Optical homodyne (single-frequency) measurement is a key technology in optical quantum information processing and key distribution, since it can realize high-speed quantum measurement with a high signal-to-noise ratio. Researchers show that photodiodes have an intrinsic noise mechanism in the photodetection process, due to distributed photocarrier generation, and the excess noise cause an effective optical loss in homodyne detection at high frequencies. They evaluate this excess loss theoretically and experimentally, showing that it depends on the photodiode’s material, and suggesting further requirements for high-efficiency, high-speed hardware.

Secure communication protocols based on quantum physics compel worldwide attention. Continuous-variable quantum key distribution (CV-QKD) uses a cost-effective detection technique instead of dedicated single-photon-counting technology, and can provide high key rates over metropolitan distances. Traditional coherent-state CV-QKD protocols, however, suffer from low tolerance of channel excess noise. The authors succeed in distributing Einstein-Podolsky-Rosen entangled states over a 50-km standard fiber with negligible excess noise, and further demonstrate CV-QKD in a high-noise environment with performance superior to that of the optimized coherent-state protocol.

Phonon lasing (coherent mechanical amplification) is a key element in phononic engineering. A one‐way phonon laser would be an indispensable tool to explore chiral light‐sound interactions or to drive chiral phonon devices, yet has remained elusive. This study proposes a strategy to achieve such a device, by coupling an optomechanical system to a spinning resonator. Through the optical Sagnac effect, enhancement or suppression of phonon lasing can be achieved by driving the coupled system from one side or the other. This strategy provides a versatile way to operate spinning devices for applications in directional phonon control, sound sensing, and topological acoustics.

Defects in solids present a scalable platform for photon-based quantum information processing, and progress here relies on improving their optical and spin properties. The authors measure the spin-coherence properties of Ga donors in ZnO using all-optical methods of spin control. A longitudinal spin-relaxation time of 0.1 s and spin-coherence time on the scale of 50 $\mu$s are observed in bulk ZnO, and it is expected that the latter can be significantly improved through chemical and isotopic purification. This work motivates further research on growth of high-purity materials, quantum device fabrication, and high-fidelity control of the donor:ZnO system for quantum technologies.

Electron microscopes can image atoms in solids, molecules, and fields, but typically a different imaging method is used for each. The authors demonstrate a technique called scanning transmission electron microscopy (STEM) holography that is highly sensitive to electric and magnetic fields, is capable of subatomic resolution, and is efficient enough to minimize degradation of easily-damaged materials. With a single STEM holographic image, one could distinguish crystallized clusters on the surface of a nanoparticle and detect an electric field due to charging of the particle. All it takes is one diffraction grating installed in any TEM with a high-throughput camera.

To promote solar-cell research, this study provides exact mathematical formulae for the efficiency of photovoltaic conversion and voltage at maximum efficiency, in terms of absorbed photon flux and rates of radiative and nonradiative recombination processes. The authors also derive universal formulae for useful energy, thermal loss, and emission loss per absorbed photon. The general relation between optimal photocarrier collection time and photocarrier lifetime shows that carrier collection processes should be faster than recombination processes by one or two orders of magnitude. This formalism is then applied to analyze GaAs devices operating near the Shockley-Queisser limit.

The ability to freely control the polarization of electromagnetic waves is of great significance, and state-of-the-art challenges include achieving efficient, wide-band polarization control with a relatively thin device in transmission geometry. To meet the challenge, this study combines slow waves (from spoof surface plasmon polariton modes) with fast waves (from waveguide modes) to handle orthogonal polarization components. Such a system can be quite thin, due to the strong dispersion between these two types of modes, with cancellation of the two nonlinearly dispersive modes yielding high-efficiency polarization conversion over a wide frequency range.

There is a growing need for unconventional computing paradigms for special tasks, such as rapid image recognition, and networks of nanoscale oscillators could be the answer. Ultrasmall oscillators based on very energy-efficient physics are desired, to minimize power dissipation. This study finds robust phase and antiphase locking in the magnetization dynamics of a pair of ferromagnetic insulators (FMIs) on a topological insulator (TI). As driving the surface electrons of a TI requires hardly any energy, the finding suggests that these systems could well serve as building blocks for unconventional, low-power computing paradigms.

Impedance spectroscopy is one of the most powerful characterization techniques available for research on dielectric materials. For the important class of ferroelectric materials, however, this method is seldom applied, as it currently has no solid foundation related to the physics of ferroelectrics. To solve this critical problem, the authors derive an equivalent circuit element from the Rayleigh law of irreversible domain-wall motion, which allows one to interpret ferroelectric impedance spectra physically. In proof of principle, this approach is successfully applied to a Si:HfO${}_2$ thin film.

Stimulated Raman amplification with compound semiconductors is important for laser amplification and wavelength conversion, in $e.g.$ all-optical high-speed telecommunication systems. Unfortunately, in a bulk compound semiconductor, anti-Stokes amplification is very weak compared to Stokes amplification, and its application range is limited. This research reveals that intermixed GaAs/AlAs superlattices can enhance anti-Stokes efficiency by up to three orders magnitude at relatively low pump intensity, where other nonlinear optical effects are also included. With anti-Stokes efficiency of similar magnitude to Stokes efficiency, the range of potential applications widens.

Band-shape luminescence thermometry is promising for remote temperature sensing, in applications ranging from bioimaging to nanoelectronics to catalysis. Thermally coupled states in Boltzmann equilibrium are generally assumed, without justification; this study shows that assumption is not generally valid. The processes important to the temperature-dependent luminescence of the widely used lanthanide ions are identified, and a general model is presented to account for both Boltzmann and non-Boltzmann statistics. These insights are crucial for understanding the useful temperature-sensing range of these sensors, and for extending that range.

Improving global navigation satellite systems (GNSS) requires a more stable basis of timekeeping. Cold-atom clocks are currently the most accurate time references, but are large. This article presents an atomic clock, “Rubiclock”, that is compact and portable, thanks to a fiber-based laser bench, isotropic light cooling, and atoms that remain motionless during the operating sequence. This configuration gives the clock an important benefit in microgravity; measurements performed on 0-$g$ flights show better stability than on the ground. The stability that Rubiclock could achieve in a satellite makes it a good candidate for the next generation of GNSS.

Liquid crystal displays (LCDs) have become ubiquitous, yet there remains room for improvement. The authors demonstrate that dual films of dye-doped antiferroelectric liquid crystals in their anticlinic chiral smectic C phase can switch transmitted light between dark and bright states. The results are important for future LCD screens that switch between dark and bright states an order of magnitude faster than present technologies, and without polarizer sheets. The authors also show how to make displays with either normally dark or bright states. This technology may find use in privacy windows, smart refrigerators, and information displays built into windshields or goggles.

Fragmentation of liquid-metal droplets by short laser pulses is a key phenomenon in producing a bright extreme-ultraviolet (EUV) light source for next-generation nanolithography. The complex multiphysics processes leading to the fragmentation are still not well understood, though. This study surprisingly reveals the emergence of either symmetric or asymmetric scenarios for droplet expansion, depending on laser intensity. This insight can be used to optimized laser-pulse characteristics to guide droplet break-up.

The instability mechanism of an electrified liquid jet shows different behavior in the viscid (having positive viscosity) and inviscid (showing no resistance to shear stress) regimes, which affects the length of the jet before it breaks up into droplets. Even after much research on electrified jets, a quantitative estimate for the breakup length is still lacking. The authors present a physical model, verified by systematic experiments, that predicts the breakup length in both the viscid and inviscid regimes. This model shows how to attain enhanced quality and resolution in electrohydrodynamic inkjet printing, for example.

Elastic metamaterials exhibiting negative values of both effective mass density and Young’s modulus in certain frequency ranges are important for controlling the propagation and path of elastic waves. However, a description of the vibrations involved in this “double negativity” is still needed. In this numerical study, the authors bring to light the microscopic mechanisms that lead to the double negativity of a double-sided metamaterial with pillars. The results will promote the computational design of devices to control waves guided in membranes and plates, such as acoustic lenses and “inaudibility” cloaks.

Self-organization of large-area micro- or nanoscale patterns, using an inexpensive one-step process, is proposed for the fabrication of low-cost, high-performance organic solar cells. The authors employ spin dewetting of a conductive polymer to fabricate an array of micro- to nanoscale ordered heterojunctions, and demonstrate improvements in the key performance indicators of the resulting organic photovoltaic devices. A theoretical study with appropriate boundary conditions is carried out to understand the pattern formation, and simulations are performed to probe the effects of varying active-layer geometry on device characteristics.

Ultrasensitive magnetic field detection based on magneto-optical rotation (the Faraday effect) is important across the sciences. Due to inherent energy-symmetry restrictions, the traditional single-beam measurement configuration suffers from low signal-to-noise ratio, which then demands sophisticated magnetic shielding, high operating temperature and laser power, and complex detectors. This work uses an optical wave-mixing technique to break the energy-symmetry blockade, enhancing signal power by nearly six orders of magnitude. This breakthrough enables applications in many fields, especially biomedical research, with $e.g.$ chip-sized biomagnetism sensors at human body temperature.

How might we measure the Dzyaloshinskii-Moriya interaction (DMI), which can be so important in spintronics? When this magnetic exchange interaction is small, knowing the actual value becomes significant, since operating speed is proportional to it in spintronic devices based on domain-wall movement. The authors develop a method for measuring a weak DMI: When current and a magnetic field are applied to a perpendicularly magnetized thin film, the characteristic morphology of the magnetic domains that form can be observed, and the DMI obtained from the angles present in the unusual domain shapes. This will improve the engineering of spintronic logic and memory applications.

The authors show that electric field enhancement and the group velocity of an electromagnetic wave, at any point in an arbitrary sequence of dielectric layers, can be calculated $analytically$ as a function of so-called “accumulated charge” $q_j$, the sum of “charge” defined for each layer from the edge of the system to the given point. This approach is general and important for designing photonic devices to enhance an electric field, or to reduce the group velocity to obtain “slow light”.

Optical homodyne (single-frequency) measurement is a key technology in optical quantum information processing and key distribution, since it can realize high-speed quantum measurement with a high signal-to-noise ratio. Researchers show that photodiodes have an intrinsic noise mechanism in the photodetection process, due to distributed photocarrier generation, and the excess noise cause an effective optical loss in homodyne detection at high frequencies. They evaluate this excess loss theoretically and experimentally, showing that it depends on the photodiode’s material, and suggesting further requirements for high-efficiency, high-speed hardware.

Soil liquefaction is a poorly understood phenomenon, yet it can be critical in civil engineering: Relative displacement of soil grains under deformation can destroy the grain-contact fabric, and if the grains then become suspended in interstitial water, the soil behaves like a liquid, unable to sustain any load. The authors create a micromechanical model to simulate grain dynamics at the pore scale in the presence of water, successfully reproducing the behavior observed during liquefaction. Interestingly, the number of contacts between grains prior to deformation acts as a critical state variable in determining the occurrence of liquefaction.

Finding compact, inexpensive terahertz sources, detectors, mixers, and other components is the prime issue in ultrahigh-speed electronics. This article proposes using the instability of plasmonic modes driven by a ballistic direct current to generate high-power THz radiation. The boundary conditions and plasmonic velocity in arrays of field-effect transistors are controlled with plasmonic stubs, narrow channel extensions that support plasmonic waves orthogonal to the plasmonic-crystal channel in a device. This approach could also be used for THz detection, mixing, frequency multiplication, and rf-to-THz conversion, opening the way to an entire THz system on a chip.

The promise of surface phononic crystals (PnCs) for $e.g.$ rf signal processing in wireless communication (like your mobile phone) has not been realized, due to the complexity of acoustic wave propagation in such structures, plus the lack of a low-loss, CMOS-compatible platform. The authors identify a wide, hypersonic surface phononic band gap in a pillar-based surface PnC. The significance of their platform lies in the reduced phononic material loss in dielectric pillars, and the use of CMOS-compatible AlN. This allows dense integration of low-loss hypersonic devices with electronics on the same die, enabling many exciting applications of surface PnCs.

Plasma recombination always follows photoionization of gas by intense femtosecond laser pulses, causing refractive-index changes via thermal and hydrodynamic effects. In gas-filled hollow-core photonic-crystal fibers, these phenomena are induced by self-compressing pulses with $\mu$J energies. Probing from the side of the fiber, the authors see refractive-index changes lasting tens of $\mu$s, and plasma-driven acoustic waves that excite MHz vibrations in the microstructure of the fiber. These results are important for the development of high-intensity, high-repetition-rate lasers where, in addition to nonlinear optics, plasma physics and optoacoustics also become relevant.

Manipulating the exchange coupling at magnetic interfaces is crucial to spintronic devices. This study shows that inserting a very thin ferromagnetic spacer at the interface of a ferromagnet/antiferromagnet Co/Ir-Mn film substantially strengthens the coupling (if there are no important interfacial defects or interdiffusion in the antiferromagnet). Furthermore, although a Fe spacer film presents the same exchange bias after annealing or He-ion irradiation, its coercivity increases threefold after irradiation. This provides a way to fine-tune the characteristics of such structures.

The band gaps of photonic crystals (PCs) play an important role in light manipulation for many applications, such as reflectors, filters, and lasers. In traditional, one-dimensional all-dielectric PCs, as the angle of incidence increases, the gaps are blueshifted in wavelength for both transverse-electric (TE) and transverse-magnetic (TM) polarizations. However, this work predicts and demonstrates that redshifted gaps can be realized in one-dimensional PCs composed of alternating hyperbolic metamaterials and dielectrics for TM polarization, while for TE polarization the gaps remain blueshifted. This property facilitates the design of polarization selectors working in a wide angle range.

The nonequilibrium Green’s function formalism is interesting for studying quantum electronic transport in nanostructures, including the treatment of polar optical (PO) phonon scattering, but the latter is usually described within a local approximation. This study uses an analytically derived rescaling technique to describe the interaction between carriers and PO phonons, including the effect of nonlocality. Simulation of double-gate transistors based on $III-V$ semiconductors shows the approach to be well-defined, very simple to implement, and effective. It seems that $III-V$ transistors will not significantly outperform Ge and Si devices, due in part to strong PO phonon scattering.

In optics and photonics, tailoring a waveguide’s dispersion and cutoff frequency is important, as these parameters govern the operating frequencies and device dimensions. The authors show that the substrate-integrated impedance surface (SIIS) enables arbitrary control of the propagation characteristics of closed-shape waveguides, and they develop a theoretical framework for the simplest form of SIIS. This SIIS-loading technique may open up possibilities for miniaturizing various waveguide-based components and networks, and for enhancing their uses in microwave sensing and nonlinear functions.

Terahertz-band (0.1—10 THz) wireless technology is envisioned as key to future communication networks. For the time being, the lack of compact THz transmitters, on-chip and at room temperature, has hampered the use of the THz band. In this paper the performance of an on-chip THz source based on the Dyakonov-Shur (DS) plasma instability is investigated analytically and with powerful numerics, revealing the importance of the termination impedances on the power of THz signals generated and radiated by a plasmonic transmitter. These results provide guidance for successful experimental realization of DS-based plasmonic THz transmitters.

In conventional materials, Cherenkov radiation (CR) due to a moving charged particle is associated with a broad frequency range and velocity threshold. Here the authors show that using a periodic grating structure with photonic bound states in the continuum (BICs) and quasi-BICs, unusual CR can be generated in a very narrow frequency band, at a particle velocity below the common threshold. This effect arises from an interesting process of light amplification in the BIC structure. These results offers a means to realize Cherenkov lasing at low electron velocity, and could find application in compact free-electron lasers.

In a bulk semiconductor, both threading dislocations (TDs) and point defects (PDs) can act as centers for nonradiative recombination of holes and electrons, but the dominant pathway is not obvious in Al-rich Al${}_x$Ga${}_{1-x}$N, an ultrawide-band-gap compound that is promising for very efficient emitters in the deep ultraviolet range. This study uses cathodoluminescence and temperature-dependent photoluminescence to clarify the pathway. TDs are clearly visible around 100 K, but disappear around room temperature, indicating that at room temperature PDs prevent carriers from reaching TDs. Thus to improve internal quantum efficiency, PDs rather than TDs should first be suppressed.

Secure communication protocols based on quantum physics compel worldwide attention. Continuous-variable quantum key distribution (CV-QKD) uses a cost-effective detection technique instead of dedicated single-photon-counting technology, and can provide high key rates over metropolitan distances. Traditional coherent-state CV-QKD protocols, however, suffer from low tolerance of channel excess noise. The authors succeed in distributing Einstein-Podolsky-Rosen entangled states over a 50-km standard fiber with negligible excess noise, and further demonstrate CV-QKD in a high-noise environment with performance superior to that of the optimized coherent-state protocol.

Two-dimensional transition-metal dichalcogenides are of great interest for nanoscale (opto)electronic device applications, but probing their electronic transport in a practical field-effect transistor (FET) is often limited by Schottky barriers and contact resistance. In addition, channel/gate-insulator interfacial states play a dominant role in electronic noise and transport properties. The authors present a systematic study of current-voltage and noise characteristics versus layer thickness in MoSe${}_2$ FETs. The results are important for understanding today’s devices, as well as for achieving improved devices tomorrow.

The authors utilize line shaping of localized oscillations in a coupled resonant system for applications in acoustic energy transformation. Perfect control over a scattering wave front is provided by an acoustic metamaterial consisting of an array of two types of cavities. The dispersion properties of binary cells are utilized to build a flat metascreen with nearly lossless spatial-filtering characteristics. The exceptional dispersive effects are also extended to build a tunable metascreen capable of constructing virtual-scattering and carpet-cloaking objects.

Fast manipulation of nanoscale magnetic textures, such as vortices and skyrmions, underlies devices proposed for next-generation magnetic memory, logic, and sensing. Understanding the switching processes in these structures requires a probe with both nanometer spatial resolution and nanosecond time resolution. This study combines time-resolved magneto-optical microscopy and single-spin magnetometry to unveil the switching of a magnetic vortex between pinning sites, in response to a short magnetic field pulse. The complex dependence of switching probability on pulse duration is explained, revealing a path toward the fast, reliable control required for future nanomagnetic technology.

In optical quantum cryptography, the afterpulse effect is important for secure key generation in quantum key distribution (QKD), because it introduces additional error and leads to great deviation from the traditional analytical model, especially for high-speed systems. This study develops a different analytical model to make QKD systems more afterpulse-compatible, by exploiting the non-Markovian nature of the afterpulse. The optimized key rate obtained here is much higher, and thus the tolerance of the afterpulse effect can be significantly improved in practical QKD systems for secure communication.

The combined increase in strength and ductility observed in high-entropy alloys broadens the palette of advanced materials. Understanding their behavior on a fundamental level facilitates the optimization of their mechanical characteristics. The atomistic theoretical approach presented herein discusses the Al-driven $B2$-type magnetochemical transition in Fe-Cr-Co-Ni-Al alloys. Diversity in the mechanical behaviors of the partially ordered and disordered phases affords opportunities to design specific properties by controlling the level of ordering in multiphase high-entropy alloys, to promote the engineering of strong, durable, and lightweight machines.

Residual stress arising from many manufacturing processes ($e.g.$ welding, forming, or additive manufacturing) can have a profound effect on mechanical performance. Strain tomography via neutron imaging promises a powerful experimental approach, but has been demonstrated in just a small number of special cases. The authors develop a general algorithm for tomographic reconstruction of residual strain fields in two dimensions, and experimentally demonstrate the technique. Strain tomography has the potential to impact a number of fields within experimental mechanics, and this study represents a significant step toward the ultimate goal: general reconstruction in three dimensions.

Finding the shortest path between two nodes in a graph has broad applications in optimization problems, such as resource allocation or navigation. The time and energy consumed by path-finding algorithms scale with graph size, however, making them prohibitive for large problems. The authors study how a circuit of memristors, which “remember” how much current has flowed through them, can find the shortest path in a massively parallel way. Simulations of realistic device models show that the time and energy consumed by this method scales only with the length of the shortest path. This scaling suggests a way to develop processors that solve large optimization problems efficiently.

Can you believe your ears? Acoustic illusions are crucial for underwater stealth techniques, but methods based on transformation optics always introduce exotic material parameters. This study designs acoustic-illusion devices by manipulating the acoustic scattering potential in the wave-vector domain, which greatly simplifies the material requirements, compared to other methods. This brings illusion devices a step closer to real-world applications like camouflage against sonar detection.

Phonon lasing (coherent mechanical amplification) is a key element in phononic engineering. A one‐way phonon laser would be an indispensable tool to explore chiral light‐sound interactions or to drive chiral phonon devices, yet has remained elusive. This study proposes a strategy to achieve such a device, by coupling an optomechanical system to a spinning resonator. Through the optical Sagnac effect, enhancement or suppression of phonon lasing can be achieved by driving the coupled system from one side or the other. This strategy provides a versatile way to operate spinning devices for applications in directional phonon control, sound sensing, and topological acoustics.

Active polarization rotators are desirable for many applications in photonics. Metasurfaces based on phase-change or magneto-optical materials can do the job, but suffer from long response time or the need for a strong magnetic field. This study reports tunable and omnidirectional polarization rotation using a pair of coupled, perforated metal screens. Here the tuning originates from the near-field coupling of the screens and its dependence on lateral displacement. For millimeter-scale displacement, continuous and efficient polarization rotation from -90 to 90 degrees can be achieved in the microwave band, and the effect could also be extended to the terahertz regime.

In a $mechanical$ $metamaterial$, the division between macrostructure and material microstructure is blurred, yielding additional functionality. Here a two-dimensional metamaterial of hinged, rigid isosceles triangles displays a Poisson ratio that changes value and sign, and even diverges during deformation. The system even “remembers” the ratio’s sign and value during an earlier shape change. This unusual behavior is scale-invariant: Deformations of polymerized phenanthrene molecules closely resemble those of actual hinged triangles. Such tunable mechanical metamaterials find application in stents for blood vessels, strain amplification, and micromanipulation of flexible structures.

For nuclear reactors, the value of the threshold displacement energy $E_d$ of graphite is an important quantity in the calculation of irradiation damage. Using atomistic simulations, this study quantifies the evolution of $E_d$ with dose by calculating displacement probabilities for pristine and irradiated graphite. An overall trend of decreasing $E_d$ with increasing irradiation dose is found, indicating that current dose estimates in reactor applications may $underestimate$ the degree of damage accumulated over a reactor’s lifetime.

An innovative spectrometer demonstrates the possibility of realizing room-temperature generation and detection of continuous-wave THz radiation spanning three octaves in frequency, and enabling molecular spectroscopy with state-of-the-art accuracy. The setup is based on a simple, reliable approach that combines robust telecom-laser components, nonlinear difference-frequency generation, and frequency-comb-assisted referencing to a primary frequency standard. The combination of ultrabroad-band coverage and microwatt power levels in a single source could pave the way to metrological-grade sources spanning most of the THz range, for countless demanding applications.

Magnetic skyrmions (arrangements of spins featuring topological properties) are candidates to implement information bits in devices for combined data storage and logic processing. A prerequisite is the further optimization of the host magnetic multilayers, to obtain sufficiently stable and mobile skyrmions below 10 nm in size. This study develops an extensive model of skyrmions in magnetic multilayers, allowing analysis or prediction of their size, magnetic structure, and dynamical behavior. This model establishes guidelines for optimizing multilayer properties on the way to applications of magnetic skyrmions.

Manipulating the scattering of light in a frequency-selective manner, within a layer that is much thinner than the wavelength, is crucial for $e.g.$ antenna applications, or wireless communication. This study use a coding metasurface to realize frequency-selective scattering based on dispersion tailoring, achieving highly efficient mirror reflection at central frequencies and low diffusive scattering in side bands. A semianalytical method reveals the underlying physical mechanisms and guides metasurface design. These results could open the way to more diverse designs for light management, such as camouflaged or invisible emitters.

Artificially patterned, periodically modulated magnetic media, popularly known as $magnonic$ $crystals$ (MCs), have attracted much attention for their huge potential in reconfigurable magnetic and spintronic devices. The authors investigate the magnetization dynamics in a two-dimensional MC, a periodic arrangement of divots in a ferromagnetic thin film, achieving active control of spin-wave dynamics at GHz frequencies by varying the strength and orientation of an external magnetic field. This tunability could be crucial to nanoscale, on-chip microwave communication technology.

Superconducting resonators and solid-state qubits such as N-$V$ centers in diamond suffer from intrinsic dissipation, which limits their coherence times. This is typically attributed to the presence of paramagnetic impurities acting as fluctuating two-level systems. In this study of surface and bulk spins in diamond, ultrasensitive magnetic force microscopy at millikelvin temperatures reveals that a high magnetic field gradient suppresses spin diffusion, enhancing relaxation times of surface spins. The technique offers a valuable tool for characterizing dilute spin systems, which could yield insight on how to reduce dissipation in qubits and other nanodevices.

SU(1,1) interferometers, based on nonlinear optical processes, potentially offer better phase sensitivity than traditional instruments. Bright-seeded variants are important for applications with large numbers of photons, yet have remained uncharacterized experimentally, for lack of a real-time technique to measure phase sensitivity. This study uses a direct intensity-detection technique to demonstrate the quantum enhancement of phase sensitivity in real time for a bright-seeded SU(1,1) system, compared to the classical shot-noise limit. It is hoped that these results will promote applications of this class of interferometer in quantum metrology.

In spintronics, lateral spin valves enable the manipulation of pure spin current without charge current. These systems are very interesting, particularly as magnetic field sensors for $e.g.$ read heads in hard disk drives. This paper uses a two-dimensional diffusive model to investigate signal and noise in such T-shaped devices. Three sources of noise are discussed, one of which is associated with possible thermal fluctuations in spin accumulation. Though their signal-to-noise ratio still needs to be improved to make these devices suitable for hard disk drives, this study offers important physical insight to guide that quest.

Manipulating the isofrequency contours (IFCs) of a metamaterial provides a remarkable ability to control light propagation and emission. Here we see a topological transition from dielectric- to metallic-type hyperbolic dispersion, and a kind of metamaterial with conical or linear-crossing IFCs at the transition point. Negative refraction and propagation along fixed directions can be realized, even for incident waves with large wave-vector components, which can be utilized for applications including optical splitting and switching, and superresolution imaging with partial cloaking.

One obstacle in developing micropumps is their extremely low efficiency, compared to their macroscale counterparts. Thus the authors explore a dynamic sealing method for microhydraulic systems, using magnetorheological (MR) fluids: To minimize backflow under particular conditions, they identify the optimal combination of parameters, which occurs at the transition from attached to detached plug flow. This leads to a design space for gear pumps that can achieve high efficiency and accuracy, using current mechanical architectures and tolerances. Also, the optimal magnetic field intensity is investigated for different working conditions, and a metric for effectiveness is proposed.

The electrical detection of spin accumulation in nonmagnetic materials is required for the operation of many spintronic devices. Typically, detection is achieved via a tunnel contact with a ferromagnetic electrode. Spin conversion in a magnetic tunnel contact is conventionally described using linear transport equations—but here the authors demonstrate that electrical spin detection in a biased ferromagnetic tunnel contact is inherently $nonlinear$. This explains a multitude of puzzling spin-transport data, including two-terminal magnetoresistance data, and truly changes our view of spintronic device physics.

Creating and controlling spin waves is a central topic in spintronics. Spatially shaped optical fields can be used to create lateral texture in the magnetization profile of a uniformly magnetized sample. When implemented with fast pulses of light, this provides a method to generate user-defined, transient magnetization texture and its accompanying band structure for spin waves. The authors demonstrate this effect using femtosecond laser pulses in the transient-grating geometry, for elastic excitation of a variety of spin waves in a modified magnetic landscape. Comparing experiments to calculations, the authors identify the various elastically activated spin-wave modes.

In spite of a long history and wide usage in commercial devices, the atomistic behavior of the trap states in $a$-Si${}_3$N${}_{4-x}$ remains elusive, which could hamper further engineering. This study uses density functional theory to investigate the atomic configurations and electronic properties of trap states identified as $K$ centers (nitrogen vacancies), and finds that the charge-neutrality condition imposed on the ensemble of $K$ centers results in apparent “negative-$U$” behavior, although each $K$ center exhibits a positive Hubbard $U$ energy. This insight into charge-trap behavior of amorphous materials could be key to improving the performance of devices based on those traps.

Controlling the motion of magnetic domain walls (DWs) in ultrathin films with perpendicular magnetic anisotropy (PMA) has opened perspectives for mass-storage applications such as “racetrack memory”. However, a crucial issue for this technology is to efficiently move and store domain walls along very narrow wires. The authors’ approach uses prepatterned substrates with regularly spaced nanoscale steps to grow high-quality films with PMA. The very low steps act as efficient pinning sites when DWs move $across$ them, yet as very efficient conduits when DWs move $along$ them. This may be a promising solution to crucial problems concerning edge damage at very small wire dimensions.

Photons in twin beams exhibit correlations in both photon number and spatial detection position. Here spatial correlations in detection positions are exploited to develop a method for reconstructing the joint photon-number distribution of a twin beam from the experimental photocount histograms. This considerably reduces experimental noise, making it superior to the usual methods. This practical reconstruction method for spatially resolved photon counting is suitable for any application that requires joint photon-number distributions of twin beams, in $e.g.$ metrology, or future quantum communication protocols.

Intrinsic bulk Ge is an indirect-band-gap semiconductor, but it can be converted to a direct-band-gap material by strain engineering, alloying with Sn, or ultrahigh $n$-type doping. The authors use all three approaches together to fabricate Ge-Sn with a direct band gap. Effective strain engineering of Ge-Sn alloys is achieved by codoping with phosphorus, plus flash-lamp annealing. Compensation of biaxial compressive strain and lattice-parameter reduction result in a band-gap change, accompanied by an enhancement of carrier mobility. The insight gained here will facilitate the design and fabrication of (opto)electronic devices based on this promising material system.

Nitrogen-vacancy center diamonds are important for quantum sensors, due to their ultrasensitivity to a magnetic field and manipulability at room temperature. A major hindrance of device sensitivity is electric field noise. This work theoretically investigates the effect of a surface covering on N-$V$-center diamonds on reducing such noise. Six covering layers currently used in experiments are studied. The results show that a covering layer is useful for decreasing surface noise, but the most suitable material depends on the device’s operational frequency range.

The chiral magnetic textures known as skyrmions are of great interest for developing highly efficient spintronic and spinorbitronic devices for handling digital data. The authors propose an approach for stabilizing immobilized skyrmion and skyrmionium states, using just the gradient of a nanoscale depression in an ultrathin anisotropic ferromagnetic film. These results show how to produce skyrmion and skyrmionium states with tunable radii in curvilinear or planar films with inhomogeneous intrinsic chiral interactions, for improved device engineering.

Hop on board: The authors present experimental evidence of particle trains in viscoelastic polymer solutions. It turns out that such lines of particles in viscoelastic liquids are easy to produce, prompting future applications in biomedical engineering and materials science—for example, in optimized flow cytometry, deterministic cell encapsulation and microfluidic fabrication of materials with enhanced localized properties. Also, the simple setup here can be coupled with other microfluidic techniques, to clarify the hydrodynamic interactions between particles.

Photoelectrochemical water splitting (“artificial photosynthesis”) to produce H${}_2$ and O${}_2$ remains stuck at an early stage, despite extensive research efforts over forty years, and disillusionment has grown. Here the authors examine 202 semiconductors that are known to either catalyze overall water splitting or reduce or oxidize water, revealing that photocorrosion stability sets a limit on the minimum possible band gap, and thus the maximum solar-to-hydrogen conversion efficiency. To overcome this 8% efficiency limit, the most promising strtegy seem to be protecting the active photocatalyst with a photocorrosion-resistant oxide coating.

Extreme pressures of several megabars are produced during compression of sample and gasket within a diamond anvil cell. This study uses finite-element simulations to analyze the effects of culet geometry, bevel angle, sample thickness, and the sample-gasket system. Multiple experimental phenomena are reproduced and interpreted. These results improve our understanding of material mechanical response under extreme pressure and large elastoplastic deformation, to benefit optimal design of the diamond anvil cells that are key to high-pressure research, and thus the search for and study of $e.g.$ metallic hydrogen.

Defects in solids present a scalable platform for photon-based quantum information processing, and progress here relies on improving their optical and spin properties. The authors measure the spin-coherence properties of Ga donors in ZnO using all-optical methods of spin control. A longitudinal spin-relaxation time of 0.1 s and spin-coherence time on the scale of 50 $\mu$s are observed in bulk ZnO, and it is expected that the latter can be significantly improved through chemical and isotopic purification. This work motivates further research on growth of high-purity materials, quantum device fabrication, and high-fidelity control of the donor:ZnO system for quantum technologies.

As necessary steps toward practical and secure quantum key distribution (QKD), identifying and patching possible attacks are important and valuable. This study discusses an avalanche-transition-region (ATR) attack on a gated-mode avalanche photodiode (APD) detector, widely used in QKD systems. Unlike other attacks, this one leaves almost no trace; the eavesdropper introduces less than 0.5% quantum bit-error rate, and the photocurrent and afterpulse probability before and after hacking are nearly identical. These findings highlight the importance of detection signals in real-life QKD systems, and are crucial to enhancing the security of practical QKD.

Thermal properties are an important ingredient of a superconducting detector of radiation, determining how an impinging photon’s energy is shared inside the detector. This study uses independent calibration of the radiation coupling losses and resistive superconductor thermometry to investigate the thermal resistance of a NbN film, the working element of a superconducting single-photon detector. An upper bound on the ratio of electron and phonon heat capacities in NbN is established, which is surprisingly close to the recent theoretical $lower$ bound for this quantity. The results will contribute to the strategy for identifying further materials for such detectors.