Category: Physics

Researchers from North Carolina State University and the University of North Carolina at Chapel Hill used chiral phonons to convert wasted heat into spin information—without needing magnetic materials. The finding could lead to new classes of less expensive, energy-efficient spintronic devices for use in applications ranging from computational memory to power grids.

Spintronic devices are electronic devices that harness the spin of an electron, rather than its charge, to create current used for data storage, communication, and computing. Spin caloritronic devices—so-called because they utilize thermal energy to create spin current—are promising because they can convert waste heat into spin information, which makes them extremely energy efficient. However, current spin caloritronic devices must contain magnetic materials in order to create and control the electron’s spin.

“We used chiral phonons to create a spin current at room temperature without needing magnetic materials,” says Dali Sun, associate professor of physics and member of the Organic and Carbon Electronics Lab (ORaCEL) at North Carolina State University.

“By applying a thermal gradient to a material that contains chiral phonons, you can direct their angular momentum and create and control spin current,” says Jun Liu, associate professor of mechanical and aerospace engineering at NC State and ORaCEL member.

Both Liu and Sun are co-corresponding authors of the research, which appears in Nature Materials.

Chiral phonons are groups of atoms that move in a circular direction when excited by an energy source—in this case, heat. As the phonons move through a material, they propagate that circular motion, or angular momentum, through it. The angular momentum serves as the source of spin, and the chirality dictates the direction of the spin.

“Chiral materials are materials that cannot be superimposed on their mirror image,” Sun says. “Think of your right and left hands—they are chiral. You can’t put a left-handed glove on a right hand, or vice versa. This ‘handedness’ is what allows us to control the spin direction, which is important if you want to use these devices for memory storage.”

The researchers demonstrated chiral phonon-generated spin currents in a two-dimensional layered hybrid organic-inorganic perovskite by using a thermal gradient to introduce heat to the system.

“A gradient is needed because temperature difference in the material—from hot to cold—drives the motion of the chiral phonons through it,” says Liu. “The thermal gradient also allows us to use captured waste heat to generate spin current.”

The researchers hope that the work will lead to spintronic devices that are cheaper to produce and can be used in a wider variety of applications.

“Eliminating the need for magnetism in these devices means you’re opening the door wide in terms of access to potential materials,” Liu says. “And that also means increased cost-effectiveness.”

“Using waste heat rather than electric signals to generate spin current makes the system energy efficient—and the devices can operate at room temperature,” Sun says. “This could lead to a much wider variety of spintronic devices than we currently have available.”

More information: Lifa Zhang, Chiral-phonon-activated spin Seebeck effect, Nature Materials (2023). DOI: 10.1038/s41563-023-01473-9. www.nature.com/articles/s41563-023-01473-9

Journal information: Nature Materials 

Provided by North Carolina State University 

Research in fundamental science has revealed the existence of quark-gluon plasma (QGP)—a newly identified state of matter—as the constituent of the early universe. Known to have existed a microsecond after the Big Bang, the QGP, essentially a soup of quarks and gluons, cooled down with time to form hadrons like protons and neutrons—the building blocks of all matter.

One way to reproduce the extreme conditions prevailing when QGP existed is through relativistic heavy-ion collisions. In this regard, particle accelerator facilities like the Large Hadron Collider (LHC) and the Relativistic Heavy Ion Collider have furthered our understanding of QGP with experimental data pertaining to such collisions.

Meanwhile, theoretical physicists have employed multistage relativistic hydrodynamic models to explain the data, since the QGP behaves very much like a perfect fluid. However, there has been a serious lingering disagreement between these models and data in the region of low transverse momentum, where both the conventional and hybrid models have failed to explain the particle yields observed in the experiments.

Against this backdrop, a team of researchers from Japan, led by theoretical physicist Professor Tetsufumi Hirano of Sophia University, undertook an investigation to account for the missing particle yields in the relativistic hydrodynamic models.

In their recent work, they proposed a novel “dynamical core-corona initialization framework” to comprehensively describe high-energy nuclear collisions. Their findings were published in the journal Physical Review C and involved contributions from Dr. Yuuka Kanakubo, doctoral student at Sophia University, (Present affiliation: postdoctoral research fellow at the University of Jyväskylä, Finland) and Assistant Professor Yasuki Tachibana from Akita International University, Japan.

“To find a mechanism that can account for the discrepancy between theoretical modeling and experimental data, we used a dynamical core-corona initialization (DCCI2) framework in which the particles generated during high-energy nuclear collisions are described using two components: the core, or equilibrated matter, and the corona, or nonequilibrated matter,” explains Prof. Hirano. “This picture allows us to examine the contributions of the core and corona components towards hadron production in the low transverse momentum region.”

The researchers conducted heavy-ion Pb-Pb collision simulations on PYTHIA (a computer simulation program) at an energy of 2.76 TeV to test their DCCI2 framework. Dynamical initialization of the QGP fluids allowed the separation of core and corona components, which were made to undergo hadronization through “switching hypersurface” and “string fragmentation,” respectively. These hadrons were then subjected to resonance decays to obtain the transverse momentum (p_T) spectra.

“We switched off the hadronic scatterings and performed only resonance decays to see a breakdown of the total yield into core and corona components, as hadronic scatterings mix up the two components in the late stage of reaction,” explains Dr. Kanakubo.

The researchers then investigated the fraction of core and corona components in the p_T spectra of charged pions, charged kaons, and protons and antiprotons for collisions at 2.76 TeV. Next, they compared these spectra with that obtained from experimental data (from the ALICE detector at LHC for Pb-Pb collisions at 2.76 TeV) to quantify the contributions from corona components. Lastly, they investigated the effects of contributions from corona components on the flow variables.

They found a relative increase in corona contributions in the spectral region of approximately 1 GeV for both 0-5% and 40-60% centrality classes. While this was true for all the hadrons, they found almost 50% corona contribution to particle production in the spectra of protons and antiprotons in the region of very low p_T (≈ 0 GeV) .

Furthermore, results from full DCCI2 simulations showed better agreement with the ALICE experimental data compared when only core components with hadronic scatterings (which neglect corona components) were compared. The corona contribution was found to be responsible for diluting the four-particle cumulants (a flow observable) obtained purely from core contributions, indicating more permutations of particles with corona contribution.

“These findings imply that the nonequilibrium corona components contribute to particle production in the region of very low transverse spectra,” says Prof. Hirano. “This explains the missing yields in hydrodynamic models, which extract only the equilibrated core components from experimental data. This clearly shows that it is necessary to extract the nonequilibrated components as well for a more precise understanding of the properties of QGP.”

More information: Yuuka Kanakubo et al, Nonequilibrium components in the region of very low transverse momentum in high-energy nuclear collisions, Physical Review C (2022). DOI: 10.1103/PhysRevC.106.054908

Provided by Sophia University

Particle accelerators are devices that use electromagnetic fields to speed up particles and collide them together or against a specific target. These devices are widely used by physicists to study particles, the forces that drive them and interactions between them.

The larger and most advanced particle accelerators in the world consume huge amounts of energy. In a paper published in Nature Physics, researchers at Technische Universität Darmstadt recently introduced a new measurement at their particle accelerator that can retain a high beam power while consuming significantly less energy.

“Particle accelerators are available in compact designs, such as those used for X-ray machines, and as large-scale facilities that are used for fundamental research,” Manuel Dutine, one of the researchers who carried out the study, told Phys.org. “The latter in particular consume an enormous amount of energy. This raises the question whether the added value justifies the realization of large research-accelerator facilities, especially from an economic and sustainable point of view.”

Future large-scale particle physics research will require accelerators with an increasingly high beam power. The key objective of the work by Dutine and his colleagues was to create an accelerator that could help to meet these needs while consuming less energy, using a technology known as energy recovery.

“The concept of energy recovery in particle accelerators is not new but under continuous development,” Dutine explained. “To achieve higher beam energies at reduced resource needs, the energy recovery concept should be combined with multi-turn acceleration, which was the purpose of our research.”

Accelerators accelerate beams, which are essentially a large set of particles, and this process unavoidably consumes energy. A high energy accelerated beam could collide with a resting target, another beam of particles or even laser light, potentially resulting in desired reactions that can then be studied. Reactions of interest, however, are very rare, and most particles in a beam end up not interacting with anything during an experiment.

“In a conventional accelerator, the remaining beam is dumped at high energy and the energy is wasted,” Dutine said. “In an ERL (energy recovery linac), the beam’s energy is recovered by decelerating the bunches. The energy is temporarily stored in the microwave fields in the accelerating structures and can afterwards be used to accelerate the next upcoming low-energy bunches.”

The ERL accelerator developed by Dutine and his colleagues has so far achieved very promising results. In initial tests, the team measured its energy recovery and recycling (i.e., the use of recovered energy for accelerating subsequent particle bunches) while accelerating an electron beam.

“By measuring energy savings, we demonstrated that the multi-turn energy recovery concept works,” Dutine said. “We measured a recovery of up to 87% of consumed beam power in the main accelerator. This power saving is financially relevant for beams with Gigawatt power or may help to reduce the footprint of energy consumption for large-scale facilities.”

The new multi-turn energy recovery accelerator created by this team of researchers could help to carry out physics studies more efficiently and sustainably, consuming less power while still achieving high beam energies. In the future, its unique design could inform the development of other accelerators with even lower fabrication costs and energy consumption, thus further promoting sustainable research. Dutine and his colleagues are now planning to upgrade their particle accelerator and also build a new one.

“The potential for energy saving is even larger for facilities with increased beam current, beam energies and number of turns,” Dutine added. “Here we are limited by the properties of our existing facility. We have started discussions with scientific partners, funding agencies, and our ministry to expand this research to a next-generation facility, in which we want to include our current findings, establish the energy recycling in a particle accelerator at larger scale, and thereby convince potential users from international science centers or industry to consider energy-recovering for their particle-accelerator needs.”

More information: Felix Schliessmann et al, Realization of a multi-turn energy recovery accelerator, Nature Physics (2023). DOI: 10.1038/s41567-022-01856-w.

Journal information: Nature Physics 

© 2023 Science X Network

In recent years, many physicists and computer scientists have been working on the development of quantum computing technologies. These technologies are based on qubits, the basic units of quantum information.

In contrast with classical bits, which have a value of 0 or 1, qubits can exist in superposition states, so they can have a value of 0 and 1 simultaneously. Qubits can be made of different physical systems, including electrons, nuclear spins (i.e., the spin state of a nucleus), photons, and superconducting circuits.

Electron spins confined in silicon quantum dots (i.e., tiny silicon-based structures) have shown particular promise as qubits, particularly due to their long coherence times, high gate fidelities and compatibility with existing semiconductor manufacturing methods. Coherently controlling multiple electron spin states, however, can be challenging.

Researchers at University of Rochester have recently introduced a new strategy to coherently manipulate either single or multiple electron spins in silicon quantum dots. This method, introduced in a paper published in Nature Physics, could open new possibilities for the development of reliable and highly performing quantum computers.

“As with many experiments in science, we were initially investigating an unrelated topic, when we started noticing all sorts of coherent oscillations popping up in our data.” John Nichol, one of the researchers who carried out the study, told Phys.org. “It took us a little while to come up with the theoretical explanation, but once we did, everything fell into place. Spin-valley coupling has been explored before many times, but never to directly mediate coherent transitions between different spin states.”

The strategy for controlling electron spins in silicon proposed by Nichol and his colleagues takes advantage of spin-valley coupling, the interaction between an electron’s spin and valley states. Electrons in silicon quantum dots have both spin and valley quantum numbers. Their spin state can be “up” or “down,” while their valley state can be + or −.

“At a certain magnetic field, the energy of the up,+ state, for example, can be almost equal in energy to the down,- state,” Nichol explained. “Because the energy difference between the + and − states depends on electric fields, we can use a voltage pulse to then bring up,+ exactly into resonance with down,−. When this happens, an electron initially prepared in an up,+ state will coherently oscillate to down,−, and back and forth. These are spin-valley oscillations.”

So far, the standard method to manipulate electron spins in silicon quantum dots entailed the use of time-varying magnetic fields. Nichol and his colleagues showed that their strategy enables the coherent manipulation of electron spins without the need to use oscillating electromagnetic fields.

“Oscillating magnetic fields can be especially difficult to generate at cryogenic temperatures, and spin-valley coupling eliminates this need,” Nichol said. “Another achievement is that the valley degree of freedom in silicon has often been regarded as a ‘bug’ rather than a feature of silicon qubits, but our work shows that it can be a very useful feature.”

The recent work by this team of researchers highlights the promise of utilizing spin-valley coupling to achieve the coherent control of qubits based on electron spins confined in silicon quantum dots. In their next papers, they hope to gain a better understanding of what characteristics of the growth, fabrication, and tuning of quantum dots can impact spin-valley coupling, as this could further inform the fabrication of electron-based quantum computing technologies.

“We would also like to explore how one could implement multi-qubit gates in this framework,” Nichol added. “One challenge is that the magnetic field needs to be tuned separately for each qubit, and we are looking into realistic ways to implement this.”

More information: Xinxin Cai et al, Coherent spin–valley oscillations in silicon, Nature Physics (2023). DOI: 10.1038/s41567-022-01870-y

Journal information: Nature Physics 

© 2023 Science X Network

Purdue University researchers found that by tailoring the film thickness of conducting nitrides and oxides, specifically plasmonic titanium nitride (TiN) and aluminum-doped zinc oxide (AZO), they can control the materials’ optical properties, most notably their epsilon near zero (ENZ) behaviors. The TiN and AZO materials developed at Purdue also feature the lowest reported optical losses. This provides novel applications for the telecommunications field and furthers the study of many optical nonlinearities.

Vladimir M. Shalaev and Alexandra Boltasseva, Purdue professors of electrical and computer engineering, and their team of researchers, led by then-postdoctoral researcher Soham Saha, investigated this method of controlling the ENZ point, the wavelength at which a material is neither dielectric nor metallic.

When light travels through an ENZ material, its group velocity slows to near zero, and it’s able to interact with the material for a longer period. This gives rise to many interesting nonlinearities. However, with most conventional materials, the ENZ point is fixed and difficult to move.

What the researchers demonstrated is that the thickness association of the optical properties is one of the easiest things to manipulate, enabling them to grow films with different ENZ properties using the same growth environment. By tailoring the thickness of material films and controlling light absorption near the ENZ regime, researchers can study absolute ENZ physics at many different wavelength ranges. The enables a wide range of nonlinear optical phenomena, including all-optical switching, time refraction and high-harmonic generation.

In their study, the researchers developed a low-loss, polycrystalline films of TiN on silicon by reactive magnetron sputtering. AZO was grown by pulsed laser deposition on the as-grown TiN. They investigated the cause of the thickness-dependent optical properties through spectroscopic ellipsometry, atomic force microscopy and transmission electron microscopy, connecting the optical properties to their structural properties.

As a proof of concept of a dynamic, tunable device, researchers demonstrated all-optical switching of the resonators using an interband pump, showing picosecond relaxation times.

The study is published in the journal Advanced Materials.

More information: Soham Saha et al, Tailoring the Thickness‐Dependent Optical Properties of Conducting Nitrides and Oxides for Epsilon‐Near‐Zero‐Enhanced Photonic Applications, Advanced Materials (2022). DOI: 10.1002/adma.202109546

Journal information: Advanced Materials 

Provided by Purdue University 

Granular materials contain large numbers of small, discrete particles, which collectively behave like uniform media. Their thermal conductivity is crucial to understanding their overall behavior—but so far, researchers haven’t considered how this value is affected by the surface roughness of their constituent particles.

Through new analysis published in The European Physical Journal B, Bo Persson at the Peter Grünberg Institute, part of the Jülich Research Centre in Germany, has discovered that when this roughness is considered, thermal conductivity in granular materials is heavily influenced by particle sizes. These findings could help physicists to better describe a wide array of granular materials: from sand and snow, to piles of rice, coffee beans, and fertilizer.

The thermal conductivity of a granular material will typically be much smaller than that of solid blocks of the same material. Its value is affected by several factors: including particle shapes and sizes; the nature of their contact between each other; and the temperature, pressure, and humidity of the gas surrounding the particles.

Most previous studies have only considered how heat is transferred between smooth, spherical particles—but in reality, particles have a surface roughness on many length scales, which heavily affects the thermal conductivity of the overall material.

In his study, Persson examines the thermal conductivity of granular materials in a humid atmosphere, where heat flows via air and water are especially relevant. Unlike previous studies, he considered particles with a mathematically-defined surface roughness—found in real particles produced through the crushing of brittle minerals.

For particles smaller than 10 microns, he found that heat transfer in humid air is dominated by water capillaries, which form in the gaps between particles. In contrast, for bigger particles, heat is mostly transferred through the air. These discoveries clarify the mechanisms driving thermal conductivity in granular materials, and will enable researchers in future studies to simulate their physical properties more closely.

The work is published in The European Physical Journal B.

More information: B. N. J. Persson, Heat transfer in granular media consisting of particles in humid air at low confining pressure, The European Physical Journal B (2023). DOI: 10.1140/epjb/s10051-023-00483-5

Journal information: European Physical Journal B 

Provided by Springer 

By using ultrafast laser flashes, scientists at the University of Rostock in collaboration with researchers of the Max Planck Institute for Solid State Research in Stuttgart have generated and measured the shortest electron pulse to date. The electron pulse was created by using lasers to remove electrons from a tiny metal tip and lasted only 53 attoseconds, that is, 53 billionths of a billionth of a second. The event has set a new speed record in man-made control of electric currents in solid materials.

The research opens new avenues for advancing the performance of electronics and information technologies, as well as developing new scientific methodologies for visualizing phenomena in the microcosm at ultimate speeds.

Ever wondered what makes your computer and your other electronic gadgets slow or fast in their performance? It is the time it takes electrons, some of the tiniest particles of our microcosm, to stream out from minute leads inside the transistors of electronic microchips and to form pulses. Methods to speed up this process are central for advancing electronics and their applications to ultimate performance limits. But what is the shortest possible streaming time of electrons from a tiny metal lead in an electronic circuit?

By using extremely short laser flashes, a team of researchers led by Professor Eleftherios Goulielmakis, head of the group Extreme Photonics of the institute for Physics at the University of Rostock, and collaborators at the Max Planck Institute of Solid State Research in Stuttgart used state-of-the art laser pulses to eject electrons from a tungsten nanotip to generate the shortest electron burst to date. This work is published in Nature.

Whereas it has long been known that light can release electrons from metals—Einstein was the first to explain how—the process is extremely hard to manipulate. The electric field of light changes its direction about a million billion times per second making it challenging to control the way it rips off electrons from the surface of metals.

To overcome this challenge, the Rostock scientists and their co-workers used a modern technology that had been previously developed by their group—light field synthesis—which allowed them to shorten a light flash to less than a full swing of its own field. In turn, they used these flashes to illuminate the tip of a tungsten needle to knock electrons free into vacuum.

“Using light pulses that comprise merely a single cycle of its field, it is now possible to give electrons a precisely controlled kick to set them free from the tungsten tip within a very short time interval,” explains Eleftherios Goulielmakis, head of the research group.

But the challenge could not be overcome unless the scientists also found a way to measure the brevity of these electron bursts. To deal with this hurdle, the team developed a new type of camera that can take snapshots of the electrons during the short time the laser is pushing them out from the nanotip and into the vacuum.

“The trick was to use a second, very weak, light flash” said Dr. Hee-Yong Kim, the leading author of the new study. “This second laser flash can gently perturb the energy of the electron burst to find out how it looks like in time. It is like the game ‘What’s in the box?’ where players try to identify an object without looking at it but just by turning it around to feel its shape with their hands.”

But how could this technology be used in electronics? “As technology advances rapidly, it is reasonable to expect the development of microscopic electronic circuits in which electrons travel in a vacuum space among closely packed leads to prevent obstacles that slow them down,” says Goulielmakis. “Using light to eject electrons and drive them among these leads could speed up future electronics by several thousand times of today’s performance.”

But the researchers believe that their newly developed methodology will be used directly for scientific purposes. “Ejecting electrons from a metal within a fraction of a light’s field cycle dramatically simplifies the experiments and allows us to use advanced theoretical methods to understand the emission of electrons in ways that were not previously possible,” says Professor Thomas Fennel, a coauthor in the new publication.

“Since our electron bursts provide excellent resolution for taking snapshots of electronic and atomic motions in materials, we plan to use them to acquire a deep understanding of complex materials to facilitate their applications in technology,” Goulielmakis says.

More information: H. Y. Kim et al, Attosecond field emission, Nature (2023). DOI: 10.1038/s41586-022-05577-1

Journal information: Nature 

Provided by University of Rostock 

Researchers have developed a new detector that can precisely measure single photons at very high rates. The new device could help make high-speed quantum communication practical.

Quantum communication uses light at the single photon level to send encoded quantum information such as encryption keys in quantum key distribution. Because of the laws of physics, data transmitted in this way is guaranteed to remain secure. Sending information at faster speeds requires a single photon detector that can not only detect photons quickly but also precisely measure their arrival times.

In an article published in Optica, researchers led by Matthew D. Shaw at NASA’s Jet Propulsion Laboratory describe and demonstrate their new detector for measuring the arrival times of photons, which they call PEACOQ (performance-enhanced array for counting optical quanta) detector.

“Our new detector is made of 32 niobium nitride superconducting nanowires on a silicon chip, which enables high count rates with high precision,” said research team member Ioana Craiciu, a postdoctoral scholar. “The detector was designed with quantum communication in mind, as this is a technological area that has been limited by the performance of available detectors.”

The detector was developed as part of a NASA program to enable new technology for space-to-ground quantum communication, which can allow sharing of quantum information across intercontinental distances in the future. This work builds upon technology developed for the NASA Deep Space Optical Communication project, which will be the first demonstration of free-space optical communication from interplanetary space.

“There is not currently another detector that can count single photons this quickly with the same timing resolution,” said Craiciu. “We know this detector will be useful for quantum communication, but we also hope that it could enable other applications that we haven’t considered.”

Faster quantum communication

Speeding up quantum communication transmission rates requires a detector on the receiving end that can make quick measurements and exhibits a short dead time so that it can contend with a high rate of arriving photons. The detector must also precisely measure the arrival time of the photons.

“Although there are detectors that can measure photon arrival times with high precision they struggle to keep up when the photons are arriving in quick succession and can miss some of the photons or get their arrival times wrong,” said Craiciu. “We designed the PEACOQ detector to precisely measure the arrival times of single photons even as they are hitting the detector at a high rate. It is also efficient—it doesn’t miss many of the photons.”

The PEACOQ detector is made of nanowires just 7.5 nm thick, or about 10,000 times thinner than a human hair. Operating it at very cold temperatures—around 1 Kelvin, or -458 °F—makes the nanowires superconducting, which means they have no electrical resistance. Under superconducting conditions, any photon that hits a wire has a good chance of being absorbed by that wire. Any absorbed photons create a hot spot that increases the wire’s electrical resistance in a detectable way. A computer along with a time-to-digital converter is used to record when the resistance changed and thus when a photon arrived at the detector.

“When the detector measures a photon, it outputs an electric pulse, and the time-to-digital converter measures the arrival time of this electrical pulse very precisely, with a resolution below 100 picoseconds or 70 million times faster than a snap of the fingers,” said Craiciu. “We developed a new time-to-digital converter that can measure up to 128 channels at once with this timing resolution, which is important because our detector needs 32 channels.”

To demonstrate the new detector, the researchers cooled it down to 1 Kelvin by installing it in a cryostat. They used a custom-built testing setup to send light into the cryostat to the detector and a chain of electronics to transmit the detector’s output signal out of the cryostat, amplify it and record it. Because there are 32 nanowires, the researchers had to use 32 sets of each component, including 32 cables and 32 of each kind of amplifier.

Unprecedented count rates

“We were very pleased with how well the detector worked,” said Craiciu. “The rate at which it can measure photons was the highest we have seen. It requires a complex setup because each of the 32 nanowires is read out individually, but for applications where you really need to measure photons at a high rate with high precision, it is worth the trouble.”

Typically, quantum information being transmitted is set to a clock, with each piece of information encoded into one photon and sent on a tick. How precisely you can measure the arrival time of the photons at the receiver determines how close together the ticks can be without making a mistake, and therefore it determines how quickly you can send the information. The new detector makes it practical to perform quantum communication with a state-of-the-art clock frequency of 10-GHz.

The researchers are still working to make improvements to the PEACOQ detector, which is currently about 80% efficient—meaning 20% of photons that hit the detector are not measured. They also plan to build a portable receiver unit that can be used for quantum communication experiments. It will contain several PEACOQ detectors along with optics, readout electronics and a cryostat.

More information: Ioana Craiciu et al, High-speed detection of 1550 nm single photons with superconducting nanowire detectors, Optica (2022). DOI: 10.1364/OPTICA.478960

Journal information: Optica 

Provided by Optica 

The standard model of particle physics tells us that most particles we observe are made up of combinations of just six types of fundamental entities called quarks. However, there are still many mysteries, one of which is an exotic, but very short-lived, Lambda resonance known as Λ(1405). For a long time, it was thought to be a particular excited state of three quarks—up, down, and strange—and understanding its internal structure may help us learn more about the extremely dense matter that exists in neutron stars.

Investigators from Osaka University were part of a team that has now succeeded in synthesizing Λ(1405) for the first time by combining a K^– meson and a proton and determining its complex mass (mass and width). The K⁻ meson is a negatively charged particle containing a strange quark and an up antiquark. The much more familiar proton that makes up the matter that we are used to has two up quarks and a down quark. The researchers showed that Λ(1405) is best thought of as a temporary bound state of the K^– meson and the proton, as opposed to a three-quark excited state.

In their study published recently in Physics Letters B, the group describes the experiment they carried out at the J-PARC accelerator. K⁻ mesons were shot at a deuterium target, each of which had one proton and one neutron. In a successful reaction, a K⁻ meson kicked out the neutron, and then merged with the proton to produce the desired Λ(1405).

“The formation of a bound state of a K^– meson and a proton was only possible because the neutron carried away some of the energy,” says Kentaro Inoue, an author of the study. One of the aspects that had been perplexing scientists about Λ(1405) was its very light overall mass, even though it contains a strange quark, which is nearly 40 times as heavy as an up quark. During the experiment, the team of researchers was able to successfully measure the complex mass of Λ(1405) by observing the behavior of the decay products.

• (Top) Measured reaction cross-section. The horizontal axis is the K^– and proton collision recoil energy converted into a mass value. Large reaction events occur at mass values lower than the sum of the K^– and proton masses, which itself suggests the existence of Λ(1405). The measured data were reproduced by scattering theory (solid lines). (Bottom) Distribution of K^– and proton scattering amplitudes. When squared, these correspond to the reaction cross-section, and are generally complex numbers. The calculated values match with the measured data. When the real part (solid line) crosses 0, the value of the imaginary part reaches its maximum value. This is a typical distribution for a resonance state, and determines the complex mass. The arrows indicate the real part. Credit: 2023, Hiroyuki Noumi, Pole position of Λ(1405) measured in d(K^-,n)πΣ reactions, Physics Letters B
• Schematic illustration of the reaction used to synthesize Λ(1405) by fusing a K^– (green circle) with a proton (dark blue circle), which takes place inside a deuteron nucleus. Credit: Hiroyuki Noumi
• (Top) Measured reaction cross-section. The horizontal axis is the K^– and proton collision recoil energy converted into a mass value. Large reaction events occur at mass values lower than the sum of the K^– and proton masses, which itself suggests the existence of Λ(1405). The measured data were reproduced by scattering theory (solid lines). (Bottom) Distribution of K^– and proton scattering amplitudes. When squared, these correspond to the reaction cross-section, and are generally complex numbers. The calculated values match with the measured data. When the real part (solid line) crosses 0, the value of the imaginary part reaches its maximum value. This is a typical distribution for a resonance state, and determines the complex mass. The arrows indicate the real part. Credit: 2023, Hiroyuki Noumi, Pole position of Λ(1405) measured in d(K^-,n)πΣ reactions, Physics Letters B
• Schematic illustration of the reaction used to synthesize Λ(1405) by fusing a K^– (green circle) with a proton (dark blue circle), which takes place inside a deuteron nucleus. Credit: Hiroyuki Noumi

“We expect that progress in this type of research can lead to a more accurate description of ultra-high-density matter that exists in the core of a neutron star,” says Shingo Kawasaki, another study author.

This work implies that Λ(1405) is an unusual state consisting of four quarks and one antiquark, making a total of five quarks, and does not fit the conventional classification in which particles have either three quarks or one quark and one antiquark. This research may lead to a better understanding of the early formation of the universe, shortly after the Big Bang, as well as what happens when matter is subject to pressures and densities well beyond what we see under normal conditions.

More information: S. Aikawa et al, Pole position of Λ(1405) measured in d(K⁻,n)πΣ reactions, Physics Letters B (2022). DOI: 10.1016/j.physletb.2022.137637

Journal information: Physics Letters B 

Provided by Osaka University 

Building off a breakthrough “anti-laser,” a team of researchers has developed a system that can direct light and other electromagnetic waves for signal processing without any unwanted signal reflections—an innovation that could advance local area networks, the field of photonics, and other applications.

The results, led by A. Douglas Stone of Yale and Philipp del Hougne of University of Rennes in France, are published in Science Advances.

A little more than a decade ago, Stone led a team in the creation of the anti-laser, or “coherent perfect absorber” (CPA). Instead of emitting a beam as a laser does, an anti-laser absorbs input light with the same precision.

In a laser, light bounces back and forth between two mirrors, each time passing through an amplifying material—known as the “gain medium”—such as gallium arsenide. Because the light is of a specific wavelength, it creates a feedback that increasingly gains in intensity. In a typical light source—an everyday lightbulb, for instance—atoms radiate independently and create light of many different wavelengths, and light goes in many directions as a result. In lasers, though, atoms radiate at the same frequency and in the same direction, creating a concentrated beam of a single wavelength.

The difference in the anti-laser is that instead of using an amplifying material, it uses one that absorbs the light—that is, a “loss medium.” In its simplest version, the anti-laser splits a single laser beam into two and directs the two beams into each other, meeting at a paper-thin silicon wafer. The light’s waves are precisely tuned to interlock with each other and become trapped. They then dissipate into heat.

For their most recent work, the researchers built off this concept and developed a device based on what they call “reflectionless scattering modes” (RSMs).

“We asked if there is some principle like this where we can guide light instead of transducing it into another form of energy,” said Stone, the Carl A. Morse Professor of Applied Physics and Physics. “Because with optical fibers and modern photonic circuits, guiding light and not having any of it reflect back is extremely valuable.”

From there, they developed the device that, instead of absorbing the waves, redirected them to specific channels. Stone worked on the theoretical side of the project, while Philipp del Hougne of University of Rennes in France built the actual device.

“Instead of having it all transduced, it could either all go into our chosen output channels or some of it could be absorbed and the rest go into the output channels,” Stone said. “In the next step, we want to make a similar device where the absorption is negligible, so that all of the energy is efficiently routed to perform its information or sensing function. There is great interest in such technologies to reduce the power consumption of cell phone networks, for example.”

The device eliminated signal reflections, which have long been a problem for signal routers, a pivotal ingredient of modern nanophotonic and radiofrequency networks. Besides causing a loss in signal power, such reflections can cause devastating unwanted reflected-signal-power echoes in the network.

More information: Jérôme Sol et al, Reflectionless programmable signal routers, Science Advances (2023). DOI: 10.1126/sciadv.adf0323

Journal information: Science Advances 

Provided by Yale University