The micropump is an integral part of any microfluidic system, for applications in diagnostics, bioengineering, drug delivery, and lab-on-chip devices. Plant leaves provide natural inspiration for designing evaporation-based passive micropumps, but development of such bioinspired pumps is limited by fabrication challenges plus a lack of understanding of the details of pumping in leaves. This study uses a simple yet scalable method to fabricate a leaf-mimicking structure from readily available materials. A model of the pumping mechanism corroborates experiment well, yielding a better understanding of the design parameters affecting the performance of the leaf-mimicking device.

Multiferroic tunnel junctions (MFTJs), featuring a ferroelectric tunnel barrier between ferromagnetic electrodes, could offer important advantages for memory and logic devices, with four nonvolatile states that can be accessed resistively. Rather than using a classic perovskite material for the barrier, the authors use the recently discovered ferroelectric Hf${}_{0.5}$Zr${}_{0.5}$O${}_2$ to fabricate standalone, highly homogeneous wire-bonded devices (at micrometer scale, in proof of principle) exhibiting four-state memory functionality. This work thus shows a clear path forward for the development of next-generation memory, logic and synaptic devices.

The rise of toroidal plasmonic metamaterial sensors has revitalized the precision of optical immunobiosensors. While detection at ultralow concentrations with conventional midinfrared plasmonic biosensors is challenging, toroidal metamaterial technology has enabled the development of ultrasensitive, label-free recognition and detection of low-weight biological objects at femto- and attomolar concentrations. The proposed metasensor is able to detect about 600 Kantrex molecules in 200 mm${}^3$ of solution. This remarkable performance stems from the exquisite sensitivity of robustly squeezed electromagnetic fields in the toroidal plasmonic metastructures.

Acoustic metasurfaces, generally comprising several types of localized meta-atoms, draw great interest for their flexibility in acoustic field manipulation. However, the unavoidable $nonlocal$ coupling between meta-atoms leads to lackluster performance for large-angle refraction. By embracing rather than ignoring nonlocality and periodicity, this research shows that acoustic metagratings can almost perfectly bend sound for very large angles (>80°), with up to 95% transmission. Besides efficiency, the suitably treated nonlocality provides another degree of freedom to manipulate sound propagation, extending the realm of acoustic metasurfaces and promoting innovative techniques.

Ultraprecise displacement measurements via laser interferometry are at the core of gravitational physics, but their use is currently limited by the complexity of the optical setups. This study seeks to use an optically simpler interferometry technique, namely $deep$ $frequency$ $modulation$, to sense picometer-level displacement at frequencies below 1 Hz. To this end, an extremely compact interferometer based on a single prism is designed and tested in an ultrastable test-mass-in-the-middle experiment, to probe the underlying noise couplings. Its demonstrated performance and optical simplicity will impact the availability, scalability, and usability of such sensitive measurements.

The emergent pyroelectricity in the ferroelectric (orthorhombic $Pca{2}_1$) phase of CMOS-compatible hafnia offers great potential for future infrared-sensing and energy-harvesting applications, but an understanding of the phenomenon in this particular compound is still lacking. The authors use first-principles calculations to show that pyroelectricity arises unexpectedly from the $secondary$ effect in ferroelectric HfO${}_2$, due to the peculiarity of its piezoelectricity. They also find an orthorhombic-to-tetragonal structural phase transition associated with a giant pyroelectric response, which can be further enhanced by doping with Si.

Turning up the heat: Treating cancer without harsh chemicals or radiation is the goal of nanoparticle-mediated magnetic hyperthermia, which is presently one of the most interesting techniques for localized treatment of malignant tissues in living bodies. This study uses nonsinusoidal driving-field waveforms of high fundamental frequency to enhance the efficiency of magnetic hyperthermia. An interesting effect of hysteresis-loop instability develops under a square-wave driving field when equilibrium conditions are not fulfilled. This approach is expected to boost the beneficial application of field-driven magnetic nanoparticles as diffuse heat sources for malignant-tissue therapy.

One of the most exciting topics in quantum technology is the development of optomechanical interfaces, to link quantum devices in hybrid systems. Efforts to bridge the radio-infrared frequency gap, to network superconducting qubits over optical fiber, have benefited from piezoelectric optomechanics, but an alternative is to use the magnetic component of electromagnetic waves to control an optomechanical resonator. This work shows that such magnetic control of a torsional resonator is phase coherent, a prerequisite for quantum operation, and that such phase control can be used for classical information transmission via phase-shift keying.

Multiferroic tunnel junctions (MFTJs), featuring a ferroelectric tunnel barrier between ferromagnetic electrodes, could offer important advantages for memory and logic devices, with four nonvolatile states that can be accessed resistively. Rather than using a classic perovskite material for the barrier, the authors use the recently discovered ferroelectric Hf${}_{0.5}$Zr${}_{0.5}$O${}_2$ to fabricate standalone, highly homogeneous wire-bonded devices (at micrometer scale, in proof of principle) exhibiting four-state memory functionality. This work thus shows a clear path forward for the development of next-generation memory, logic and synaptic devices.

The micropump is an integral part of any microfluidic system, for applications in diagnostics, bioengineering, drug delivery, and lab-on-chip devices. Plant leaves provide natural inspiration for designing evaporation-based passive micropumps, but development of such bioinspired pumps is limited by fabrication challenges plus a lack of understanding of the details of pumping in leaves. This study uses a simple yet scalable method to fabricate a leaf-mimicking structure from readily available materials. A model of the pumping mechanism corroborates experiment well, yielding a better understanding of the design parameters affecting the performance of the leaf-mimicking device.

Efficient, broadband terahertz detection through electro-optic sampling is an enabling technology for numerous applications, including time-resolved spectroscopy of condensed matter systems and fundamental investigations of the quantum vacuum. The technique is inherently sensitive, but ultimately limited by the intrinsic optical properties of the known nonlinear crystals. In this study the authors imprint a phase grating on the surface of a nonlinear crystal, to define a noncollinear detection geometry inside the crystal. This surface grating allows improved phase-matching conditions, which lead to a doubling of detection bandwidth and efficiency.

While nonreciprocal devices such as light isolators and circulators are becoming indispensable components in classical and quantum information processing, nonreciprocity in cavity magnon systems, which offer distinct advantages, still needs investigation. This study proposes an intrinsically tunable two-cavity magnon system that can achieve nonreciprocal light transmission, based on the magnon Kerr effect. By adjusting the external magnetic field, even one-way transmission can be obtained. These results point the way to microscale magnonic structures for potential applications in light diodes, on-chip light control, and optical communication.

Hybrid organic-inorganic heterojunctions have emerged as low-cost alternatives for fabricating optoelectronic devices, but their charge-transport mechanism has not been fully established. In this work, the charge transport in PEDOT:PSS/silicon heterojunctions is found to be governed by a twofold mechanism: injection-limited under reverse bias, space-charge-limited under forward bias. This study provides physical insight into the interpretation of the characteristic current-voltage curves of these and similar hybrid devices.

Terahertz waves are useful in many fields, including imaging, spectroscopy, and telecommunication. However, an effective source of THz radiation source for real-world applications is still lacking. This study proposes using the approximately equidistant helical edge states in topological-insulator quantum dots for frequency down-conversion, to produce light covering most of the “THz gap”, from 3 to 10 THz. Such a topological-insulator quantum-dot array can form a continuous-wave THz laser that works at room temperature, with an output power of 28 mW. This study expands both the application of topological phenomena and THz device physics.

Converting charge to spin in the two-dimensional electron gas (2DEG) at an interface of oxides has shown promise for electrical manipulation of a ferromagnet’s magnetization via spin-orbit torque. However, the only efficient system so far is SrTiO${}_3$/LaAlO${}_3$, the making of which requires complex pulsed laser ablation and intricate pressure and temperature regulation. This work shows that good conversion can also be realized in Ar${}^{+}$-etched SrTiO${}_3$ 2DEGs simply fabricated by magnetron sputtering, with about 90% efficiency at room temperature for an etched SrTiO${}_3$/Ni-Fe structure—an efficient, easily achieved spin source for spin-orbit-torque applications.

The Dzyaloshinskii-Moriya Interaction (DMI) is a key ingredient for the observation of skyrmions and Néel domain walls in films with perpendicular magnetic anisotropy. The dynamic control of DMI is, therefore, of great interest for developing chirality-dependent spintronics. This study demonstrates the nonvolatile modification of DMI and magnetic anisotropy through electric-field-induced ionic migration in Pt/Co/HfO${}_2$/liquid-gate structures. As the ionic liquid gate can be removed after applying the electric field, this is not only a step toward ionic control of DMI in devices, but also a simple post-growth method for materials processing.

What if scanning probe microscopy (SPM) could be used in water? $In$ $vivo$ or $in$ $operando$ detection of the electrical properties of functional systems in polar liquids by SPM is in high demand, in many research domains. Progress has been thwarted, though, because the physical mechanism behind the imaging difficulties in such an environment is still ambiguous. This study clarifies the physics by simulating electric field screening based on the formation of electric double layers at the solid-liquid interfaces of the metal tip and sample. Visualizing the electromechanical coupling on ferroelectrics experimentally confirms the screening effect in electrolytes.

Manipulation of nonlinear processes of orbital angular momentum (OAM) modes in optical fibers is an emerging challenge, providing the potential for a wide variety of applications in parametric amplification, high-dimensional entanglement generation, and frequency conversion. The authors demonstrate low-crosstalk intramodal processes for four-wave mixing between OAM modes in an optical-vortex fiber. By engineering the relative group velocity within a spatial dimension, the phase-matching conditions for OAM modes can be tailored. The proposed scheme may also illuminate further possibilities for generating and manipulating OAM states in fiber systems.

Unintentional doping of the active layer is detrimental to an organic solar cell’s performance, but is often overlooked. This work clarifies the impact of doping on charge-collection efficiency in low-mobility solar cells. By analytical derivation and numerical simulation, the authors find the collection efficiency of photogenerated charge carriers in a doped active layer to be independent of light intensity, but distinctly dependent on voltage, resulting in an electric field dependence of the photocurrent. The results will help to overcome the effect of unwanted doping, and to distinguish between different recombination loss mechanisms in these important photovoltaics.

High-performance thermoelectric materials often possess special characteristics in their electronic structure, but predicting materials with such desirable features is quite difficult. This first-principles study reveals that ternary oxides and nitrides crystallized in the antiperovskite $A_{3}B$O structure can exhibit strong thermoelectric performance, thanks to a band structure with unusually many exotic aspects. This represents a rather unique playground in which to seek hidden high-performance thermoelectric compounds for tomorrow’s waste-heat harvesters and specialty heaters, coolers, and power generators.

The emerging field of $straintronics$ may be able to sustain Moore’s Law by replacing transistors with switches in which electrically generated strain flips the state of a nanomagnet—an extremely energy-efficient platform for Boolean logic and memory. Can it really work, though? Considering realistic nanomagnets with localized or extended structural defects, the authors find that those flaws increase the switching-error probability by orders of magnitude, making logic applications inconceivable. Even memory applications, more forgiving of errors, are dubious. This calls into question the viability of straintronics for Boolean computing.

Films of the ferromagnetic alloy Co-Fe-B in general perform very nicely at room temperature and have been widely used, but not in magnetoelectric devices or microwave applications, because of unfavorable magnetostriction, Gilbert damping, and thermal tolerance. Here the authors instead use carbon-doped Co${}_{0.5}$Fe${}_{0.5}$ to create a Co-Fe-C film with high magnetostriction, low Gilbert damping, and high thermal stability. This work provides insight into the basic aspects of Co-Fe-C as a promising candidate for use in voltage-tunable magnetoelectric devices and microwave devices.

In the growing field of $magnonics$, spin waves (magnons) are manipulated as information carriers, particularly for data processing. Thus controlling the propagation of spin waves is of paramount interest here. This investigation considers a ferromagnetic bilayer with dipolar coupling as a potential nonreciprocal magnonic device, in which frequency dispersion is reciprocal for parallel yet nonreciprocal for antiparallel orientation of layer magnetizations. This is progress in the technological implementation of magnonic devices with asymmetric transmission properties, since reconfigurability can be easily achieved just by switching, without any rotation of the applied magnetic field.

Wind power is the fastest-growing source of renewable energy, and a major contributor in the portfolio. Wind is a highly intermittent source of energy, though, exhibiting strong non-Gaussian characteristics that persist in the aggregated power supply. The process governing how wind turbulence translates to power fluctuations is not fully understood, and is the subject of significant research effort. This analysis provides insight into the benefits and limitations of a nonlinear dynamical model of a wind turbine, which could lead to improvements in control and modeling to better replicate fluctuations in wind power—and possibly to mitigate them.

Ignoring the bumps along the road: The study of topological properties of wave propagation has become intense, due to the appealing prospect of transmission that is robust against defects. Designing simple structures that exhibit nontrivial topological phases is essential for innovative device applications, such as splitters and isolators. Here the authors show that topological edge waves can exist in a simple acoustic network of air channels. A theoretical model from condensed matter physics and corresponding numerical simulations are provided in great detail to analyze the topological edge waves seen in experiments.

Spin waves (magnons) are a promising medium for tomorrow’s information processing, but high-fidelity control of spin-wave propagation is still a bottleneck in realizing magnonic devices. This study focuses on the manipulation of magnetostatic surface spin waves via electric fields. With magnetoelectric coupling in the form of an effective Dzyaloshinskii-Moriya interaction, applying an electric field influences the surface-wave dispersion relations, and this effect can be used to manipulate the propagation and transfer of spin waves in waveguides. This approach is used to design an experimentally feasible, high-fidelity nanoscale spin-wave directional coupler.

CO${}_2$ dissolution in brine is an important trapping mechanism in the context of permanent, safe storage of CO${}_2$ in geologic formations. An innovative high-pressure cell enables visualization of the convective mixing due to CO${}_2$ gas dissolving in the underlying saturated porous medium. The ability to monitor both gas volume and pressure, which both evolve with dissolution, yields insight into the dynamics of convection under conditions that reflect actual geological systems. These results elucidate how the presence of two separate phases in a confined system, as well as different salt types, alter the short- and long-term scaling behavior of solute convection in porous media.

Sequences of random numbers are crucial resources for computer simulation and cryptocommunication. Appropriate measurements of the quadratures of a laser’s field have been shown to enable efficient generation of high-security, unbiased, truly random numbers. The authors extend this technique to realize a real-time quantum random-number generator, where the source of entropy does not need to be trusted and can be controlled by a malicious third party. By measuring a pair of conjugate quadratures, secure random numbers useful for cryptocommunication can be extracted. The technique is tested on different entropy sources, including thermal and squeezed states of light.

The rise of toroidal plasmonic metamaterial sensors has revitalized the precision of optical immunobiosensors. While detection at ultralow concentrations with conventional midinfrared plasmonic biosensors is challenging, toroidal metamaterial technology has enabled the development of ultrasensitive, label-free recognition and detection of low-weight biological objects at femto- and attomolar concentrations. The proposed metasensor is able to detect about 600 Kantrex molecules in 200 mm${}^3$ of solution. This remarkable performance stems from the exquisite sensitivity of robustly squeezed electromagnetic fields in the toroidal plasmonic metastructures.

As a representative indirect imaging technique, $ghost$ $imaging$ reconstructs object information from the calculated intensity correlation of two beams, though neither of the beams can yield the object image independently. By introducing three reconstruction algorithms for ghost imaging, the authors demonstrate that different algorithms not only have different imaging efficiencies, but also can affect the phenomena of positive versus negative ghost imaging. Simulations and experiments show that a positive or negative ghost image can be reconstructed by modulating the monotonicity of the bucket object signal function in each algorithm.

Superconducting nanowire single-photon detectors (SNSPDs) are widely used in quantum information science and quantum optics because of their excellent properties, yet much of their dynamical behavior still is not fully understood. This study identifies characteristic time and resistance scales for the SNSPD detection process, and uses them to develop a universal model of SNSPD behavior. The authors furthermore clarify how different detector parameters affect the output signal, specifically exploring the effects of photon number, detector length, and bias current. These results will have an impact on SNSPD fabrication and encourage innovative applications of these sensors.

Acoustic metasurfaces, generally comprising several types of localized meta-atoms, draw great interest for their flexibility in acoustic field manipulation. However, the unavoidable $nonlocal$ coupling between meta-atoms leads to lackluster performance for large-angle refraction. By embracing rather than ignoring nonlocality and periodicity, this research shows that acoustic metagratings can almost perfectly bend sound for very large angles (>80°), with up to 95% transmission. Besides efficiency, the suitably treated nonlocality provides another degree of freedom to manipulate sound propagation, extending the realm of acoustic metasurfaces and promoting innovative techniques.

Spin-orbit coupling gives rise to the spin Hall effect (SHE) in nonmagnetic heavy metals, and to the anomalous Hall effect (AHE) in ferromagnetic metals. This article proposes a mechanism to switch the perpendicular magnetization in a magnetic trilayer, without an assisting magnetic field, by utilizing the SHE or AHE plus out-of-plane spin polarization. The authors derive the analytic expression for the resulting spin torque, and perform macrospin simulation to validate the field-free switching. The proposed switching mechanism combines well-established spin-valve technology with the spin Hall effect, potentially enhancing switching efficiency in practical devices.

In recent years the efficiency of photovoltaic cells (PVCs) has developed rapidly. Even though its quantum efficiency can approach 100%, Shockley and Queisser famously showed that a conventional PVC’s efficiency is limited by unavoidable losses, due to $e.g.$ spontaneous radiation. This work presents an alternative PVC design to overcome those limits: a two-dimensional van der Waals heterojunction photovoltaic cell (2D-PVC) combined with an interference-based light-trapping structure. The authors demonstrate that this 2D-PVC exhibits much lower spontaneous-radiation loss, with an efficiency limit that can be 10% larger than for a traditional PVC.

Resistance-based thermometry is a simple and widely used technique, though we still lack a single sensor that spans a broad temperature range without losing sensitivity. This article presents an alloy that is useful for thermometry from room temperature down to about 100 mK with little variation in its dimensionless sensitivity. Its resistivity varies logarithmically from approximately 200 K down to approximately 200 mK, allowing for simple calibration, and it can be patterned on a chip alongside experimental devices. This material might be quite useful in producing a chip-integrated thermometer operating at stable sensitivity over three decades in temperature.

Ultraprecise displacement measurements via laser interferometry are at the core of gravitational physics, but their use is currently limited by the complexity of the optical setups. This study seeks to use an optically simpler interferometry technique, namely $deep$ $frequency$ $modulation$, to sense picometer-level displacement at frequencies below 1 Hz. To this end, an extremely compact interferometer based on a single prism is designed and tested in an ultrastable test-mass-in-the-middle experiment, to probe the underlying noise couplings. Its demonstrated performance and optical simplicity will impact the availability, scalability, and usability of such sensitive measurements.

Front-surface recombination is one of the prominent loss mechanisms in some silicon solar cells, so understanding the passivation provided by fixed charges in dielectric layers is crucial to improving efficiency, particularly for devices with submicrometer and high-aspect-ratio surface features. The authors analyze field-effect passivation in textured and planar surfaces, through simulations and models accounting for the interaction between the opposing facets. The change in field-effect passivation upon texturing is significant for moderately large fixed-charge densities, yet negligible for very small or large densities.

Materials featuring negative thermal expansion (NTE) attract attention due to their numerous applications in printed circuit boards, machinery, optical-fiber reflective grating devices, and high-precision mirrors. Unfortunately, most large samples of NTE material are very sensitive to external magnetic fields. To address this stumbling block, the authors chemically modify CrAs, which can turn antiferromagnetic and thus field-insensitive, and obtain a large NTE coefficient across a large range that includes room temperature. Electrons in $3d$ orbitals are responsible for NTE, accompanied by a first-order antiferromagnetic-paramagnetic transition.

Much effort has been devoted to anticounterfeiting technologies, and recent advances in metasurfaces have provided further opportunities to realize holograms with unique properties. This work demonstrates a multichannel metasurface device that can reconstruct helicity-multiplexed holographic images and hide a grayscale image in the polarization profile of a light beam. The feasibility study of a multichannel device based on independent control of phase and polarization shows this technique to be very attractive for compact optical devices with multiple functionalities for encryption and high-level anticounterfeiting.

Origami has been widely used in designing mechanical metamaterials, providing various intriguing properties and functionalities that could lead to disruptive engineering applications for mechanical waves. Here the authors extend the concept of origami to acoustic metamaterial design, for manipulating sound waves. They design and fabricate a reconfigurable, origami-inspired acoustic metamaterial with tunable functionalities that include acoustic focusing, splitting, localization, and one-way transmission. Their approach opens an avenue to practical applications based on the control of sound.

Magnetization, the magnetic moment per unit volume, is the basic quantity for describing a magnetic system. Generally, magnetic imaging techniques show the relative magnitude of magnetization, and its direction. Determining the $absolute$ value of magnetization requires separate measurement with a reference sample, though. This study describes the stripe state of magnetization, in which the absolute value can be measured simply by magneto-optical imaging. In particular, this approach is applicable to samples so small that they cannot be measured by conventional magnetometers.

The phenomenon of $acoustic$ $levitation$ has lately found application in noncontact particle manipulation in research laboratories, but is not mature enough for commercial use, as the dynamics of a levitated particle are not understood well enough. The authors derive a refined model for these particle dynamics, and use it with parametric excitation in real time to control the oscillations of one specific particle in a levitated group. This insight brings acoustic-levitator design a step closer to commercial reality.

The emergent pyroelectricity in the ferroelectric (orthorhombic $Pca{2}_1$) phase of CMOS-compatible hafnia offers great potential for future infrared-sensing and energy-harvesting applications, but an understanding of the phenomenon in this particular compound is still lacking. The authors use first-principles calculations to show that pyroelectricity arises unexpectedly from the $secondary$ effect in ferroelectric HfO${}_2$, due to the peculiarity of its piezoelectricity. They also find an orthorhombic-to-tetragonal structural phase transition associated with a giant pyroelectric response, which can be further enhanced by doping with Si.

While there has been exciting progress in the emerging area of nonlinear acoustic nonreciprocity, one of the main challenges is to realize $passive$ nonreciprocal acoustic waveguides with predictable, reliable broadband operation. Here the authors demonstrate an entirely passive nonreciprocal acoustic waveguide that enables transmission of acoustic waves in a preferred direction. Moreover, due to its strong nonlinearity, this nonreciprocity is passively adaptive to energy and can be predictively tuned by the intensity of the applied broadband excitation. Related applications span telecommunication, optics, and biomedical imaging.

Excitons as a physical phenomenon have been crying out for application since their discovery, but their subtle nature is a drawback here. Small binding energies restrict the existence of excitons at room temperature, and these quasiparticles are far from stable in time, too. Nevertheless, these shortcomings can be viewed instead as $sensitivity$, and so this study explores excitons as a probe for characterizing semiconductor nanostructures and optimizing their growth. Advanced numerical modeling, leading-edge growth techniques, and precise optical measurements are combined to obtain a fresh perspective on these important optoelectronic systems.

The authors significantly extend the optical technique of coherent population trapping (CPT) using three photons, based on a dark resonance of trapped Ca${}^{+}$ ions. CPT with three photons is an important approach for high-precision measurements, as it opens up the use of large ensembles of trapped ions, rather than individual atoms. Furthermore, an atomic terahertz frequency reference based on three visible lasers can be derived from this technique. Terahertz spectroscopy is a rapidly growing field with a host of applications, and recent studies point to a clear need for frequency standards in this range.

Vector beams in optics play a vital role in a wide variety of fundamental studies and applications, including high-capacity communication, super-resolution imaging through biological tissue, quantum information encryption, surveillance, and sensing. Here the densely coded information and inseparability of spatial and polarization degrees of freedom complicate full characterization, and the lack of a technique for complete beam metrology hinders applications. The authors demonstrate a direct tomographic protocol that can reveal unambiguously the complete transverse field structure of a fully polarized vector beam, all with a single measurement.

The interaction of laser pulses with magnetic materials has become a major topic in modern magnetism, with ultrashort pulses providing unique windows to view ultrafast phenomena. It is still a challenge, though, to extend spatial resolution, to study magnetic nanostructures and complex spin textures. Here the high spatiotemporal resolution of Lorentz ultrafast transmission electron microscopy (Lorentz UTEM) reveals remarkable features and key details of the magnetic transient states in Mn-Ni-Ga after a femtosecond laser pulse. This work showcases Lorentz UTEM as a means of directly imaging ultrafast magnetization phenomena at the nanoscale.

Transition-metal dichalcogenides (TMDs) are promising electronic materials for next-generation transistor and CMOS technologies. Here many challenges remain, though, including the ability to precisely control the type and density of free carriers in TMDs. This study uses first-principles calculations to provide important understanding of the experimentally observed large variation in attainable TM dopant concentration in bulk WSe${}_2$. The chemical trend found for the TM doping efficiency in the bulk compound should also be relevant to understanding doping and identifying more effective dopants in mono- or few-layer WSe${}_2$, and in other TMDs.

In secure communication, reference-frame calibration is critical to ensuring a secure key rate in quantum key distribution (QKD) systems. This process also increases the complexity of the system, which is not conducive to practical use. Although the reference-frame-independent (RFI) QKD protocol can simplify alignment, it requires three bases to prepare six states. Here the authors present instead a three-state RFI QKD protocol, and verify its security using a recently developed proof based on semidefinite programming. A proof-of-principle experiment is conducted to demonstrate the feasibility of the scheme. This work is sure to boost the practical application of the RFI QKD protocol.

Acoustic asymmetric transport is important for noise control and communication acoustics. Recent progress on non-Hermitian systems has yielded a scheme to realize asymmetric transport without nonlinear effects or mode conversion. The authors report the experimental realization of an asymmetric diffraction grating based on a cleverly designed passive $PT$-symmetric medium, in which loss is introduced as an extra modulation factor, and the energy distribution can be adjusted more freely. Moreover, the designed structure is helpful for further research on non-Hermitian acoustic systems.

Turning up the heat: Treating cancer without harsh chemicals or radiation is the goal of nanoparticle-mediated magnetic hyperthermia, which is presently one of the most interesting techniques for localized treatment of malignant tissues in living bodies. This study uses nonsinusoidal driving-field waveforms of high fundamental frequency to enhance the efficiency of magnetic hyperthermia. An interesting effect of hysteresis-loop instability develops under a square-wave driving field when equilibrium conditions are not fulfilled. This approach is expected to boost the beneficial application of field-driven magnetic nanoparticles as diffuse heat sources for malignant-tissue therapy.

One of the most exciting topics in quantum technology is the development of optomechanical interfaces, to link quantum devices in hybrid systems. Efforts to bridge the radio-infrared frequency gap, to network superconducting qubits over optical fiber, have benefited from piezoelectric optomechanics, but an alternative is to use the magnetic component of electromagnetic waves to control an optomechanical resonator. This work shows that such magnetic control of a torsional resonator is phase coherent, a prerequisite for quantum operation, and that such phase control can be used for classical information transmission via phase-shift keying.

The discovery of alternate methods of producing energy that do not contribute to global climate change is a compelling problem of our time. This study develops a means of generating electricity from small temperature differences by exploiting a first-order ferroelectric phase transformation. The authors model the thermodynamics of energy conversion using the Landau free energy, and experimentally demonstrate the direct conversion of heat to electricity using single-crystal BaTiO${}_3$. A key role is played by the influence of electric field on transformation temperature, which has the effect of opening up a mixed-phase region in the temperature-entropy diagram.

Enhancing the thermal conductivity of a multilayered system is seen as a remedy for critical thermal issues in nanoelectronics. While confinement in nanodevices can generate hot spots and spoil performance, thermal conduction via surface electromagnetic waves (SEWs) in multilayers can provide an additional channel for energy transport, to spread out the heat. The authors present analytic solutions for SEWs in three basic glass multilayered structures, and optimize them to obtain SEW thermal conductivity of 1.27 W m${}^{-1}$ K${}^{-1}$, similar to that of bulk glass. This insight will promote engineering solutions to alleviate thermal issues in nanodevices.

The isotopic ratio ${}^{13}$C/${}^{12}$C of CO${}_2$ is of particular importance in environmental and biomedical contexts. Mass spectrometry usually works, but for ${}^{13}$C/${}^{12}$C there can be near mass coincidences due to ${}^{13}$C${}^{16}$O${}_2$ and ${}^{16}$O${}^{12}$C${}^{17}$O, which can confound the results. The authors use a frequency-comb source plus a high-resolution dispersive spectrometer to exploit the nuclear-mass-dependent frequency shifts of molecular spectra, to accurately measure the natural abundance ratio for CO${}_2$. This technique allows clear differentiation of species with similar atomic mass, without sacrificing any other desirable technical features, and is generally applicable to other gases.

The diffraction limit, which originates from the loss of high-spatial-frequency evanescent waves, constrains the attainable resolution of conventional imaging methods to the scale of the light’s wavelength. This study unveils $spatial$ $spectrum$ $sampling$ as a method for retrieving the broadband evanescent information about targets by spoof surface plasmons, which allows almost-real-time far-field imaging at subwavelength resolution, for terahertz and microwave frequencies. This work has potential for imaging applications at longer wavelengths, such as nondestructive testing, quality control in manufacturing, and medical diagnosis.

Utilizing spin in condensed matter is an important means to create or enhance technologically important phenomena. However, this route has rarely been used to produce relevant functional advantages for the generation, detection, or manipulation of light. This work shows that the spin Seebeck effect (SSE), in which a pure spin current is generated from magnetization and a thermal gradient, can detect light in Pt/Y${}_3$Fe${}_5$O${}_{12}$ devices at wavelengths from ultraviolet to near-infrared. The extremely flat, broadband responsivity of these SSE devices paves the way for a generation of spin-based optical detectors.

Elusive Majorana bound states (MBSs) could enable fault-tolerant topological quantum computing. However, for such applications it is crucial to demonstrate MBS tunability and exchange, which yet remains to be realized. This work proposes a platform to address this challenge: a semiconductor-superconductor hybrid nanowire atop magnetic stripe domains. Realistic calculations and initial experiments indicate that here MBSs are robust and can be tuned by electric field or strain, due to synthetic spin-orbit coupling. This discovery points to a potentially important path to tunable MBSs, and also new tricks involving the magnetic stripe domains commonly used in spintronics.

To date, ghost imaging (in which correlation of photons is used to locate an object and enhance its picture) has struggled with low imaging speed. The authors construct an LED array that displays light patterns at a modulation rate as high as 100 MHz, which enables imaging at more than 1.4 million frames per second, with the capability of color imaging. They further extend this method to high-speed imaging in low light by introducing multiple single-photon detectors. This technique offers a major step for the application of ghost imaging.

Transparent conducting oxides such as gallium-doped zinc oxide (GZO) are of increasing importance for displays and photovoltaic applications. For manufacturing, an atmospheric-pressure plasma jet (with no vacuum chamber) is suitable for continuous inline processing; however, deposition becomes more susceptible to oxygen quenching, reducing the plasma’s reactivity and the formation of oxide. Here the authors show that the electrical conductivity and deposition rate of GZO films are both enhanced by using a different nozzle feature, to lessen oxygen quenching. These findings may be extended to other areas where vacuum-based technologies are not applicable, such as biomedical applications.

Go with the flow: Extracting useful work from an external diffusive flux at the microscale is challenging. The authors propose a prototype for a microturbine that is powered purely by an external thermal or chemical gradient, due to fore-and-aft asymmetric phoresis. The turbine’s rotation is $perpendicular$ to the diffusive flux, and thus precise alignment is unnecessary. This perpendicular-axis phoretic turbine offers possibilities for exploiting ubiquitous thermal or chemical energy at small length scales.

Under certain conditions, dissipative “breather” solitons can suddenly explode, and then recur. Getting a handle on this behavior is important to understanding wave instabilities in many physical systems that support dynamical solutions, but to study this phenomenon we first have to be able to produce it reliably. This study uses a mode-locked fiber laser and a fast detection technique to study this fascinating phenomenon. It turns out that breathing dissipative soliton explosions occur far below the mode-locking threshold, and nonlinearity-mediated $Q$-switching stimulates this—a result that could facilitate the design of stable ultrashort-pulse lasers.

Quantum key distribution requires photon-pair sources with high spectral correlation. Meanwhile, by applying wavelength-division multiplexing, quantum information capacity can be increased massively. This work experimentally achieves an efficient, high-quality multichannel photon-pair source that matches the standard International Telecommunication Union frequency grid. Strong, uniform spectral correlation over multiple channels is demonstrated by reconstructing the joint spectral intensity with high-resolution, probe-swept stimulated four-wave mixing.

Traveling-wave parametric amplifiers (TWPAs) are widely used in quantum information science for amplification and measurement at the quantum limit, but their development is held back by a lack of understanding of their susceptibility to internal loss. The authors use both lumped-element and distributed-loss models to describe the output of a lossy TWPA, and identify a surprising, strong dependence on the symmetry of loss between signal and idler modes. This insight will have immediate impact on the design of TWPAs as both quantum-limited amplifiers and sources of squeezed radiation.

This study theoretically predicts and experimentally demonstrates the capability for standoff measurement of local vector magnetic fields, based on depolarization of microwave scattering. Since scattering only occurs after formation of a localized plasma via an ultrashort laser pulse, the measurement has both high temporal and spatial resolution, and can be performed remotely. This approach, which could be extended to other frequencies, is expected to interest those working on electric propulsion, dynamic magnetic field confinement, and low-temperature physics, where the spatial distribution of a magnetic field is often complex, time-varying, and not measurable with conventional probes.

All shook up: Ultrasound is widely employed in medicine, chemistry, and industry. The effects of ultrasound include mechanical shaking and the thermal effect and shear forces produced by acoustic cavitation, all of which are intensity-dependent. Unfortunately, it has been very difficult to produce uniform ultrasound intensity, especially in the acoustic near field—but it just got easier. The authors present an innovative transducer that can completely eliminate the intensity variation in the near field, producing a three-dimensional region of uniform ultrasound, for $e.g.$ more reliable experimental data and improved sonochemical and therapeutic ultrasound.

Microresonator frequency combs continue to draw great interest, due to the nonlinear physics that they can illustrate, and their relevance to field-deployable optical clocks. To push the boundaries, the authors study them at cryogenic temperatures, where the thermorefractive coefficient is decreased 100-fold. This gives easy access to the phase-stable soliton regime, and brings experiment closer to theoretical ideality. By varying temperature, thermorefractive effects are controllably reintroduced, elucidating the competition with Kerr effects and validating theory. Cryogenic operation also yields a dramatic drop in the thermodynamic noise that currently limits microcomb stability.

The identification of unconventional physical systems for neuroinspired information processing comes with much-desired capabilities for powerful data analysis. Moreover, it offers the possibility to replicate complex dynamical behavior under closed-loop operation. Robust, reliable autonomous replication of the structure of complex dynamics has remained elusive, though. The authors experimentally demonstrate the advantage of pretraining a system with additive input noise, achieving robust operation over an extended parameter range, and introduce quantifiers for the quality of replication.

Generating indistinguishable single photons is often a key step in photonic quantum information processing. To this end, the design of a nonlinear crystal’s poling profile involves many parameters, while performance is also restricted by the pump spectrum, and traditional optimization algorithms cannot efficiently keep up. However, with the benefit of an elegant machine-learning framework, the authors can simultaneously optimize poling profile and pump spectrum to produce high-purity single photons over the whole telecommunication band. Periodic peaks in the machine-learning poling profile cleverly compensate for group-velocity mismatch and other nonidealities.

Vortex waves have potential applications in wireless communication, because of their advantages in improving channel-transmission efficiency. However, the peculiar divergence of vortex waves seriously limits their propagation distance for communication. Thus the authors propose an innovative method for collimating vortex waves at microwave frequencies, based on transformation optics. Their all-dielectric device can reduce the divergence angle of vortex waves over a wide bandwidth, and collimate vortex beams with multiple modes.

A special class of many-body quantum systems can be simulated by probabilistic algorithms running on digital computers, but generating correlated random numbers is computationally expensive. The authors propose an asynchronous probabilistic coprocessor that uses a slightly modified cell structure in the emerging magnetoresistive RAM technology, which should accelerate such probabilistic algorithms by several orders of magnitude, in terms of sampling speed and energy. This approach complement existing efforts to simulate quantum systems by using scalable, room-temperature building blocks.