Category: Physics

by Ingrid Fadelli , Phys.org

Non-perturbative interactions (i.e., interactions too strong to be described by so-called perturbation theory) between light and matter have been the topic of numerous research studies. Yet the role that quantum properties of light play in these interactions and the phenomena arising from them have so far remained widely unexplored.

Researchers at Technion–Israel Institute of Technology recently introduced a new theory describing the physics underpinning non-perturbative interactions driven by quantum light. Their theory, introduced in Nature Physics, could guide future experiments probing strong-field physics phenomena, as well as the development of new quantum technology.

This recent paper was the result of a close collaboration between three different research groups at Technion, led by principal investigators Prof. Ido Kaminer, Prof. Oren Cohen and Prof. Michael Krueger. Students Alexey Gorlach and Matan Even Tsur, co-first authors of the paper, spearheaded the study, with support and ideas from Michael Birk and Nick Rivera.

“This was a major scientific journey for us,” Prof. Kaminer and Gorlach told Phys.org. “We began thinking about high harmonic generation (HHG) and its quantum features already in 2019. Back then, the light in all HHG experiments was explained classically and we wanted to find when quantum physics starts to play role there.

“Frankly, it bothered us that several foundational phenomena in physics were each explained by a completely different theory, thus it was not possible to relate them. For example, HHG was based on a theory that contradicted the theory usually applied to calculate spontaneous emission—each explained on a different basis.”

HHG is a highly nonlinear physical processes that entails a strong interaction between light and matter. Specifically, it occurs when intense pulses of light applied to matter cause it to emit so-called high-harmonics of the driving intense light pulse.

For a few years, Prof. Kaminer and his research group have been trying to devise a single quantum theory-based framework that would collectively account for all photonics phenomena, including HHG. Their first paper on this topic, published in Nature Communications in 2020, introduced a proposed version of this unifying framework, analyzing HHG in the language of quantum optics.

“This study contributed to opening the now-rising field of quantum HHG,” Prof. Kaminer and Gorlach explained. “Still, all HHG experiments were driven by classical laser fields. It even seemed like there cannot be any quantum light intense enough to create HHG. However, works by Prof. Maria Chekhova showed that it is possible to create intense enough quantum light in a form known as bright squeezed vacuum. This motivated our new investigation.”

As part of their new study, Prof. Kaminer, Gorlach and their colleagues devised a full framework describing strong-field physics processes driven by quantum light. To theoretically validate their framework, they applied it to HHG, predicting how this process would change if driven by quantum light.

“We showed that on the contrary to expectations, many important features like the intensity and spectrum, all change as a result of using a driving light source with different quantum photon statistics,” Prof. Kaminer and Gorlach said. “The paper we wrote also predicts experimentally feasible scenarios that cannot be explained in any other means except by considering the photon statistics. These upcoming experiments will be of even greater impact and importance to this rising field of strong-field quantum optics.”

So far, the work carried out by this team of researchers is purely theoretical. Their paper introduces the very first theory of non-perturbative processes driven by quantum light, while also theoretically demonstrating that the quantum state of light affects measurable quantities, such as the emitted spectrum.

“The way our theory works is by splitting the driving light into one of two representations called the generalized Glauber distribution or the Husimi distribution, and then using the conventional simulations of the HHG field, the time-dependent Schrodinger equation (TDSE), to simulate separately the parts of the distribution, before combining the simulations together to derive the overall result,” Prof. Kaminer and Gorlach said.

“This connection of the standard tools of the community into such a quantum-optical calculation scheme is what made our work powerful and useful—applicable to an arbitrary quantum state of light and an arbitrary system of emitters.”

The new theory derived by Prof. Kaminer, Gorlach and their colleagues could soon inform studies in different areas of physics. In fact, their paper envisions taking the idea beyond HHG, to a wide range of non-perturbative processes, which can all be driven by non-classical light sources.

This theoretical prediction could soon be tested and validated in experimental settings. For instance, the team’s theory can be directly applied to the generation of attosecond pulses via HHG, a process that can underpin the functioning of quantum sensing and quantum imaging technologies.

In that regard, the team published a recent theory paper in Nature Photonics that proposed controlling the attosecond pulse profiles using the quantum nature of light, for example showing promising conditions using a mixture of classical light and quantum squeezed light.

In addition, their theory could be applied to other phenomena based on strong-field physics, such as the Compton effect, a process used to generate X-ray pulses.

“We recently published a follow-up paper on this application in Science Advances, which ended up appearing earlier due to delays in the peer-review process,” Kaminer and Gorlach added about the Compton effect. “We are now working toward performing the experiment theoretically discussed in our paper.

“Another ambitious goal will be to generalize the developed theory beyond HHG, and to investigate quantum effects in various materials driven by intense light, which connects our new developments in quantum optics to the frontiers of condensed matter physics.”

More information: Alexey Gorlach et al, High-harmonic generation driven by quantum light, Nature Physics (2023). DOI: 10.1038/s41567-023-02127-y

Matan Even Tzur et al, Photon-statistics force in ultrafast electron dynamics, Nature Photonics (2023). DOI: 10.1038/s41566-023-01209-w

Majed Khalaf et al, Compton scattering driven by intense quantum light, Science Advances (2023). DOI: 10.1126/sciadv.ade0932

Journal information: Science Advances  , Nature Communications  , Nature Photonics  , Nature Physics 

© 2023 Science X Network

by Light Publishing Center, Changchun Institute of Optics, Fine Mechanics And Physics, CAS

Non-Hermitian systems with their spectral degeneracies known as exceptional points (EPs) have been explored for lasing, controlling light transport, and enhancing a sensor’s response.

A ring resonator can be brought to an EP by controlling the coupling between its frequency degenerate clockwise and counterclockwise traveling modes. This has been typically achieved by introducing two or more nano-tips into the resonator’s mode volume.

While this method provides a route to study EP physics, the basic understanding of how the nano-tips’ shape and size symmetry impact the system’s non-Hermicity is missing, along with additional loss from both in-plane and out-of-plane scattering. The limited resonance stability poses a challenge for leveraging EP effects for switches or modulators, which requires stable cavity resonance and fixed laser-cavity detuning.

In a paper published in eLight, a team of scientists, led by Professor Tingyi Gu from the Department of Electrical and Computer Engineering, University of Delaware, U.S., and co-workers have developed lithographically-defined asymmetric and symmetric Mie-scatterers.

This enables subwavelength control of wave transmission and reflections without deflecting to additional radiation channels. They show that those pre-defined meta-units can bring the system to an EP without post-tuning and enable chiral light transport within the resonator.

Counterintuitively, a geometric defect, named meta-unit, results in an enhanced quality factor measured on the transmission port by coherently suppressing the backscattering from surface roughness.

The proposed device platform enables pre-defined chiral light propagation and backscattering-free resonances needed for various applications such as frequency combs, solitons, sensing, and other nonlinear optical processes such as photon blockade, and regenerative oscillators

The scientists say, “Our work not only opens a new direction for chiral silicon photonics but is also significant for the following four reasons. First, it reveals the critical role of spatial asymmetry of the nano-tip and Mie scatterers for bringing the system towards EP. Second, the scatter geometry-controlled pathway of driving the non-Hermitian system towards and away from an EP is illustrated in detail.”

“Third, our system is mechanically stable. It allows reliable comparison between transmission and reflection spectra for the perturbed micro-resonator, which reveals the nano-tip/scatter’s contribution to the diagonal terms. This is in contrast to the conventional way of achieving EP with two nano-tips. Fourth, enhancement of the empirical quality factor extracted from the transmission spectra is demonstrated for the first time.”

More information: Hwaseob Lee et al, Chiral exceptional point and coherent suppression of backscattering in silicon microring with low loss Mie scatterer, eLight (2023). DOI: 10.1186/s43593-023-00043-5

Journal information: eLight 

Provided by Light Publishing Center, Changchun Institute of Optics, Fine Mechanics And Physics, CAS

by Ingrid Fadelli , Phys.org

Quantum computers, technologies that perform computations leveraging quantum mechanical phenomena, could eventually outperform classical computers on many complex computational and optimization problems. While some quantum computers have attained remarkable results on some tasks, their advantage over classical computers is yet to be conclusively and consistently demonstrated.

Ramis Movassagh, a researcher at Google Quantum AI, who was formerly at IBM Quantum, recently carried out a theoretical study aimed at mathematically demonstrating the notable advantages of quantum computers. His paper, published in Nature Physics, mathematically shows that simulating random quantum circuits and estimating their outputs is so-called #P-hard for classical computers (i.e., meaning that is highly difficult).

“A key question in the field of quantum computation is: Are quantum computers exponentially more powerful than classical ones?” Ramis Movassagh, who carried out the study, told Phys.org. “Quantum supremacy conjecture (which we renamed to Quantum Primacy conjecture) says yes. However, mathematically it’s been a major open problem to establish rigorously.”

Researchers have recently been trying to demonstrate the advantages of quantum computers over classical computers in various ways, both via theoretical and experimental studies. A key to demonstrating this mathematically would be to prove that it is hard for classical computers to achieve the results of quantum computers with high precision and small margins of error.

“In 2018 a colleague gave a talk at MIT on, at the time, a recent result which tried to provide evidence for the hardness of random circuit sampling (RCS),” Movassagh explained. “RCS is the task of sampling from the output of a random quantum circuit and Google had just proposed it as the lead candidate for demonstrating quantum primacy. I was in the audience and had never worked on quantum complexity before; in fact, I remember that as a grad student I even vowed I’d never work in this field!”

The mathematical proof that Movassagh’s colleague presented at MIT in 2018 did not conclusively solve the long-standing problem of demonstrating quantum primacy yet it was a considerable advancement toward this goal. The proof was achieved via a series of approximations and so-called truncation of series; thus, it was somewhat indirect and introduced unnecessary errors.

“I love to bridge mathematics to solve big open problems, especially if the math is direct, less known to the experts in that field, and is beautiful,” Movassagh said. “In this case, I felt that I could probably find a better proof, and naively thought that if I solved the problem the right way then I might solve the big open problem. So, I set out to work on it.”

The mathematical proof presented by Movassagh greatly differs from those introduced so far. It is based on a new set of mathematical techniques that collectively show that the output probabilities of an average case (i.e., random quantum circuit) are as hard as the worst-case (i.e., most contrived).

“The idea is that you can use the Cayley path proposed in the paper to interpolate between any two arbitrary circuits, which in this case is taken to be between the worst-case and average-case,” Movassagh said. “Cayley path is a low-degree algebraic function. Since the worst -case is known to be #P hard (i.e., a very hard problem), using the Cayley path one can interpolate to the average-case and show that the random circuits are essentially as hard as the worst case with high probability.”

In contrast with other mathematical proofs derived in the past, Movassagh’s proof does not involve any approximations and is quite direct. This means that it allows researchers to explicitly bound involved errors and quantify its robustness (i.e., its tolerance to errors).

Since Movassagh first came up with the proof, both his research group and other teams have further tested it and improved its robustness. It could thus soon inform additional studies aimed at improving the proof or using it to highlight the potential of quantum computers.

“We realized direct proofs of the hardness of estimating the output probabilities of quantum circuits,” Movassagh said, “These provide computational barriers for the classical simulation of quantum circuits. The new techniques such as the Cayley path and rational function version of Berlekamp-Welch are of independent interest for quantum cryptography, computation and complexity, and coding theory. Currently, this is the most promising path toward eventual refutation of Extended-Church Turing thesis, which is an imperative goal of quantum complexity theory.”

The recent work by Movassagh greatly is a key contribution to ongoing research efforts exploring the advantages of quantum computers over classical computers. In his future studies, he plans to build on his current proof to mathematically demonstrate the huge potential of quantum computers for tackling specific problems.

“In my next studies, I hope to bridge this work to the hardness of other tasks to better map out the (in)tractability of quantum systems,” Movassagh added. “I am investigating the applications of this work in quantum cryptography among others. And last but not least, I hope to prove the quantum primacy conjecture and prove that the Extended Church-Turing thesis is false!”

More information: Ramis Movassagh, The hardness of random quantum circuits, Nature Physics (2023). DOI: 10.1038/s41567-023-02131-2

Journal information: Nature Physics 

© 2023 Science X Network

by Technion – Israel Institute of Technology

Researchers at the Technion—Israel Institute of Technology have developed a coherent and controllable spin-optical laser based on a single atomic layer. This discovery is enabled by coherent spin-dependent interactions between a single atomic layer and a laterally confined photonic spin lattice, the latter of which supports high-Q spin-valley states through the photonic Rashba-type spin splitting of a bound state in the continuum.

Published in Nature Materials and also featured in the journal’s Research Briefing, the achievement paves the way to study coherent spin-dependent phenomena in both classical and quantum regimes, opening new horizons in fundamental research and optoelectronic devices exploiting both electron and photon spins.

Can we lift the spin degeneracy of light sources in the absence of magnetic fields at room temperature? According to Dr. Rong, “Spin-optical light sources combine photonic modes and electronic transitions and therefore provide a way to study the exchange of spin information between electrons and photons and to develop advanced optoelectronic devices.”

“To construct these sources, a prerequisite is to lift the spin degeneracy between the two opposite spin states either in their photonic or electronic parts. This is usually accomplished by applying magnetic fields under a Faraday or Zeeman effect, although these approaches generally require strong magnetic fields and cannot produce miniaturized sources. Another promising way takes advantage of artificial magnetic fields for photonic spin-split states in momentum space, underpinned by a geometric phase mechanism.”

“Unfortunately, previous observations of spin-split states have relied heavily on propagation modes with low quality factors, which impose undesired limitations on spatial and temporal coherence of the sources. This approach is also hindered by the spin-controllable properties of a bulk laser gain material being unavailable or nontrivial to access for active control of the sources, especially in the absence of magnetic fields at room temperature.”

To achieve high-Q spin-split states, the researchers constructed photonic spin lattices with different symmetry properties, which comprise an inversion-asymmetry core and inversion-symmetry cladding integrated with a WS₂ monolayer to create laterally confined spin-valley states. The essential inversion-asymmetry lattice the researchers use has two important properties.

1. A controllable spin-dependent reciprocal lattice vector due to space-variant geometric phases from its constituting inhomogeneous-anisotropic nanoholes. This vector splits a spin-degenerate band into two spin-polarized branches in momentum space, being referred to as the photonic Rashba effect.
2. A pair of high-Q symmetry-enabled (quasi-) bound states in the continuum, that is, ±K (corners of the Brillouin zone) photonic spin-valley states, at the band edges of the spin-split branches. Moreover, the two states form a coherent superposition state with equal amplitudes.

Professor Koren noted that, “We used a WS₂ monolayer as the gain material because this direct-bandgap transition metal dichalcogenide possesses unique valley pseudospins, which have been widely investigated as an alternative information carrier in valleytronics. Specifically, their ±K’ valley excitons (radiated as in-plane spin-polarized dipole emitters) can be selectively excited by spin-polarized light according to a valley-contrasted selection rule, thus enabling active control of spin-optical light sources without magnetic fields.”

In the monolayer-integrated spin-valley microcavities, ±K’ valley excitons couple to ±K spin-valley states owing to polarization matching, and spin-optical excitonic lasing is achieved at room temperatures through strong optical feedback. Meanwhile, ±K’ valley excitons (initially without a phase correlation) are driven by the lasing mechanism to find the minimum-loss state of the system, which leads them to re-establish a phase-locked correlation according to the opposite geometric phases of ±K spin-valley states.

This lasing-mechanism-driven valley coherence removes the need for cryogenic temperatures to suppress the intervalley scattering. Moreover, the minimum-loss state of the Rashba monolayer laser can be regulated to be satisfied (broken) via a linear (circular) pump polarization, which provides a way to control the lasing intensity and spatial coherence.

“The unveiled photonic spin valley Rashba effect provides a general mechanism to construct surface-emitting spin-optical light sources. The demonstrated valley coherence in the monolayer-integrated spin–valley microcavity makes a step towards achieving entanglement between ±K’ valley excitons for quantum information by means of qubits,” explains Professor Hasman.

“For a long time, our group has been working on developing spin optics to harness photonic spin as an effective tool to control the behavior of electromagnetic waves. In 2018, we were attracted by valley pseudospins in two-dimensional materials, and therefore began a long-term project to study the active control of atomic-scale spin-optical light sources in the absence of magnetic fields. We initially tackled the challenge of coherent geometric phase pickup from individual valley excitons by using a non-local Berry-phase defect mode.”

“However, the underlying coherent addition of multiple valley excitons of the realized Rashba monolayer light sources remained unsolved, owing to the lack of a strong synchronizing mechanism between the excitons. This issue inspired us to think about high-Q photonic Rashba modes. Following innovations in new physical approaches, we achieved the Rashba monolayer laser described here.”

The achievement paves the way to study coherent spin-dependent phenomena in both classical and quantum regimes, opening new horizons in fundamental research and optoelectronic devices exploiting both electron and photon spins.

The study was conducted in the research group of Professor Erez Hasman, head of the Atomic-Scale Photonics Laboratory, in collaboration with Professor Elad Koren, head of the Laboratory for Nanoscale Electronic Materials and Devices in the Department of Materials Science and Engineering, and Professor Ariel Ismach at Tel Aviv University.

The two groups at the Technion are in association with the Helen Diller Quantum Center and Russell Berrie Nanotechnology Institute (RBNI). Dr. Kexiu Rong conducted and led the research, and collaborated with Dr. Xiaoyang Duan, Dr. Bo Wang, Dr. Vladimir Kleiner, Dr. Assael Cohen, Dr. Pranab K. Mohapatra, Dr. Avinash Patsha, Dr. Subhrajit Mukherjee, Dror Reichenberg, Chieh-li Liu, and Vladi Gorovoy.

The fabrication was performed at the Micro-Nano Fabrication & Printing Unit (MNF&PU) of the Technion.

More information: Kexiu Rong et al, Spin-valley Rashba monolayer laser, Nature Materials (2023). DOI: 10.1038/s41563-023-01603-3

A spin-optical monolayer laser based on a photonic spin lattice, Nature Materials (2023). DOI: 10.1038/s41563-023-01623-z

Journal information: Nature Materials 

Provided by Technion – Israel Institute of Technology 

by José Tadeu Arantes, FAPESP

A study conducted at the Center for Research, Education and Innovation in Vitreous Materials (CeRTEV) in São Carlos, São Paulo state, Brazil, shows for the first time that including niobium oxide (Nb₂O₅) in silicate glass results in silica network polymerization, which increases bond density and connectivity, enhancing the mechanical and thermal stability of specialty glass.

The study was reported in an article published in the journal Acta Materialia.

The first author of the article, Henrik Bradtmüller, is a postdoctoral researcher at the Federal University of São Carlos’s Center for Exact Sciences and Technology (CCET-UFSCar). His supervisor is Edgar Dutra Zanotto, director of CeRTEV and last author of the article. CeRTEV is hosted by UFSCar.

“Our study combined experimental observations using nuclear magnetic resonance spectroscopy and Raman spectroscopy with computational modeling. Besides the results mentioned, we found that higher levels of niobium led to Nb₂O₅, clustering, and heightened electronic polarizability, with a significant impact on the optical properties of the glass,” Bradtmüller said.

It is worth recalling that Raman spectroscopy provides precise information on the molecular structure of materials, while nuclear magnetic resonance (NMR) spectroscopy additionally explores the magnetic properties of their atomic nuclei.

“Our strategy based on these two observational techniques plus computational modeling can be used to study functional elements of many other types of glass, including optical materials, bioactive glass and glassy fast-ion conductors. This will facilitate the development of innovative glass formulations adapted for various applications,” Bradtmüller said.

Alongside the everyday applications of ordinary glass in containers, windows and so on, high-quality glass has also become almost ubiquitous in today’s world, Bradtmüller noted. It is present in the microscopes and telescopes used by scientists, for example, in the optical fibers used to carry data and power, and in the glass-ceramic orthotic devices increasingly used in medicine. “In recognition of the role played by glass in contemporary society, the United Nations declared 2022 to be the International Year of Glass,” he said.

For advanced high-tech applications, materials scientists are using machine learning software and other computational resources to design glass with customized properties, but to do so they require reliable databases and structural parameters that take into account the physicochemical complexity of glass.

This is the relevance of the study by Bradtmüller and colleagues. “Glass intermediate oxides play a strategic role in this new technological moment. They don’t form glass under standard cooling in the laboratory, but they can make a positive contribution in the presence of other oxides by helping to build oxygen bridges and giving glass the properties of interest. Niobium oxide is a good example,” he explained.

Glass that contains niobium (Nb) is valued for its non-linear optical properties, with potential applications in optoelectrical devices, and for mechanical properties relevant to the fabrication of bioactive materials. “Although studies had been conducted using Nb₂O₅ before our own, the structural role of Nb remained obscure, owing mainly to lack of systematic spectroscopic characterization data. We set out to fill this knowledge gap in our study,” he said.

“We discovered through spectroscopy that the addition of Nb causes ‘polymerization’ of the silica-oxygen network, increasing the connectivity of the glass’s components. This clarified the role of Nb as a ‘network former’. Another highlight of the study is our demonstration that a new NMR technique we developed in 2020 using other materials is applicable to glass. This technique, which is called W-RESPDOR, can be used to measure the distance between two elements—in this case, lithium and Nb, which has such a challenging nucleus that it had never been measured with similar techniques.”

Computational modeling showed that lithium ions are randomly distributed in silica-based glass at the nanometric scale (5–10 nanometers), while Nb tends to form clusters at higher concentrations of Nb₂O₅, he explained, adding that this kind of structural arrangement had never been reported in the literature and is an original contribution of the study.

“In a broader perspective, the study points to an experimental and computational strategy to investigate the role played in glass by intermediate oxides with active nuclei for NMR spectroscopy,” Zanotto said.

More information: Henrik Bradtmüller et al, Structural impact of niobium oxide on lithium silicate glasses: Results from advanced interaction-selective solid-state nuclear magnetic resonance and Raman spectroscopy, Acta Materialia (2023). DOI: 10.1016/j.actamat.2023.119061

Journal information: Acta Materialia 

Provided by FAPESP 

by Helmholtz Association of German Research Centres

Freeze casting processes can be used to produce highly porous and hierarchically structured materials that have a large surface area. They are suitable for a wide variety of applications, as electrodes for batteries, catalyst materials or in biomedicine.

Now a team led by Prof. Ulrike G. K. Wegst, Northeastern University, Boston, MA, U.S. and Dr. Francisco García Moreno from the Helmholtz-Zentrum Berlin have now used the newly developed X-ray tomoscopy technique at the Swiss Light Source of the Paul Scherrer Institute to observe in real time and at high resolution how the process of structure formation takes place during freezing. A sugar solution served as the model system.

Freeze-casting requires several steps: First, substances are dissolved or suspended in a solvent and then frozen in a mold with a cooling rate applied to the bottom (directional solidification). After freezing, the solid solvent phase is removed by sublimation. What remains are the previously dissolved solute molecules and suspended particles, which form the cell walls of the resulting complex, highly porous architecture.

Freeze-cast materials can be used for many applications: for instance, due to their enormous internal surface areas as battery electrodes or catalysts or because of their aligned porosity in biomedical applications for example as scaffolds for peripheral nerve repair. However, exactly how the ice templates the complex architecture during freezing, and how the desired honeycomb-like aligned porosity and the cell walls with their various surface features are formed, has remained little understood until now.

Dr. Francisco García Moreno and his team at Helmholtz-Zentrum Berlin have developed a method to observe these highly dynamic processes in detail. “Using X-ray tomoscopy, we can image the formation of structures in situ with high spatial and temporal resolution and even observe transient phenomena and transitional structures,” explains the physicist.

Using an ultrafast turntable, intense X-rays, an extremely fast detector and software for rapid analysis of the X-ray data, the HZB team, together with colleagues at the Swiss Light Source of the Paul Scherrer Institute, studied freeze casting on a model system and demonstrated the high performance of the method.

“For this study, we developed a new measuring cell with sensors to precisely record the temperature gradient,” says Dr. Paul Kamm (HZB), lead author of the study. A 3D tomogram with a spatial resolution of 6 µm per second was generated. The entire freezing process was documented over 270 seconds.

Prof. Ulrike G. K. Wegst from Northeastern University, U.S., had suggested an aqueous sugar solution as a polymeric model system, since this system can be simulated computationally, and because aqueous solutions still dominate the freeze casting process. “We are now able to experimentally observe for the first time the dynamics of directional ice crystal grow from the liquid phase,” says Wegst.

“In doing so, the images document how instabilities form during crystal growth, how these shape the sugar phase and how characteristic, organic-looking structures are formed on the cell walls that are reminiscent of jellyfish and tentacles.” It is also interesting to note that some of these structures may disappear again.

The paper is published in the journal Advanced Functional Materials.

More information: Paul H. Kamm et al, X‐Ray Tomoscopy Reveals the Dynamics of Ice Templating, Advanced Functional Materials (2023). DOI: 10.1002/adfm.202304738

Journal information: Advanced Functional Materials 

Provided by Helmholtz Association of German Research Centres

by Karyn Hede, Pacific Northwest National Laboratory

The humble neutrino, an elusive subatomic particle that passes effortlessly through normal matter, plays an outsized role among the particles that comprise our universe. To fully explain how our universe came to be, we need to know its mass. But, like so many of us, it avoids being weighed.

Now, an international team of researchers from the United States and Germany leading an ambitious quest called Project 8 reports that their distinctive strategy is a realistic contender to be the first to measure the neutrino mass. Once fully scaled up, Project 8 could help reveal how neutrinos influenced the early evolution of the universe as we know it.

In 2022, the KATRIN research team set an upper bound for how heavy the neutrino could possibly be. That milestone was a tour-de-force accomplishment that has been decades in the making. But these results simply narrow the search window. KATRIN will soon reach and may one day even exceed its targeted detection limits, but the featherweight neutrino might be lighter still, begging the question: “What’s next?”

In their most recent study, the Project 8 team reports in Physical Review Letters that they can use a brand-new technique to reliably track and record a natural occurrence called beta decay. Each event emits a tiny amount of energy when a rare radioactive variant of hydrogen—called tritium—decays into the three subatomic particles: a helium ion, an electron, and a neutrino.

The ultimate success of Project 8 hinges on an ambitious plan. Rather than try to detect the neutrino—which effortlessly passes through most detector technology—the research team has instead gone after a simple measurement strategy that can be summarized as follows:

We know the total mass of a tritium atom equals the energy of its parts, thanks to Einstein. When we measure a free electron generated by beta decay, and we know the total mass, the “missing” energy is the neutrino mass and motion.

“In principle, with technology developments and scale up, we have a realistic shot at getting into the range necessary to pin down the neutrino mass,” said Brent VanDevender, one of the principal investigators of Project 8 at the Department of Energy’s Pacific Northwest National Laboratory.

Why Project 8?

These researchers chose to go after an ambitious strategy because they have worked through the pros and cons and concluded that it could work.

Talia Weiss is a nuclear physics graduate student at Yale University. She and her Project 8 colleagues have spent years figuring out how to accurately tease out the electron signals from electronic background noise. Christine Claessens is a postdoctoral associate at the University of Washington who earned her Ph.D. on Project 8 at the University of Mainz, Germany. Weiss and Claessens performed the two final analyses that placed limits on the neutrino mass derived from the new technique for the first time.

“The neutrino is incredibly light,” said Weiss. “It’s more than 500,000 times lighter than an electron. So, when neutrinos and electrons are created at the same time, the neutrino mass has only a tiny effect on the electron’s motion. We want to see that small effect. So, we need a super precise method to measure how fast the electrons are zipping around.”

Project 8 relies on just such a technique, one conceived over a decade ago by physicists Joe Formaggio and Ben Monreal, then working at Massachusetts Institute of Technology. An international team rallied around the idea and formed Project 8 to convert the vision into a practical tool. The resulting method is called Cyclotron Radiation Emission Spectroscopy (CRES).

It captures the microwave radiation emitted from newborn electrons as they spiral around in a magnetic field. These electrons carry away most—but not all—of the energy released during a beta decay event. It’s that missing energy that can reveal the neutrino mass. This is the first time that tritium beta decays have been measured, and an upper limit placed on the neutrino mass, with the CRES technique.

The team is only interested in tracking these electrons because their energy is key to revealing the neutrino mass. While this strategy has been used previously, the CRES detector measures that crucial electron energy with the potential to scale up beyond any existing technology. And that scalability is what sets Project 8 apart. Elise Novitski is an assistant professor at the University of Washington and has led many aspects of the newly published work.

“Nobody else is doing this,” Novitski said. “We’re not taking an existing technique and trying to tweak it a little bit. We’re kind of in the Wild West.”

In their most recent experiment, built at the University of Washington in Seattle, the team tracked 3,770 tritium beta decay events over an 82-day trial window in a sample cell the size of a single pea. The sample cell is cryogenically cooled and placed in a magnetic field that traps the emerging electrons long enough for the system’s recording antennas to register a microwave signal.

Crucially, the team registered zero false signals or background events that could be confused for the real thing. That’s important because even a very small background can obscure the signal of neutrino mass making interpretation of useful signal more difficult.

From chirps to signals

A subset of Project 8 researchers, led by PNNL experimental physicist Noah Oblath, but involving a dozen others across multiple institutions, have also developed a suite of specialized software—each delightfully named after various insects—to take the raw data and convert them to signals that can be analyzed. And project engineers have put their tinkering hats on to invent the various parts that make Project 8 come together.

“We do have engineers who are crucial to the effort,” Novitski said. “It’s kind of out there from an engineer’s point of view. Experimental physics is kind of at the boundary of physics and engineering. You have to get particularly adventuresome engineers and practical-minded physicists to collaborate, make these things come into being because this stuff is not in the textbooks.”

Getting to the finish line

Now that the team has shown their design and experimental system works using molecules of tritium, they have another pressing task ahead. A subset of the full team is now working on the next step: a system that will produce, cool and trap individual atoms of tritium. This step is tricky because tritium, like its more abundant cousin hydrogen, prefers to form molecules. Those molecules would make the ultimate goals of the Project 8 team unachievable. The researchers, led by physicists at the University of Mainz, are developing a testbed to create and trap atomic tritium with intricate arrays of magnets that will keep it from even touching the walls of the sample cell—where it is almost certain to revert to molecular form.

This technology advance, and scaling up the whole apparatus, will be the critical steps to reaching and ultimately exceeding the sensitivity achieved by the KATRIN team.

For now, the research team, which has contributing members from ten research institutions, is working on testing designs for scaling up the experiment from the pea-size sample chamber to one a thousand times larger. The idea there is to capture a lot more beta decay events using a bigger listening device—going from the size of a pea to a beachball.

“Project 8 is not only a bigger and better CRES experiment, it is the first CRES experiment and was the very first to ever use this detection technique,” Oblath said. “It had never been done before. Most experiments have a 50- or 100-year history, at least of the detection technique that they’re using, whereas this is really brand new.”

More information: A. Ashtari Esfahani et al, Tritium Beta Spectrum Measurement and Neutrino Mass Limit from Cyclotron Radiation Emission Spectroscopy, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.131.102502

Journal information: Physical Review Letters

Provided by Pacific Northwest National Laboratory

by Louise Lerner, University of Chicago

Scientists with the University of Chicago have demonstrated a way to create infrared light using colloidal quantum dots. The researchers said the method demonstrates great promise; the dots are already as efficient as existing conventional methods, even though the experiments are still in early stages.

The dots could someday form the basis of infrared lasers as well as small and cost-effective sensors, such as those used in exhaust emissions tests or breathalyzers.

“Right now the performance for these dots is close to existing commercial infrared light sources, and we have reason to believe we could significantly improve that,” said Philippe Guyot-Sionnest, a professor of physics and chemistry at the University of Chicago, member of the James Frank Institute, and one of three authors on the paper published in Nature Photonics. “We’re very excited for the possibilities.”

The right wavelength

Colloidal quantum dots are tiny crystals—you could fit a billion into the period at the end of this sentence—that emit different colors of light depending on how big you make them. They’re very efficient and easy to make and are already being used in commercial technology; you might already have bought a quantum-dot TV without knowing it.

However, those quantum dots are being used to make light in the visible wavelength—the part of the spectrum humans can see. If you wanted quantum dot light in the infrared wavelength, you’ve mostly been out of luck.

But infrared light has a lot of uses. In particular, it is very useful for making sensors. If you want to know whether harmful gases are coming out of your car exhaust, or test whether your breath is above the legal alcohol limit, or make sure methane gas isn’t coming out of your drill plant, for example, you use infrared light. That’s because different types of molecules will each absorb infrared light at a very specific wavelength, so they’re easy to tell apart.

“So a cost-effective and easy-to-use method to make infrared light with quantum dots could be very useful,” explained Xingyu Shen, a graduate student and first author on the new study.

Infrared lasers now are made through a method called molecular epitaxy, which works well but is labor- and cost-intensive. The scientists thought there might be another way.

Guyot-Sionnest and his team have been experimenting with quantum dots and infrared technology for years. Building on their previous inventions, they set out to try to recreate a “cascade” technique that is widely used to make lasers, but had never been achieved with colloidal quantum dots.

In this “cascade” technique, researchers run an electrical current across a device, which sends millions of electrons traveling across it. If the architecture of the device is just right, the electrons will travel through a series of distinct energy levels, like falling down a series of waterfalls. Each time the electron falls down an energy level, it has the chance to emit some of that energy as light.

The researchers wondered if they could create the same effect using quantum dots. They created a black “ink” of trillions of tiny nanocrystals, spread it onto a surface and sent an electrical current through.

“We thought it would be likely to work, but we were really surprised by how well it worked,” said Guyot-Sionnest. “Right away, from the first time we tried it, we saw light.”

In fact, they found that the method was already as efficient as other, conventional ways to produce infrared light, even in exploratory experiments. With further tinkering, the scientists said, the method could easily surpass existing methods.

Potential applications

They hope the discovery could lead to significantly cheaper infrared lights and lasers, which could open up new applications.

“I think it’s one of the best examples of a potential application for quantum dots,” said Guyot-Sionnest. “Many other applications could be achieved with other materials, but this architecture really only works because of the quantum mechanics. I think it’s pushing the field forward in a really interesting way.”

More information: Xingyu Shen et al, Mid-infrared cascade intraband electroluminescence with HgSe–CdSe core–shell colloidal quantum dots, Nature Photonics (2023). DOI: 10.1038/s41566-023-01270-5

Journal information: Nature Photonics 

Provided by University of Chicago 

A team of chemists and physicists with members from Kyoto University, the Nishina Center for Accelerator-Based Science, RIKEN, Rikkyo University and Tohoku University, all in Japan, have for the first time observed electron scattering from radioisotopes that do not occur naturally. The study is published in the journal Physical Review Letters.

Ever since the discovery in the 1950s that atomic nuclei have a finite size—on the femtometer scale—researchers have been looking for ways to create pictures of atomic nuclei to learn more about their structure. Such a device would necessarily have to be a type of femtoscope. In this new effort, the research team built a system that represents the realization of such a device.

The research team began with a particle accelerator—it was used to energize a group of electrons, which were directed to smash into a block of uranium carbide. This resulted in the production of a stream of cesium-137 ions. The ions were then directed to what the team describes as a self-confining radioactive-isotope ion target (SCRIT) system.

Their system trapped the ions in a three-dimensional space aligned with an electron beam. An overlap was created between the ions and electrons in the beam, which allowed for collisions between them. The researchers then used a magnetic spectrometer to record the interference patterns that developed—a means for recording the electron scattering.

The system marks the first time that such scattering has been recorded and observed. It also opens the door to new research avenues as the same system can be used to study scattering with other types of nuclei, most particularly those that are short lived. The research team also points out that their system was able to demonstrate the properties of a femtoscope—an oscilloscope that operates on the femtometer scale. They suggest that it could ultimately be used to develop a common, unified theory to describe the structure of atomic nuclei.

More information: K. Tsukada et al, First Observation of Electron Scattering from Online-Produced Radioactive Target, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.131.092502

Journal information: Physical Review Letters 

© 2023 Science X Network

Window glass, at the microscopic level, shows a strange mix of properties. Like a liquid, its atoms are disordered, but like a solid, its atom are rigid, so a force applied to one atom causes all of them to move.

It’s an analogy physicists use to describe a quantum state called a “quantum spin-glass,” in which quantum mechanical bits (qubits) in a quantum computer demonstrate both disorder (taking on seemingly random values) and rigidity (when one qubit flips, so do all the others). A team of Cornell researchers unexpectedly discovered the presence of this quantum state while conducting a research project designed to learn more about quantum algorithms and, relatedly, new strategies for error correction in quantum computing.

“Measuring the position of a quantum particle changes its momentum and vice versa. Similarly, for qubits there are quantities which change one another when they are measured. We find that certain random sequences of these incompatible measurements lead to the formation of a quantum spin-glass,” said Erich Mueller, professor of physics in the College of Arts and Sciences (A&S). “One implication of our work is that some types of information are automatically protected in quantum algorithms which share the features of our model.”

“Subsystem Symmetry, Spin-glass Order, and Criticality From Random Measurements in a Two-dimensional Bacon-Shor Circuit” published on July 31 in Physical Review B. The lead author is Vaibhav Sharma, a doctoral student in physics.

Assistant professor of physics Chao-Ming Jian (A&S) is a co-author along with Mueller. All three conduct their research at Cornell’s Laboratory of Atomic and Solid State Physics (LASSP).

“We are trying to understand generic features of quantum algorithms—features which transcend any particular algorithm,” Sharma said. “Our strategy for revealing these universal features was to study random algorithms. We discovered that certain classes of algorithms lead to hidden ‘spin-glass’ order. We are now searching for other forms of hidden order and think that this will lead us to a new taxonomy of quantum states.”

Random algorithms are those that incorporate a degree of randomness as part of the algorithm—e.g., random numbers to decide what to do next.

Mueller’s proposal for the 2021 New Frontier Grant “Autonomous Quantum Subsystem Error Correction” aimed to simplify quantum computer architectures by developing a new strategy to correct for quantum processor errors caused by environmental noise—that is, any factor, such as cosmic rays or magnetic fields, that would interfere with a quantum computer’s qubits, corrupting information.

The bits of classical computer systems are protected by error-correcting codes, Mueller said; information is replicated so that if one bit “flips,” you can detect it and fix the error. “For quantum computing to be workable now and in the future, we need to come up with ways to protect qubits in the same way.”

“The key to error correction is redundancy,” Mueller said. “If I send three copies of a bit, you can tell if there is an error by comparing the bits with one another. We borrow language from cryptography for talking about such strategies and refer to the repeated set of bits as a ‘codeword.'”

When they made their discovery about spin-glass order, Mueller and his team were looking into a generalization, where multiple codewords are used to represent the same information. For example, in a subsystem code, the bit “1” might be stored in 4 different ways: 111; 100; 101; and 001.

“The extra freedom that one has in quantum subsystem codes simplifies the process of detecting and correcting errors,” Mueller said.

The researchers emphasized that they weren’t simply trying to generate a better error protection scheme when they began this research. Rather, they were studying random algorithms to learn general properties of all such algorithms.

“Interestingly, we found nontrivial structure,” Mueller said. “The most dramatic was the existence of this spin-glass order, which points toward there being some extra hidden information floating around, which should be useable in some way for computing, though we don’t know how yet.”

More information: Vaibhav Sharma et al, Subsystem symmetry, spin-glass order, and criticality from random measurements in a two-dimensional Bacon-Shor circuit, Physical Review B (2023). DOI: 10.1103/PhysRevB.108.024205

Journal information: Physical Review B 

Provided by Cornell University 

by Kate Blackwood, Cornell University