In quantum optics, is it possible to operate a single-photon source without having to tweak its alignment every day? In principle, yes, but usually such sources suffer tremendous losses and poor performance. The authors show that it is possible to build a single-photon source that retains high efficiency and good performance in an alignment-free package, by appropriately engineering the source chip and permanent coupling to optical components. Their device delivers laboratory-grade performance in a stable package that is easy to use, reliable, and compact, thus bridging the gap between highly equipped labs and real-world applications.

Detecting single photons is essential for numerous technologies, from quantum computing to observations of the faintest objects in the universe. The authors propose a detector based on graphene’s dual abilities to absorb light across a huge range of wavelengths, and to experience an extreme rise in electronic temperature by absorbing just one photon. Simulations show that these properties allow for high detection efficiency, when the graphene is coupled to superconductors in a Josephson junction at cryogenic temperatures.

Sensors based on magnetic domain walls are key nonvolatile components for “true power on” devices, but their development has been hampered by the challenges inherent to controlling domain-wall dynamics. To realize such devices, the authors study domain-wall propagation and nucleation under applied magnetic fields, in industrially fabricated structures. They show the strong dependence of nucleation field strength on geometry provides a knob for tuning device characteristics, such as a sensor’s electrical resistance, enabling simple and fast industrial testing and production.

The authors demonstrate on-chip diamond nanophotonics with a high-efficiency fiber-optic interface, achieving >90% power coupling at visible wavelengths. They use this approach to create a bright source of narrowband single photons, based on a silicon-vacancy color center embedded in a waveguide-coupled diamond photonic-crystal cavity. Their quantum nanophotonic interface yields a high flux of coherent single photons into a single-mode fiber, enabling possibilities for quantum networks that couple multiple emitters, either on the same chip or separated by long distances.

Spintronics continues its march from the laboratory to everyday devices, with control via electric rather than magnetic fields a key goal. This study harnesses electrically modulated interfacial spin-orbit interaction in a magnetic multilayer to manipulate the microwave spectral characteristics of the resulting spin-current auto-oscillator. The observed enhancement of electrostatic gating in the strongly nonlinear regime of oscillations is explained quantitatively. These effects can be exploited for frequency mixing, synchronization, and logic gates in spin-wave-based magnonic devices.

Devices employing surface acoustic waves (SAWs) are of major interest in sensor applications, due to their ease of manufacturing, sensitivity, small size, and wireless interrogation. This study shows that the response of a magnetic SAW device is much more complex than as is often reported, and that a detailed understanding of its magnetic properties is essential to controlling the magnetoacoustic response. For example, sensing can be extended to out-of-plane fields, beyond the traditional in-plane measurements.

Understanding the complex physics of high-pressure arc discharges, important for many applications, is challenging; work in decades past was limited by the available laser sources and digital processing capabilities. The authors apply spatial carrier-wave interferometry to measure heavy-particle and electron densities independently in a circuit breaker arc, at high resolution. Their observations shed light on the process of current interruption, and will permit the calibration of computational fluid dynamics simulations used to study such arcs.

In a quantum computer, superconducting transmission lines link qubits, control their states, and read them out—and are hampered by stray magnetic fluxes in the substrate or at interfaces. Various types of low-frequency flux noise limit the fidelity of quantum-information hardware, and are described by the flux-flux correlator. The author considers flux noises and their effects on different computational protocols, demonstrating nontrivial dependences on frequency and size for different sources. This provides a way forward for characterizing these mechanisms, and mitigating their effects on coherence.

A central challenge in building a scalable quantum computer with superconducting qubits is simultaneously achieving long coherence times and fast two-qubit gates. Currently, fast (tunable) gates are obtained at the expense of coherence time. The authors propose a “flatsonium” qubit, based on the highly nonlinear fluxonium qubit, that yields both coherence and tunability—jackpot. Dephasing due to global flux noise is reduced by engineering flux-insensitive sweet spots in the qubit spectrum at the frequencies of interest, while leakage errors are eliminated by the typically large anharmonicity of its energy spectrum.

In semiconductor physics, conventional $p-n$ junctions are bipolar, featuring of both $p$- and $n$-type carriers ($i.e.$ holes and electrons). In this study, $p-n$ junction rectification is achieved with $unipolar$, purely $n$-type (Al,In)GaN structures. This is significant, as the direct-gap group-$III$ nitrides offer the most viable approach to high-frequency, high-power electronics, yet progress has been thwarted by the high resistivity of $p$-type GaN, which is totally absent from the authors’ device. Furthermore, their approach can be used for other strongly polarized semiconductors, such as ZnO.

With low leakage current, and thus low power consumption and high ON/OFF ratio, nanoelectromechanical switches offer a promising basis for next-generation electronics. They also tolerate radiation, temperature variations, and external electric fields, which makes them suitable for extreme environments, as in space exploration or nuclear disaster recovery. Considering the unique elastic and electrical properties of carbon nanotubes (CNTs), the authors present analytical expressions for the characteristic “pull-in” and “pull-back” voltages of double-pole CNT nanorelays, plus a generic model and design rules for such devices.

Optical quantum-communication networks require the development of practical technology, particularly sources of entangled photon pairs, to mitigate the impact of photon loss on error rates in long-distance transmission. Sources generally do not operate at telecom wavelengths, are incompatible with existing fiber networks, or suffer from classical photon statistics—a potential threat to security. By using a semiconductor quantum dot, the authors demonstrate a quantum relay that is compatible with standard telecom infrastructure and at the same time intrinsically secure.

Controlling the magnetic fine structure between two domains in a wire forms the basis of many promising applications, such as “racetrack” memory, sensors, and logic concepts for magnetic computing. This paper presents a study of V-shaped soft-magnetic nanowires, with regard to the influence of wire width, thickness, and bending angle on the prevalent type of domain wall. Domain patterns are imaged at high resolution via scanning electron microscopy with polarization analysis (SEMPA), and compared to micromagnetic simulations. Based on the results, a phenomenological model enables the predictable tuning of remanent wall type in such a nanowire.

Quantum information processing is seen as the key to technological advancement, but an easily accessible and scalable scheme is required to fulfill its potential. In this regard, the authors investigate electron transport through an integrated quantum device, consisting of a quantum point contact coupled to a tunable electronic cavity. They observe anomalous interference features due to coupling between the two systems; this is related to the well-known Fano effect. Their prototype device could offer a step toward the quantum analog of amplitude modulation, or phase modulation.

Occasionally, pure wonder and practicality can both be addressed in a single study. The authors observe axial banding in a bed of two types of granules, differing only in density, tumbling in a horizontally rotating drum. By manipulating the drum’s internal geometry, one may direct the banding to produce an arbitrary number of predetermined segregation patterns. This insight into an unexplained physical phenomenon has direct implications for the separation or sorting of particulate matter in industry, spanning the agricultural, pharmaceutical, automotive, mining, construction, and environmental sectors.

Due to their very short de Broglie wavelengths, matter waves could be used for high-resolution patterning, competing with $e.g.$ e-beam or helium-ion lithography for producing features smaller than 10 nm. Binary holography is presently the only method available for mask-based atom lithography, but its theoretical framework has remained quite limited. The authors present an optimal method for making binary holograms that can shape atom beams arbitrarily, allowing for free selection of the geometrical properties of the generated masks, and the resulting structures.

For quantum information processing with superconducting circuits, efficient amplification of weak microwave signals is often required. For this purpose, Josephson parametric amplifiers (JPAs) are commonly used, due to their quantum-limited noise performance. The authors show that flux-driven JPAs exhibit hysteresis, even for negligible screening parameters, and that their amplification properties depend strongly on the resonator characteristics. These phenomena are important for a full understanding of the magnetic field response and parametric amplification in JPAs, and thus their application.

The authors explore the concept of “acoustic jets” for the superfocusing of sound waves. Compared to conventional diffraction-limited systems, 50% better spatial resolution of ultrasound images can be achieved using a ball-shaped polymer lens. Excellent agreement is found between the experimental data and both analytical and numerical predictions. Potential applications extend beyond medical ultrasound to nondestructive testing, acoustic microscopy and sensing, microparticle manipulation, and sonar.

Group-$IV$ semiconductor nanowires (NWs) are promising for thermoelectric energy harvesting, due to their remarkably reduced thermal conductivity. Recent experiments have reported a hexagonal crystalline phase in Si and Ge NWs, indicating the presence of polytypes, which needs further investigation. A combination of density functional theory and Boltzmann transport calculations reveals that the thermoelectric properties of Si and Ge NWs may be tuned by changing the stacking sequence along the growth direction. These results offer a means to engineer high-performance thermoelectric materials.

In quantum computing, metrology, single-photon counting, and nanomechanics, weak electronic signals at extremely low temperatures need amplification. To this end, the authors build and study silicon-germanium heterojunction bipolar transistors, which offer excellent amplifier characteristics, integrability with silicon quantum electronics, low cost, and manufacturability. Their research at the junction of physics and electrical engineering is a step toward next-generation integrated circuits at the 90-nm scale for this temperature regime.

How far can we miniaturize magnetic recording media? Reducing grain size and layer thickness nearly to the atomic level requires finer handling than in typical micromagnetic calculations. The authors develop atomistic simulation of spin dynamics to study magnetization reversal in complex granular media, to optimize performance and push areal density above 1 Tbit/in${}^2$. They demonstrate the importance of magnetostatic interactions and inter- and intralayer exchange couplings, and visualize switching at the atomic level to reveal the reversal mechanism. This study transfers knowledge from a university setting directly to industry and real-world problems.

Sensors based on magnetic domain walls are key nonvolatile components for “true power on” devices, but their development has been hampered by the challenges inherent to controlling domain-wall dynamics. To realize such devices, the authors study domain-wall propagation and nucleation under applied magnetic fields, in industrially fabricated structures. They show the strong dependence of nucleation field strength on geometry provides a knob for tuning device characteristics, such as a sensor’s electrical resistance, enabling simple and fast industrial testing and production.

Josephson parametric amplifiers continue to attract attention for monitoring and readout of superconducting qubits in quantum information processing, as well as other applications. The authors continue their research on these systems, presenting a comprehensive study of nondegenerate amplifiers that addresses nonlinear gain, parametric instability, amplification processes (two- and four-mode linear processes plus nonlinear corrections, with signal-to-noise ratios for all cases), and frequency conversion. Quantum fluctuations and squeezing are considered as well.

This study demonstrates a simple, effective, and tunable technique to control the lateral migration of ellipsoidal microparticles by combining microscale shear flows with a uniform magnetic field. Adjusting the direction of the magnetic field results in versatile control of symmetric and asymmetric rotation of the particles, and their shape-dependent lateral migration of the particles. This approach has great potential for selective microfluidic manipulation of micrometer-sized biological objects such as cells, bacteria, and viruses, by means of their nonspherical shapes.

Acoustic traps, particularly glass capillaries actuated by piezoelectric transducers, are used to capture and handle suspended microparticles or cells in microfluidic applications, yet comprehensive modeling and characterization of such systems are needed. The authors develop a three-dimensional model of ultrasound pressure and displacement fields in a liquid-filled capillary, and use it to explain the existence of localized levitating resonance modes. Certain channel geometries lead to modes with less spatial variation, which could be experimentally advantageous.

In quantum optics, is it possible to operate a single-photon source without having to tweak its alignment every day? In principle, yes, but usually such sources suffer tremendous losses and poor performance. The authors show that it is possible to build a single-photon source that retains high efficiency and good performance in an alignment-free package, by appropriately engineering the source chip and permanent coupling to optical components. Their device delivers laboratory-grade performance in a stable package that is easy to use, reliable, and compact, thus bridging the gap between highly equipped labs and real-world applications.

Detecting single photons is essential for numerous technologies, from quantum computing to observations of the faintest objects in the universe. The authors propose a detector based on graphene’s dual abilities to absorb light across a huge range of wavelengths, and to experience an extreme rise in electronic temperature by absorbing just one photon. Simulations show that these properties allow for high detection efficiency, when the graphene is coupled to superconductors in a Josephson junction at cryogenic temperatures.

Unsurprisingly, considering its significance to electronics, the impact of copper impurities on the electrical properties of silicon has been widely studied. Surprisingly, there is still marked disagreement, both among and between experiments and calculations. The remarkable disagreement between various theoretical studies is mainly due to the band-gap error in basic versions of density functional theory. This study revisits the problem using the nuanced HSE06 hybrid functional. Its model for the electronic structure of Cu in Si yields results that agree well with experimental transition levels and solubilities.

Using strain to manipulate magnetization at the nanoscale is attractive, due to its relatively high energy efficiency, but typically produces 90° reorientation, rather than the 180° flipping desired for spintronic applications. Using a compositionally graded film of the ferrimagnetic alloy Tb-Fe, the authors achieve strain-induced 180° magnetization rotation. The applied strain induces magnetic anisotropy in the Tb-dominant region, reversing the magnetic moment of the exchange-coupled Fe-dominant region. This is a promising development for spintronics based on voltage-controlled “exchange springs”.

Frequency conversion of lasers is used in many applications, but as beam intensity continues to increase, one must reconsider exactly what occurs within the target material. Based on data for short-pulse second-harmonic generation by a periodically poled nonlinear optical crystal, the authors develop a two-temperature model of laser-matter interaction in a dielectric. This extended description of frequency conversion and beam propagation in the crystal accounts for the effects of photoinduced plasma generation, thermal and plasma-induced dephasing, and related phenomena of optical breakdown and filamentation.

The authors demonstrate on-chip diamond nanophotonics with a high-efficiency fiber-optic interface, achieving >90% power coupling at visible wavelengths. They use this approach to create a bright source of narrowband single photons, based on a silicon-vacancy color center embedded in a waveguide-coupled diamond photonic-crystal cavity. Their quantum nanophotonic interface yields a high flux of coherent single photons into a single-mode fiber, enabling possibilities for quantum networks that couple multiple emitters, either on the same chip or separated by long distances.

The $p-n$ junction has been a building block of electronics for many decades, but the introduction of graphene allows still more device modalities. The authors study a multilayered structure in which carrier concentrations in graphene are dictated by the behavior of an underlying ferroelectric. They analyze pronounced hysteresis of ferroelectric polarization and graphene charge in response to periodic gate voltage, as well as size effects and the dominance of electric boundary conditions for polarization. These top-gated structures seem promising as next-generation modulators and rectifiers.

Despite extensive research, the technology of resistive random-access memory (RRAM) continues to be held back by insufficient understanding of the underlying physics. Using electrostatic and physical-kinetic approaches that are independent of microscopic structural details, the authors make two key observations that lead to a phenomenological theory of RRAM operation. This insight provides closed-form solutions for steady and transient states in terms of material parameters, which should have a significant impact on RRAM engineering.

Developing better batteries primarily depends on identifying better cathode materials, which in turn means understanding their physical properties and behavior. This study addresses eldfellite, NaFe(SO${}_4$)${}_2$ , which features mobile Na${}^{+}$ rather than the ubiquitous Li${}^{+}$. Using first-principles calculations, the authors elucidate possible mechanisms of ion diffusion and electron transfer in both Na-deficient and -rich modifications of the basic compound. The results are promising for inexpensive, nontoxic, high-performance power storage.

The authors describe an alternative to digital quantum computation that uses natural quantum dynamics for information processing. $Quantum$ $reservoir$ $computing$ does not require fine tuning of parameters, is robust against noise, and is based on existing devices. Simulations suggest that with this approach, a system of just 5 to 7 qubits is as powerful as a recurrent neural network with hundreds of nodes. This framework for artificial intelligence powered by quantum physics enables $temporal$ machine-learning tasks, such as natural language processing and predicting the stock market.

Color centers in diamond and related wide-band-gap semiconductors are the leading candidates for single-photon sources under ambient conditions, but their behavior under electrical control is poorly understood. The authors present a comprehensive theory to address single-photon emission from electrically pumped color centers. Self-consistent simulations furthermore reproduce the experimentally measured emission characteristics, creating a backbone for the development of practical single-photon sources for applications of quantum optics.

In the rapidly evolving field of high-throughput materials screening, compounds are rated according to their potential for use in applications. For photovoltaics, unfortunately, the selection metric that is currently in wide use does not distinguish in a thermodynamically correct way between internal $material$ properties and external $device$ properties. This study suggests a thermodynamically correct selection metric, extending the Shockley-Queisser approach to calculate efficiency limits from intrinsic bulk material properties, which should help to zero in on the truly most promising needles in the haystack of candidates.

At optical frequencies, the magnetic interaction of light and matter is negligible, compared to its electric counterpart. Enhancing the local magnetic field would enable one to see magnetic dipolar transitions in matter, for applications in spectroscopy and microscopy. This comprehensive study of a class of magnetic nanoantennas made of gold nanospheres presents two important figures of merit, and a formula to estimate the natural magnetic-resonance frequency for an arbitrary number of spheres. The results are of significant practical importance for the optical study of weak magnetic transitions in molecules.

The spin Hall effect (SHE) is a means to electrically control the magnetization of a ferromagnet—one of the prime goals in spintronics. The most-studied material for exploiting the SHE is platinum, but in CMOS electronics the preferred metal is copper, which has a weak SHE. The authors show that Pt-Cu alloy with just 28% Pt can be as efficient as pure Pt at generating spin current. This alloy can also withstand high annealing temperatures, and thus can be readily incorporated into CMOS technology, nudging spintronics that much closer to widespread use.

Photonic entanglement and heralded single photons are vital for optical quantum information processing, and are realized by spontaneous parametric down-conversion (SPDC) in chiefly just two materials: LiNbO${}_3$ and KTiOPO${}_4$. The particular properties of these materials, however, strongly restrict advanced techniques. Thus the authors study four other nonlinear optical materials, plus KTiOPO${}_4$, and configurations in which they can be used to generate pure photon states and entanglement of polarization or frequency. The team finds a broad variety of promising SPDC setups that are impossible to implement using the traditional materials.