Three-dimensional (3D) reconstruction of a scene hidden from the direct line of sight is an emerging challenge, with applications in defense, security, and surveillance. The authors demonstrate 3D full-color retrieval of a hidden scene, from high-resolution time-of-flight information provided by a single-pixel camera. By combining a high-efficiency single-photon avalanche diode (SPAD) detector and a high-refresh-rate digital mirror device (DMD), this technique allows the 3D recovery of a hidden scene, with acquisition times of less than a second, and with no mechanical scanning parts.

The precise control of ultrasound focused into the central nervous system (CNS) is limited mainly by strong phase aberrations due to refraction and attenuation by the skull. This study proposes the use of acoustic holograms to correct these phase aberrations, and also to conform arbitrary acoustic images inside the brain corresponding to CNS structures, such as the hippocampus. Experiments using a skull phantom are in excellent agreement with theory and simulations. Ultrasonic focusing can be observed for various target structures simultaneously, using an inexpensive 3D-printed acoustic holographic lens. These results open paths to innovative biomedical ultrasound applications.

Solid-state quantum electronics relies on efficient cooling of the electrons in nanoscale devices, which is typically done indirectly, via the insulating substrate of the chip. The weak electron-phonon coupling at low temperatures is, then, a roadblock on the route to ultralow temperatures. The authors demonstrate that on-chip nuclear demagnetization, using indium as the nuclear refrigerant, bypasses this bottleneck, owing to the high integration density and strong electron-nucleus coupling of In. The results showcase an enabling technology for physical discoveries and applications at ultralow electron temperatures, in or below the millikelvin regime.

With quantum computing technologies nearing the era of quantum supremacy, and of commercialization, near-term devices need to be tested with application-driven benchmarks. Here the authors contribute a comprehensive assessment of the readiness of quantum optimization for a real industrial problem: fault diagnosis in digital circuits. From the perspective of physics and application, they investigate the impact of next-generation quantum annealers in direct comparison to state-of-the-art classical heuristics. Although more challenging in nature, these real-world problems provide insight on the real-world performance of quantum optimization machines.

Self-absorption of internally radiated photons (“photon recycling”) is common in high-quality, direct-gap semiconductors, and can yield increased photovoltage in solar cells and enhanced emission efficiency in LEDs. For perovskite semiconductors, photon recycling is relatively unexplored, especially for devices under operating conditions. The authors develop a model to quantify the extent of photon recycling in state-of-the-art perovskite solar cells of varying nonradiative loss and geometry. They present clear experimental targets for material optoelectronic quality to harness photon recycling in solar cells, with major implications for achieving low-threshold lasing and efficient LEDs.

Bioelectrochemical systems are an emerging technology for the recovery of electrical energy and chemicals from aqueous organic streams. Organic compounds are converted by bacteria that form a biofilm on the electrodes. To improve performance, a better understanding of the physical processes in these biofilms is needed. This study uses physics-based modeling to gain a theoretical understanding of the coupled transport and conversion processes of operating bioelectrochemical systems. Transport of organic molecules and electrons inside the biofilm does $not$ limit current production significantly, but the accumulation of protons inside the biofilm can be a limiting factor.

The use of matter-wave interferometry to detect inertial forces is already a powerful tool in precision measurement, but extending it beyond the lab to navigation applications requires optimization for compact setups. Here researchers characterize point-source atom interferometry (PSI) in a centimeter-scale cell, using an expanding cloud of cold atoms and an optical Raman interaction for simultaneous measurement of the system’s acceleration along the Raman laser beams plus its rotation vector projected into the plane perpendicular to the beams. The sensitivity measurements presented here inform the further development of quantum inertial sensing that balances size and performance.

The familiar Hodgkin-Huxley (HH) model describes transmission of a nerve impulse through an axon’s membrane in terms of the dynamics of connected electrical circuits, featuring capacitors, voltage sources, and memristors. The recent quantization of the memristor’s dynamics now allows study of a $quantum$ version of the HH circuit. The authors reproduce the behavior of the single-ion-channel HH circuit in the quantum regime, and exploring the possibility of this setup to be the building block for bioinspired quantum neural networks. Their results will impact the fields of superconducting devices, neuromorphic quantum computing, and hardware-based solutions for quantum machine learning.

Size-based particle sorting is desirable for industrial and biomedical applications, but scalability remains a challenge. This study demonstrates the use of capillary forces to design a tunable soft filter that passively separates microparticles based on size through dip coating. Indeed, when a substrate is withdrawn from a suspension, only small particles are entrained, while large ones remain in the suspension. This separation technique is well suited for high-throughput applications, given the simple and scalable nature of dip coating, and may even be applied to bioparticles and microorganisms.

Three-dimensional (3D) reconstruction of a scene hidden from the direct line of sight is an emerging challenge, with applications in defense, security, and surveillance. The authors demonstrate 3D full-color retrieval of a hidden scene, from high-resolution time-of-flight information provided by a single-pixel camera. By combining a high-efficiency single-photon avalanche diode (SPAD) detector and a high-refresh-rate digital mirror device (DMD), this technique allows the 3D recovery of a hidden scene, with acquisition times of less than a second, and with no mechanical scanning parts.

Recent years have witnessed the flourishing of experimental $Ising$ $machines$, special-purpose computational devices that promise to solve the world’s toughest optimization problems in record times. Evaluating an Ising machine’s performance is problematic, though, as it poses two seemingly contradictory requirements: On the one hand, the generated problem instances should be hard to solve, yet on the other hand they should have verifiable solutions. This study provides a methodology for generating random optimization-problem sets from linear systems of equations that possess both desired properties, thereby allowing direct, unbiased benchmarking of these physical optimization devices.

A Faraday-rotation atomic magnetometer typically employs a resonant, circularly polarized pump beam and an off-resonant, linearly polarized probe beam. Using two light beams is an obstacle to miniaturization, though. This study develops a single-beam scheme that converts a linearly polarized three-color beam into a circularly polarized carrier with orthogonally linearly polarized side bands. In a magnetic field, the carrier prepares anisotropic atoms, which then symmetrically rotate the polarizations of the side bands in opposite directions. This approach should enable miniature devices, and the sensing technologies that depend on them.

Solid-state quantum electronics relies on efficient cooling of the electrons in nanoscale devices, which is typically done indirectly, via the insulating substrate of the chip. The weak electron-phonon coupling at low temperatures is, then, a roadblock on the route to ultralow temperatures. The authors demonstrate that on-chip nuclear demagnetization, using indium as the nuclear refrigerant, bypasses this bottleneck, owing to the high integration density and strong electron-nucleus coupling of In. The results showcase an enabling technology for physical discoveries and applications at ultralow electron temperatures, in or below the millikelvin regime.

Two-dimensional (2D) materials) offer great potential for battery applications, due to their large surface areas, ability to accommodate various ions and molecules between their layers, and open channels for ion transport. The authors demonstrate that heterostructures made of an $MX$ene and functionalized graphene are promising candidates to control and manipulate the generation and diffusion of Li${}^{+}$ at the interface of the dissimilar materials. Combining doped graphene with an $MX$ene enhances storage capacity and markedly limits the volume change that is critical for eliminating restacking and maintaining cyclic stability.

While quantum efficiency of radiation a fundamental physical property of a semiconductor, the $external$ quantum efficiency (EQE) reflects the complicated dynamics of photoexcited carriers, and an accurate method for calculating it from photoluminescence measurements of real crystals, with defects, is an open problem. The authors present a general form for EQE by considering spatial carrier dynamics and self-absorption in the sample. They derive a simple, useful analytical formula for EQE, and find that it is affected by two varieties of light-extraction efficiency. The results provide a means of reliably quantifying the defects in crystals prepared for use in $e.g.$ power electronics.

Multiferroic tunnel junctions (MFTJs) have seen time in the spotlight lately, due to their high tunneling electroresistance and nonvolatility, which are appealing for digital memory devices. In this theoretical analysis of MFTJs, time-dependent perturbation theory yields the relation between ferroelectric polarization and ferromagnetic order at the ferroelectric/ferromagnetic interface. Furthermore, parameters for simulations are estimated from density functional theory and experimental results, and a comprehensive examination of tunneling electroresistance and tunneling magnetoresistance is conducted.

With quantum computing technologies nearing the era of quantum supremacy, and of commercialization, near-term devices need to be tested with application-driven benchmarks. Here the authors contribute a comprehensive assessment of the readiness of quantum optimization for a real industrial problem: fault diagnosis in digital circuits. From the perspective of physics and application, they investigate the impact of next-generation quantum annealers in direct comparison to state-of-the-art classical heuristics. Although more challenging in nature, these real-world problems provide insight on the real-world performance of quantum optimization machines.

Terahertz generation from liquid, in particular water, was long considered impossible, because of strong absorption. While terahertz radiation from liquid water was finally observed in 2017, the mechanism has remained unclear, and the efficiency low. Here experiments show that when a column of flowing water is used, the efficiency can be enhanced by three orders of magnitude—as high as for a typical two-color air plasma source, yet using a single-color laser pump here. The authors explain the mechanism with a model of current induced by laser ponderomotive force, which is supported by particle-in-cell simulations.

As spintronics continues its march to replace conventional electronics, spin-orbit-torque (SOT) -induced magnetization switching attracts much attention, due to its relatively low switching current density, and scalability. SOT requires the breaking of time-reversal symmetry, and is typically assisted by an external magnetic field, or additional antiferromagnetic layers in a device. However, this study presents SOT-driven switching $without$ an external field, in multilayers consisting of two ferromagnetic Co layers with mixed anisotropies, separated by a Pt spacer acting as a source of spin current. This structure has potential in particular for applications in neuromorphic computing.

Heat transport along a thin film can be severely affected by phonon scattering at surfaces or interfaces. Here researchers take advantage of this phenomenon to develop an extremely sensitive technique for examining in real time the growth of thin films, from the very early stages of island growth up to cluster coalescence and film formation. In addition, the high sensitivity of phonons as probes of the surface state enables the development of strategies to tune thermal transport at the atomic level.

Controlling thermal conduction is critical in fields such as thermoelectric power conversion and cooling, and bolometric radiation detection. A very effective way to influence heat flow is the coherent modification of phonon properties by using periodic phononic crystals, with the surprising prediction that structures of larger and larger period lead to vanishing conductance. This study demonstrates that, in practice, the effect works only up to a certain point, beyond which scattering due to surface roughness reverses the trend and conductance increases. This means that optimal designs for low thermal conductance exist, and can be improved through better control of surface roughness.

Though polarization-conversion (PC) metasurfaces have been thoroughly explored in the microwave and optical regimes, they have seldom been used in everyday applications, such as improving the overall performance of antennas for wireless communication. This study not only focuses on the strategy for designing an efficient PC metasurface, but also applies it to construct a high-performance Cassegrain antenna. This metasurface is capable of simultaneous polarization manipulation and arbitrary phase tailoring, while the antenna has the advantages of low profile, high directivity, broad bandwidth, and high polarization purity.

Plasmas produced from microdroplets of liquid tin provide light at an extreme ultraviolet (EUV) wavelength of 13.5 nm, for state-of-the-art nanolithography that will enable the continuation of Moore’s law in shrinking transistors. Currently CO${}_2$ gas lasers are used to drive such plasma; transitioning to modern solid-state lasers would have significant advantages, if the efficiency of converting laser energy into 13.5-nm radiation were sufficiently competitive. This study quantifies the radiation efficiency of solid-state-laser-driven tin plasma. High conversion efficiencies are obtained, and paths toward higher efficiencies using 1-$\mu$m solid-state lasers are identified.

The optical response of a two-dimensional material is given by its in-plane optical-conductivity tensor, which can be anisotropic. In the extreme, a $hyperbolic$ $material$’s tensor includes opposite signs in the two directions, offering the potential for a wide variety of applications. This work shows that atomically thin black phosphorus can be efficiently tuned to become hyperbolic over the entire visible spectrum into the ultraviolet, via bias voltage, strain, number of layers, or optical pumping. This approach will enable actively modulated nanophotonics and optoelectronics for sensing, wavefront control, nanoscale heat transfer, and enhancement of spontaneous-emission rate.

Superconducting quantum circuits are a leading candidate technology for large-scale quantum computing and simulation. Future scaling and improvements in device performance hinge upon a more detailed understanding of the sources of dielectric loss in these systems, yet standard techniques cannot separate the contributions from distinct dielectric regions to the aggregate device performance. This study presents a method for assessing the separate loss contributions from each material interface and bulk dielectric within such a circuit, enabling targeted improvements in performance by both informing device design and providing feedback to assess microfabrication techniques.

The pursuit of quantum computing in silicon is motivated by the facts that (1) Si can be isotopically engineered into a nuclear-spin-free material, yielding long spin lifetimes, and (2) CMOS fabrication technology can be leveraged for manufacturing qubits in the large numbers required for fault-tolerant quantum computing. Establishing wafer-scale ${}^{28}$Si has been seen as a major bottleneck, but here the authors integrate the isotope into a state-of-the-art CMOS fab. The quantum transport properties of the two-dimensional electron gas obtained at the ${}^{28}$Si/${}^{28}$SiO${}_2$ interface support the use of wafer-scale ${}^{28}$Si as a material platform for industrial spin qubits.

Ultrasound-directed self-assembly (DSA) enables the arranging of particles into user-specified patterns, but not explicitly accounting for particle orientation has long hampered the potential for high-aspect-ratio particles. The authors demonstrate accurate control over the orientation of such particles, by maximizing the curvature of the acoustic radiation potential orthogonal to the chosen orientation. This knowledge makes it possible to use ultrasound DSA as a processing method for engineered materials based on $e.g.$ fibers in a polymer matrix.

Light emission stemming from V impurities in $4H$-SiC is recorded at 1.28 and 1.33 $\mu$m, in the telecommunication $O$ band, which gives hope for the creation of efficient single-photon sources in existing telecommunication networks, ultimately paving the way for secure long-range quantum communication networks. Combined with the available electronic and nuclear degrees of freedom, vanadium presents all of the required ingredients for a highly efficient spin-photon interface. These V centers are reminiscent of the Mo defect in SiC and the Si-$V$ complex in diamond, but work at practical wavelengths for telecommunication.

The precise control of ultrasound focused into the central nervous system (CNS) is limited mainly by strong phase aberrations due to refraction and attenuation by the skull. This study proposes the use of acoustic holograms to correct these phase aberrations, and also to conform arbitrary acoustic images inside the brain corresponding to CNS structures, such as the hippocampus. Experiments using a skull phantom are in excellent agreement with theory and simulations. Ultrasonic focusing can be observed for various target structures simultaneously, using an inexpensive 3D-printed acoustic holographic lens. These results open paths to innovative biomedical ultrasound applications.

Self-absorption of internally radiated photons (“photon recycling”) is common in high-quality, direct-gap semiconductors, and can yield increased photovoltage in solar cells and enhanced emission efficiency in LEDs. For perovskite semiconductors, photon recycling is relatively unexplored, especially for devices under operating conditions. The authors develop a model to quantify the extent of photon recycling in state-of-the-art perovskite solar cells of varying nonradiative loss and geometry. They present clear experimental targets for material optoelectronic quality to harness photon recycling in solar cells, with major implications for achieving low-threshold lasing and efficient LEDs.

Bioelectrochemical systems are an emerging technology for the recovery of electrical energy and chemicals from aqueous organic streams. Organic compounds are converted by bacteria that form a biofilm on the electrodes. To improve performance, a better understanding of the physical processes in these biofilms is needed. This study uses physics-based modeling to gain a theoretical understanding of the coupled transport and conversion processes of operating bioelectrochemical systems. Transport of organic molecules and electrons inside the biofilm does $not$ limit current production significantly, but the accumulation of protons inside the biofilm can be a limiting factor.

The use of matter-wave interferometry to detect inertial forces is already a powerful tool in precision measurement, but extending it beyond the lab to navigation applications requires optimization for compact setups. Here researchers characterize point-source atom interferometry (PSI) in a centimeter-scale cell, using an expanding cloud of cold atoms and an optical Raman interaction for simultaneous measurement of the system’s acceleration along the Raman laser beams plus its rotation vector projected into the plane perpendicular to the beams. The sensitivity measurements presented here inform the further development of quantum inertial sensing that balances size and performance.

Obtaining room-temperature magnetic semiconductors is a big challenge in science, and for spintronic applications. Using density functional theory, this study predicts Cr${}_2$Ge${}_2$Se${}_6$ to be a stable two-dimensional (2D) ferromagnetic semiconductor, for which the Curie temperature can be above room temperature, by applying slight strain. In addition, the anomalous Hall conductivities of 2D Cr${}_2$Ge${}_2$Se${}_6$ and Cr${}_2$Ge${}_2$Te${}_6$ are predicted to be an order of magnitude greater than that of the conventional dilute magnetic semiconductor Ga(Mn,As). The microscopic mechanism to obtain room-temperature magnetic semiconductors by strain is highlighted.

Concentrating electromagnetic fields in a tight region of space can be useful for energy harvesting, or to enhance nonlinear effects. Nonreciprocal unidirectional guides offer unique opportunities in this context, as they may be used to “stop” a wave, leading to the formation of an electromagnetic energy sink. Here the authors unveil the topological origin of this effect, showing that it is due to the breakdown of the bulk-edge correspondence in electromagnetic continua. Similar energy sinks may also occur in fully reciprocal platforms with a $parity-time-duality$ symmetry, thus providing a practical path to realize energy sinks without a magnetic bias.

Surface acoustic waves (SAWs) serving as universal interfaces are intriguing for hybrid systems with classical and quantum components. This study demonstrates a method for designing a high-quality SAW resonator using phononic band structure engineering. The SAW resonator presents a quality factor $Q$ > 104 at gigahertz frequencies, yielding a figure of merit (the product of frequency and $Q$) of 10${}^{13}$ at room temperature. Improvements in $Q$ at cryogenic temperatures are also observed. The presented methodology paves the way for hybrid classical-quantum phonon networks.

Topology-optimized 3D-printed magnets are interesting for Larmor spin-rotators in neutron optics in general, and in neutron interferometry in particular. Using 3D-printed magnets instead of magnetic coils avoids heat dissipation, which is the the main cause of loss in fringe visibility, due to temperature gradients in the interferometer. This study applies the technique to implement an arbitrary neutronic phase gate, for rotations of up to $4\pi$ of the neutron’s spinor wave function in one arm of the interferometer. This is achieved by varying the distance between the 3D-printed magnets, while maintaining homogeneity of the magnetic action over the neutron beam’s profile.

Optical systems provide a versatile platform for realizing different types of bound states in the continuum (BICs), thanks to advanced nanofabrication for photonic structures on demand. Optical BICs exhibit ultrahigh-$Q$ resonances, which can enhance light-matter interaction by orders of magnitude. Forming BICs in photonic crystals and metamaterials is usually associated with in-plane symmetry breaking, but here the authors break the $out-of-plane$ symmetry in a dielectric metasurface’s unit cell to control Fano resonances originating from quasi-BICs. This idea is found to be general, and confirmed experimentally for lattices of particle clusters of different symmetries.

Artificial magnetic micro- and nanomachines, and their steering by weak rotating magnetic fields, have been extensively studied for biomedical applications, such as drug delivery. This work theoretically investigates the driven propulsion of planar magnetic microstructures that can be mass-produced by standard photolithography, and thus are of practical interest. Actuation by a conically rotating magnetic field (in contrast to a planar rotating field) can yield efficient unidirectional propulsion of a two-dimensional ⋁-shaped sructure magnetized in its plane. Surprisingly, even though the symmetrical ⋁ rotates with the actuating field, its propulsion velocity is frequency-independent.

Singlet-triplet spin states of a quantum dot support promising semiconductor-based qubits, yet often suffer from poor state preparation and measurement, due to low-lying excited states. Engineering a large energy splitting is impeded by the inability to accurately measure both large and small energy splittings with the device biased to nominal operation. The authors present a measurement and fitting approach that accurately extracts both large and small splittings in this regime. They also find evidence that both orbital and valley degrees of freedom may set this energy separation, significantly affecting which paths to pursue in device design.

Is it possible to make a solid that supports only shear waves and offers unique elastic wave functions over a broad frequency range? The authors show that, with delicate design of the elasticity tensor, the wave polarization can be engineered in a special type of elastic metamaterial, which can be made to carry only shear or longitudinal waves along a targeted direction. Simulations and experiments validate broadband wave-polarization control even when the unit cell is less than 2% of the wavelength. Such broadband polarization engineering of elastic waves is important for potential applications in structural monitoring, elastic-wave communication, and ultrasonic elastography.

In optics, the two main mechanisms for enhancing the Goos-Hänchen (GH) shift of a reflected light beam have a common shortcoming: The maximum shift is located exactly at the reflectance dip, which makes the reflected beam hard to detect. Here the authors tune the excitation of guided modes in a compound grating-waveguide structure, to realize quasibound states in the continuum (quasi-BICs) with ultrahigh $Q$-factors. Assisted by these quasi-BICs, the GH shift at the reflectance peak can be greatly enhanced. This giant GH shift with high reflectance can be used for $e.g.$ ultrasensitive sensors, wavelength-division (de)multiplexers, optical switches, and polarization beam splitters.

Charge injection and collection in organic optoelectronic devices are highly sensitive to the dielectric properties of the commonly used transparent electrodes. This characteristic has been somewhat overlooked, though, as experimental results can be adequately fitted by tuning other device parameters. This study introduces a correction to charge injection from the transparent electrode into the organic layer, which is found both experimentally and theoretically to cause a drastic reduction in the injected current. The authors argue that this characteristic must be considered in designing electrodes, and also must be included in models and parameter analysis to obtain reliable results.

Transmission lines with high characteristic impedance optimize the energy transfer among quantum systems spaced at micrometer distances, allowing for efficient wiring-up of quantum circuits on a chip, and for a strong coupling between semiconductor qubits and microwave photons. The authors propose a type of low-loss high-impedance transmission line based on the plasmonic response of materials in the quantum Hall regime that are capacitively coupled to external electrodes. The ability to manufacture these structures extends the toolkit of quantum Hall devices, which could provide a critical boost to semiconductor-based quantum information processing.

The manner in which an electrified jet breaks up is a key point in many applications, such as direct writing, electrospinning, and electrospray mass spectrometry. Controlling the instability mode of an electrified jet is always challenging, though, as there are many governing parameters. This study presents a robust, easy-to-use model to predict the transition from the axisymmetric (varicose) mode to the nonaxisymmetric (whipping) mode. The authors derive scaling laws based on competition of surface stresses in different electrohydrodynamic operating regimes, as a function of fluid properties and flow rate, and experiments on a variety of liquids reveal a unified threshold value.

Conventional acoustic metamaterials feature velocity-momentum coupling and pressure-strain coupling, in analogy to standard materials. $Willis$ $materials$, on the other hand, are a class of bianisotropic acoustic metamaterials that exhibit strain-velocity and pressure-momentum couplings, which do not exist in conventional elastic materials. A simple device consisting of two membrane resonators with slightly different decorating platelets, and separated by a sealed air column, not only exhibits the largest Willis coefficient by far, but also complies with the Kramers-Kronig relations demanded by causality.

Compact ultracold-atom sensors using a single vacuum chamber have their stability ultimately constrained by vacuum dynamics. To speed evacuation, the straightforward approach is to increase the pumping speed, which implies increasing pump size and sensor volume. This investigation of the dynamics of sputtered ion pumps (SIPs) describes the main physical mechanisms with a nonlinear model for ion-current pump-down in the low-pressure regime. One pumping process suggests that a structured cathode could curtail the increase in pump size, or at least maintain the trapping cross section of the electrode—a result that may be important in the design and development of miniature ion pumps.

The coiling of an electrified jet during dispensing can potentially benefit the printing and fabrication of fibers. However, due to a lack of understanding of the rich dynamics involved, some subtle and useful effects cannot be explained or predicted, limiting the technique’s utility for manipulating viscous fluids. This work considers the profile of the electric field as a control parameter for electrically induced coiling. Just by adjusting the field’s profile, coiling can be triggered or suppressed on demand, which is promising for $e.g.$ printing nanostructures, or fluid mixing.

Over the past decade, the quest for better scintillation detectors of ionizing radiation has led to the study of BaBrCl:Eu. Codoping with even 0.1% AuBr${}_3$ increases light yield and suppresses the undesirable long tail of emission, but the mechanism is not well understood. Codoping is a general method for improving scintillators, and explaining it in this case, where the Au seems not to be incorporated into the crystal, could be particularly interesting. Here time-resolved absorption spectroscopy plus first-principles calculations of self-trapped excitons and defects in BaBrCl illuminate the mechanisms of scintillation in BaBrCl:Eu, and of the effect of AuBr${}_3$ codoping.

Leaky-wave antennas (LWAs), which exploit traveling waves with phase velocity greater than the speed of light, have attracted much attention for their miniaturizability and easy fabrication. However, the polarization states of leaky-wave radiation are generally hard to design at will. This study presents a spoof-surface-plasmon waveguide with bilateral tilted grooves to tailor the polarization of its emission. The design is simple and feasible for realizing arbitrary polarization of leaky waves, just by changing the relative displacement of grooves on either side of the waveguide, and is expected to impact advanced microwave circuits and antennas.

The familiar Hodgkin-Huxley (HH) model describes transmission of a nerve impulse through an axon’s membrane in terms of the dynamics of connected electrical circuits, featuring capacitors, voltage sources, and memristors. The recent quantization of the memristor’s dynamics now allows study of a $quantum$ version of the HH circuit. The authors reproduce the behavior of the single-ion-channel HH circuit in the quantum regime, and exploring the possibility of this setup to be the building block for bioinspired quantum neural networks. Their results will impact the fields of superconducting devices, neuromorphic quantum computing, and hardware-based solutions for quantum machine learning.

The accuracy and speed of qubit readout can greatly affect the performance of quantum computers, which are held back by the lack of a more adaptive, accurate method for determining the system’s quantum state. This study uses field-programmable gate arrays for machine-learning-assisted methods of single-qubit readout on a Yb${}^{+}$ ion-trap system, achieving 99.53% average fidelity within 171 $\mu$s per sample. The proposed scheme shows considerable advantages over traditional methods in fidelity, speed, and robustness, and is compatible with real-time readout and feedback control of qubit states.

Porous conducting materials with ionizable surface groups provide flexible control of membrane selectivity, but a thorough physical understanding is still needed. In this work, researchers develop a pore model to capture the underlying physics, and achieve good agreement with experimental data. This insight will help to characterize and develop membranes for applications such as reverse electrodialysis and especially desalination, which becomes ever more important for ensuring the world’s water supply.

The importance of ferromagnetic resonance, discussed more than half a century ago by Charles Kittel, is ever-increasing, as we use it to evaluate various forms of spin-orbit torque in spintronics. However, a widely employed method can lead to significant overestimation of the torque in some systems. The authors offer a scheme that allows the quantification of spin-orbit torques, free from spurious signals—a welcome contribution to the worldwide effort to create the next generation of ultralow-power devices for data storage and processing.

Electric-field control of magnetism is especially interesting for its potential to boost energy efficiency in device applications. Strain-mediated effects in magnetostrictive-piezoelectric hybrid materials, such as ferromagnetic/ferroelectric heterostructures, offer some of the strongest voltage modulations of magnetic properties. For such a system in an electric field, not only strain-mediated effects arise, but also electronic charging and oxygen migration may simultaneously occur. Sorting out these contributions, as the authors do in this study, is crucial to gaining insight into the control of magnetism by voltage, and the design of next-generation magnetoelectronic devices.

The nitrogen-vacancy center in diamond offers a platform for high-sensitivity temperature sensing at the nanoscale. Extension of such thermometry to low temperatures has been limited, though, as techniques based on spin resonances lose sensitivity. The authors show that an alternative, all-optical technique works from room temperature down to liquid-nitrogen temperature, with no deterioration in thermal sensitivity. Using an array of diamond nanoparticles embedded in a polymer membrane, they measure a temperature gradient on the surface of yttrium iron garnet under local heating, which suggests that this technique could reveal crucial spatiothermal details in spin-caloritronic systems.

Understanding the magnetization dynamics in lattices of antidots (holes, rather than bumps) is interesting for applications in spintronics and data-storage technology. Magnetic antidot lattices have potential as magnetophotonic crystals, due to their higher spin-wave velocities (steeper dispersion), compared to dot arrays. This study shows that, by tuning the shapes of the holes (and hence the anisotropy) in the arrays, one can control the spin-wave spectra and engineer the domain structure in a thin film of Co${}_2$Fe${}_{0.4}$Mn${}_{0.6}$Si. This work should have significant impact on magnetic filters, splitters, and other magnonic devices.

Single-flux-quantum (SFQ) digital logic is an attractive candidate for classical control and monitoring of large-scale quantum processors based on superconducting qubits. The authors describe a control approach based on repeated irradiation of the qubits with short registers of classical bits, which can be stored locally in compact SFQ registers and streamed to the quantum array at a high rate. Numerical simulations show achievable gate fidelity in excess of 99.99%; moreover, the approach allows control of many qubits resonating at different frequencies with a single, global SFQ clock. As a result, the scheme is well matched to the control of a scalable two-dimensional surface code.

Efficient, reliable determination of a system’s state is at the heart of quantum science. Directly measuring any desired density-matrix elements is a unique challenge for both theorists and experimentalists, as standard methods have always suffered from the rapid increase in complexity of measurements and reconstruction algorithms. This study presents an efficient scheme for directly measuring arbitrary density matrices with only one strong measurement, or a weak measurement, of each qudit. This method is important for characterizing large-scale quantum systems, and the technology that will be derived from it will be easy to expand and integrate, for example, on a quantum chip.

Topological insulators continue to be a hot topic in research, particularly for their backscattering immunity. While previous works have restricted consideration to identical primitive unit cells, this work demonstrates an $aperiodic$ valley topological insulator, with robust selection of $C_{3v}$ symmetric scatterers in adjacent sonic crystals. This study also presented a functional device, the valley-selective cross-waveguide beam splitter, in the proposed acoustic system. The ability to select scatterers found in this work may illuminate further possibilities for designing acoustic topological-insulator-based devices with flexible structures.

Precise measurement of the polarization of relativistic spin-polarized electron beams is crucial for experiments in nuclear physics and high-energy physics. However, the usual methods are not applicable for low-repetition-rate, dense ultrarelativistic electron beams, such as those produced by strong laser pulses. The authors develop a theory of polarimetry based on nonlinear Compton scattering, which can determine the polarization of such a beam via single-shot interaction with a strong laser pulse, by detecting the asymmetry in high-energy-photon spectra. Statistical precision better than 0.3% can be achieved within femtoseconds.

Optical vortices bearing orbital angular momentum (OAM) with fractional topological charge have many applications in particle guiding and transport, anisotropic edge enhancement, high-dimensional quantum entanglement, and free-space optical communication. A fractional optical vortex (FOV) breaks OAM orthogonality, though, making precise measurement complex and difficult. This article shows how to measure the charge of a FOV by using a two-dimensional multifocal array of vortices with different integer charges. With an error of FOV detection below 2.5%, this technique is sufficient for a wide range of real-time requirements for using FOV in optical manipulation, imaging, or quantum optics.

Effective detection of an individual molecule’s fluorescence in a nanostructured environment is crucially important for DNA sequencing and single-molecule spectroscopy. The physics of single-molecule fluorescence in a nanoenvironment is very complicated, a fact that has hindered the design of nanodevices for near-field detection. Here analytical calculations and numerical simulations reveal that “leaky” plasmonic waves can substantially enhance a molecule’s radiation, and the specifics of this insight might have real impact on detector optimization.

Based on numerical simulations, the authors offer a midinfrared pulse source that uses hollow-core optical fiber filled with gas. Such a source could find applications across a large variety of sectors in science and technology (particularly biomedical), due to its conceptual simplicity. The approach taken in this work resolves all major issues related to pulse generation in the midinfrared spectral region. The system promises to be highly robust against noise, and has a major advantage over existing devices, in using a pump in the 1-$\mu$m wavelength range, which is readily available.

Many biological and medical applications of microfluidic devices involve handling blood at different concentrations (hematocrit). However, there is a lack of understanding of how high hematocrit influences device performance. The authors study the effect of hematocrit on the performance of sorting devices that are based on deterministic lateral displacement, through extensive experiments and simulation. They successfully identify several physical mechanisms that are responsible for changes in performance with hematocrit.

Improving the coherence of superconducting qubits is a crucial step toward the goal of fault-tolerant quantum processors based on them. An ongoing experimental challenge is to diagnose and mitigate the dominant decoherence mechanisms. In this work, the authors demonstrate a method of probing the decoherence induced by nonequilibrium superconducting quasiparticles, and show that improved filtering of quasiparticle-generating radiation can improve the energy-relaxation time $T_1$ of superconducting qubits, reaching an average of 200 $\mu$s in the measured device. Furthermore, the measurement technique is relevant for ultralow-noise sensing in general.

Circularly polarized (CP) light is typically obtained through either polarization conversion or structural (geometric or material) chirality. Here the authors reveal a fundamentally different mechanism of CP thermal radiation from a pair of nonequilibrium antennas, coupled via near-field interactions. Practically speaking, this mechanism enables the temperature-based reconfigurability of the polarization state that is lacked by most CP light sources. Fundamentally, it reveals a surprising connection between thermal nonequilibrium and the angular momentum of emitted radiation, without using any magnetic field.

Short, brilliant pulses of MeV-level gamma rays with controllable orbital angular momentum (OAM) and small divergence angle would be useful for precision microscopy, nuclear imaging, radiography, micromanipulation, and more. These pulses are still unavailable, though, due to the unavoidable laser-induced damage associated with traditional methods. The authors show how to make such a gamma-ray pulse (shown in red), based on tailored interaction of an intense OAM-carrying Laguerre-Gaussian laser pulse (green) with underdense plasma. The topological structure, divergence angle, and OAM of the resulting gamma-ray pulse can be controlled by selecting the topological charge of the laser.

Compositional variation in (In,Ga)N alloys is a controversial topic, still debated in the literature for its influence on the performance of light-emitting diodes. This study accounts for the presence of nanometer-scale indium clustering in an LED, and describes its impact on the optical properties of the device. Clustering induces substantial redshift and broadening of the emission spectrum; furthermore, the temperature dependence of the radiative coefficient derived for the nonuniform structures is in good agreement with experiments that show a trend opposite that expected from standard theoretical considerations.

Nonmagnetic disorder in a topological insulator (TI) often dopes the material with extra electrical charges, creating a normal electron fluid coexisting with the exotic Dirac electron fluid. As both of these fluids conduct electricity, conventional transport measurements are not convenient for studying the Dirac electron fluid. Therefore the authors develop a technique in which a TI with disorder (here Bi${}_2$Se${}_3$ with Se vacancies) is used to inductively couple two coils. With this setup they are not only able to specifically identify the contribution from currents induced in the Dirac fluid, but also unravel a complex, temperature-dependent interplay between the two electron fluids.

Thermionic emission critically influences the performance of graphene-based electronics, optoelectronics, and energy converters, but modeling it often relies on the Dirac-cone approximation of graphene’s electronic structure, which becomes invalid for electrons in high-energy states. The authors present a theory of high-energy thermionic electron emission in graphene based on the $full-band$ tight-binding electronic structure. The emitted electrical and heat current densities predicted by the Dirac approximation can deviate by more than 50% from the more accurate full-band model. The model developed here provides an improved avenue for analysis and design of graphene-based devices.

Traumatic brain injury (TBI) is a major public health concern, affecting millions of people each year in the USA. Understanding the patterns of brain movement and deformation during head impact is one of the most powerful strategies for shedding light on the physical causes of TBI. This study uses modal analysis and advanced finite-element simulations to characterize the dynamical behavior of the human brain during head impact. The authors find evidence of geometrical nonlinear effects in the deep white matter of the brain, suggesting a link between the onset of injury and the existence of local nonlinearity in brain tissue.

In secure communication, maintaining system stability is crucial for practical quantum key distribution (QKD). To date, “scanning-and-transmitting” programs have been adopted to stabilize all QKD systems, reducing efficiency in key transmission. For this reason, the authors turn to a machine-learning model to predict variations in physical parameters and actively exercise real-time control over corresponding QKD devices, dramatically increasing the efficiency of key transmission. This approach should also be applicable to other QKD systems using any coding scheme or QKD protocol, and thus should impact large-scale application of quantum communication networks in the near future.

Many applications involving optical cavities call for long photon lifetime, measured by the quality factor $Q$, in a small volume $V$. However, these constraints tend to be incompatible. In the case of a whispering-gallery resonator, light can circulate for a long time only if the radius is sufficiently large; below a certain size, bending losses soar and $Q$ quickly degrades. Here the authors suppress those fundamental losses using a scheme that can be implemented with existing fabrication technologies. By maintaining large $Q$ with small $V$, cavity performance could be boosted and limits extended in all related applications, from cavity quantum electrodynamics to biosensing.

Cavity optomagnonics and magnomechanics form a promising basis for an integrated platform for quantum information and sensing. For applications to blossom, proper comprehension and addressing of spin waves beyond the uniform precession mode is necessary. However, the traditional approach to identifying spin-wave modes shows ambiguities, potentially leading to misinterpretation and improper control of devices. This article presents a fresh approach: structural imaging of spin waves in a magnetized sphere by magnetic induction tomography. Until now, there has been no way to image and robustly identify the spin-wave modes in these macroscopic magnetized structures.

All-optical photoacoustic sensing is emerging as a sensitive technique for inspecting small volumes of fluids, for applications such as liquid biopsies, drug discovery, development of contrast agents for photoacoustic imaging, and analysis of gases. The authors implement a whispering-gallery-mode microbubble resonator as a multifunctional component for all-optical photoacoustic sensing. A laser pulse triggers photoacoustic activation of a sample in the cavity of the resonator, and the transient deformation of the cavity wall as a resonant oscillation is monitored. This configuration promises high sensitivity to optical absorbance, and removes the need for impedance-matched media.

A $pyroelectric$ material’s electric polarization depends on temperature, and this effect can play an important role in converting waste heat to electricity. However, the electricity actually converted by such a device is often confused with the external applied power. The authors present a thermodynamic analysis that specifically and unambiguously eliminates this confusion, and propose a pyroelectric figure of merit that separates the contributions from material properties and device parameters. Their insight will promote both materials development for and optimal design of devices for pyroelectric energy harvesting.