Near-field heat transfer is essential in thermal nanolithography, scanning thermal microscopy, and thermophotovoltaics, for example, but applications are hindered by low heat-current amplitude. This work shows that the heat current can be significantly boosted in the presence of localized zero-energy edge states, which offers a means of thermal switching via externally tuning for the presence or absence of such states. Surprisingly, heat transfer exhibits nonmonotonic behavior with respect to gap distance, at these length scales; it does not simply increase as the gap closes. These insights could change the way we approach thermal management at the nanoscale.

In semiconductor heterostructures with internal polarization that breaks inversion symmetry, resonant tunneling of electrons is very interesting for high-power ultrafast oscillators and THz quantum cascade lasers, but the effects of the broken symmetry on the magnitude and phase of the electron resonances are unclear. Here those transport quantities are measured by systematically controlling quantum inference effects, and a general quantum transport model for polar heterostructures explains all of the experimental features arising from broken inversion symmetry. This represents a significant step in understanding resonant tunneling, and impacts the design of $III$-nitride quantum devices.

The $Yablonovitch$ $limit$ has long provided an upper bound for how much enhancement can be achieved through surface texturing in solar cells. In this study, the authors develop a similar approximate limit for dilute arrays of absorbing “metaparticles”, and interestingly apply the result to arrays of buoys for harvesting the energy of oceanic waves. This result is applicable to a broad range of scattering problems; in particular it bridges the gap between two seemingly different problems in energy extraction that nonetheless feature similar underlying physics.

Spin-exchange relaxation-free (SERF) optical atomic magnetometers are a promising alternative to superconducting quantum interference devices (SQUIDs) for biomedical applications, as they do not require liquid-helium cryogenics or a shielded room. However, their sensitivity in the 0.1–100 Hz range is undermined by $1/f$ noise. The authors present a vector SERF magnetometer with suppressed $1/f$ noise, due to the ac response along each of its two sensitive axes. The improved long-term stability of this system offers the possibility of precise gradiometry with several independent sensors.

Wavelengths in the telecommunication window (ca.1.25–1.65 $\mu$m) are ideal for quantum communication, due to the low transmission loss in optical-fiber networks. To realize quantum networks operating at these wavelengths, we need long-lived quantum memories that couple efficiently to telecom-band photons. This study proposes using optical tweezers to couple neutral ytterbium atoms, which have a strong telecom-wavelength transition, to a silicon photonic-crystal cavity. The combination of high system efficiency, telecom-band operation, and long coherence times makes this platform well suited for quantum optics on a silicon chip and long-distance quantum communication.

Terahertz accelerators can achieve potential gradients beyond 200 MV/m. Gyrotrons are the only power sources capable of producing megawatt-level, microsecond-long pulses in this frequency range, yet nanosecond-long pulses are required for breakdown-free operation. The authors explore the possibility of using a single GaAs wafer to enable production of the required pulse widths, with rise and fall times that closely track the illuminating laser’s pulse length. They demonstrate and quantify the reflective properties and laser-induced photoconductive effect of GaAs in the millimeter-wave regime, for use in a laser-operated shutter.

$III$-N emitters have enabled the technological revolution of LED lighting, but some applications remain hindered by two phenomena, of debated origin, that degrade quantum efficiency at high power in long-wavelength LEDs (the “green gap”). In this study of carrier dynamics, the authors discover that two effects are intertwined, and correlated with defect density. Beyond the commonly known effect, defective samples suffer from an additional form of nonradiative recombination at high current, and this trend is exacerbated at long wavelengths. These findings provide guidance for developing more efficient LEDs, at all powers and wavelengths.

Spin-photon interfaces based on an (In,Ga)As quantum dot coupled to a waveguide are a promising avenue toward scalable quantum information processing, but coherent control of the spin state is challenging, because of the complicated near-to-far-field polarization transformation induced by the waveguide. The authors search for a particular polarization of light that excites a circular dipole in the quantum dot, and use this polarization for the coherent control of an electron’s spin. They obtain a coherence time of 2.2 ns, comparable to the typical value in bulk media. The authors’ method for polarization-controlled excitation can be readily applied to other nanophotonic structures, too.

Nanoscale microwave generators based on magnetization precession require a narrow and tunable spectral band, high absolute amplitude, and high-frequency tunability—a challenging combination. The authors show that this combination can be achieved under ultrafast optical excitation of a nanolayer of galfenol, a ferromagnetic alloy of iron and gallium. In a film several nanometers thick, a femtosecond laser pulse excites only the ground spin-wave mode, not the higher-order modes that would broaden the precession spectral band. The extremely narrow spectral response remains easily detectable above 100 GHz, which is a boon for applications in nanomagnetism and spintronics.

Near-field heat transfer is essential in thermal nanolithography, scanning thermal microscopy, and thermophotovoltaics, for example, but applications are hindered by low heat-current amplitude. This work shows that the heat current can be significantly boosted in the presence of localized zero-energy edge states, which offers a means of thermal switching via externally tuning for the presence or absence of such states. Surprisingly, heat transfer exhibits nonmonotonic behavior with respect to gap distance, at these length scales; it does not simply increase as the gap closes. These insights could change the way we approach thermal management at the nanoscale.

Efficient control of the motion of domain walls (DWs) is of great interest for high-performance “racetrack” memory and magnetic logic. However, the current density required for spin-orbit-torque-induced domain-wall motion is still too high for low-power devices. The authors experiment with helicity-dependent domain-wall motion by synchronizing polarized femtosecond laser pulses with short current pulses: A domain wall stays pinned under one helicity but is depinned under the other, and the critical current density is greatly reduced. This offers a path to energy-efficient spintronic-photonic devices, beyond conventional all-optical or all-current switching approaches.

A plasmonic nanoantenna offers directionality and a large photonic density of states to a nearby emitter. Here researchers create antennas based on silver columnar thin film for bright, directional emission from N-$V$ centers in nanodiamonds (NDs). By simply drop-casting the NDs from aqueous solution on top of the film, order-of-magnitude enhancement of emittance is attained. The silver nanocolumns couple to an N-$V$ center’s emission via the optical near field, and out-couple the excitation energy effectively into the far field. As the approach is not restricted to these emitters, it offers a broad way to practical implementation of solid-state single-photon sources.

The breakdown of Fourier’s law in nanoscale thermal transport, which has become (ahem) a $hot$ topic lately due to improvements in experimental precision, presents a serious problem for analysis of heat flow. The authors show that the observed discrepancies can be naturally explained by adopting a hydrodynamic extension to Fourier’s law. With parameters calculated $ab$ $initio$, this equation shows remarkable agreement with experiments on films of “holey” Si. The simplicity of this multiscale model allows numerical solutions for arbitrary geometries using finite-element methods, providing a good tool for interpreting recent experiments with complex setups.

Phase-sensitive x-ray imaging techniques offer significant advantages over conventional absorption radiography, especially for low-$Z$ samples (like your body), for which absorption contrast is weak. While standard for dedicated synchrotron beamlines, these techniques remain challenging for compact lab-based systems. The authors present a laboratory setup for x-ray phase-contrast imaging, based on a high-power rotating-anode source and confocal multilayer mirror, which at once attains high flux, spatial and temporal coherence. This detailed description of an innovative system plus examples of biomedical applications is expected to have real impact on the x-ray-imaging community.

Understanding the relationship of photoexcited carrier lifetimes, mobilities, and recombination to structure and processing of photovoltaic absorber materials is critical to designing efficient solar cells. Using three complementary techniques, the authors elucidate photoexcited carrier dynamics in a high-quality kesterite crystal. These dynamics help to explain limitations in charge-carrier collection observed in a device’s $J-V$ relation and external quantum efficiency. Combining ultrafast spectroscopy and device measurements in this way can lead to a more detailed understanding of the performance-limiting photophysical processes, accelerating the improvement of solar cells.

Superconducting nanowires have been critical in $e.g.$ sensing and digital logic, offering certain advantages where traditional Josephson junctions fall short, but nanowire-based electronics have been limited to binary operations, due to the uncontrolled dynamics of the resistive hotspot. This work explores a nanowire device that can controllably trap flux in discrete quantities, can respond proportionally to the strength of its input, and can be operated to achieve multilevel output with distinguishable states. This may offer a platform for nonbinary nanowire technologies, such as multilevel memory, or tunable elements for multilevel logic operations.

This study presents $experimental$ evidence of nonclassical optical bistability, in the context of photon-pair generation via spontaneous four-wave mixing in a silicon microring resonator. The hysteresis results greatly benefit on-chip quantum optics, to build a bridge between classical and potential nonclassical applications of optical bistability in microcavities. This work also improves traditional resonance-locked strategies by balancing power-induced heating and active cooling; since no precise pump tuning is required, the proposed resonance-locked regime can be more widely applied in fixed-wavelength optical communication systems.

Imperfections on the surfaces of optical components can cause lots of damage to a high-power laser facility, due to the diffraction pattern they produce. However, actual defect specifications do not take this phenomenon into consideration, and so the impact is under- or possibly overestimated. The authors present a method using an old phenomenon, the Poisson spot, to explain how surface imperfections are able to generate huge light intensifications. It is found that a few simple parameters are sufficient to describe hotspot formation, and are also useful in controlling hotspot generation by considering particular phase objects.

Focused ultrasound is important in clinical medical applications, to destroy tumor cells or treat a variety of neurological diseases, for example. The focused ultrasound field projected by a transducer array includes traditionally unavoidable side lobes, however, which may cause harmful heating in nontargeted regions. This work implements an active acoustic metasurface, comprising 16×16 elements of subwavelength thickness, which can completely eliminate such side lobes. Experiments convincingly demonstrate that this active metasurface with programmable phase and amplitude distributions offers high performance in side-lobe-proof, steerable ultrasound focusing.

Coherent energy transfer in mechanical systems is important because of its potential in on-chip signal processing. Although dynamical protocols are an efficient means to this end, a primary challenge in coherent control of transport is ameliorating the dephasing due to noise. This study uses the geometric phase, which depends only on the global properties of the evolution path, to achieve noise-resilient control of energy transfer between two mechanical modes. A high rate of energy transfer, comparable to that for dynamical protocols, can be achieved with this fresh insight on the geometric aspect of classical mechanical systems.

A “forest” of treelike carbon nanostructures offers two-level roughness, reminiscent of the surface of the famously nonwetting lotus leaf. The authors explore the wettability of this material system, along with its dependence on the geometry of the nanotrees. Such a forest is superhydrophobic, with extreme nonsticking properties for water. This study informs the design of superhydrophobic materials with similar structures for $e.g.$ self-cleaning surfaces.

The Loss-DiVincenzo quantum computer, introduced two decades ago, still has not been fully realized, due in part to the lack of an effective method for qubit initialization. Here the authors propose a method for very fast initialization of qubits defined on electron spins trapped in electrostatic quantum dots. This method is significantly faster than others currently in use, and offers an initialization time two orders of magnitude shorter than the decoherence time. The proposed nanodevice should be easy to implement, since it is based on the mature technology of gated quantum wires.

Acoustic imaging is important in diverse applications, such as target tracking, clinical treatment, and structural health monitoring, but is limited by hardware complexity, as an acoustic camera relies on a group of detectors to image sound sources. This study uses spatially encoded structures to solve the problem of single-detector planar acoustic imaging at audio frequencies: A space-coiling metamaterial with high anisotropy is proposed to realize a single-detector acoustic camera. This design outperforms the conventional acoustic camera in terms of dimensions, bandwidth, and cost, and is expected to have real impact on engineering in single-detector acoustic imaging.

Josephson parametric amplifiers (JPAs) are key devices in superconducting quantum circuits, as they ultimately dictate quantum efficiency and speed of measurement, but they still suffer from low saturation power. This work shows that a JPA’s saturation power can be increased by using a Josephson-junction array, rather than a single-junction amplifier. Modeling such an array as a nonlinear $LC$ resonator reproduces the observed amplification and saturation effects very well. This use of arrays to fight low power saturation is easy to implement, and can be directly combined with impedance-engineered environments to enhance dynamic range even more.

Neuromorphic computing based on stochastic spintronic units has attracted intense attention, but controlling such a stochastic system with high energy efficiency remains a challenge. The authors propose a voltage-controlled spintronic device that enables low-energy neuromorphic computing, with the stochastic behavior of the device being managed by an applied electric field via voltage-controlled magnetic anisotropy. These results will advance the quest to create energy-efficient spintronic systems for brainlike cognitive computing.

To suppress gate leakage current in nanoscale field-effect transistors, quantum mechanical tunneling of carriers through the oxide in a metal-oxide-semiconductor structure must be minimized. Here the first-principles scattering-state approach is used to study tunneling through ultrathin oxide layers in Ge/$a$-GeO${}_2$/Ge structures. This work demonstrates the strong correlation among atomic structure, band-gap alignment, tunneling spectra, and tunneling current densities with respect to oxide thickness, interfacial structures, and Ge orientations. The informative results will impact MOSFET design and should be of great interest to the diverse community of semiconductor researchers.

In the quest for ultimate control of wave propagation, the authors propose a design scheme that allows them to efficiently construct reconfigurable nonreciprocal components with optimal performance and minimal structural complexity. The method utilizes an effective time-independent Hamiltonian formalism within the framework of Floquet-scattering engineering. It allows the user to design driving protocols with optimal nonreciprocal response. This scheme is applicable in a variety of physical settings, ranging from acoustics and thermal transport to electromagnetic and matter waves.

Using spin-orbit torque (SOT) to flip magnetic bits has emerged as a leading strategy for next-generation digital storage and logic. To achieve deterministic magnetization switching, most SOT studies use an assisting magnetic field in the direction of the applied electric current, but this work examines SOT-driven switching with an assisting field that is not aligned with the current. Asymmetry between “up-to-down” and “down-to-up” switching processes is observed, and is explained by the transverse SOT effective field. This asymmetry is an important feature in understanding the SOT switching process, and in designing delicate spintronic devices with selective switching capability.

Improving the $p$-type doping of wide-band-gap nitride and oxide semiconductors is important for optoelectronic applications such as lasers and LEDs, but is still challenging, as the underlying physics is still not fully understood. This study uses first-principles calculations to systematically investigate Mg and Be doping in nitride semiconductors, to unravel how chemical and strain effects influence the ionization states of these dopants. The results should have an impact on engineering $p$-type semiconductor devices.

Droplet-based microfluidics has shown potential in applications like high-throughput pharmaceutical screening. However, selective breakup of droplets into smaller daughter droplets is important, for success in conducting permutations of reactions. When a droplet interacts with a bypass channel, the droplet will either break or not, and determining the transition region between the two outcomes would be quite important. This combined experimental and numerical study of the forces at play at the knife edge of droplet breakup/persistence contributes significantly to the development of microfluidic technology.

$Quantum$ $reservoir$ $computing$ provides a scheme for exploiting the natural dynamics of quantum systems as a computational resource. An NMR spin-ensemble system is a realistic candidate for implementing the framework, which is currently available in laboratories. Considering realistic experimental constraints, the authors propose a spatial multiplexing technique to effectively boost the platform’s computational power. This scheme exploits disjoint dynamics of multiple, different quantum systems driven by common input streams in parallel. This allows one to prepare a huge number of qubits from individually small quantum systems, which are easy to handle in experiments.

In the field of spin caloritronics, direct observations of the anisotropic magneto-Peltier effect and the anomalous Ettingshausen effect (magneto-thermoelectric effects appearing in magnetic materials) have recently been reported. This work establishes a procedure for the pure detection of these phenomena in thin films, and clarifies the important role of sample dimensions and thermal boundary conditions in the measurements. The findings reported here will invigorate research on the physics and materials science of spin caloritronics, and will develop nanoscale thermal-management technologies based on magneto-thermoelectric effects.

Measuring optical forces at the nanoscale with optimal resolution of both position and force is crucial in optical-trapping physics. This study presents a method to measure radiation pressure on metallic nanoparticles down to the thermal limit, with a large dynamic range and fine resolution in position. This progress is made possible by a careful assessment of the conditions of stability through an Allan-based deviation analysis. The capacity to measure femtonewton forces is important in the context of optical spin-orbit interactions and chiral forces.

While organic electronic devices are multifunctional and offer high performance, they are nonetheless confronted with serious interface problems in their metal/organic-semiconductor contacts. In this work, a cobalt film is magnetron sputtered onto a P3HT film to simulate the Co/P3HT interface in a device. With decreasing Co thickness, the magnetoresistance is reversed from negative to positive, due to the increasing influence of the Co/P3HT interfacial layer. This result clearly shows the importance of the metal/organic interface in organic spin valves for spintronic applications.

Accurate, $in$ $situ$, real-time measurements of flow rate are critical for virtually every application of microfluidics that requires precise control over the system’s operating conditions. Despite this, existing measurement techniques require unrealistically precise information about system geometry and related parameters, making them inoperative at the nL/min scale. The authors present an approach to measure flow rates based on the relationship between scaling laws and the rate of fluorescence bleaching. Interestingly, this approach scales uncertainty in the measurement so that relative errors are approximately constant and on the order of a few percent, down to a few nL/min.

Optical parametric generation is important for the creation and control of classical and quantum light. Here LiNbO${}_3$ microresonators have not yet lived up to their potential, due to the need for careful design of both phase matching and light extraction in high-$Q$ resonators. This study achieves cavity-enhanced second-harmonic generation with a conversion efficiency of 1500% W${}^{-1}$, by using a high-$Q$ Z-cut LiNbO${}_3$ microring resonator with a single bus waveguide to couple the phase-matched modes. Difference-frequency generation is also observed. This work is an important step toward efficient wavelength conversion and optical signal processing in integrated photonic circuits.

Directional amplification is essential in superconducting quantum information experiments, to measure small signals without disturbing potentially fragile sources. While several classes of amplifiers provide directionality and good noise figures, they generally have low levels of saturation. Optomechanical amplifiers would not suffer from this limitation. This work demonstrates a microwave directional amplifier, based on the directional interference effect between two optomechanical amplifiers. The study reveals this amplifier’s robustness to typical nonidealities, and allows the formulation of guidelines for designing multimode optomechanical devices for applied or fundamental physics.

Doubly clamped resonators could be interesting for magnetic field sensors, due to a magnetoelastic stress-induced shift in the resonant frequency, but an understanding of how to optimize the figure of merit for this geometry is needed. This study elucidates the effects of shape and stress anisotropy on the magnetic field sensitivity of an electrodeposited beam. The device’s complex structure strongly affects the local magnetization-reversal process and domain behavior in the film. The observed nonsaturating frequency shift at high magnetic fields is expected to impact applications where it is necessary to detect small fluctuations on top of a high bias field, as in NMR or MRI systems.

Since 2009, diamond magnetic microscopy has been used to image magnetic phenomena in condensed matter and biological systems, but observing single nanoparticles at room temperature is a challenge. Of particular interest are paramagnetic hemozoin nanocrystals that grow inside the parasites that transmit malaria. The authors use diamond magnetic microscopy to study individual hemozoin nanocrystals from infected blood, as well as synthetic hemozoin. Data from over 100 nanocrystals yield histograms of their magnetic properties. Translating this platform to study $living$ malaria-infected cells could shed light on their hemozoin formation dynamics, and interaction with antimalarial drugs.

In quantum information processing, the shortcut-to-adiabaticity (STA) protocol has been proposed to accelerate a system’s adiabatic evolution by introducing an additional, counterdiabatic Hamiltonian. This protocol is particularly suitable for realizing the controlled-$Z$ ($CZ$) gate, but a counterdiabatic Hamiltonian requires a variable, complex coupling of qubits. By introducing a representation transformation and a rescaling method, this work uses the STA protocol to obtain a high-fidelity $CZ$ gate with two coupled transmon qubits. This method affords high flexibility in the evolution time and control waveform, and is expected to be directly useful in other quantum systems as well.

While relatively successful theoretical models exist to link the signal of an acoustic biosensor to the mass and viscoelasticity of film-forming adsorbates, this is not the case for surfaces discretely covered by analytes such as proteins or DNA. This work extends and examines in detail the underlying physics of a previously published theory that links the response of microbalance sensors to the shapes and sizes of attached molecules, shedding light on the hydrodynamics and energy-dissipation processes. Their analysis offers further physical insight, and could be exploited to improve detection approaches relevant to biology and biomedicine.

In semiconductor heterostructures with internal polarization that breaks inversion symmetry, resonant tunneling of electrons is very interesting for high-power ultrafast oscillators and THz quantum cascade lasers, but the effects of the broken symmetry on the magnitude and phase of the electron resonances are unclear. Here those transport quantities are measured by systematically controlling quantum inference effects, and a general quantum transport model for polar heterostructures explains all of the experimental features arising from broken inversion symmetry. This represents a significant step in understanding resonant tunneling, and impacts the design of $III$-nitride quantum devices.

The $Yablonovitch$ $limit$ has long provided an upper bound for how much enhancement can be achieved through surface texturing in solar cells. In this study, the authors develop a similar approximate limit for dilute arrays of absorbing “metaparticles”, and interestingly apply the result to arrays of buoys for harvesting the energy of oceanic waves. This result is applicable to a broad range of scattering problems; in particular it bridges the gap between two seemingly different problems in energy extraction that nonetheless feature similar underlying physics.

Understanding the physics of coupling between surface and volume electromagnetic fields in highly overmoded complex structures (“effective metadielectrics”) is important for a wide spectrum of applications. This theoretical analysis of periodic surface lattices is validated by both numerical simulations and experiments. The resulting insight should lead directly to design innovations ranging from higher-power radiation sources ($e.g.$ free-electron lasers) to more efficient radiation absorbers ($e.g.$ solar cells). The principles here can be scaled over decades of frequency, from microwaves to visible light, providing wide-ranging impact.

In the quest to realize a quantum computer, physicists are dealing with signals that can be as weak as a single microwave photon. To detect such signals with sufficient quantum efficiency to implement $e.g.$ quantum error correction, they must be amplified within the cryostat, and the amplifier itself must be a quantum device. To this end, the authors present a broadband, nearly quantum-limited parametric amplifier based on the tunneling of quasiparticles in a superconducting junction. This development may allow the generation of microwave continuous-variable cluster states, and is expected to have great impact on quantum information science.

Heat transport across compositionally graded interfaces can be a key factor in microelectronic devices, where lifetime can be dramatically affected by small changes in operating temperature. Such heat flow is seldom included in the design phase, though, due to poor understanding of the underlying physics, and a lack of predictive models to treat the problem. This study uses a first-principles-based approach to phonon transmission plus a Monte Carlo simulator to account for all relevant bulk and interfacial phenomena, yielding quantitative predictions that can help guide device design. Surprisingly, alloy disorder has a larger impact on heat transport at interfaces than does reflection.

Strongly correlated perovskite transition-metal oxides have received considerable attention due to their intriguing physical properties, including a metal-insulator transition that can be used in devices. Key questions remain, though, including whether in-plane tensile strain helps or hinders the transition. This study uses both static and dynamic lattice strain to manipulate the transport properties of NdNiO${}_3$ films systematically. The electric-field-tunable ferroelastic/piezoelectric strain approach can be used not only to obtain deeper insight into the intrinsic properties of nickelate films, but also as a simple, energy-efficient means to construct multistate resistive memory.

Multilayer architecture plays a pivotal role in energy-storage devices, such as lithium metal batteries. Interfacial deposition and dissolution lead to volumetric changes, which could adversely affect battery performance. This study theoretically addresses the mechanics and transport interaction under asymmetric volumetric changes in a solid/solid multilayer configuration. The analytical formalism is applied to an all-solid-state lithium metal battery, which allows design guidelines for the interfacial mechanical stability of such a layered architecture.

Rigid magnetic micropropellers, driven in an external magnetic field, are a model system for envisioned biomedical applications, but currently the potential of such propellers is neither fully used nor fully understood. This study uses the swimming characteristics of randomly shaped propellers to investigate these possibilities, and their dynamics. In this regard, propellers with frequency-induced reversal of swimming direction (FIRSD) are a key example that might help to extend even further the range of applications of magnetic micropropellers, especially for multitargeting or swarm control.

Spin-exchange relaxation-free (SERF) optical atomic magnetometers are a promising alternative to superconducting quantum interference devices (SQUIDs) for biomedical applications, as they do not require liquid-helium cryogenics or a shielded room. However, their sensitivity in the 0.1–100 Hz range is undermined by $1/f$ noise. The authors present a vector SERF magnetometer with suppressed $1/f$ noise, due to the ac response along each of its two sensitive axes. The improved long-term stability of this system offers the possibility of precise gradiometry with several independent sensors.

Superresolution microscopy based on stimulated Raman scattering (SRS) is appealing for label-free quantitative imaging of biological and biomedical systems with higher resolution than conventional SRS microscopy, which is diffraction-limited. This study experimentally demonstrates a saturated SRS (SSRS) technique to break the diffraction limit for vibrational imaging of biological samples, such as chloroplasts and Hela cells. The technique holds great promise for studying living cells.

A precise understanding of the excitonic fine structure of all-inorganic perovskite nanocrystals is essential, in view of applications at the single-object scale in fields such as nanophotonics and quantum optics. This class of colloidal materials with defect-tolerant behavior has emerged recently as a potential alternative to $II-VI$ semiconductor nanocrystals. Here calculations clearly show that the interplay of shape anisotropy and crystalline phase define the energy splitting between the fine-structure components, and thus the emission characteristics, of the nanocrystal. This study provides key information for optimizing nanophotonic devices based on these inorganic perovskites.

Optical spatial differentiation is important for applications in massively parallel computing and real-time image processing. This study makes use of the generalized spin Hall effect of light to perform optical spatial differentiation. Importantly, this scheme generally accompanies oblique light reflection and refraction at $any$ planar interface, regardless of material compositions or angles of incidence. Moreover, this spin-optical approach offers robust image processing to extract the boundaries of objects, where two distinct images can be stored in two different polarization states.

Wavelengths in the telecommunication window (ca.1.25–1.65 $\mu$m) are ideal for quantum communication, due to the low transmission loss in optical-fiber networks. To realize quantum networks operating at these wavelengths, we need long-lived quantum memories that couple efficiently to telecom-band photons. This study proposes using optical tweezers to couple neutral ytterbium atoms, which have a strong telecom-wavelength transition, to a silicon photonic-crystal cavity. The combination of high system efficiency, telecom-band operation, and long coherence times makes this platform well suited for quantum optics on a silicon chip and long-distance quantum communication.

The development of on-chip optical isolators for integrated photonic circuits has been actively pursued for several decades now, especially since the advent of optical-fiber telecommunication. This article reports the experimental realization, practical implementation, and performance of a $topological$ $edge-state$ $isolator$, based on the Su-Shrieffer-Heeger model in the optical regime. Such isolators have been predicted to deliver superior isolation ratios (up to -50 dB at telecom wavelengths), due to the existence of a topological edge state in the forward propagation direction, and its destruction in the reverse direction via the magneto-optical nonreciprocal phase-shift effect.

Yttrium iron garnet (YIG) is a material with unique properties that make it very attractive for use in microwave applications, such as frequency-selective limiters and logic processing. However, the decay of a spin wave’s amplitude as it propagates in the material curbs the use of YIG in practical applications. To surmount this limitation, in this study the surface spin currents of a topological insulator are used to enhance the spin-wave amplitude in adjacent YIG. In addition, interesting properties of magnons are observed and investigated, to advance spintronics.

Detecting the local amplitude and phase of magnetization excitations (spin waves) is important for coherent spin information processing. Phase detection traditionally is either via electrically rectified signals, or by pump-probe optical and x-ray methods. In this work, the authors demonstrate tabletop, heterodyne optical detection of the local phase of ferromagnetic resonance, with the capability to extract spin-orbit torques by quantifying an additional precessional phase in a heavy-metal/ferromagnet bilayer. The results offer spin-torque magnetometry with spatial and phase resolution, for studying electrically driven magnetization dynamics in spintronic nanodevices.

In $valleytronics$, local maxima and minima in a semiconductor’s electronic band structure are exploited for processing bits of information. Here a model of the $AB$-stacked WS${}_2$/MoS${}_2$ bilayer heterostructure, in which inversion symmetry is designed to be ideal for Berry-curvature engineering, shows that four-bit valley manipulation can be achieved, going beyond the two-bit standard. This ingenious management leads to the proposal of a $valley$ $valve$, a device that realizes nonlocal resistances at multiple levels, being analogous to the spin valve of spintronics.

Capitalizing on the spatial degree of freedom for highly multimode quantum memories demands a flexible method, analogous to those using spatial light modulators. The authors demonstrate arbitrary one-dimensional phase modulation of a coherent spin-wave state, stored in a wave-vector-multiplexed quantum memory, via the ac Stark effect. This technique can be valuable for practical implementations of innovative protocols in quantum information and communication.

Superconducting coplanar microwave resonators may serve as a quantum bus to transfer information from superconducting qubits to natural spin systems, such as ultracold atomic vapors, for long-lived quantum memory. The authors investigate a stacked two-chip superconducting device, consisting of a lumped-element resonator side-coupled to a coplanar-waveguide transmission line. By moving the chips relative to each other, the device can be operated in different regimes, enabling control over both the resonance frequency and the coupling. This system has the advantage of providing tunable coupling to clouds of ultracold atoms that must be operated at millitesla-level magnetic fields.

In spintronics research, the actual dynamics of magnetoelectric magnetization switching in an antiferromagnet is often overlooked in experimental work, even though it is important to establish whether multidomain effects play a role. The authors analyze the speed and energy limits of the spatial mechanisms governing electrically controlled domain switching in antiferromagnetic insulators. This study provides valuable insight into the performance of such materials for applications in ultralow-power nonvolatile memory and logic processing.

Terahertz accelerators can achieve potential gradients beyond 200 MV/m. Gyrotrons are the only power sources capable of producing megawatt-level, microsecond-long pulses in this frequency range, yet nanosecond-long pulses are required for breakdown-free operation. The authors explore the possibility of using a single GaAs wafer to enable production of the required pulse widths, with rise and fall times that closely track the illuminating laser’s pulse length. They demonstrate and quantify the reflective properties and laser-induced photoconductive effect of GaAs in the millimeter-wave regime, for use in a laser-operated shutter.

In quantum cryptography, the twin-field (TF) quantum key distribution (QKD) protocol could extend theoretical key rates through a lossy channel, but most of its security proofs require phase postselection, which complicates practical systems and severely limits secure key rates. This study presents a truncated TF-QKD protocol without any postselection, gives a general security proof, and shows that this scheme may beat other protocols in terms of key rate, even when only finite decoy states are employed. By showing that a medium-range QKD system can reach much higher key rates, this protocol will make QKD systems more applicable in tomorrow’s cybersecurity.

Water, water, everywhere… Ocean waves are a source of renewable energy at the $terawatt$ level. Optimizing array layouts for wave-energy converters is tricky, though, as the performance of each device is impacted by the presence of its neighbors. The excitation forces acting on each harvester can be increased or decreased via the complicated interference between waves scattered by the array. Based on small-scale experiments, the authors develop a one-dimensional interference model to predict the force amplifications resulting from such multiple-wave interference, and find that regular arrays may not always be optimal.

Plates that can fully recover their shape after extreme bending and folding could allow a large structure to be stored in a small volume, or to survive dramatic collisions. The authors theoretically and experimentally study nanoscale-plate mechanical metamaterials that simultaneously offer enhanced bending stiffness and folding robustness. Recoverable, sharp bending is due to a nonmonotonic moment-curvature relation, caused by structural geometric nonlinearity, even though the local material response remains completely elastic. Such metamaterials could be optimized for $e.g.$ the wings of microflyers, or expandable aerospace components that need to be deployed after takeoff.

Thermal metamaterials usually rely on the asymmetric interaction between embedded particles and the surrounding matrix. This study instead propose tailoring the symmetric interaction between particles arranged in periodic lattices. Consequentially, thermal transparency is also realized. The authors find that the Maxwell-Garnett and Bruggeman formulas can explain periodic systems with large area fractions of particles, and that they depend on each other. This work provides an avenue to exploration of periodic interparticle interactions, either to reduce thermal stress concentration or cheat infrared detection.

The microarchitecture of mechanical metamaterials is often based on lattice structures made of beamlike elements (struts). Analytical geometry-property relationships could greatly facilitate the rational design of such metamaterials, but are generally only available for struts of uniform cross section. Here the authors present geometry-property relationships for structures of beamlike components of $variable$ cross section. This is of particular interest for pentamode metamaterials, a special class of extremal materials exhibiting unusually high resistance to deformation in certain directions. This work promotes $e.g.$ the design of artificial bone for medical implants.

To improve spin-transfer-torque magnetic random-access memory (STT-MRAM) devices, high thermal stability of magnetic states and efficient switching are required, both of which depend on the energy barrier for switching. In characterizing that energy barrier, this study reveals an asymmetry in the transition between the two magnetic states that depends on the initial state, which would not occur in a uniform magnetic field. The asymmetry is likely associated with inhomogeneities of the fringe magnetic field at the edge of the device. This result is expected to be important in engineering tomorrow’s spintronic memory.

Topological band gaps lead to protected edge states and robust transport in electronics, photonics, and even mechanics. In this work, the authors design a one-dimensional mechanical topological band gap hosting a protected twist edge state, using a simple, analytical mass-spring model and finite-element simulations of a three-dimensional chiral beam microstructure. Such a beam can transform a small push-pull excitation at the one end (provided by $e.g.$ a piezoelectric transducer) into a large rotation at the other end. The system could potentially be utilized as a high-frequency resonant mechanical laser-beam scanner.

Quantum random-number generators (QRNGs) are important for numerical simulations, communication protocols, and numerous algorithms, as the true randomness coming from quantum mechanics is indispensable. Commercial QRNGs are being held back, though, because many schemes require photon sources and high-precision measurements, which increase cost and limit portability and stability. In this study, the variable time interval between quantum tunneling events in silicon diodes yield a small, reliable, chip-integrable device that churns out random numbers at 6.89 Mb/s.

Although focusing sound waves beyond the diffraction limit offers possibilities for many applications, such as acoustic detection, sensing, and imaging, experimental realization remains a challenge. This study offers a straightforward way of superfocusing sound, through modulating spoof surface acoustic waves along a gradient-index metasurface. The same system also allows subwavelength acoustic imaging. Intriguingly, the enhanced and confined sound field is measurable even away from the metasurface, allowing “visualization” of the entire sound-field evolution process at subwavelength scales. This capability suggests an enhanced platform for observing many phenomena in wave physics.

The topic of latency in superconducting nanowire single-photon detectors (SNSPDs, the fastest type of such sensors) is of interest for understanding the absolute limits of timing performance in these devices, but so far no model of the detection mechanism could quantitatively explain recent measurements of intrinsic jitter and latency. This analysis of the connection between detector latency and timing jitter in the presence of Fano fluctuations and spatial nonuniformity provides a framework for studying the intrinsic jitter in SNSPDs, and suggests that intrinsic jitter could be reduced by enhancing scattering to suppress superconductivity as quickly as possible.

The Leidenfrost phenomenon (as when a bead of water skates atop trapped steam across a hot griddle) receives much interest for transporting droplets with low friction. While linear motion is eased by low friction, changing direction becomes difficult, without constraint by walls. The authors use the concept of negative feedback by design of substrate structure to achieve transport along complex pathways, without walls. These paths furthermore allow control of a droplet’s position, while maintaining speed and low friction. The intrinsic self-centering and correction via negative feedback offers exciting potential for designing complex paths and tracks for levitated droplets to follow.

VO${}_2$, with its reversible metal-insulator transition accompanied by drastic change in optical properties, is a promising material for numerous applications, including energy-saving smart windows. The transition occurs near 68 °C, which is too high for building-type applications, but this temperature can be tuned by varying the Au core size in Au/VO${}_2$ core-shell nanoparticles—and the physical mechanism is a bit unexpected. This research points the way to optimizaton of this thermochromic material for switching devices, optical waveguides and limiters, sensing components, tunable metamaterials, and smart windows.

Channels for high-speed propagation are interesting for magnonic devices, where spin waves can be used as the information carriers. Unfortunately, exchange-dominated perpendicular standing waves suffer from low group velocities. This study uses a Y${}_3$Fe${}_5$O${}_{12}$/Ni${}_{81}$Fe${}_{19}$ bilayer in a waveguide structure to enhance the group velocities of spin-wave modes. Other bilayer systems are also explored to optimize the enhancement. This work paves the way to high-performance directional couplers and multifrequency magnonic devices that provide multiple spin-wave modes propagating with high group velocities.

Efficient, fully deterministic current-induced generation and displacement of magnetic skyrmions (quasiparticles with topologically protected chiral spin texture) is a prerequisite for a whole class of spintronic devices. One of the key ingredients for fast dynamics is a material with low magnetic damping, and Heusler alloys are interesting in this respect. The authors use N-$V$ center magnetometry to study the creation and movement of skyrmions in a Co-based Heusler alloy, and show that skyrmion generation is facilitated by application of an in-plane magnetic field. Pinning effects due to structural defects limit skyrmion dynamics, even in this favorable material.

In photonic quantum information processing, spectrally uncorrelated biphoton states generated from nonlinear crystals are a fundamental resource, but a rare one, and researchers have only used a few kinds of crystals to produce such biphoton states, over a limited wavelength range. This study reveals that crystals of a family of phosphate and arsenate salts similar to potassium dihydrogen phosphate (KDP) can be used to generate spectrally pure (as high as 98%) states from near-infrared to telecommunication wavelengths. The key is to engineer group-velocity matching.

High-energy electron radiography is an emerging technique for imaging matter at high energy densities ($i.e.$ under extreme temperature and pressure) with high spatiotemporal resolution, but its development is hindered by the technical difficulty of creating advanced electron optics. This study explores the combination of state-of-the-art high-brightness electron sources with an innovative cascaded imaging system to make progress on the problem. Interestingly, the energy spread of the electron source seems to be the main obstacle to further improvement of the resolving power of this technology.

In the context of emerging quantum technology and energy harvesting, there has been a recent surge of interest in nanoscale thermoelectric transport. The authors present a near-field imaging technique to study such phenomena, and demonstrate its great potential by revealing in a quantum point contact a localized state that is invisible to standard transport measurements. This approach should help, for example, to resolve the long-lived debate over the conductance and thermoelectric properties of quantum point contacts (the 0.7 and zero-bias anomalies), as it is particularly efficient in detecting bound states.

Nanoscale devices based on molecular electronics are of interest, but cannot be fabricated by methods standard to the semiconductor industry. Self-assembly of films is an appealing alternative, but what actually comes out? The characterization of self-assembled molecular networks by scanning tunneling spectroscopy is often hampered by instabilities within the molecular assembly. The authors show that a statistical analysis of differential-conductance images provides insight into the coupling between conformers. Their approach can be applied to nonperiodic features in periodic two-dimensional networks.

Two-dimensional, $\delta$-doped Si:P structures are of interest for qubits, high-performance electronics, and quantum metamaterials. However, a lack of low-resistance, high-yield Ohmic contacts to these subsurface, atomically thin systems has hampered applications. This study demonstrates Pd${}_2$Si contacts to Si:P with a yield near 100%, which introduce only a small parasitic resistance and are compatible with the low-temperature processing needed for an abrupt Si:P delta layer. This technology enables reliable fabrication of large numbers of devices, and provides the low resistances that are critical for coherent measurements with Si:P quantum devices.

The conventional approach to transformation optics starts with a virtual space and determines a complicated material profile in physical space, to achieve unconventional phenomena. The authors take a reverse approach, and find that the conformal singularities in the refractive-index profile are equivalent to topological defects. Optical splitting and illusion effects are confirmed. They furthermore fabricate a device with a positive topological defect, and demonstrate its light-bending functionality. Their methods could be used to connect conventional geometric optics with on-chip applications.

Understanding the relationship of microscopic dynamics to mechanical response in soft matter carries far-reaching implications for industrial applications and materials design. Light scattering coupled with rheology could help here, but conventional scattering methods lack sufficient resolution, and cannot distinguish various dynamical behaviors. Thus the authors offer a setup for rheology plus space- and time-resolved dynamic light scattering, in a geometry that allows the contributions of microscopic rearrangements to be seen. Testing on a biogel shows rearrangements in time and space similar to a stick-slip process, even when the $macroscopic$ rheological response is linear.

The authors propose a graphene-based subwavelength plasmonic switch, in which gating of a surface plasmon polariton is operated by inducing a local plasmon resonance within a $p$-type Si(100) layer. The resulting sharpness of its switching is the main advantage of this device, for example allowing a response to even a modest carrier-density modulation, such as has been demonstrated recently by others. Putting the pieces together here could strongly advance all-optical plasmonic switching applications, particularly in information processing.