The orbital angular momentum (OAM) spectrum of light, produced by superposition of vortex beams, is important for applications in optical metrology and classical and quantum communication. Nonetheless, manipulation of the OAM spectrum remains challenging. The authors propose a technique for control of the OAM spectrum using structured pinhole plates, which can generate both wide and narrow OAM spectra. In particular, a rather simple pinhole plate can produce an interesting series of discrete, equally spaced OAM modes: an OAM comb, akin to a frequency comb. This approach could be extended beyond photonics to work with rf, acoustic, electron, or neutron composite vortices.

Piezoacoustic drop-on-demand inkjet printing is widely applied in high-end digital printing, due to its remarkable precision and reproducibility. The stability of such printing can be compromised, however, by stochastic entrainment of bubbles in the ink channel. Here bubble nucleation, translation, and growth in an experimental printhead are visualized at microsecond resolution, using high-speed imaging triggered by changes in the ink channel’s acoustics. Impurities in the ink are trapped in the vortical flow close to the nozzle, and can induce bubble nucleation at the oscillating meniscus. These insights should lead to better waveform and nozzle designs, for improved inkjet printing.

The optimal design of permanent-magnet systems is crucial for applications such as electric motors and generators, accelerator magnets, and MRI scanners. One challenge in achieving best performance is to compute the optimal segmentation into uniformly magnetized blocks, but typical numerical techniques cannot handle three-dimensional (3D) problems. The authors demonstrate a versatile analytical method, using a surprising connection to the problem of tessellating a 2D spherical surface to study 3D problems. This insight is of interest beyond the development of permanent magnets, bringing geometric methods to bear on a widely studied problem in magnetostatics.

Understanding the interactions that couple gate-defined quantum-dot qubits is an important step to using such devices in quantum computing. For double-quantum-dot qubits with an effective charge dipole moment, a capacitive dipole-dipole interaction can yield coherent coupling between neighboring qubits. Here researchers reveal the tunability of this capacitive-coupling energy with applied gate voltages in a quadruple-quantum-dot array, tuning the coupling energy from 15 to 32 GHz. Modeling the system as a network of charge nodes joined by capacitors, the authors demonstrate how the capacitive-coupling energy between pairs of double dots depends on the various capacitances in the network.

$Reversible$ $computing$ envisions conservative information processing with zero energy dissipation, but the large sizes and energy costs of previously proposed information carriers have impeded the development of practical systems. This study proposes a scalable solution for reversible computing based on magnetic skyrmions, nanoscale whirls of magnetization that can be propagated with minimal energy to perform nonvolatile logical operations. By applying a simple, global clocking scheme to synchronize skyrmion motion, Boolean and quantum logic gates can be directly cascaded and integrated into large-scale, pipelined reversible-computing systems.

By combining phased-array technology with recent developments in acoustic holograms, the authors enhance the capabilities of ultrasonic particle manipulation while reducing the cost. A 3D-printed hologram enables them to create high-fidelity force fields and manipulate small objects along complex paths. Attached to this hologram is an array of independent sound sources; controlling the output delay (phase) of these sources allows movement of the force field in space, yielding noncontact “tweezers” that can trap particles of almost any shape. Applications include 3D-bioprinting of cells to create designer tissues, as well as holding microorganisms in place for careful biological study.

The acoustic radiation force and associated trapping by “acoustic tweezers” is an efficient means of particle manipulation in a variety of applications, although this trapping is highly sensitive to parameters of the object and beam. The authors show how to extract and manipulate droplets from a fluid interface without mechanical contact, using near-field acoustic tweezers, with the near-field pressure gradient allowing automatic trapping after extraction. The strong, flexible trapping here is an advantage over traditional tweezers based on focused beams or standing waves, and can be of great importance in both laboratory and industrial settings.

Controlling the direction of fluid flow is challenging, since the flow direction follows the applied external forces. To this end, lately $hydrodynamic$ $metamaterials$ have emerged, enabling arbitrary control of the fluidic forces based on transformation hydrodynamics, for spatial viscosity mapping. This study theoretically and experimentally demonstrates fluid flow rotors that can arbitrarily manipulate flow direction using transformation hydrodynamics. The metamaterial microfluidic device developed in this study is expected to inspire academic and industrial endeavors involving fluid dynamics.

Using heat that otherwise would be lost is important for efficient use of solar energy, but continues to be held back for lack of an ultrathin photovoltaic absorber that permits both sufficient absorption of light and ultrafast extraction of “hot” carriers before they lose their kinetic energy. This article presents a concept to enable heat recovery in a solar cell $without$ the need for ultrafast carrier extraction. This thermoelectric approach should impact engineering solutions for further improvement in silicon solar cells, beyond the Shockley-Queisser limit.

A particular type of semiconductor photocathode with a nanostructured surface is shown to increase photoemission yield by more than a factor of three, compared to a flat photocathode. This improvement stems from the excitation of Mie resonances, which serve to reduce reflectivity and enhance light absorption near the surface, where electrons are more efficiently emitted. Nanostructured photocathodes, which can be described as “electrically bright” yet “optically dark”, could benefit many applications, including high-current electron cooling of hadron beams in particle accelerators.

Antiferromagnets are insensitive to external magnetic fields and exhibit ultrafast magnetization dynamics, which are desirable properties for advanced data storage. The Napos{e}el-order spin-orbit torque is a means to manipulate the antiferromagnetic state, and thus could serve as the writing mechanism in future spintronic memory cells. This study investigates in depth the switching characteristics of a sputtered antiferromagnetic alloy, and presents a general scheme for analyzing the switching of arbitrary antiferromagnets. The results highlight the significance of thermal activation in this switching mechanism, which is crucial for designing and optimizing devices.

Reducing the write-error rate (WER) is one of the main concerns for commercialization of spin-transfer-torque magnetic random-access memory (STT-MRAM). While most studies have been focused on finding proper material, this work discusses the use of limited stochastic (LS) current, for which WER is controlled. By solving the Fokker-Planck equation, an analytical expression for the LS current for a given WER is obtained. Most importantly, by using the LS current and optimizing it together with the related pulse duration, the authors find the optimum combination of current amplitude and pulse duration, which can reduce the energy consumption of STT-MRAM by up to 75%.

The nitrogen-vacancy center (NVC) in diamond is an exciting candidate system for building a quantum computer, because of its long coherence time at the relatively high temperature of 5 K. Scaling up to a useful quantum computer requires having an array of many individual NVCs in one diamond; such arrays can be created, but suffer from much shorter coherence times than in natural NVCs. This study uses laser writing to create arrays of single NVCs with “natural” spin coherence times, at the right depth for use with optical cavities, enabling arrays of 200,000 coherent NVCs per diamond.

Practical, large-scale quantum computers require active error correction, but for this to work the components must attain fidelities that are very challenging. Here the authors customize the leading “surface code” approach, layering it on top of a second, simpler code to detect the most common type of noise. Information from the lower-level code is fed into a specialized, advanced controller for the higher-level code so that it can make smarter choices. Numerical simulations confirm that the authors’ code is superior over a wide range of parameters, in terms of noise and hardware connectivity. This work offers a potential path to scaling up quantum hardware under biased noise.

The orbital angular momentum (OAM) spectrum of light, produced by superposition of vortex beams, is important for applications in optical metrology and classical and quantum communication. Nonetheless, manipulation of the OAM spectrum remains challenging. The authors propose a technique for control of the OAM spectrum using structured pinhole plates, which can generate both wide and narrow OAM spectra. In particular, a rather simple pinhole plate can produce an interesting series of discrete, equally spaced OAM modes: an OAM comb, akin to a frequency comb. This approach could be extended beyond photonics to work with rf, acoustic, electron, or neutron composite vortices.

Full waveform inversion (FWI) of the shapes of impenetrable obstacles has broad value in $e.g.$ ultrasonic characterization of cracks and voids, seismic-wave inversion of salt bodies, and sonar, as well as parallel topics for electromagnetic waves. The success of FWI for shape reconstruction has been rather limited, though, due to the difficulty of realizing target convergence. The authors propose an FWI scheme to directly invert the geometrical parameters of an impenetrable scatterer, using elastic-wave finite-element formulations. Their iterative approach provides a powerful means to reconstruct the shapes of impenetrable scatterers from full waveform measurements in an automated way.

Invisibility cloaks designed by optical conformal mapping work only at some discrete frequencies, due to the phase-delay problem. The present work solves the phase-delay problem by using an optical null medium in the lower Riemann sheet. This approach has many advantages, such as a continuous working-frequency band (without geometrical dispersion), omnidirectionality, robustness under frequency shift, and no need for complex materials that are anisotropic and inhomogeneous. The technique offers a realistic means to an omnidirectional conformal cloak.

Optically pumped magnetometers are serious candidates to replace SQUIDs, and thus avoid the need for cryogenics, in applications like magnetoencephalography. In building magnetometer arrays for imaging, a compact yet multiaxis architecture is highly desirable. Here the authors present a scheme based on atomic alignment that only requires one optical access point, bearing both a pump beam and a probe beam at a small angle to it, and that allows measurement of two magnetic field components. Measuring the third component should be possible via partial depolarization of the pump beam. Such a compact scheme opens interesting perspectives for magnetometer arrays for medical imaging.

Plasma-based electron/positron wakefield acceleration has made great strides in the past decade. Currently, a major challenge to its application in coherent light sources and linear colliders is the relatively large beam-energy spread, dominated by the energy chirp. The authors propose a nearly ideal dechirper based on a low-density hollow channel plasma to reduce the beam energy spread down to the 0.1% level or even lower, while maintaining beam emittance. This approach may significantly improve the beam quality of plasma-based accelerators, paving the way for compact free-electron lasers and colliders.

Charge trapping across multiple interfaces is a universal, important process in semiconductor devices such as high-$\kappa$ MOSFETs and nonvolatile memory. However, a straightforward simulation framework for such complicated physical processes, and systematic studies of them, are lacking. The authors combine first-principles calculations and Marcus charge-transfer theory in an optimized simulation framework, and apply it to Si/SiO${}_2$/HfO${}_2$ gate stacks to characterize hole trapping in high-$\kappa$ gate transistors. This insight and framework could be helpful for studying charge trapping in other semiconductor devices as well.

Implementing high-fidelity entangling operations between qubits is a key challenge in quantum computing. Multiple sources of error and a large parameter space make optimization of control schemes for high-fidelity operations a complex technical challenge. This study presents a complete recipe for calibrating a high-fidelity cross-resonance gate implemented between two dispersively coupled transmon qubits, showing that even large amounts of crosstalk can be effectively canceled. This approach should be useful as a general tool for those working on experiments with coupled quantum systems, in particular for computing.

One-way edge states are emerging as a robust way to control light in $topological$ $photonics$, in which a high gap Chern number is essential for generating multiple one-way edge states for multiplexing and demultiplexing circuits. Contrary to approaches using gyrotropic photonic crystals, this study shows how reciprocal continuous media with a gauge field and chirality can be used to generate a gap Chern number greater than 1, and to support a maximum of 4 one-way edge states. Such media can be further used to construct one-way signal combiners and splitters, in which branching and transmission efficiencies can be accurately controlled.

Transient changes in polarizability alter the phase-matching conditions in nonlinear optical processes in a gas-filled hollow-core photonic crystal fiber. Understanding these polarizability changes is crucial for describing light-matter interactions in $e.g.$ high-field laser science. Although techniques to detect these changes exist, they are complex and have not been proven to work in confined geometries. In this study, the wavelength shift of the dispersive wave emitted by a soliton in a fiber is used to probe the plasma at the temporal focus of a counterpropagating pump soliton. By varying the delay between pump and probe, the buildup of plasma is monitored and its spatial profile reconstructed.

For simplicity and efficiency, metasurfaces need to be constructed of passive and lossless material. On the contrary, a coherent perfect absorber (CPA), and its time-reversal device the laser, require lossy active materials. Thus ideas from CPA design have seldom been extended to metasurfaces. This study shows that the combination of CPA and laser can also work as an efficient wavefront-manipulating device, if the wave energy absorbed by the CPA can be somehow recovered and then re-emitted by the L]laser. Not only could this approach greatly simplify metasurface design, but it could also bridge research efforts on metasurfaces and non-Hermitian systems.

Highly nonlinear, low-loss, 50-Ω transmission lines are highly desirable for the development of microwave quantum optics, and could be used directly as ultralow-noise traveling-wave parametric amplifiers. Unfortunately, an accessible, widespread fabrication technique is lacking. This study shows that a simple, affordable process based on SQUID arrays is possible. The authors present detailed characterization of such transmission lines and demonstrate $in$ $situ$ tuning of the characteristic impedance. This result should promote research in microwave quantum optics worldwide by broadly increasing access to the necessary traveling-wave structures.

Undergraduate coursework familiarizes us with heat engines—but not like these. This study identifies an innovative type of radiative heat engine and derives its fundamental efficiency and power bounds. The findings include the surprising possibility that certain classes of radiative heat engines, such as thermoradiative diodes, can exceed the Landsberg efficiency limit and approach the Carnot limit. Conventional radiative heat engines have a converter coupled to a cold reservoir and absorbing external hot (e.g. solar) radiation to produce work, but here the situation is flipped, with the converter on the hot side generating work from its own radiative emission to the cold side.

Piezoacoustic drop-on-demand inkjet printing is widely applied in high-end digital printing, due to its remarkable precision and reproducibility. The stability of such printing can be compromised, however, by stochastic entrainment of bubbles in the ink channel. Here bubble nucleation, translation, and growth in an experimental printhead are visualized at microsecond resolution, using high-speed imaging triggered by changes in the ink channel’s acoustics. Impurities in the ink are trapped in the vortical flow close to the nozzle, and can induce bubble nucleation at the oscillating meniscus. These insights should lead to better waveform and nozzle designs, for improved inkjet printing.

What if we could use small devices to protect buildings from earthquakes? Unfortunately, it is very difficult to trap complex elastic waves over a broad band of low frequencies. This paper addresses the problem by using an omnidirectional concentrator designed via a special transformation method. Experimental results demonstrate that a prototype just 20 cm in diameter can absorb complex elastic waves efficiently in the range of 10–800 Hz. Thus the longstanding dream of seismic protection might be realized by using the “elastic black hole” presented here.

Measuring the concentration of low-abundance biological nanoparticles or biomolecules suspended in liquid is both important and challenging. The authors show how viruslike particles can be detected by using their ability to induce aggregation of fluorescently labeled lipid vesicles of two different colors. This approach allows a detection limit of a few picomolar in a mere 30 minutes, employing a simple videomicroscope for readout.

To scale up the number of qubits in a quantum processor, it is important that each qubit, or pair of qubits, can be controlled individually. Flux crosstalk between frequency-tunable superconducting qubits means that assumptions about addressability may no longer be valid. In this study, the authors detail several methods for measuring flux crosstalk between transmon qubits. These crosstalk metrics can then be used to predict $simultaneous$ performance of two-qubit gates, and thus employed as engineering milestones on the path to truly scalable quantum computers.

Transparent conducting oxides and graphene have dominated research on optical modulation in recent years. Regrettably, studies are often unrealistic, which hinders a clear understanding of the state of the art. The authors revisit both material technologies to present an equitable, quantitative comparison on common grounds, rigorously modeling their shared physical principles. Both inline and resonant configurations are examined from the ground up, using a silicon photonic platform as underlying structure. The proposed modulators are thoroughly compared in terms of performance, demonstrating high-quality switching with rates exceeding 100 GHz.

Generation of “squeezed” light is a key technology for quantum information processing with continuous variables. In this field, continuous-variable quantum sampling is a promising candidate for near-term demonstration of quantum advantage. Despite many years of progress, though, a squeezed-light source suitable for quantum sampling has not been demonstrated. This work finally provides a blueprint for squeezed-light sources that can be used for large-scale quantum sampling applications, and thus will have an impact on advancing photonic quantum technology for practical deployment.

The crystallographic characterization of phosphorene is essential for sample preparation, edge-geometry classification and identification, and transport measurements of nanostructures made from this two-dimensional material that lies “beyond graphene”. This study reveals that there may be more varieties of phosphorene nanostructures than were previously thought to exist, including a type of nanoribbon with interesting atomic configurations at its edges. This insight is expected to enable engineering solutions in phosphorene-based nanoelectronics.

While nonpairwise couplings are hard to implement in electronic systems, because the apparent interactions are two-body, such higher-order interactions could be beneficial in various applications. Studying a system of coupled artificial atoms (imagined as flux qubits), the authors show that $four-body$ local interactions appear via virtual coupler excitations, and that they can even be tuned into the strong-interaction regime. This theoretical exploration should serve as a starting point for circuit implementations of higher local interactions, especially in the context of adiabatic quantum computing.

Ultrathin electromagnetic cloaks to reduce backward scattering at microwave wavelengths are in demand, for their promise in $e.g.$ radar detection, as well as concealment. Concealing wavelength-scale objects with sufficient bandwidth at tiny cloak thicknesses is still challenging, though. This study experimentally demonstrates a composite strategy to design an ultrathin microwave conformal metasurface cloak for efficiently reducing backscattering from a wavelength-scale cylinder, with improved bandwidth. These results will have an impact on many applications, such as electromagnetic isolation and camouflage.

This proposal opens a way to applications of the recently demonstrated exciton-polariton topological insulator by suggesting a more practical and compact device design. The authors suggest replacing bulky external magnets with magnetic quantum dots embedded close to the layers of the quantum wells in the system; the proximity of these quantum dots to the excitons provides a local magnetic field that is strong enough to create unidirectional, topologically protected exciton-polariton edge modes. This approach offers significant practical advantage by eliminating the strong external magnetic field and cryogenics required in the previous experiments.

Nanoscale electrooptical modulation is key to making active metasurfaces, nanophotonic sensors, filters, and switches offering simple structures, easy production, and low loss. Progress has been stymied, though, because of the huge loss associated with using metals, and adverse effects caused by extra interlayers. Taking advantage of the chemical affinity between ZnSe and a particular liquid crystal (LC), perfect vertical alignment in LC cells is readily attained using nanoscale ZnSe films as aligning layers. This study provides guidance for designing low-loss active metasurfaces and could promote the engineering of surface-plasmon-based integrated circuits and photoelectric detectors.

The authors propose $transformation$ $caustics$, by applying coordinate transformation to the caustic effect (a result of reflection or refraction that yields a curved region of concentrated light intensity along the curve to which each ray is tangent). The boundaries of caustics could be used to design devices offering asymmetric light propagation or confinement, providing a further degree of freedom for metamaterials. Such effects would promote device applications like optical switches, energy concentrators, and microcavities.

The Dzyaloshinskii-Moriya interaction (DMI) is a very interesting phenomenon for creating devices to exploit spin chirality, but enhancing DMI in artificial structures is still very challenging. This comprehensive investigation presents several possible approaches to manipulating interfacial DMI in magnetic multilayers. These results are expected to facilitate the engineering of large DMI for spintronic applications.

Electrostatic force microscopy (EFM) is a versatile tool that provides valuable surface information, including local surface potentials, work functions, and doping profiles. Here the authors show that EFM provides a fingerprint of the constituent layered materials in complex van der Waals heterostructures of graphene, hexagonal boron nitride, and transition-metal dichalcogenides. In addition to providing visually striking images of buried layers, the technique is also useful for probing the electrical properties of the constituent layers. This imaging approach will be helpful in advancing reliable, high-throughput device architectures for various optoelectronic applications.

Magnetic skyrmions and skyrmionium (a pair of skyrmions with a net topological charge of zero) in ferromagnetic systems continue to be studied intensively, due to their potential for application in spintronic devices. However, the physics and possible applications of their $antiferromagnetic$ (AFM) counterparts still need to be explored. This combined analytical and numerical investigation of the current-driven dynamics of these AFM topological structures reveals the similarities and subtle differences in the influence of AFM texture inertia on skyrmion and skyrmionium dynamics.

The optimal design of permanent-magnet systems is crucial for applications such as electric motors and generators, accelerator magnets, and MRI scanners. One challenge in achieving best performance is to compute the optimal segmentation into uniformly magnetized blocks, but typical numerical techniques cannot handle three-dimensional (3D) problems. The authors demonstrate a versatile analytical method, using a surprising connection to the problem of tessellating a 2D spherical surface to study 3D problems. This insight is of interest beyond the development of permanent magnets, bringing geometric methods to bear on a widely studied problem in magnetostatics.

Efficient generation of spin current in ferromagnet/heavy-metal systems is critical to spin orbitronics, but the interface limits the efficiency of transmitting spin current, and weakens the effective spin Hall angle of the heavy-metal layer. In this study the authors enhance the spin-mixing conductance in such a multilayer by inserting an atomically thin layer of $\alpha$-W at the interface, and propose a corresponding model that considers the spin-backflow current. This approach is expected to further reduce the charge-current density and realize power-efficient spin-orbit-torque devices based on the important Co-Fe-B/W system.

Sensors based on the Delta-E effect can detecting low-frequency and low-amplitude magnetic fields, which makes them potentially interesting for applications in biomagnetism, which require high sensitivity and low noise levels. This work analyzes the high magnetic sensitivity for different resonance modes of cantilever sensors. It is found that the Delta-E effect significantly depends on the resonance, with second-order bending modes increasing the magnetic sensitivity compared to first-order modes. The weighting of local properties by mode shape is not only key to understanding and modeling the Delta-E effect, but also an important factor in improving sensitivity.

Quantum technologies, including those based on superconducting quantum circuits, require high-fidelity high-speed detection of the quantum state of a qubit. Standard quantum nondemolition readout of a superconducting qubit is based on its dispersive coupling to a resonator, but this method suffers from the Purcell effects: Purcell decay, critical photon number, and qubit-dependent Kerr nonlinearity. The authors propose a method in which both speed and fidelity of flux-qubit readout can avoid all three Purcell limitations.

Ultrathin asymmetrically sandwiched ferromagnetic films can support magnetic skyrmions and skyrmion bubbles, which are eyed as functional units for tomorrow’s spintronic logic and memory devices. The type of object and its size and mobility are set by intrinsic magnetic parameters, which are largely unknown. The authors demonstrate that the exchange stiffness of ultrathin films can be determined by analysis of the temperature dependence of magnetization. This simple yet robust approach is crucial not only for the experimental community but also for theorists, as it determines domain-wall thickness, the period of a Dzyaloshinskii spiral, and the characteristic magnetic length.

Charge transport in organic semiconductors, especially photovoltaics, has predominantly been analyzed using nearest-neighbor hopping (nnH) models. Available variable-range hopping (VRH) models suffer from significant “user-unfriendliness”, and the errors due to ignoring long-range hopping remain unknown. Here the authors propose a VRH model, calibrated with kinetic Monte Carlo simulations, that consistently describes the transport in both hole- and electron-only devices; thus a threshold for the applicability of nnH models is established. Surprisingly, there is no clear correlation between the established energetic disorder in the active material and photovoltaic-device performance.

The exotic transport properties of topological insulators are very appealing for device applications, but the details of the physics in particular systems still need to be established. Using the theory of equilibrium Green’s functions and topological phase transitions, the authors design several types of spintronic devices based on the properties of topological edge states in silicenelike nanoribbons, finding good spin-filter performance for these three-segment nanojunctions. Of particular interest are the discovery of Fano and Fabry-Perot resonances in these devices, and of course robustness against disorder and dephasing.

Life in an imperfect world: While fault-tolerant quantum computing is an ultimate goal, it is far off, so for now we need to find ways to mitigate the errors that creep into quantum computations. Quantum error correction is of key interest, both theoretically and practically. The authors advance the field by considering error-corrected quantum simulation of geometrically local fermionic systems, providing a systematic way to construct error-correcting code using methods from lattice gauge theory, with the gauge operators serving as stabilizers. This method solves two major problems (geometry locality and error mitigation) in near-term quantum simulations of lattice fermion problems.

Superconducting thin films underpin the function of a large variety of devices. As a promising and intriguing system with tunable electronic properties, lab-grown diamond films exhibit superconductivity upon heavy boron doping. This work reveals an anomalous superconducting anisotropy in such films: In contrast to other superconducting thin films featuring a larger in-plane upper critical field, these show a larger $out-of-plane$ upper critical field, due to quantum confinement correlated with crystallite geometry. This study provides physical insight for developing nanodiamond-based superconducting quantum devices, by exploiting grain or twin boundaries.

For effective quantum communication, the security of the photon source is particularly important in the era of measurement-device-independent quantum key distribution (MDI-QKD) and twin-field QKD (TF-QKD). In practice, the security of the source can still be cracked by an adversary. This study experimentally demonstrates that a practical source based on a semiconductor laser diode is vulnerable to a laser-seeding attack, in which light injected from the communication line into the laser yields increased intensities of the prepared states. Theory shows that the unnoticed intensity increase compromises the security of the prepare-and-measure decoy-state BB84 and MDI-QKD protocols.

In optical quantum cryptography, alignment error is important in secure key generation for quantum key distribution (QKD), being one of the roots of error events. However, theoretical modeling typically only reflects the behavior of a single photon, which is incompatible with a weak coherent source. This study develops a realistic model to include the propagation of multiphoton pulses in the misalignment-error analysis by defining the $leakage$ $light$ $ratio$, which can be obtained conveniently in practical QKD systems. In addition, double-click events can be depicted precisely, and thus the gap between the model and real systems is narrowed.

Just around the corner… Unscrambling the image reflected from a scattering surface is a real challenge, with numerous potential applications along the lines of seeing past obstructions. Two methods exist to tackle this problem: In one a coherence property of light is used to recover the image from apparently random reflected intensity data, while the other uses the time-of-flight information of the photons to triangulate the object. The authors combine these approaches, implementing coherent control of the input laser beam for the time-of-flight reconstruction, and preserving the advantages of both techniques.

Current-induced spin textures in antiferromagnets (AFMs) are of great importance and interest, not only for a detailed understanding of the underlying physical phenomena, but also for magnetization switching using AFMs in spintronics. Direct visualization of such spin textures in AFMs has remained elusive, however. This study uses a scanning photovoltage microscope to image current-induced spin accumulation in (001)- and (111)-oriented IrMn${}_3$. In contrast to the conventional spin Hall phenomena, significant out-of-plane spin accumulation is observed at the top surface in the (001) case. This work promotes AFM-based devices exploiting spin-orbit torque without a magnetic field.

Hybrid spin systems such as the N-$V$ center in diamond are promising candidates for building quantum devices,as they combine useful properties like fast operation based on electron spins, and long storage times based on nuclear spins. Their properties also pose challenges, including the slow response of nuclear spins to external control fields. Here the authors experimentally implement a scheme that allows them to control the nuclear spins of an N-$V$ center system indirectly, via control operations applied to the electron spin. This versatile approach can be implemented over a wide range of magnetic field strengths, and at any temperature.

Interleaving fin structures allow adaptive thermal control, and thus offer a way forward to innovative, energy-efficient devices such as heat switches in hyperpolarized MRI systems, redundancy systems for space technologies, and cryogenic fixation of cells. Unfortunately, an accurate, physics-based prediction of heat transfer in these structures, accounting for the interplay of geometry and temperature-dependent material properties, is lacking. The authors identify that the finite conductivity of the fins can result in a strong reduction in heat-transfer rate. They develop a one-dimensional model to describe this effect, and identify a dimensionless number to predict it.

Understanding the interactions that couple gate-defined quantum-dot qubits is an important step to using such devices in quantum computing. For double-quantum-dot qubits with an effective charge dipole moment, a capacitive dipole-dipole interaction can yield coherent coupling between neighboring qubits. Here researchers reveal the tunability of this capacitive-coupling energy with applied gate voltages in a quadruple-quantum-dot array, tuning the coupling energy from 15 to 32 GHz. Modeling the system as a network of charge nodes joined by capacitors, the authors demonstrate how the capacitive-coupling energy between pairs of double dots depends on the various capacitances in the network.

Single-atom transistors hold great promise for reducing semiconductor devices to their ultimate scale, approaching atomic dimensions. Typically these transistors use dopant atoms embedded in a semiconductor to form quantum-dot switches; unfortunately, in this approach shallow potential wells restrict device operation to cryogenic temperatures. This work extends the operation of single-atom transistors to $room$ $temperature$, by embedding dopant atoms in SiO${}_2$ tunnel barriers to form deep quantum wells. Nearby dopant atoms can communicate electrostatically, forming double quantum dots, opening a route toward practical, room-temperature atomic-scale quantum electronics.

The authors present a family of compact, versatile, and deterministic sources of quantum light with user-selected properties. These photon guns (“PhoGs”) will enhance performance in many quantum technologies by providing a ready alternative to attenuated quantum coherent states, which are normally used for their convenience. PhoG devices are based on engineered nonlinear loss in dissipatively coupled optical waveguide networks, with the “cheap” attenuated coherent states as input. In different regimes, a PhoG acts either as a deterministic source of highly sub-Poissonian light, or as a source of entangled photons in different state configurations.

Spontaneous orientation polarization and the resulting giant surface potential of many organic molecules, especially electron-transport materials, have been known for years, yet various aspects of their influence on device performance have not been investigated thoroughly. In this study, the authors apply the concept of “dipolar doping” (diluting a polar species in a nonpolar matrix) to systematically tune the orientation polarization of a prototypical OLED layer stack. They find that a low concentration of a polar species (within the range of usual dye doping in OLEDs) can enhance carrier injection by an order of magnitude.

$Reversible$ $computing$ envisions conservative information processing with zero energy dissipation, but the large sizes and energy costs of previously proposed information carriers have impeded the development of practical systems. This study proposes a scalable solution for reversible computing based on magnetic skyrmions, nanoscale whirls of magnetization that can be propagated with minimal energy to perform nonvolatile logical operations. By applying a simple, global clocking scheme to synchronize skyrmion motion, Boolean and quantum logic gates can be directly cascaded and integrated into large-scale, pipelined reversible-computing systems.

The exotic spin textures known as skyrmions are important for memory and logic applications, due to their robustness against external perturbations. Skyrmions have been considered to exist only in perpendicular magnetic domains, but this study shows that they may also exist in $in-plane$ domains, as seen by employing the rotation transformation (a type of continuous transformation). The peculiar feature of in-plane skyrmions is that textures of opposite “charge” can coexist within the same domain. This work notes the possibility of double-bit transfer on a single track in a memory device.

By combining phased-array technology with recent developments in acoustic holograms, the authors enhance the capabilities of ultrasonic particle manipulation while reducing the cost. A 3D-printed hologram enables them to create high-fidelity force fields and manipulate small objects along complex paths. Attached to this hologram is an array of independent sound sources; controlling the output delay (phase) of these sources allows movement of the force field in space, yielding noncontact “tweezers” that can trap particles of almost any shape. Applications include 3D-bioprinting of cells to create designer tissues, as well as holding microorganisms in place for careful biological study.

A high-efficiency nonlinear source of visible light in a multiple-quantum-well (MQW) metasurface is in high demand for photonic quantum technology, but existing systems suffer from the limited conduction-band offset of MQWs and high dissipative losses of metal, which severely limit applicability in the visible range. This study presents a simple, reliable system without metal losses, by utilizing the interband excitonic transitions of MQWs and the Mie resonances of the metasurface’s structure. This shows a viable path toward a coherent, nonlinear light source for use in the visible region and beyond with high conversion efficiency, to enable nanophotonic quantum information processing.

The passive technology described in this paper is important for reducing the thermoelastic response of silicon-based high-precision accelerometers and gravimeters. This study uses the angular strain induced by mismatched coefficients of thermal expansion to passively compensate for the sag displacement in Si-based spring-mass systems in environments with fluctuating temperature. The hundredfold reduction in temperature sensitivity allows a top acceleration sensitivity of less than 10${}^{-9}$ $g$. Such a temperature-compensated seismic accelerometer has already been used as a science payload of the InSight Mars lander, which touched down in November 2018, for marsquake monitoring.

A coherent interface between temporal and spatial degrees of freedom (DOFs) empowers photons to take advantage of both DOFs in quantum information processing. Such interfaces have either suffered from a typical loss of 1–3 dB or disturbed the polarization DOF in the conversion process. The authors realize a low-loss, polarization-independent, coherent interface between temporal and spatial DOFs (specifically time-bin and path modes), which promises applications like the postselection-loophole-free Bell inequality test, and can be put into a cavity to boost the generation rates of multiphoton polarization-entangled sources, or to enhance the scalability of sequential quantum metrology.

All the noise, noise, noise, noise… Attenuating broadband, low-frequency acoustic noise remains a longstanding societal need. Helmholtz resonators are often employed for sound abatement, yet their rigid enclosures yield narrowband sound absorption. This report investigates a soft resonator in which the structural response, acoustic field, and material properties synergistically combine to induce broadband, low-frequency, nearly total sound absorption. Analysis and experiment uncover the mechanisms that create such multiphysics coupling and subwavelength sound attenuation in soft matter, and the findings may inspire further concepts for lightweight sound-absorbing materials.

Optical trapping is important in assessing the deformability of red blood cells (RBCs), providing quick, noncontact force measurement in liquid medium with piconewton resolution. Traditional techniques, though, can involve complicated processes or cause photodamage. This study employs “tug-of-war” (TOW) optical tweezers with object-adapted optical potentials, to simply and stably trap, stretch, and squeeze a single RBC under different osmotic conditions, thus enabling the assessment of RBC deformability. The large forces of the TOW tweezers can cause larger deformations than traditional dual-trap tweezers at the same power conditions, which can greatly reduce photodamage.

Although two-dimensional (2D) semiconductors are the focus for next-generation field-effect transistors, it is still difficult to produce good, simple electrical contacts with these materials. The authors use density functional theory to study the Schottky-barrier height under the influence of a vertical external electric field, and demonstrate that NbS${}_2$ is a promising electrode for achieving $p$-type Ohmic contact with WSe${}_2$ and BP monolayers, with trivial dependence on external field. This suggests that with metallic NbS${}_2$ monolayer as the electrode, the influence of gate voltage on electrode-channel interfaces can be greatly suppressed, which has real impact on circuit design.

Orbital angular momentum (OAM) of electrons in free space must be omnipresent, but unfortunately we lack efficient equipment and techniques to measure the OAM state of an electron beam. This work demonstrates that a nanofabricated diffraction grating can be used as a device to address this problem. The authors present a protocol for obtaining an electron beam’s OAM spectrum, and experimentally demonstrate how this approach can be used in a variety of applications, including those with magnetic materials. The results will have an impact on a great range of studies of electron OAM, and of other quantum waves, too.

Strong enhancement of sound scattering is important for some applications of acoustic metamaterials, such as sensing and acoustic antennae. Here the authors create a meta-rod structure to achieve the $sound$ $superscattering$ effect. The near-degeneracy of resonances in multiple channels of the meta-rod can break the so-called single-channel limit of subwavelength structures, and thus achieve extremely strong sound scattering, even in the deeply subwavelength regime. The mechanism behind superscattering of sound turns out to be more robust than that for light, so there are not stringent fabrication requirements for the proposed structure, which is important for practical applications.

The photoemission process is generally surface sensitive, and therefore is not typically applied to the examination of buried structures or interfaces. Using photoemission electron microscopy with deep-ultraviolet excitation, this study exploits the formation of optical standing waves to enable visualization of atomically thin MoS${}_2$ tens of nanometers below the surface of dielectric films. This approach is potentially useful for imaging buried nanostructures or nanomaterials, a keenly sought capability in diagnostics for optoelectronic components.

While frequency combs have become the state of the art in spectroscopy and high-precision measurements, they remain notoriously hard to produce in miniaturized silicon ring resonators at telecommunication wavelengths. The authors propose the generation of frequency combs using coupled-cavity waveguides, where the spectral properties can be engineered to produce anomalous dispersion and enhancement of optical nonlinearity by “slow light”. Results show that silicon devices of this kind may efficiently produce combs at telecom wavelengths, even in the presence of nonlinear losses, and thus hold great promise for integrated silicon photonics.