Hybrid quantum networks of stationary and “flying” qubits are essential for distributed quantum information processing. In superconducting quantum computation, two-qubit gates are currently realized by the interaction between neighboring qubits. The authors propose a gate comprising a superconducting “atom” and a microwave photon, in which gate operation is completed $deterministically$ (not probabilistically) upon reflection of the photon. This gate’s type can be continuously varied $in$ $situ$, enabling $remote$ entanglement of many “atoms” via a single photon, or creation of a quantum domino effect.

The efficiency of (In,Ga)N light-emitting diodes decreases from blue to green—the so-called “green gap”, which thwarts the engineering of white-light LED modules. While there is a fundamental limit due to increasing internal fields, the authors discover that increasing losses at defects are similarly important. This shifts the focus for improving solid-state lightning to optimizing growth conditions during the manufacture of green LEDs.

Measuring minuscule forces with atomic force microscopy (AFM) requires the subtraction of spurious forces acting on a macroscopic transducer. Compensating for this “background” is crucial to correctly interpreting AFM images in terms of the surface’s nanoscale viscoelastic response. Little has been done to measure or understand these forces, partly because traditional dynamic AFM simply does not contain enough information to separate background and tip-surface forces. The authors’ multifrequency lock-in measurement method, on the other hand, enables this separation.

Major challenges in thermoelectric energy harvesting include improving the efficiency and output power of devices, which usually are studied in terms of elastic transport processes in the linear-response regime. Departing from convention, the authors show that $nonlinear$ transport effects can considerably enhance thermoelectric efficiency and power in devices with $inelastic$ transport. The key mechanism is the unbounded increase of the Bose distribution in the nonlinear regime, for the bosons involved in the inelastic processes. These findings offer important guidance for developing tomorrow’s high-performance thermoelectric systems.

Transforming the dynamical behavior of a nonlinear mechanical oscillator is promising for applications as diverse as high-bandwidth resonators, tunable frequency references, and filters. The authors present a method, based on three-dimensional femtosecond laser-matter interaction, to locally alter the dynamics of a glass micromechanical resonator. They not only continuously tune the resonator’s response from nonlinear to linear, but also switch the “stiffness” of its nonlinear dynamics from “hard” to “soft”.

Many of us use lithium-ion batteries every day, and sodium-ion cells could be even better, once the kinks have been worked out. Layered oxides with formula Na$M$O${}_2$ and $P2$ stacking seem promising here, yet suffer from limited capacity and cycling, so combinations of transition metals $M$ are pursued. The authors’ simulations provide a rational framework to tune transition-metal mixing for optimal sodium diffusivity and enhanced rate capability in this class of cathode materials.

A basic challenge in photonics is to reproduce the key functions of electronics, while adding the parallelism and simplified circuitry of optics. One major hurdle is the lack of a generic, diode-like optical element. To address this difficulty, the authors offer proof of principle for a miniaturized optical device capable of being only “on” or “off”. Combining photoinduced optical circuitry in quadratic electro-optic crystals with localized structures in broad-area semiconductor cavities, they create a possible building block for future digital optics.

The silicon-on-insulator waveguide is one of the most promising platforms for scalable quantum information processing. The authors present multiplexed energy-time, time-bin, and polarization-entanglement photon sources from a single silicon nanowire on a chip. These sources offer high brightness and high entanglement quality across more than 10 correlated channels, and are fully compatible with dense-wave-division multiplexing (DWDM). They could be used for quantum key distribution, teleportation, entanglement swapping, and many other applications in quantum communication and computation.

Hybrid quantum networks of stationary and “flying” qubits are essential for distributed quantum information processing. In superconducting quantum computation, two-qubit gates are currently realized by the interaction between neighboring qubits. The authors propose a gate comprising a superconducting “atom” and a microwave photon, in which gate operation is completed $deterministically$ (not probabilistically) upon reflection of the photon. This gate’s type can be continuously varied $in$ $situ$, enabling $remote$ entanglement of many “atoms” via a single photon, or creation of a quantum domino effect.

The efficiency of (In,Ga)N light-emitting diodes decreases from blue to green—the so-called “green gap”, which thwarts the engineering of white-light LED modules. While there is a fundamental limit due to increasing internal fields, the authors discover that increasing losses at defects are similarly important. This shifts the focus for improving solid-state lightning to optimizing growth conditions during the manufacture of green LEDs.

Lithium-bearing transition-metal oxides such as LiCoO${}_2$, a cathode material in commercial batteries, are usually $p$-type semiconductors with conductivity mediated by hole polaron hopping—and therefore subject to any stress in the cathode. To this end, the authors derive an $analytical$ expression to calculate the electronic conductivity of a semiconductor as a function of strain. This result can be used to estimate the electronic conductivity of a polarizable semiconductor or insulator under any given strain field, at low cost in computational power and time, to facilitate battery design.

Are we really sure about the electronic properties of zigzag silicon carbide nanoribbons (NRs)? Density-functional calculations at the mean-field level have predicted them to be intrinsically half-metallic. However, the authors’ refined results, including electronic correlations and excitonic effects, show that wide NRs are semiconductors with spin-polarized states at the band gap, which could be useful for spintronic devices. Meanwhile, narrow NRs have strongly bound excitons that make them interesting for optoelectronics.

As lithium niobate shows good biocompatibility plus pyroelectric and photovoltaic effects, it is of interest for lab-on-a-chip biomedical applications. In this context, its most important function would be controlling microdroplets via light-induced changes in its material properties. Most investigations, however, have involved $nonpolar$ dielectric liquids, unlike the water that is the basis of biofluids. This study demonstrates successful photovoltaic manipulation of water microdroplets, using a LiNbO${}_3$ substrate coated with a hydrophobic film.

Understanding the origin of intrinsic limits of alloy disorder and localization is important for engineering band gap and quantum confinement in semiconductor heterostructures. The authors propose a set of attributes, relatively easy computed from first principles, to characterize and compare disorder in different alloys. This leads to a quantitative explanation for the stronger localization in dilute bismide alloys versus dilute nitrides. The authors also emphasize a link between intrinsic disorder and performance limitations in optical-gain materials for telecommunication applications.

Heusler alloys are attractive for spintronic applications, but for them it is challenging to realize enhanced spin polarization with very low magnetic damping, independent of other parameters such as thickness and spin-wave frequency. By choosing a suitable underlayer of chromium for the deposition of Heusler alloy on MgO substrate, the authors obtain well-ordered Heusler alloy with very low thickness- and frequency-independent magnetic damping, without spoiling its intrinsic properties. This is an important advance for device fabrication.

Superconducting and magnetic materials can be used together to access the spintronic aspects of the superconducting order. By inserting either a magnetic or nonmagnetic alloy layer into ferromagnetic Josephson junctions, the authors reveal a qualitative change in the characteristics of critical-current oscillation that is not explained by conventional theory. Their observations indicate that superconducting memory based on a spin valve cannot be properly understood or designed without an added phase offset.

In the quest for all-optical data processing, $nonreciprocal$ components are currently magnetism-based and cannot be integrated into CMOS-compatible systems. As an alternative, the authors establish a theoretical framework for achieving optical nonreciprocity and breaking of time-reversal symmetry in general, multimode optomechanical systems. Their work provides both general insight and guidelines for design optimization, showing the way to isolators and gyrators compatible with integrated, low-noise nanophotonic systems.

Josephson junctions are the cornerstones of numerous applications, and fabricating them from high-temperature superconductors has received much attention in recent years. The authors exploit the local field-effect doping of a cuprate superconducting film by a ferroelectric overlayer, where domain structure is “written” at will using an atomic force microscope. The key advantage of this approach is that it creates devices that are planar and can be reconfigured by “rewriting” the ferroelectric.

In a polar material, a laser pulse can trigger nonlinear optical phenomena, such as second-harmonic generation (SHG). The authors investigate the variation of SHG in BiCoO${}_3$ upon irradiation with femtosecond terahertz pulses. Applying a THz pulse, they observe enhancement of SHG by more than 50% at room temperature, the response being so fast that it simply follows the THz pulse. This huge, ultrafast enhancement, explained in terms of a photon dressed state, could be used to control nonlinear optical properties at the femtosecond timescale.

Rapid, accurate quantum gates are needed for an efficient quantum computer. Among the various physical platforms for logic gates, neutral atoms excited to high-lying states have met with much attention, but have been fundamentally limited by the gate protocol based on the well-known “blocking” method. Using a Rydberg interaction to tailor a generalized Rabi oscillation frequency, this study presents a class of exceedingly rapid and accurate two-bit quantum-gate protocols, with implications for quantum control across a wide range of platforms featuring two-body interactions.

Measuring minuscule forces with atomic force microscopy (AFM) requires the subtraction of spurious forces acting on a macroscopic transducer. Compensating for this “background” is crucial to correctly interpreting AFM images in terms of the surface’s nanoscale viscoelastic response. Little has been done to measure or understand these forces, partly because traditional dynamic AFM simply does not contain enough information to separate background and tip-surface forces. The authors’ multifrequency lock-in measurement method, on the other hand, enables this separation.

Much of our cultural heritage is recorded in irreplaceable artifacts made of paper. Studying the structures of delicate organic-matter samples $nondestructively$ is essential to understanding and diagnosing their inevitable degradation, and conserving them. THz light is particularly good for probing the hydrogen bonding that is key to the supramolecular arrangements in cellulose, but ancient paper is so thin that it does not absorb much at those wavelengths. Thus the authors develop a transmission-mode THz probe for very thin samples of low refractive index, by exploiting Fabry-Pérot interference from multiple reflections inside the sample.

Research on spin-torque oscillators (STOs) has been one of the hottest topics in spintronics for the past 20 years, yet STOs still have not actually seen use in microwave systems, because their phase noise is too great. Improving the phase stability of STOs is of paramount importance to their practical application. The authors build a phase-locked-loop circuit to dynamically stabilize an STO. Analysis of the residual phase error clarifies that the thermal stability of the STO under free-running oscillation sets the theoretical limit for phase error under phase-locked oscillation.

For quantum sensing and information processing, nitrogen-vacancy centers in diamond are not the only tool in the box. Silicon-vacancy centers in SiC are also of keen interest, but for successful applications, we must be able to reliably control where these color centers form in a device. Through ion implantation, the authors succeed in generating an array of single-photon emitters in SiC, with an efficiency of 19±4%. This ability could enable significant progress in spintronic and photonic quantum technologies.

Metal-insulator-metal tunnel junctions (MIMTJs) have become a fundamental enabling technology for microelectronics, and obtaining a well controlled, atomically thin, high-quality insulating tunnel barrier is key to their progress. Through experiment and simulations, the authors establish an atomic-layer deposition process to make ultrathin Al${}_2$O${}_3$ barriers of superior quality to industry-standard AlO${}_x$. Their method can improve magnetic tunnel junctions or Josephson junctions, for example, and thus a host of applications.

The authors propose applying a microwave-frequency electric field to a ferromagnetic/dielectric nanowire to excite propagating spin waves in the wire, providing a path to energy-efficient spintronic signal processing. This scenario should not be confused with the “parallel parametric pumping” discussed previously, as the mechanism of parametric coupling is completely different: via out-of-plane dynamic magnetization, not precession ellipticity.

Schneider famously noted that “Nature abhors a gradient”, but interesting counterexamples can arise. This study shows a case from electrokinetics in which, to preserve electroneutrality, an ionic circuit with an ion-selective gate must approach a stationary state with an inhomogeneous concentration profile. Scaling theory verifies that the selected gate current is universal, being independent of ionic strength, channel dimension, and membrane material. Beyond allowing lab-on-a-chip separation of analytes, this generic membrane-gating mechanism may be how living cells maintain electroneutrality in the face of a cross-membrane concentration gradient.

Precise measurement is at the heart of physical science, and taking advantage of quantum effects, where practicable, affords sensitivity beyond the reach of classical methods. Rather than increasing photon number to reduce the de Broglie wavelength and achieve superresolution via quantum optics, the authors use quantum frequency up-conversion to realize 525-nm maximally entangled photons. This two-photon NOON state offers a visibility better than the threshold for beating the standard quantum limit, and could be used for high-precision metrology in many applications.

To optimize a device, you must know what it’s actually made of. Cu${}_2$ZnSnS${}_4$ (CZTS) is appealing as a photovoltaic absorber, but small structural variations in a real sample can have a large impact on device performance and reliability. To understand the structure and stoichiometry of CZTS nanoparticles, the authors measure the extended x-ray absorption fine structure of the constituent metals. They find that as-synthesized nanoparticles are not always of the intended composition, but that copper-poor CZTS does retain the kesterite (or stannite) structure, with the possibility to fine-tune functional properties by controlling antisite defects.

So-called “photovoltaic tweezers” are an emerging tool for trapping, manipulating, and sorting objects at the micro- and nanoscales. This technology exploits the strong electric fields generated inside some ferroelectrics under illumination, without any electrodes, voltage supply, or lithography. As these tweezers have been used mostly for electrically neutral objects, this study extends their applicability to $charged$ objects. This is significant for handling $e.g.$ DNA molecules in biotechnology. This article’s conclusions could also inform other techniques that use electric fields for particle manipulation.

Electrically detected magnetic resonance (EDMR) is a very sensitive method for reading out the spin states of electron donors in silicon, which are a promising platform for spin-based quantum technologies. However, care must be taken: The authors show that the properties of the spectra measured via EDMR depend strongly on the optical excitation used. Different subpopulations of spins can contribute to the signal under different excitations, and the characteristic timescales of EDMR’s spin-dependent recombination process depend on excitation wavelength.

The proverbial “canary in a coal mine” is truly a thing of the past; nowadays zinc oxide is a key material in solid-state gas sensors, yet the mechanism by which doping can improve its performance is not fully understood. The authors use two-dimensional photoelectron diffraction to study thin films of tungsten-doped ZnO, and find clear evidence for substitution of segregated W atoms into Zn sites in the subsurface layer, which drastically improves the sensing response. This insight into the surface’s structure-function relationship suggests how best to prepare these films for this important application.

The authors show that paired, nonidentical transmission lines in a geometry inspired by butterfly wings couple four waveguide modes, leading to gigantic enhancement of rf electromagnetic field in a circular waveguide. This butterfly structure with a fourth-order flat-top dispersion profile emulates a complex photonic crystal (PC) to control the propagation of light. The authors’ alternative approach is particularly appealing for applications with strict spatial constraints, where traditional stacked PCs are impractical.

The authors show that the photogenerated polarons in the PTB7/PCBM copolymer blend, used in organic photovoltaic (OPV) solar cells, couple strongly to six vibrational modes that can be described by the amplitude-mode model. From the relative strengths of the doping-induced absorption of the polaron vibrations and electronic transitions, the polaron’s effective mass is obtained, and its influence on carrier mobility unraveled. This understanding of the photophysics of such blends is the next step in engineering lightweight, low-cost, high-efficiency OPV solar panels.

Liquid-crystal display technology is ubiquitous, but these materials can also be used to make electrically tunable components for optics and photonics. The authors demonstrate voltage control of the wavelength of flexoelectric domains in a transmissive diffraction grating, which enables dynamic light steering. They prove that the mechanism of pattern onset differs from that of switching between flexodomain states, and they explain the surprising finding that the response to increasing voltage is much slower than that to decreasing voltage.

(In,Ga)N/GaN quantum wells (QWs) have great potential in optoelectronics, and are increasingly used in high-power lasers and LEDs. Here localized electronic states play a crucial role in device efficiency, and could be investigated via the partially polarized light that nonpolar nitride QWs emit. The authors develop a technique to spatially map the polarization of near-field photoluminescence from these QWs, unambiguously identifying the band states formed by local fluctuations in alloy composition. The effective mass of holes in the second valence band is found to be much smaller than typically assumed, which could have real impact on device engineering.

The manipulation of $light$ via engineered structures (metamaterials) continues to inform the manipulation of $sound$. The authors show analytically that a periodic chain of perforated metallic shells can transform an audible sound of wide frequency spectrum into two monochromatic waves that propagate along the chain in opposite directions. Such a chain may serve not only as a splitter and redirector of acoustic waves, but also as a passive antenna that points the way back to the source of the incoming sound.

Micro- and nanofluidic devices operating in ionic solutions can yield a variety of chemical and physical signals, but it is crucial to translate those signals into electric currents and potentials, for interfacing with solid-state components. Translation requires circuitry with robust, predictable electrical coupling to the solution. The authors study the connectivity between nanofluidic diodes and conventional capacitor systems using various porous membranes and input potential signals, arriving at a model that describes well the electrical circuit’s behavior over a wide range of experimental conditions.

Preventing stray magnetic interactions is important to improving technology that relies upon manipulating an electron’s charge and spin. To this end, the authors investigate a binary intermetallic that has an internal magnetic field, yet does not affect the spin of surrounding components. As-grown epitaxial thin films of Mn${}_3$Al on GaAs(001) present the the ideal D0${}_3$ Heusler phase. This compound should be quite interesting for optimizing tomorrow’s spintronic devices.