacancy (NV) centers. These NV centers are modifications to a diamond’s lattice of carbon atoms in which a carbon is replaced by a nitrogen atom, and adjacent to it is an empty space, or vacancy, in the molecular structure. Diamonds with NV centers are one of the few tools that can measure changes in magnetic fields at the scale and speed needed for critical experiments in quantum technology and condensed matter physics.

While a single NV center allowed scientists to take detailed readings of magnetic fields, it was only when de Leon’s team worked out a method to harness multiple NV centers simultaneously that they were able to measure the spatial structure of noise in a material. This opens the door to understanding the properties of materials with bizarre quantum behaviors that until now have been analyzed only theoretically, said de Leon, the senior author of a paper describing the technique published online Dec. 22 in the journal Science.

“It’s a fundamentally new technique,” said de Leon. “It’s been clear from a theoretical perspective that it would be very powerful to be able to do this. The audience that I think is most excited about this work is condensed matter theorists, now that there’s this whole world of phenomena they might be able to characterize in a different way.”

One of these phenomena is a quantum spin liquid, a material first explored in theories nearly 50 years ago that has been difficult to characterize experimentally. In a quantum spin liquid, electrons are constantly in flux, in contrast to the solid-state stability that characterizes a typical magnetic material when cooled to a certain temperature.

“The challenging thing about a quantum spin liquid is that by definition there’s no static magnetic ordering, so you can’t just map out a magnetic field” the way you would with another type of material, said de Leon. “Until now there’s been essentially no way to directly measure these two-point magnetic field correlators, and what people have instead been doing is trying to find complicated proxies for that measurement.”

By simultaneously measuring magnetic fields at multiple points with diamond sensors, researchers can detect how electrons and their spins are moving across space and time in a material. In developing the new method, the team applied calibrated laser pulses to a diamond containing NV centers, and then detected two spikes of photon counts from a pair of NV centers—a readout of the electron spins at each center at the same point in time. Previous techniques would have taken an average of these measurements, discarding valuable information and making it impossible to distinguish the intrinsic noise of the diamond and its environment from the magnetic field signals generated by a material of interest.

“One of those two spikes is a signal we’re applying, the other is a spike from the local environment, and there’s no way to tell the difference,” said study coauthor Shimon Kolkowitz, an associate professor of physics at the University of Wisconsin-Madison. “But when we look at the correlations, the one that is correlated is from the signal we’re applying and the other is not. And we can measure that, which is something people couldn’t measure before.”

Kolkowitz and de Leon met as Ph.D. students at Harvard University, and have been in touch frequently since then. Their research collaboration arose early in the COVID-19 pandemic, when laboratory research slowed, but long-distance collaboration became more attractive as most interactions took place over Zoom, said de Leon.

Jared Rovny, the study’s lead author and a postdoctoral research associate in de Leon’s group, led both the theoretical and experimental work on the new method. Contributions by Kolkowitz and his team were critical to designing the experiments and understanding the data, said de Leon. The paper’s coauthors also included Ahmed Abdalla and Laura Futamura, who conducted summer research with de Leon’s team in 2021 and 2022, respectively, as interns in the Quantum Undergraduate Research at IBM and Princeton (QURIP) program, which de Leon cofounded in 2019.

The article, “Nanoscale covariance magnetometry with diamond quantum sensors,” was published online Dec. 22 in Science.

More information: Jared Rovny et al, Nanoscale covariance magnetometry with diamond quantum sensors, Science (2022). DOI: 10.1126/science.ade9858

Journal information: Science 

Provided by Princeton University 

Will an electron escaping a molecule through a quantum tunnel behave differently depending on the left- or right-handedness of the molecule?

Chemists have borrowed the phrases “left-handed” and “right-handed” from anatomy to describe molecules that are characterized by a particular type of asymmetry. To explore the concept of chirality, look at your hands, palms up. Clearly, the two are mirror images of one another. But try as we might to superimpose them, they will not overlap completely. Such objects, termed “chiral,” can be found at all scales in nature, from galaxies down to molecules.

Each day, we experience chirality not only when we grab an object or put on our shoes but also when we eat or breathe: our taste and smell can distinguish two mirror images of a chiral molecule. In fact, our body is so sensitive to chirality that a molecule can be a medicine and its mirror image a poison. Chirality is thus crucial in pharmacology, where 90 percent of synthesized drugs are chiral compounds.

Chiral molecules have particular symmetry properties that make them great candidates for the investigation of fundamental phenomena in physics. Recently, the research teams led by Prof. Yann Mairesse from CNRS / Bordeaux University and Prof. Nirit Dudovich of the Weizmann Institute’s Department of Physics of Complex Systems used chirality to shed new light on one of the most intriguing quantum phenomena: the tunneling process.

Tunneling is a phenomenon in which quantum particles cross seemingly impossible-to-cross physical barriers. Since this motion is forbidden in classical mechanics, it is very difficult to establish an intuitive picture of its dynamics. To create a tunnel in chiral molecules, the researchers exposed them to an intense laser field. “The electrons of the molecules are naturally bound around the nuclei by an energy barrier,” explains Mairesse. “You can imagine the electrons as air trapped inside an inflatable balloon. The strong laser fields have the ability to reduce the thickness of the balloon enough for some air to tunnel through it, even though there’s no hole in the balloon.”

Mairesse, Dudovich and their teams set out to study an as-of-yet-unexplored aspect of tunneling: the moment in which a chiral molecule meets a chiral light field, and the way in which their brief encounter affects electron tunneling. “We were very excited to explore the connection between chirality and tunneling. We were keen to learn more about what tunneling would look like under these particular circumstances,” says Dudovich.

It only takes a few hundred attoseconds for an electron to escape an atom or molecule. Such minuscule time frames characterize many of the processes studied in Mairesse’s and Dudovich’s labs. The two teams asked the following question: How does the chirality of a molecule affect the escape of an electron?

“We used a laser field that rotates in time to spin the barrier around the chiral molecules,” says Mairesse. “To follow up with the balloon metaphor, if the laser field rotates horizontally, you expect the air to exit the balloon on the horizontal plane, following the direction of the laser field. What we found is that if the balloon is chiral, the air exits the balloon flying towards the floor or the ceiling, depending on the rotation direction of the laser. In other words, the electrons emerge from the chiral tunnel with a memory of the rotation direction of the barrier. This is very much like the effect of a corkscrew, but at the nanometer and attosecond scales.”

The two teams thus discovered that the likelihood that an electron will undergo tunneling, the phase at which the electron tunnels out and the timing of the tunneling event depend on the chirality of the molecule. These exciting results lay the groundwork for additional studies that will use the unique symmetry properties of chiral molecules to investigate the fastest processes occurring in light-matter interaction.

The paper is published in the journal Physical Review X.

More information: E. Bloch et al, Revealing the Influence of Molecular Chirality on Tunnel-Ionization Dynamics, Physical Review X (2021). DOI: 10.1103/PhysRevX.11.041056

Journal information: Physical Review X 

Provided by Weizmann Institute of Science 

Researchers have uncovered a previously hidden heating process that helps explain how the atmosphere that surrounds the sun called the “solar corona” can be vastly hotter than the solar surface that emits it.

The discovery at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) could improve tackling a range of astrophysical puzzles such as star formation, the origin of large-scale magnetic fields in the universe, and the ability to predict eruptive space weather events that can disrupt cell phone service and black out power grids on Earth. Understanding the heating process also has implications for fusion research.

First clear 3D explanation

“Our direct numerical simulation is the first to provide clear identification of this heating mechanism in 3D space,” said Chuanfei Dong, a physicist at PPPL and Princeton University who unmasked the process by conducting 200 million hours of computer time for the world’s largest simulation of its kind. “Current telescope and spacecraft instruments may not have high enough resolution to identify the process occurring at small scales,” said Dong, who details the breakthrough in the journal Science Advances.

The hidden ingredient is a process called magnetic reconnection that separates and violently reconnects magnetic fields in plasma, the soup of electrons and atomic nuclei that forms the solar atmosphere. Dong’s simulation revealed how rapid reconnection of the magnetic field lines turns the large-scale turbulent energy into small-sale internal energy. As a consequence the turbulent energy is efficiently converted to thermal energy at small scales, thus superheating the corona.

“Think of putting cream in coffee,” Dong said. “The drops of cream soon become whorls and slender curls. Similarly, magnetic fields form thin sheets of electric current that break up due to magnetic reconnection. This process facilitates the energy cascade from large-scale to small-scale, making the process more efficient in the turbulent solar corona than previously thought.”

When the reconnection process is slow while the turbulent cascade is fast, reconnection cannot affect the transfer of energy across scales, he said. But when the reconnection rate becomes fast enough to exceed the traditional cascade rate, reconnection can move the cascade toward small scales more efficiently.

It does this by breaking and rejoining the magnetic field lines to generate chains of small twisted lines called plasmoids. This changes the understanding of the turbulent energy cascade that has been widely accepted for more than half a century, the paper says. The new finding ties the energy transfer rate to how fast the plasmoids grow, enhancing the transfer of energy from large to small scales and strongly heating the corona at these scales.

The new discovery demonstrates a regime with an unprecedentedly large magnetic Reynolds number as in the solar corona. The large number characterizes the new high energy transfer rate of the turbulent cascade. “The higher the magnetic Reynolds number is, the more efficient the reconnection-driven energy transfer is,” said Dong, who is moving to Boston University to take up a faculty position.

200 million hours

“Chuanfei has carried out the world’s largest turbulence simulation of its kind that has taken over 200 million computer CPUs [central processing units] at the NASA Advanced Supercomputing (NAS) facility,” said PPPL physicist Amitava Bhattacharjee, a Princeton professor of astrophysical sciences who supervised the research. “This numerical experiment has produced undisputed evidence for the first time of a theoretically predicted mechanism for a previously undiscovered range of turbulent energy cascade controlled by the growth of the plasmoids.

“His paper in the high-impact journal Science Advances completes the computational program he began with his earlier 2D results published in Physical Review Letters. These papers form a coda to the impressive work that Chuanfei has done as a member of the Princeton Center for Heliophysics, a joint Princeton and PPPL facility. We are grateful for a PPPL LDRD [Laboratory Directed Research & Development] grant that facilitated this work, and to the NASA High-End Computing (HEC) program for its generous allocation of computer time.”

The impact of this finding in astrophysical systems across a range of scales can be explored with current and future spacecraft and telescopes. Unpacking the energy transfer process across scales will be crucial to solving key cosmic mysteries, the paper said.

More information: Chuanfei Dong et al, Reconnection-driven energy cascade in magnetohydrodynamic turbulence, Science Advances (2022). DOI: 10.1126/sciadv.abn7627

Journal information: Physical Review Letters  , Science Advances 

Provided by Princeton Plasma Physics Laboratory 

The breakthrough came in an impossibly small slice of time, less than it takes a beam of light to move an inch. In that tiny moment, nuclear fusion as an energy source went from far-away dream to reality. The world is now grappling with the implications of the historic milestone. For Arthur Pak and the countless other scientists who’ve spent decades getting to this point, the work is just beginning.

Pak and his colleagues at Lawrence Livermore National Laboratory are now faced with a daunting task: Do it again, but better—and bigger.

That means perfecting the use of the world’s largest laser, housed in the lab’s National Ignition Facility that science-fiction fans will recognize from the film “Star Trek: Into Darkness,” when it was used as a set for the warp core of the starship Enterprise. Just after 1 a.m. on Dec. 5, the laser shot 192 beams in three carefully modulated pulses at a cylinder containing a tiny diamond capsule filled with hydrogen, in an attempt to spark the first fusion reaction that produced more energy than it took to create. It succeeded, starting the path toward what scientists hope will someday be a new, carbon-free power source that will allow humans to harness the same source of energy that lights the stars.

Pak, who joined the Lawrence Livermore lab outside San Francisco in 2010, woke at 3 a.m. that day, unable to resist checking the initial results from his San Jose home. He’d tried staying awake for the shot itself, finally giving up as the experiment’s painstaking preparations dragged late into the night. “If you stayed up for every shot, every time for 10 years, you’d go insane,” he said.

For the last several months, it was clear his team was getting close, and in the pre-dawn dark, he checked for a key number that could show whether they’d succeeded—a count of neutrons the blast produced.

“When I saw that number, I was blown away,” he said.

“You can work your whole career and never see this moment. You’re doing it because you believe in the destination, and you like the challenge,” said Pak, leader for diagnostics on the experiment. “When humans come together and work collectively, we can do amazing things.”

The team at Lawrence Livermore—a government-funded research lab—will likely run its next test in February, with several more experiments to come in the months after. The goal will be to keep increasing the amount of energy that’s produced in the reaction. The means more tinkering: Use more laser energy. Fine-tune the laser blast. Generate more X-rays within the target—a key step of the process—using the same amount of energy. Maybe, eventually, upgrade the facility itself, a decision that would require buy-in from the Energy Department and a huge amount of funding.

All of that will take years, if not decades, starting with the Lawrence Livermore lab’s bite-sized experiments that last just nanoseconds.

“We need to figure out: Can we make it simpler? Can we make this process easier and more repeatable? Can we begin to do it more than one time a day?” said Kim Budil, director of the Lawrence Livermore lab. “Each of these is an incredible scientific and engineering challenge for us.”

Most experts forecast that the world is still at least 20 to 30 years away from fusion technology becoming viable on a scale that’s large and affordable enough to produce commercial power. That timeline places fusion beyond the scope of significantly being used to reach the world’s net-zero emissions goals by 2050. In that sense, fusion could be the carbon-free energy source of the future, but not of the current global energy transition that’s faced continuous hurdles.

Fusion has captured the scientific imagination for decades. It’s already used to give modern nuclear weapons their devastating power, but the dream is taming it for civilian energy demand. If it can be brought to scale, it would lead to power plants that supply abundant electricity day and night without emitting greenhouse gases. And unlike the nuclear power of today, sparked through a process called fission, it wouldn’t create long-lived radioactive waste. Entire generations of scientists have pursued it. President Joe Biden’s chief science advisor, Arati Prabhakar, spent a summer working on the lab’s laser-fusion program as a 19-year-old college student in bell bottoms—in 1978.

“This is such a tremendous example of what perseverance can achieve,” she said at a press conference last week. “This is how you do really big, hard things.”

Merging atoms

The successful laser shot produced fusion reactions generating 3.15 megajoules of power, topping the 2.05 megajoules imparted by the laser. It was a major threshold, the first time more energy came out than went in from the laser. But the equation needs to tilt much more in the direction of how much comes out to become commercially viable.

While today’s nuclear power plants employ fission, splitting atoms apart, fusion merges atoms together. Fusion researchers have followed two primary tracks. Lawrence Livermore, using a process called inertial confinement, blasts targets with laser beams, imploding a small amount of hydrogen until it fuses into helium. A commercial plant using this approach would need to repeat the process over and over again, extremely rapidly, to generate enough energy to power the electric grid.

Numerous companies are developing inertial confinement systems, though there are significant differences. Some are looking at different materials for the target, while others use particle accelerators instead of lasers, triggering the fusion reaction by slamming atoms together.

The main competing idea is called magnetic confinement, with systems that create a cloud of plasma, superheated to hundreds of millions of degrees, which can trigger a fusion reaction. Powerful magnets control the plasma and sustain the reaction. This approach has not yet achieved a net-energy gain, and the approach faces challenges including developing better magnets and creating materials that can withstand superhot temperatures and be used for the container to contain the plasma.

About $5 billion in funding has gone into fusion companies to date, with the vast majority aimed at magnetic confinement technologies, according to the Fusion Industry Association trade group.

Inertial confinement may be better suited to proving that fusion can work, said Adam Stein, director of nuclear energy innovation at The Breakthrough Institute, an Oakland, California-based research group. But in the longer run when it comes to commercialization, “plasma magnetic confinement is more likely to succeed,” he said.

‘Be an optimist’

Years were spent refining each part of the process at the Lawrence Livermore lab.

A lot of the success came down to precision. The fuel capsules all contain minute imperfections that can make a significant difference in how the reaction proceeds. So can the frozen hydrogen inside, a mix of the isotopes deuterium and tritium. The team would often produce the hydrogen ice, melt it back down and try again several times before a shot, hoping to get the best possible target and increase the chances of success.

Everyone working on fusion “has to be an optimist,” said Denise Hinkel, a physicist who focuses on improving the predictive ability of the program’s computer simulations and who has worked at Lawerence Livermore for 30 years. “Otherwise, you wouldn’t stay in the field.”

By this summer, the giant laser will be able to deliver about 8% more energy than it did during this month’s shot, according to Jean-Michel Di Nicola, chief engineer for the National Ignition Facility’s laser. Michael Stadermann, the target fabrication program manager, said that the lab is also developing a computer program that can examine the fuel-capsule shells for flaws much faster than humans can. They’re also working with capsule maker on improving the fabrication process.

It’s possible that the Lawrence Livermore breakthrough will remain just a moment of scientific history, and not mark the beginning of a new fusion industry powering the globe. Bridging the gap from experiment to commercialization could take decades, if it happens at all. And magnetic confinement could eventually be the fusion method that wins out, providing the world abundant clean energy. Pak, a soft-spoken man with a swoop of brown hair and a quick wit, said that outcome wouldn’t disappoint him.

“They can learn from us—we can learn from them,” Pak, 40, said. “When I’m an old man, I’m going to be really satisfied with my contributions.”

2022 Bloomberg L.P. Visit bloomberg.com. Distributed by Tribune Content Agency, LLC.

The precise control of micro-mechanical oscillators is fundamental to many contemporary technologies, from sensing and timing to radiofrequency filters in smartphones. Over the past decade, quantum control of mechanical systems has been firmly established with atoms, molecules, and ions in the first wave of development and superconducting circuits in the second quantum revolution.

This has been in particular catalyzed by cavity optomechanics. The field has allowed us to control mesoscopic mechanical objects with electromagnetic radiation pressure force. This has substantially improved our understanding of their quantum nature, which has enabled a host of advances including ground-state cooling, quantum squeezing, and remote entanglement of mechanical oscillators.

Pioneering theoretical studies have predicted that significantly richer physics and novel dynamics can be accessed in optomechanical lattices, including quantum collective dynamics and topological phenomena. But reproducing such devices experimentally under high control, as well as and building optomechanical lattices that can host multiple coupled optical and mechanical degrees of freedom has been a challenge.

Researchers in the group of Tobias J. Kippenberg at EPFL’s School of Basic Sciences have now built the first large-scale and configurable superconducting circuit optomechanical lattice that can overcome the scaling challenges of quantum optomechanical systems. The team realized an optomechanical strained graphene lattice and studied non-trivial topological edge states using novel measurement techniques. This work is now published in Nature.

The key element, which is a part of the single site of the lattice, is a so-called “vacuum-gap drumhead capacitor,” which is made of a thin aluminum film suspended over a trench in a silicon substrate. This constitutes the vibrating part of the device and, at the same time, forms a resonant microwave circuit with a spiral inductor.

“We developed a novel nanofabrication technique for superconducting circuit optomechanical systems with high reproducibility and extremely tight tolerances on the parameters of the individual devices,” says Amir Youssefi, who led the project. “This enables us to engineer the different sites to be virtually identical, like in a natural lattice.”

The graphene lattice is well known to exhibit non-trivial topological properties and localized edge states. The researchers observed such states in what they call an “optomechanical graphene flake,” consisting of twenty-four sites.

“Thanks to the built-in optomechanical toolkit, we were able to directly and non-perturbatively image the collective electromagnetic mode shapes in such lattices,” says Andrea Bancora who contributed to the research. “This is a unique feature of this platform.”

The team’s measurements closely match the theoretical predictions, showing that their new platform is a reliable testbed for studying topological physics in one and two-dimensional lattices.

“By having access to both the energy levels and mode shapes of these collective excitations, we were able to reconstruct the full underlying Hamiltonian of the system, allowing for the full extraction of disorder and coupling strengths in a superconducting lattice for the first time,” says Shingo Kono, another member of the research team.

The demonstration of optomechanical lattices not only provides access to studying many-body physics in such realizations of condensed matter lattice models but will also provide a route towards novel hybrid quantum systems when combined with superconducting qubits.

More information: Tobias Kippenberg, Topological lattices realized in superconducting circuit optomechanics, Nature (2022). DOI: 10.1038/s41586-022-05367-9. www.nature.com/articles/s41586-022-05367-9

Journal information: Nature 

Provided by Ecole Polytechnique Federale de Lausanne 

After over three years of upgrade and maintenance work, the Large Hadron Collider began its third period of operation (Run 3) in July 2022. Since then, the world’s most powerful particle accelerator has been colliding protons at a record-breaking energy of 13.6 TeV. The ATLAS collaboration has just released its first measurements of these record collisions, studying data collected in the first half of August 2022.

The researchers measured the rates of two well-known processes: the production of top-quark pairs and the production of a Z boson, which proceed through strong and electroweak interactions, respectively. The ratio of their cross-sections is sensitive to the inner structure of the proton, and their measurement sets constraints on the relative probabilities that reactions are initiated by quarks and gluons.

These early measurements also validate the functionality of the ATLAS detector and its reconstruction software, which underwent many improvements in preparation for Run 3.

Physicists focused on Z-boson decays to electron and muon pairs, and on top-quark decays to a W boson and a jet—collimated sprays of particles—originating from a bottom quark. The W boson subsequently decays into one electron or muon and an invisible neutrino. As the analysis uses very early Run 3 data, physicists relied on preliminary calibrations of the leptons, jets and luminosity. These were derived promptly after the first data became available.

ATLAS measured a top-quark pair to Z boson production ratio that is consistent with the Standard Model prediction within the current experimental uncertainty of 4.7%.

The calibration and corresponding uncertainties will be improved as more data is processed. Future updates of the calibration will allow researchers to measure the cross sections with greater precision.

To validate their results, physicists performed a series of cross-checks. These included measuring the ratio of the cross section each time the LHC was injected with a new fill of protons for a data-taking run.

More analyses using the Run 3 data will follow, exploiting the unprecedented energies and the increased LHC data set.

Provided by CERN 

We see the world around us because light is being absorbed by specialized cells in our retina. But can vision happen without any absorption at all—without even a single particle of light? Surprisingly, the answer is yes.

Imagine that you have a camera cartridge that might contain a roll of photographic film. The roll is so sensitive that coming into contact with even a single photon would destroy it. With our everyday classical means there is no way there’s no way to know whether there’s film in the cartridge, but in the quantum world it can be done. Anton Zeilinger, one of the winners of the 2022 Nobel Prize in Physics, was the first to experimentally implement the idea of an interaction-free experiment using optics.

Now, in a study exploring the connection between the quantum and classical worlds, Shruti Dogra, John J. McCord, and Gheorghe Sorin Paraoanu of Aalto University have discovered a new and much more effective way to carry out interaction-free experiments. The team used transmon devices—superconducting circuits that are relatively large but still show quantum behavior—to detect the presence of microwave pulses generated by classical instruments. Their research was recently published in Nature Communications.

An experiment with added layer of ‘quantumness’

Although Dogra and Paraoanu were fascinated by the work done by Zeilinger’s research group, their lab is centered around microwaves and superconductors instead of lasers and mirrors. “We had to adapt the concept to the different experimental tools available for superconducting devices. Because of that, we also had to change the standard interaction-free protocol in a crucial way: we added another layer of ‘quantumness’ by using a higher energy level of the transmon. Then, we used the quantum coherence of the resulting three-level system as a resource,” Paraoanu says.

Quantum coherence refers to the possibility that an object can occupy two different states at the same time—something that quantum physics allows for. However, quantum coherence is delicate and easily collapses, so it wasn’t immediately obvious that the new protocol would work. To the team’s pleasant surprise, the first runs of the experiment showed a marked increase in detection efficiency. They went back to the drawing board several times, ran theoretical models confirming their results, and double-checked everything. The effect was definitely there.

“We also demonstrated that even very low-power microwave pulses can be detected efficiently using our protocol,” says Dogra.

The experiment also showed a new way in which quantum devices can achieve results that are impossible for classical devices—a phenomenon known as quantum advantage. Researchers generally believe that achieving quantum advantage will require quantum computers with many qubits, but this experiment demonstrated genuine quantum advantage using a relatively simpler setup.

Potential applications in many types of quantum technology

Interaction-free measurements based on the less effective older methodology have already found applications in specialized processes such as optical imaging, noise-detection, and cryptographic key distribution. The new and improved method could increase the efficiency of these processes dramatically.

“In quantum computing, our method could be applied for diagnosing microwave-photon states in certain memory elements. This can be regarded as a highly efficient way of extracting information without disturbing the functioning of the quantum processor,” Paraoanu says.

The group led by Paraoanu is also exploring other exotic forms of information processing using their new approach, such as counterfactual communication (communication between two parties without any physical particles being transferred) and counterfactual quantum computing (where the result of a computation is obtained without in fact running the computer).

More information: Shruti Dogra et al, Coherent interaction-free detection of microwave pulses with a superconducting circuit, Nature Communications (2022). DOI: 10.1038/s41467-022-35049-z

Journal information: Nature Communications 

Provided by Aalto University 

In recent years, many physicists and material scientists have been studying superconductors, materials that can conduct direct current electricity without energy loss when cooled under a particular temperature. These materials could have numerous valuable applications, for instance generating energy for imaging machines (e.g., MRI scanners), trains, and other technological systems.

Researchers at Fudan University, Shanghai Qi Zhi Institute, Hong Kong University of Science and Technology, and other institutes in China have recently uncovered a new mechanism to generate anisotropically-enhanced in-plane upper critical field in atomically thin centrosymmetric superconductors with topological band inversions. Their paper, published in Nature Physics, specifically demonstrated this mechanism on a thin layer of 2M-WS₂, a material that has recently attracted much research attention.

“In 2020, a paper by our theoretical collaborator Prof. K.T. Law proposed that 2D centrosymmetric superconductors with a topological band inversion, such as 1T′-WTe₂ exhibit a distinct type of superconductivity, called spin-orbit-parity coupled (SOPC) superconductivity,” Enze Zhang, one of the researchers who carried out the study, told Phys.org.

“SOPC is predicted to produce novel superconductivity near the topological band crossing with both largely enhanced and anisotropic spin susceptibility with respect to in-plane magnetic field directions. At that time, we were conducting research on the superconducting properties of atomically thin 2M-WS₂, so after talking with Prof. K.T. Law, we felt that the emergent van der Waals superconductor 2M-WS₂ would most likely be a promising candidate for spin-orbit-parity coupled superconductivity.”

The structure of monolayer 2M-WS₂ is identical to that of 1T′-WTe₂, the material previously investigated by Prof. Law and his team. 2M-WS₂, however, has a unique stacking mode, which distinguishes it from other transition metal dichalcogenides.

The researchers previously found that in its bulk form, this material exhibit a high superconducting transition temperature T_C of 8.8 K. In addition, theoretical calculations suggested that atomically thin layers of 2M-WS₂ hold topological edge states with band inversion.

In their experiments, Zhang and his colleagues measured the in-plane upper critical field at a high magnetic field and confirmed the violation of the Pauli limit law. They also observed a strongly anisotropic two-fold symmetry in the material, in response to the in-plane magnetic field direction.

“Tunneling experiments conducted under high in-plane magnetic fields also showed that the superconducting gap in atomically thin 2M-WS₂ possesses an anisotropic magnetic response along different in-plane magnetic field directions, and it persists much above the Pauli limit,” Zhang explained. “Using self-consistent mean-field calculations, our theoretical collaborators conclude that these unusual behaviors originate from the strong spin-orbit-parity coupling arising from the topological band inversion in 2M-WS₂.”

The researchers’ experiments spanned across several steps. Firstly, the team performed magneto-transport measurements on atomically thin 2M-WS₂ and found that its in-plane upper critical field is not only far beyond the Pauli paramagnetic limit, but also exhibits a strongly anisotropic two-fold symmetry in response to the in-plane magnetic field direction.

Subsequently, they used tunneling spectroscopy to collect measurements under high in-plane magnetic fields. These measurements revealed that the superconducting gap in atomically thin 2M-WS₂ possesses an anisotropic magnetic response along different in-plane magnetic field directions, which persists much above the Pauli limit.

Finally, the researchers performed a series of self-consistent mean-field calculations to better understand the origin of the unusual behaviors they observed in their sample. Based on their results, they concluded that these behaviors originate from the strong spin-orbit-parity coupling arising from the topological band inversion in 2M-WS₂, which effectively pins the spin of states near the topological band crossing and renormalizes the effect of external Zeeman fields anisotropically.

“We uncovered a new mechanism for generating an anisotropically-enhanced in-plane upper critical field in atomically thin centrosymmetric superconductors with topological band inversions, highlighting 2D 2M-WS₂ as a wonderful platform for the study of exotic superconducting phenomena such as higher-order topological superconductivity and further device applications,” Zhang said.

“The novel properties found here are highly nontrivial as they directly reflect a strong SOPC inheriting from the topological band inversion in the normal state of 2M-WS₂, which had been ignored for many years in previous studies of centrosymmetric superconductors.”

In recent years, more research teams worldwide have been exploring the properties and mechanisms of centrosymmetric superconducting transition metal dichalcogenides (TMDs), such as monolayer superconducting 1T′-MoS₂, and 1T′-WTe₂, due to the characteristic co-existence of topological band structure and superconductivity within them.

The recent paper by Zhang and his colleagues could pave the way towards the exploration of large enhanced and strongly anisotropic in-plane upper critical fields, which could further improve the current understanding of these materials’ exotic physics.

“We now plan to explore the usual superconducting properties (such as the in-plane upper critical field and tunneling spectroscopy behavior at high magnetic field) of more atomically thin centrosymmetric superconductors with topological band inversions,” Zhang added.

More information: Enze Zhang et al, Spin–orbit–parity coupled superconductivity in atomically thin 2M-WS2, Nature Physics (2022). DOI: 10.1038/s41567-022-01812-8

Ying-Ming Xie et al, Spin-Orbit-Parity-Coupled Superconductivity in Topological Monolayer WTe2, Physical Review Letters (2020). DOI: 10.1103/PhysRevLett.125.107001

Journal information: Physical Review Letters  , Nature Physics 

© 2022 Science X Network

Luttinger liquids are usually paramagnetic materials exhibiting non-Fermi liquid behavior, such as molybdenum oxides. These “liquids” and their fascinating properties had so far been only observed in 1D and quasi-1D compounds, such as blue bronze A_0.3MoO₃ (A= K, Rb, Tl) and purple bronze Li_0.9Mo₆O₁₇.

Researchers at Tsinghua University, ShanghaiTech University, and other institutes in China recently observed prototypical Luttinger liquid behavior in η-Mo₄O₁₁,a charge-density wave material with a quasi-2D crystal structure. Their findings, published in Nature Physics, could pave the way for the exploration of non-Fermi liquid behavior in other 2D and 3D quantum materials.

“In our previous work, we identified the Luttinger liquid phase in the normal state of blue bronzes, which is not surprising due to its quasi-1D nature,” Lexian Yang and Yulin Chen, two of the researchers who carried out the study, told Phys.org.

“We then noticed that a large family of Molybdenum oxides share common construction unit: Mo-O octahedron chains. But in some of them, such as η-Mo₄O₁₁, quasi-1D chains cross each other and weave into a quasi-2D structure.”

Materials with quasi-2D structures, such as the material examined by Yang, Chen and their colleagues, have attracted considerable research attention, with physicists debating on whether they might preserve some properties of 1D materials, including Luttinger liquid behavior. Initially, the researchers did not expect to observe this behavior, thus they were very surprised when they did.

In their experiments, they used quasi-2D η-Mo₄O₁₁ samples with a layered structure. The advantage of using these samples is that they can be easily cleaved to expose large and flat surfaces, facilitating their examination.

“To protect our samples from contamination, we studied the sample in an ultra-high vacuum environment by exciting electrons inside the crystals using monochromatic light,” Yang and Chen explained. “We then collected these excited electrons, or photoelectrons and analyze their energy and momentum to deduced their initial status back inside the sample.”

To examine their samples, Yang and his colleagues used a spectroscopic technique known as angle-resolved photoemission spectroscopy (ARPES), which allows researchers to directly visualize the electronic structure of materials. This technique can be applied to countless different types of materials, and was previously also used to examine high-temperature superconductors, topological quantum materials, and transition metal dichalcogenides.

“We showed that Luttinger liquid physics, which was previously considered as prototype 1D behavior, can be extended to quasi-2D systems,” Yang and Chen said. “This extension may help us to understand other puzzling non-Fermi liquid behaviors in 2D or even 3D systems. Luttinger liquid behavior is a rare example of an exactly solvable model for interacting systems. Although it has long been regarded as the ‘standard model’ for 1D metals, theorists have proposed that it is related to the non-Fermi liquid behaviors in different systems such as the normal state of high-temperature cuprate superconductors.”

The recent findings gathered by this team of researchers represent a significant step towards achieving a unified understanding of non-Fermi liquid behaviors in 2D and 3D systems. Their work could thus soon inspire new studies exploring Luttinger liquid behavior and other non-Fermi liquid states in other materials.

“Our future research is already underway,” Yang and Chen added. “Our first step will be to explore and find more materials systems (low-dimensional Molybdenum oxides and beyond) featuring presumable Luttinger liquid. Secondly, knowing the common Luttinger liquid behavior in different materials, their similarities and differences will help unveil the physics laws underneath. Thirdly and more interestingly, the interactions between other degrees of freedom and the Luttinger liquid that could lead to long-range ordered states deserve a thorough exploration.”

More information: X. Du et al, Crossed Luttinger liquid hidden in a quasi-two-dimensional material, Nature Physics (2022). DOI: 10.1038/s41567-022-01829-z

L. Kang et al, Band-selective Holstein polaron in Luttinger liquid material A0.3MoO3 (A = K, Rb), Nature Communications (2021). DOI: 10.1038/s41467-021-26078-1

Journal information: Nature Communications  , Nature Physics 

© 2022 Science X Network