We present the physical model for the entropy source of a quantum-random-number-generator chip based on the quantum fluctuations of the photon number emitted by light-emitting diodes. This model, combined with a characterization of the chip, estimates a quantum min-entropy of over 0.98 per bit without postprocessing. Finally, we show with our model that the performances in terms of security are robust against fluctuations over time.

We report the magnetostatically induced formation of an easy-cone magnetic state in free layers of magnetic tunneling junction (MTJ) with only first-order perpendicular magnetic anisotropy. By means of micromagnetic modeling, we study easy-cone angle dependence on the first-order anisotropy and its impact on the microwave sensitivity of the unbiased spin-torque diode. Considered magnetization tilt can be effectively engineered by choosing the ratio of the shape and first-order surface anisotropy, defined by the free layer’s thickness, in-plane MTJ shape, and interlayer magnetostatic interactions. Through micromagnetic modeling over a broad range of input powers and free-layer thicknesses, we demonstrate a significant role of nonlinearity and inhomogeneous magnetization dynamics in achieving high sensitivity levels. Our modeling for typical state-of-the-art parameters of MTJ shows a possibility to reach a record-breaking unbiased sensitivity of 1100 mV/mW and 4650 mV/mW after impedance matching, which is far beyond the parameters of commercial Schottky diodes. These results will be very helpful for the development of efficient spintronic ambient energy harvesters.

Bound states in continuum (BICs) are resonances with zero width (infinite lifetime) without any leakage into the surrounding media. Their fascinating properties and potential applications have attracted a great deal of interest. In this paper, we give an analytical, numerical, and experimental demonstration of BICs in simple acoustic structures based on either a single solid layer or a triple solid-liquid-solid layer inserted between two liquids. These modes are an intrinsic property of the inserted structure (solid layer or solid-liquid-solid triple layer) with free surfaces and are independent of the surrounding media. Two kinds of BICs are discussed: (i) Fabry-Perot (FP) BICs exist as the consequence of the intersection of the local resonances induced by inserted structure intersect the transmission zeros induced by the solid layers. (ii) Symmetry-protected (SP) BICs occur when appear at normal incidence due to the decoupling of the transverse modes in the solid layer from the longitudinal modes that propagate in the solid and solid-liquid multilayer media. When the incidence angle departs slightly from the BIC conditions, the latter transform into Fano resonances characterized by an asymmetric line shape in the transmission spectra. In addition, we show that the transmission zeros give rise to negative delay times and therefore acoustic superluminal effect. The theoretical results are obtained by means of the Green’s function method, whereas the experimental measurements are carried out in ultrasonic domain using plexiglass plates in water. These results may have important applications to realize subsonic and acoustic superluminal phenomena as well as acoustic filters and sensors.

Group-V substitution at the $\mathrm{Te}$ site, $X_{\mathrm{Te}}$ (X = $\mathrm{P}$, $\mathrm{As}$, $\mathrm{Sb}$), under $\mathrm{Cd}$-rich conditions is an effective way to enhance the hole density and, in the meantime, suppresses the dominant nonradiative carrier recombination center in $\mathrm{Cd}\mathrm{Te}$, thus, improving the performance of $\mathrm{Cd}\mathrm{Te}$ thin-film solar cells. However, it is not clear which group-V dopant, X, is the most effective dopant, because it is expected that ${\mathrm{P}}_{\mathrm{Te}}$ will have the shallowest acceptor level due to its high electronegativity, whereas ${\mathrm{Sb}}_{\mathrm{Te}}$ will have the smallest formation energy due to its small size mismatch with $\mathrm{Te}$. Our systematic first-principles study shows that the hole concentration contributed by the acceptor $X_{\mathrm{Te}}^{-}$ is limited by the related compensating $A{X}^{+}$ center that increases simultaneously with $X_{\mathrm{Te}}^{-}$ as the chemical potential of dopant X increases. However, the ratio of $X_{\mathrm{Te}}^{-}$ acceptors to the $A{X}^{+}$ donors can be significantly increased if the sample is grown at high temperature and then annealed to room temperature, achieving a high hole density and low Fermi level $(E_F)$. We find that all group-V ($\mathrm{P}$, $\mathrm{As}$, and $\mathrm{Sb}$) dopings can achieve maximum hole densities of about ${10}^{17}{\mathrm{cm}}^{-3}$, which are consistent with previous experimental results. Despite the relatively deep acceptor level of 150 meV, $\mathrm{Sb}$ doping can achieve a considerable hole density due to the low formation energy of substituting $\mathrm{Sb}$ for $\mathrm{Te}$ with similar atomic radii. $\mathrm{P}$ doping can achieve a higher hole density than that of $\mathrm{Sb}$ due to its shallow transition energy at ε(0/−) = 70 meV. However, the highest hole density is achieved through $\mathrm{As}$ doping, which is attributed to its balanced defect level at ε(0/−) = 80 meV and relatively small formation energy.

We report on the demonstration of an effective, nonlinearity-induced nonreciprocal behavior in a single nonmagnetic multi-mode Taiji resonator. Nonreciprocity is achieved by a combination of an intensity-dependent refractive index and of a broken spatial reflection symmetry. Continuous wave power-dependent transmission experiments show nonreciprocity and a direction-dependent optical bistability loop. These can be explained in terms of the unidirectional mode coupling that causes an asymmetric power enhancement in the resonator. The observations are quantitatively reproduced by a numerical finite-element theory and physically explained by an analytical coupled-mode theory. This nonlinear Taiji resonator has the potential of being the building block of large arrays for the study of topological and/or non-Hermitian physics. This represents an important step towards the miniaturization of nonreciprocal elements for photonic integrated networks.

Hybrid integrated quantum photonic circuits have the potential to scale up the number of quantum nodes and to build a network with distributed quantum-information-processing units at affordable resources. A central need of such systems is to have near-unity success-rate state transfer between the quantum nodes. This turns out to be a grand challenge since stringent conditions of spatial mode matching and time-reversal symmetry between the sending and receiving nodes have to be simultaneously satisfied. Here we devise a type of hybrid quantum node consisting of a single solid-state quantum system and a coupled-cavity structure toward achieving perfect state transfer between two distant nodes connected via waveguides. The hybrid node possesses flexibility in tailoring the temporal profile of the emitted single-photon wave packet without any dynamic modulation. In particular, we show it could emit time-reversal symmetric single-photon wave packets and fully couple the emission to the waveguide, fulfilling the conditions for perfect quantum state transfer. We develop a complete theoretical framework and provide a clear physical picture of the protocol for transferring a superposition state. We show ideally the simple node structures with one, two, and three chain-coupled microring resonators could lead to total success rates of 90.5%, 97.5%, and 99.3%, respectively. We then further discuss the influence of imperfections and experimental realizations of our scheme with integrated quantum photonics.

Quantum sensing employs quantum resources of a sensor to attain a smaller estimation error of physical quantities than the limit constrained by classical physics. To measure a quantum reservoir, which is significant in decoherence control, a nonunitary-encoding sensing scheme becomes necessary. However, previous studies showed that the reservoir-induced degradation to quantum resources of the sensor makes the errors divergent with the increase of encoding time. We here propose a scheme to use $N$ two-level systems as the sensor to measure a quantum reservoir. A threshold, above which the shot-noise-limited sensing error saturates or even persistently decreases with the encoding time, is uncovered. Our analysis reveals that it is due to the formation of a bound state of the total sensor-reservoir system. Solving the outstanding error-divergency problem in previous studies, our result supplies an insightful guideline in realizing a sensitive measurement of quantum reservoirs.

In general, singular acoustic beams travel in a straight line and expand during propagation, which limits the scope of their applications. We present a method for generating paraxial zero-order Bessel-like acoustic beams with arbitrary trajectories in water. We then add a vortex phase to obtain a first-order Bessel-like acoustic beam. This beam exhibits a combination of features of vortex, Bessel, and Airy beams. Moreover, it maintains a dark “hole” in the center, preserving its angular momentum, and displays resistance to diffraction, self-healing, and self-bending during propagation. Furthermore, we experimentally realize the beam using a three-dimensionally printed phase mask and observe the acoustic field distribution using the Schlieren imaging method. Finally, we realize particle trapping and transport in a curved trajectory in water.

We carry out a comprehensive series of experiments to understand the effect of multiple-contact miscibility on drainage from a porous medium. Our experiments are performed using a microfluidic porous medium consisting of a random distribution of cylindrical pillars to create a two-dimensional heterogeneous network of pores. A mixture of ethanol and 1-hexanol is injected into the porous medium as a wetting reservoir phase. A mixture of water and ethanol is subsequently injected to displace the reservoir phase. Depending on the precise composition of these two phases, they can be immiscible, develop miscibility over time through mass transfer between them, or be immediately miscible. We describe the distinct behaviors observed depending on the composition and injection flow rate and compare and contrast them with the classical parameter regime diagram for drainage displacement of R. Lenormand et al. [J. Fluid Mech. 189, 165 (1988)]. When the system is immiscible, the injection rate is the dominant control and the behavior is broadly classical. In systems that develop miscibility for the compositions we explore, the fluids remain immiscible at the injection tip and develop miscibility further upstream. Trapped regions of reservoir fluid are mobilized over time through the development of miscibility. For systems that nearly develop miscibility, trapped regions are mobilized at sufficiently high flow rates. We also show that the porosity has limited impact on the behavior and phase-saturation level. Our work has applications to carbon-dioxide enhanced oil recovery.

Forming a desired optical-field distribution from a given source requires precise spatial control of a field’s amplitude and phase. Low-loss metasurfaces that allow extreme phase and polarization control of optical fields have been demonstrated over the past few years. However, metasurfaces that provide amplitude control have remained lossy, utilizing mechanisms such as reflection, absorption, or polarization loss to control amplitude. Here, we describe the amplitude and phase manipulation of optical fields without loss, by using two lossless phase-only metasurfaces separated by a distance. We first demonstrate a combined beam former and splitter optical component using this approach. Next, we show a high-quality computer-generated three-dimensional hologram. The proposed metaoptic platform combines the advantage of lossless complex-valued field control with a physically small thickness. This approach could lead to low-profile, three-dimensional holographic displays, compact optical components, and high precision optical tweezers for micro-particle manipulation.

Electrical readout of metallic antiferromagnet (AFM) memories is typically realized by measuring the anisotropic magnetoresistance (AMR), but the mechanisms for enhanced AMR are not yet established. We study AMR of single crystals of AFM ${\mathrm{Fe}}_2\mathrm{As}$ from $T=5$ K to above the Néel temperature, $T_N\approx 353$ K. With an applied magnetic field $B$ rotating in the (001) plane, we observe a peak-to-peak AMR change of 1.3% for $B>1$ T at $T=5$ K, one order of magnitude larger than reported in $\mathrm{Cu}\mathrm{Mn}\mathrm{As}$, a widely studied candidate for AFM spintronics. The AMR varies strongly with temperature, decreasing by a factor of approximately 10 at $T\approx 200$ K. Our results suggest that large AMR in easy-plane AFMs may require Néel temperatures that greatly exceed room temperature.

This paper presents a reconfigurable metasurface that is able to dynamically and independently control the gain and propagation direction of the radiated field, with reduced biasing complexity and power consumption at a low profile. Moreover, the proposed metasurface is guided-wave fed thus leading to a compact tunable leaky-waveguide structure. Heretofore, reported reconfigurable metasurfaces have mainly demonstrated dynamic tailoring of free-space waves by redistributing their reflection or transmission phase profiles at a fixed amplitude. In contrast, we demonstrate dynamic transformation of a guided wave into an aperture field with controlled amplitude such that the gain and scan angle of the corresponding radiated field are dynamically and independently controlled. In particular, the aperture field is digitally synthesized by the proposed tunable Huygens’ metasurface whose local transmission coefficient is able to be dynamically tuned as two digital bits of $-|T_o|$ and $+|T_o|$, where $T_o$ represents a user-defined constant. This digital synthesis and the deliberate utilization of a nonbianisotropic type Huygens’ metasurface simplify biasing requirements, and make the proposed design more feasible. Through simulations and experiments, we show dynamic steering of a beam from $-{40}^{\circ }$ to $+{40}^{\circ }$, and two broadside radiations with different radiation gains at a fixed operating frequency of 5 GHz.

Nanomechanical circuits for transverse acoustic waves promise to enable new approaches to computing, precision biochemical sensing, and many other applications. However, progress is hampered by the lack of precise control of the coupling between nanomechanical elements. Here, we demonstrate virtual-phonon coupling between transverse mechanical elements, exploiting tunneling through a zero-mode acoustic barrier. This allows the construction of large-scale nanomechanical circuits on a silicon chip, for which we develop a scalable fabrication technique. As example applications, we build mode-selective acoustic mirrors with controllable reflectivity and demonstrate acoustic spatial-mode filtering. Our work paves the way toward applications such as fully nanomechanical computer processors and distributed nanomechanical sensors, and exploration of the rich landscape of nonlinear nanomechanical dynamics.

Optical implementation of artificial neural networks has been attracting great attention due to its potential in parallel computation at speed of light. Although all-optical deep neural networks (AODNNs) with a few neurons have been experimentally demonstrated with acceptable errors recently, the feasibility of large-scale AODNNs remains unknown because error might accumulate inevitably with increasing number of neurons and connections. Here, we demonstrate a scalable AODNN with programmable linear operations and tunable nonlinear activation functions. We verify its scalability by measuring and analyzing errors propagating from a single neuron to the entire network. The feasibility of AODNNs is further confirmed by recognizing handwritten digits and fashions, respectively.

In this paper, we propose a subwavelength closed surface consisting of radial gradient grooves for achieving localized acoustic rainbow trapping. Similar to conventional rainbow trapping based on a plane structured surface, we demonstrate that rainbow trapping can also be realized by a closed surface in the deep-subwavelength scale. In particular, the trapped spatial positions on the closed surface can be freely tuned by changing the structural parameters. Based on the advantages of the multiband response and subwavelength dimension of this structure, we propose an acoustic barrier consisting of an array of closed surfaces to achieve multiband sound insulation and high ventilation simultaneously. Investigated results indicate that this kind of acoustic barrier possesses the features of subwavelength scale, and multiband and omnidirectional responses for blocking incident acoustic waves, yet has high ventilation. Finally, we discuss the influence of the channel width on the magnitude and bandwidth of the transmission loss spectrum. The experimental results agree with our theoretical prediction that the material losses can broaden the response bandwidth, and form broadband sound insulation. The acoustic barrier consisting of localized rainbow trapping structures may have many applications in infrastructure requiring air-permeability and sound-proofing.

Wide-field magnetometry can be realized by imaging the optically detected magnetic resonance of diamond nitrogen-vacancy ($\mathrm{N}$-$V$) center ensembles. However, $\mathrm{N}$-$V$ ensemble inhomogeneities significantly limit the magnetic field sensitivity of these measurements. We demonstrate a double-double quantum (DDQ) driving technique to facilitate wide-field magnetic imaging of dynamic magnetic fields at a micron scale. DDQ imaging employs four-tone radio-frequency pulses to suppress inhomogeneity-induced variations of the $\mathrm{N}$-$V$ resonance response. As a proof of principle, we use the DDQ technique to image the dc magnetic field produced by individual magnetic nanoparticles tethered by single DNA molecules to a diamond-sensor surface. This demonstrates the efficacy of the diamond $\mathrm{N}$-$V$ ensemble system in high-frame-rate magnetic microscopy, as well as single-molecule biophysics applications.

We report on the magnetic damping properties of polycrystalline $\mathrm{Fe}\text{−}\mathrm{V}$ alloy thin films that are deposited at room temperature. By varying the concentration of $\mathrm{V}$ in the alloy, the saturation magnetization can be adjusted from that of $\mathrm{Fe}$ to near zero. We show that exceptionally low values of the damping parameter can be maintained over the majority of this range, with a minimum damping at approximately 15%–20% $\mathrm{V}$ concentration. Such a minimum is qualitatively reproduced with ab initio calculations of the damping parameter, although at a concentration closer to 10% $\mathrm{V}$. The measured intrinsic damping has a minimum value of (1.53 ± 0.08) × ${10}^{-3}$, which is approximately a factor of 3 higher than our calculated value of 0.48 × ${10}^{-3}$. From first-principles theory, we outline the factors that are mainly responsible for the trend of the damping parameter in these alloys. In particular, the band structure and resulting damping mechanism is shown to change at $\mathrm{V}$ concentrations greater than approximately 35% $\mathrm{V}$ content.

The advent of reliable nanoscale memristive components is promising for next-generation compute-in-memory paradigms; however, the intrinsic variability in these devices has prevented widespread adoption. Here, we show coherent electron wave functions play a pivotal role in the nanoscale transport properties of these emerging nonvolatile memories. By characterizing both filamentary and nonfilamentary memristive devices as disordered Anderson systems, the switching characteristics and intrinsic variability arise directly from the universality of electron transport in disordered media. Our framework suggests that localization phenomena in nanoscale solid-state memristive systems are directly linked to circuit-level performance. We discuss how quantum conductance fluctuations in the active layer set a lower bound on device variability. This finding implies that there is a fundamental quantum limit on the reliability of memristive devices and that electron coherence will play a decisive role in surpassing or maintaining Moore’s law with these systems.