Twin-field (TF) quantum key distribution (QKD) can overcome fundamental secret-key-rate bounds on point-to-point QKD links, allowing us to reach longer distances than ever before. Since its introduction, several TFQKD variants have been proposed, and some of them have already been implemented experimentally. Most of them assume that the users can emit weak coherent pulses with a continuous random phase. In practice, this assumption is often not satisfied, which could open up security loopholes in their implementations. To close this loophole, we propose and prove the security of a TFQKD variant that relies exclusively on discrete phase randomization. Remarkably, our results show that it can also provide higher secret-key rates than an equivalent continuous-phase-randomized protocol.

We propose and experimentally demonstrate a mechanism for focusing two-dimensional (2D) vortex for airborne sound beyond diffraction limit on an ultrathin structured surface in free space. A simple design of unit cell is presented as a practical implementation, which is capable of exciting evanescent wave and modulating its propagation phase over full 0-to-2π range. By analytically deriving the dispersion relationship and desired azimuthal distribution of effective parameters, we elucidate how to guide the propagation of excited evanescent waves in the vicinity of an unbounded surface decorated with designed structures and focus such surface vortex within a target region despite its compactness and nonaxisymmetry. The effectiveness of our mechanism is demonstrated numerically and experimentally via production and confinement of 2D vortex on a square surface at subwavelength spatial resolution. We anticipate our methodology with no need of a 2D system, bulky device size, geometric symmetry, active elements, or complicated phased array to offer new possibilities for the miniaturization and integration of planar vortex devices and may promote their on-chip applications in various fields such as by significantly boosting the information density and manipulation precision.

Nonlinear and hysteretic electrical devices are needed for applications from circuit protection to next-generation computing. Widely-studied devices for resistive switching are based on mass transport, such as the drift of ions in an electric field, and on collective phenomena, such as insulator-metal transitions. We ask whether the large photoconductive response known in many semiconductors can be stimulated in the dark and harnessed to design electrical devices. We design and test devices based on photoconductive $\mathrm{Cd}\mathrm{S}$, and our results are consistent with the hypothesis that resistive switching arises from point defects that switch between deep- and shallow-donor configurations: defect-level switching. This electronic device design principle, photoconductivity without photons, leverages decades of research on photoconductivity and defect spectroscopy. It is easily generalized and will enable the rational design of nonlinear hysteretic devices for future electronics.

On the basis of real-time ab initio calculations, we study the nonperturbative interaction of two-color laser pulses with $\mathrm{Mg}\mathrm{O}$ crystal in the strong-field regime to generate isolated attosecond pulses from high-harmonic emissions from $\mathrm{Mg}\mathrm{O}$ crystal. In this regard, we examine the impact of the characteristics of the incident pules, such as its shape, intensity, and ellipticity, as well as the consequences of the crystal anisotropy on the emitted harmonics and their corresponding isolated attosecond pulses. Our calculations predict the creation of isolated attosecond pulses with a duration of approximately 300 as; in addition, using elliptical driving pulses, we show the generation of elliptical isolated attosecond pulses. Our work prepares the path for all-solid-state compact optical devices offering perspectives beyond traditional isolated attosecond pulses emitted from atoms.

Quantum annealing offers an approach to finding the optimal solutions for a variety of computational problems, where the quantum annealing controls influence the observed performance and error mechanisms by tuning the underlying quantum dynamics. However, the influence of the available controls is often poorly understood, and methods for evaluating the effects of these controls are necessary to tune quantum computational performance. Here we use portfolio optimization as a case study by which to benchmark quantum annealing controls and their relative effects on computational accuracy. We compare empirical results from the D-Wave $2000{\mathrm{Q}}^{\mathrm{TM}}$ quantum annealer to the computational ground truth for a variety of portfolio optimization instances. We evaluate both forward and reverse annealing methods and we identify control variations that yield optimal performance in terms of probability of success and probability of chain breaks.

Recent developments in Nernst-Ettingshausen (NE) physical phenomena combined with advances in the performance of rare-earth permanent magnets make thermomagnetic (TM) cryocoolers well suited for practical applications. The device performance of a NE cryocooler depends on both the material and the geometric shape of the device. Despite continued progress in TM materials, the optimum shape is still based on a simplified infinite-stage model derived in 1963 by Harman [Adv. Energy Convers. 3(4), 667–676 (1963)]. Harman's model assumes several nonrealistic assumptions, such as temperature-independent material properties and constant current density. We relax such assumptions and derive a fully-temperature-dependent numerical model to accurately solve for the thermomagnetic features of a NE cooler with arbitrary geometry. We correct Harman's analytical function and compare its performance with the performance of devices of various shapes. The corrected shape has a higher coefficient of performance (COP) at higher temperature differentials, which indicates that when the material resistivity is a strong function of the temperature, the corrected infinite-stage device can provide better performance than Harman's geometry. Moreover, the corrected infinite-shape device can provide higher heat flow density under a similar optimum-COP condition. A case study based on a state-of-the-art TM material, $\mathrm{Bi}$-$\mathrm{Sb}$ alloy, is presented, and the critical parameters for designing an efficient thermomagnetic cooler are discussed in detail.

The observation of electrically tunable and highly confined plasmons in graphene has stimulated the exploration of interesting properties of plasmons in other two-dimensional materials. Recently, hyperbolic plasmon resonance modes have been observed in exfoliated ${\mathrm{W}\mathrm{Te}}_2$ films, a type-II Weyl semimetal with layered structure, providing a platform for the assembly of plasmons with hyperbolicity and exotic topological properties. However, the plasmon modes were observed in relatively thick and small-area films, which restrict the tunability and application for plasmons. Here, large-area (approximately cm) ${\mathrm{W}\mathrm{Te}}_2$ films with different thicknesses are grown by the chemical vapor deposition method, in which plasmon resonance modes are observed in films with different thicknesses down to about 8 nm. Hybridization of plasmon and surface polar phonons of the substrate is revealed by mapping the plasmon dispersion. The plasmon frequency is demonstrated to be tunable by changing the temperature and film thickness. Our results facilitate the development of a tunable and scalable ${\mathrm{W}\mathrm{Te}}_2$ plasmonic system for revealing topological properties and towards various applications in sensing, imaging, and light modulation.

Topological characteristics, including invariant topological orders, band inversion, and the topological edge mode (TEM) in photonic insulators, have been widely studied. Whether intriguing topological modes can be taken advantage of in simple one-dimensional systems to implement some practical applications is an issue, which is of increasing concern. In this work, based on a photonic dimer chain composed of ultrasubwavelength resonators, we verify experimentally that the TEM in the effective second-order parity-time (PT) system is immune to the inner disorder perturbation, and can be used to realize the long-range wireless power transfer (WPT) with high transmission efficiency. To intuitively show the TEM can be used for WPT, a power signal source is used to excite the TEM. It can be clearly seen that two light-emitting diode (LED) lamps with 0.5 W at both ends of the structure are lit up with the aid of TEMs. In addition, in order to solve the special technical problems of standby power loss and frequency tracking, we further propose that a WPT system with effective third-order PT symmetry can be constructed by using one topological interface mode and two TEMs. Inspired by the long-range WPT with TEMs in this work, the use of more complex topological structures is expected to achieve energy transmission with more functions, such as the WPT devices whose direction can be selected flexibly in the quasiperiodic or trimer topological chains.

In spaceborne gravitational-wave detectors, the stringent requirements on the magnetic cleanliness of the test masses (TMs) make the development of ground-based characterization techniques of their magnetic properties important. An electrostatically controlled torsion pendulum has been built to measure the magnetic properties of TianQin-like solid TMs. The remanent magnetization $m_r$ and the magnetic susceptibility $\chi$ of TMs can be measured at the level of 1 nA ${\mathrm{m}}^2$ and $8\times {10}^{-7}$, respectively. The precisions of measurements are mainly limited by the mechanical sensitivity of silicon fiber. The experimental results from a solid tungsten TM as an example show that the apparatus provides a feasible way to investigate the bulk effect of a full-size TM for the TianQin mission with enough resolution.

Electric generators convert external energy, such as mechanical, thermal, nuclear, chemical, and so forth, into electricity and are the foundation of power station and energy harvesting operations. Inevitably, the external source supplies a force per unit charge (commonly referred to as an impressed electric field) to free or bound charge, which produces ac electricity. In general, the external impressed force acts outside Maxwell’s equations and supplies a nonconservative electric action generating an oscillating electrodynamic degree of freedom. In this work we analyze the electrodynamics of ideal free- and bound-charge electricity generators by introducing a time-dependent permanent polarization, which exists without any applied electric field, necessarily modifying the constitutive relations and essential to oscillate free or bound charge in a lossless way. For both cases, we show that Maxwell’s equations and, in particular, Faraday’s law are modified, along with the required boundary conditions through the addition of an effective impressed magnetic current boundary source and the impressed electric field, related via the left-hand rule. For the free-charge case, we highlight the example of an electromagnetic generator based on Lorentz force, where the impressed force per unit charge that polarizes the conductor comes from mechanical motion of free electrons due to the impressed velocity of the conductor relative to a stationary dc magnetic field. In contrast, the bound-charge generator is simply an idealized permanently polarized bar electret, where the general case of a time-dependent polarized electret is the underlying principle behind piezoelectric nanogenerators. In the open-circuit state, both bound- and free-charge electricity generators are equivalent to idealized Hertzian dipoles, with the open-circuit voltage equal to the induced electromotive force (emf). Analyzing the short-circuit responses, we show that the bound-charge electricity generator has a capacitive source impedance. In contrast, we show that, for the ideal free-charge ac electricity generator, the back emf from the inductance of the loop that defines the short circuit directly cancels the source emf, so the voltage across the inductor is solely determined by the magnetic current boundary source. Thus, we find that the magnetic current boundary source best describes the output voltage of an ac generator, rather than the electric field.

Hybrid halide perovskites have great potential for application in optoelectronic devices. However, an understanding of some basic properties, such as charge-carrier transport, remains inconclusive, mainly due to the mixed ionic and electronic nature of these materials. Here, we perform temperature-dependent pulsed-voltage space-charge-limited current measurements to provide a detailed look into the electronic properties of methylammonium lead tribromide (${\mathrm{MA}\mathrm{Pb}\mathrm{Br}}_3$) and methylammonium lead triiodide (${\mathrm{MA}\mathrm{Pb}\mathrm{I}}_3$) single crystals. We show that the background carrier density in these crystals is orders of magnitude higher than that expected from thermally excited carriers from the valence band. We highlight the complexity of the system via a combination of experiments and drift-diffusion simulations and show that different factors, such as thermal injection from the electrodes, temperature-dependent mobility, and trap and ion density, influence the free-carrier concentration. We experimentally determine effective activation energies for conductivity of (349 ± 10) meV for ${\mathrm{MA}\mathrm{Pb}\mathrm{Br}}_3$ and (193 ± 12) meV for ${\mathrm{MA}\mathrm{Pb}\mathrm{I}}_3$, which includes the sum of all of these factors. We point out that fitting the dark current density-voltage curve with a drift-diffusion model allows for the extraction of intrinsic parameters, such as mobility and trap and ion density. From simulations, we determine a charge-carrier mobility of 12.9 cm${}^2$/Vs, a trap density of 1.52 × 10${}^{13}$ cm${}^{-3}$, and an ion density of 3.19 × 10${}^{12}$ cm${}^{-3}$ for ${\mathrm{MA}\mathrm{Pb}\mathrm{Br}}_3$ single crystals. Insights into charge-carrier transport in metal-halide perovskite single crystals will be beneficial for device optimization in various optoelectronic applications.

We observe bursting, crown or jet formation, and atomization phenomena when electric discharge is confined at the base of a liquid droplet. Emulating an electrowetting-on-dielectric setup, a hemispherical droplet is placed on a cathode-isolator platform while a pointed anode from the top stimulates the discharge inside the drop. Beyond a critical intensity of the applied electric field, the dielectric layer under the droplet suffers breakdown to generate the discharge. Within a few milliseconds, the electric discharge stimulates crown or jet formation or bursting of the droplet. A lower (higher) salt loading in the droplet at lower (higher) field intensity leads to the jetting (bursting). The crown formation happens only for an intermediate window of salt loadings and electric field intensities. The energy conversion efficiency is found to be approximately $25\mathrm{\%}$–$30\mathrm{\%}$ for droplet bursting while the same is less than 2% for jet or crown formation. The experiments and simulations uncover that the location of the pointed electrode in the droplet and the separation distance of electrodes can be some crucial factors in varying the location of discharge, which eventually leads to the aforementioned flow morphologies. Further, the interplay between surface tension, viscous, and electric field forces inside the droplet is found to play crucial roles to engender the flow patterns. The setup transiently shows up the formation of end pinching of jets, microjets, high-aspect-ratio liquid threads, upward moving jet, liquid spikes riding on a crown rim, and capillary breakup of jets. Such electric-discharge-mediated droplet disintegration resembles closely the previously reported laser or plasma-induced phenomena. Further, with proper tuning of the aforementioned control parameters, the proposed methodology is expected to find its application in several cutting-edge technologies, such as inkjet printing, tissue ablation, or electroporation etc.

We describe a direct gradiometer using optical pumping with opposite circular polarization in two ${}^{87}\mathrm{Rb}$ atomic ensembles within a single multipass cell. A far-detuned probe laser undergoes a near-zero paramagnetic Faraday rotation due to the intrinsic subtraction of two contributions exceeding 3.5 rad from the highly polarized ensembles. We develop analysis methods for the direct gradiometer signal and measure a gradiometer sensitivity of 10.1 $\mathrm{fT}/\mathrm{cm}\sqrt{\mathrm{Hz}}$. We also demonstrate that our multipass design, in addition to increasing the optical depth, provides a fundamental advantage due to the significantly reduced effect of atomic diffusion on the spin-noise time-correlation, in excellent agreement with the theoretical estimate.

Imaging the magnetic configuration of thin films has been a long-standing area of research. The emergence of two-dimensional ferromagnetic materials in the last few years calls for innovation in the field of magnetic imaging. As the magnetic moments are extremely small, standard techniques like superconducting quantum interference devices, torque magnetometry, magnetic force microscopy, and Kerr effect microscopy are challenging and often lead to the detection of parasitic magnetic contributions or spurious effects. In this work, we report a magnetic microscopy technique based on the combination of magnetic circular dichroism and the Seebeck effect in a ferromagnet-semiconductor bilayer. We implement this method with perpendicularly magnetized $\mathrm{Co}$/$\mathrm{Pt}$ multilayers sputtered on $\mathrm{Ge}$(111). We further show that the electrical detection of magnetic circular dichroism is more sensitive than the Kerr magnetometry, especially in the ultrathin film regime, which makes it particularly promising for the study of emergent two-dimensional ferromagnetic materials.

Scattering-type scanning near-field optical microscope (s-SNOM) is a modern technique for subdiffractional optical imaging and spectroscopy. Over the past two decades, tremendous efforts have been devoted to modeling complex tip-sample interactions in s-SNOM, aimed at understanding the electrodynamics of materials at the nanoscale. However, due to complexities in analytical methods and the limited computation power for fully numerical simulations, compromises must be made to facilitate the modeling of tip-sample interaction, such as using quasistatic approximation or unrealistic tip geometries. $\mathrm{In}$ this paper, we apply a variety of widely utilized machine-learning methods, including k nearest neighbor and feed-forward neural network etc. to study the phase-resolved spectroscopic near-field response. With only a small set of training data, which is simulated using the finite-dipole model, we demonstrate that the relation between the experimental near-field signal and sample optical constant can be one to one mapped without the need for tip modeling: for a given material with a moderate dielectric function, its complex near-field spectrum can be accurately determined within the mid-IR spectral range, and vice versa. Our preliminary study sets the stage for future exploration using real experimental data. Our method is beneficial for processing the increasing amount of data accumulated across many research groups and especially useful for user facilities such as synchrotron-based national laboratories where a large amount of data is generated on a daily basis.

Pyroelectric energy converters are functional capacitors that use a pyroelectric material as the dielectric layer. By utilizing a first-order phase transformation of the material, a pyroelectric device can generate an adequate amount of electricity from small temperature fluctuations. However, most pyroelectric capacitors leak during energy conversion. In this paper, we analyze the thermodynamics of pyroelectric energy conversion with consideration of electric leakage. Our thermodynamic model is verified by experiments using three phase-transforming ferroelectric materials with different pyroelectric properties and leakage behaviors. We demonstrate that the impact of leakage on electric generation is prominent, and sometimes may be confused with the actual power generation by pyroelectricity. We discover a material candidate, $(\mathrm{Ba},\mathrm{Ca})(\mathrm{Ti},\mathrm{Zr},\mathrm{Ce}){\mathrm{O}}_3$, that exhibits a large pyroelectric current and an extremely low leakage current. A pyroelectric converter made of this material generates a pyroelectric current density of $1.95\mu \mathrm{A}/{\mathrm{cm}}^2$ and a pyroelectric work density of $0.2\mathrm{J}/{\mathrm{cm}}^3$ even after 1389 thermodynamic-conversion cycles.

X-ray ptychotomography in the near-field regime is a promising technique for high-resolution, quantitative, and nondestructive investigations in materials science, paleontology, and biomedicine. X-ray near-field ptychography has been previously demonstrated in projection and tomography mode, but the quantitativeness of the reconstructed data has never been discussed in detail. Here, we use measurements of a sample made of aluminum and nickel microparticles to evaluate the quantitativeness of the volumetric mass-density data. Moreover, we propose an algorithm (VortRem) for the removal of phase vortexes, a type of artifact that frequently occurs in holographic methods. VortRem and the results presented here may be fundamental for extending the applicability of this emerging technique to quantitative three-dimensional characterization studies of light as well as dense samples down to the nanoscale.

Harnessing the spontaneous emission of incoherent quantum emitters is one of the hallmarks of nano-optics. Yet, an enduring challenge remains—making them emit vector beams, which are complex forms of light associated with fruitful developments in fluorescence imaging, optical trapping, and high-speed telecommunications. Vector beams are characterized by spatially varying polarization states whose construction requires coherence properties that are typically possessed by lasers—but not by photons produced by spontaneous emission. Here, we show a route to weave the spontaneous emission of an ensemble of colloidal quantum dots into vector beams. To this end, we use holographic nanostructures that impart the necessary spatial coherence, polarization, and topological properties to the light originating from the emitters. We focus our demonstration on vector vortex beams, which are chiral vector beams carrying nonzero orbital angular momentum, and argue that our approach can be extended to other forms of vectorial light.

Non-Hermitian skin effect (NHSE) has led to interesting physics and sophisticated applications beyond the conventional framework. However, NHSE studies have been limited to an individual dimension due to the difficulty of constructing nonreciprocal coupling in higher-dimensional systems. With the concept of synthetic dimension, we realize two-dimensional (2D) NHSE in a synthetic photonic lattice. The synthetic photonic lattice is composed of a spatial dimension and a synthetic frequency dimension, which is constructed by introducing gain and loss and dynamically modulating the complex refractive index in a one-dimensional ring-resonator array. In the synthetic 2D NHSE system, we can manipulate the spatial position and frequency mode of the light in real time, thus enabling programmable light propagation and frequency conversion. Specifically, the incident light can be localized in the spatial dimension and the frequency dimension simultaneously, leading to second-order corner modes in the synthetic 2D NHSE lattice. Besides, the 2D NHSE system exhibits an excellent robustness for the disorder and point defect in the lattice. This work generalizes the non-Hermitian skin effect in synthetic dimension to control the light in different dimensions, which shows great promise for potential applications in on-chip light manipulation, frequency synthesis, and information processing.