The effectiveness and accuracy of parameter estimation are guarantees for the high-performance and high-safety operation of practical continuous-variable quantum key distribution systems. A scheme based on Bayesian estimation is proposed to tackle the fluctuations of channels. In free-space channel, the Bayesian random-effects model is employed to construct complete prior information, and the channel state information extracted through pilot signals is applied to rectify the prior information. Compressed sensing technology is applied due to the sparsity of the free-space channel to reduce the cost of the system. Experimental results show that the proposed scheme has higher estimation accuracy under the free space, and the system performance is also enhanced. Moreover, the results under fiber show higher estimation accuracy and better stability than that of traditional methods. In conclusion, the scheme can create practical conditions for the construction of future global quantum networks.

Single-crystal growth, magnetic properties, and magnetocaloric effect of the $S=3/2$ kagome ferromagnet ${\mathrm{Li}}_9{\mathrm{Cr}}_3({\mathrm{P}}_2{\mathrm{O}}_7{)}_3({\mathrm{PO}}_4{)}_2$ (trigonal, space group: $P\overline{3}c1$) are reported. Magnetization data suggest dominant ferromagnetic intraplane coupling with a weak anisotropy and the onset of ferromagnetic ordering at $T_C\simeq 2.6$ K. Microscopic analysis reveals a very small ratio of interlayer to intralayer ferromagnetic couplings ($J_{\perp }/J\simeq 0.02$). Electron spin-resonance data suggest the presence of short-range correlations above $T_C$ and confirms the quasi-two-dimensional character of the spin system. A large magnetocaloric effect characterized by isothermal entropy change of $-\mathrm{\Delta }{S}_m\simeq 31\mathrm{J}{\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ and adiabatic temperature change of $-\mathrm{\Delta }{T}_{\mathrm{ad}}\simeq 9\mathrm{K}$ upon a field sweep of 7 T is observed around $T_C$. This leads to a large relative cooling power of $\mathrm{RCP}\simeq 284\mathrm{J}{\mathrm{kg}}^{-1}$. The large magnetocaloric effect, together with negligible hysteresis render ${\mathrm{Li}}_9{\mathrm{Cr}}_3({\mathrm{P}}_2{\mathrm{O}}_7{)}_3({\mathrm{PO}}_4{)}_2$ a promising material for magnetic refrigeration at low temperatures. The magnetocrystalline anisotropy constant $K\simeq -7.42\times {10}^4\mathrm{erg}{\mathrm{cm}}^{-3}$ implies that the compound is an easy-plane-type ferromagnet with the hard axis normal to the $a$$b$ plane, consistent with the magnetization data.

High-fidelity control of quantum systems is crucial for quantum information processing, but is often limited by perturbations from the environment and imperfections in the applied control fields. Here, we investigate the combination of dynamical decoupling (DD) and robust optimal control (ROC) to address this problem. In this combination, ROC is employed to find robust shaped pulses, wherein the directional derivatives of the controlled dynamics with respect to control errors are reduced to a desired order. Then, we incorporate ROC pulses into DD sequences, achieving a remarkable improvement of robustness against multiple error channels. We demonstrate this method in the example of manipulating nuclear spin bath via an electron spin in the nitrogen-vacancy center system. Simulation results indicate that ROC-based DD sequences outperform the state-of-the-art robust DD sequences. Our work has implications for robust quantum control on near-term noisy quantum devices.

Thermal noise in amorphous coatings is a limitation for a wide range of precision experiments such as gravitational-wave detectors (GWDs). Mirrors for GWDs are composed of multiple thin layers of dielectric materials deposited on a substrate: the stack is made of layers with a high-refractive index interleaved with layers of a low-refractive index. The goal is to obtain high reflectivity and low thermal noise. In this paper we report on the optical and mechanical properties of ion-beam-sputtered aluminium fluoride (${\mathrm{Al}\mathrm{F}}_3$) thin films that have one of the lowest refractive indexes among the known coating materials and we discuss their application in current and future GWDs.

The transport behaviors of topological guiding modes for classical waves in heterostructures with width degree of freedom have attracted much attention recently. With the width degree of freedom, topological waveguides are endowed with increased capacity in energy transport and improved flexibility in interfacing existing devices. However, to date, such topological waveguides are only realized in hexagonal lattices, by virtue of the known valleys existing at the Brillouin zone corners. Here, we report on the implementation of topological waveguides in heterostructures of a square lattice, consisting of three domains of acoustic crystals: the middle domain has Dirac points and the two side domains have valleys of opposite valley Chern numbers. A pair of counterpropagating topological waveguide modes, with amplitudes that are uniform in the central domain and attenuated sidewards in the two side domains, are supported in the heterostructure. We experimentally verify that the topological waveguide modes are valley locked and backscattering immune, and they have a high capacity for energy transport. We also demonstrate experimentally the conversion between the valley waveguide states and the usual valley edge states. More interestingly, when the central domain of the Dirac points is applied with a transverse structural gradient mirror symmetrically, a different kind of topological waveguide mode emerges. We verify that the waveguide modes possess the same topological properties.

Disorder in ultrathin magnetic films can significantly hinder domain-wall motion. One of the main issues on the path toward efficient domain-wall-based devices remains the characterization of the pinning landscape at the nanoscale. In this paper, we study domain-wall motion in $\mathrm{W}$/$\mathrm{Co}$-$\mathrm{Fe}$-$\mathrm{B}$/$\mathrm{MgO}$ thin films with perpendicular magnetic anisotropy crystallized by annealing at $400{}^{\circ }\mathrm{C}$ and a process based on $\mathrm{He}$${}^{+}$ irradiation combined with moderated temperatures. The magnetic properties are similar for the whole series of samples, while the magnetic domain-wall mobility is critically improved in the irradiated samples. By using an analytical model to extract the nanoscale pinning parameters, we reveal important variations in the disorder of the crystallized samples. This work offers an opportunity to selectively analyze the effects of disorder on the domain-wall dynamics, without the contribution of changes in the magnetic properties. Our results highlight the importance of evaluating the nanoscale pinning parameters of the material when designing devices based on domain-wall motion, which in return can be a powerful tool to probe the disorder in ultrathin magnetic films.

Topological insulators, protected by nontrivial band topology, exhibit backscattering-immune edge states, conducive to robust waveguiding with high efficiency. However, routing such robust edge states has been restricted by the isotropy in conventional unit cells respecting crystalline symmetries, such as $C_{4v}$ symmetry in a square lattice or $C_3$ symmetry in a hexagonal lattice. We effectively tackle this issue by introducing anisotropic coupling into a square lattice. With theoretical prediction from the discrete mechanical model, we experimentally demonstrate that such anisotropy can enable distinctive topological phases along different directions, giving rise to directional edge states. In addition, when the bands along the two directions are topologically identical and untrivial, the coexisting edge states have distinctive frequency ranges, giving rise to the frequency-routed properties. Our work offers an effective strategy for the robust steering, filtering, detection, and transmission of elastic waves through tactical edge state routing.

Size-dependent continuous microparticle separation is important for various applications in sensing and drug delivery to particle manufacturing. Several invasive, noninvasive, active, and passive methods have been developed, spanning operation across a wide range of system scales, from bulk-macroscale devices to precise-microscale systems. However, devices in these wide system scales have contradictory benefits. Bulk methods have limitations with the size of particles that can be manipulated, while microscale methods have limitations with processing volumes. Here, we present a method to continuously separate micron- and submicron-sized particles using low-frequency vibrations (of the order of 10 Hz). We generate capillary waves in an open channel with a continuous particle-laden flow perpendicular to the vibration direction. The size-based response of the ensuing flow field aligns particles above a critical size along the center of the channel, while the remaining particles remain in the bulk and undergo downstream separation. A key feature of this mechanism is that the separated particle sizes can be controlled by changing the vibration amplitude. The proposed mechanism and design provides a semibulk method to distinguish submicron-sized particles with robust in situ control using a simple millimeter-scale setup and operation.

Voltage-induced dynamic switching in magnetic tunnel junctions (MTJs) is a writing technique for voltage-controlled magnetoresistive random access memory (VCMRAM), which is expected to be an ultimate nonvolatile memory with ultralow power consumption. In conventional dynamic switching, the width of subnanosecond write voltage pulses must be precisely controlled to achieve a sufficiently low write-error rate (WER). This very narrow tolerance of pulse width is the biggest technical difficulty in developing VCMRAM. Heavily damped precessional switching is a writing scheme for VCMRAM with a substantially high tolerance of pulse width, although the minimum WER has been much higher than that of conventional dynamic switching with an optimum pulse width. In this study, we theoretically investigate the effect of MTJ shape and the direction of the applied magnetic field on the WER of heavily damped precessional switching. The results show that the WER in elliptical-cylinder MTJs can be several orders of magnitude smaller than that in usual circular-cylinder MTJs when the external magnetic field is applied parallel to the minor axis of the ellipse. The reduction in the WER is due to the fact that the demagnetization field narrows the component of the magnetization distribution perpendicular to the plane direction immediately before the voltage is applied.

Binding, magnetic, and electron transport properties of $3d$ transition-metal (TM) adatoms ($\mathrm{Cr}$-$\mathrm{Cu}$) on an antiferromagnetic armchair graphene nanoribbon heterojunction are studied by means of density-functional-theory (DFT) calculations and nonequilibrium Green function (NEGF) formalism. The heterojunction emulates a system of two symmetric potential barriers and one well with resonances (quasibound states). Binding energies show a strong interaction between $3d$ TM adatoms and heterojunction hollow sites, where $3d$ TM adatoms prefer the edges rather than the middle of the graphene heterojunction. As for magnetic properties, heterojunctions with Cr, $\mathrm{Mn}$, and $\mathrm{Fe}$ have a ferromagnetic behavior with a total magnetic moment $\ge 4{\mu }_B$, while $\mathrm{Co}$, $\mathrm{Ni}$, and $\mathrm{Cu}$ adatoms induce antiferromagnetism in the full system with total magnetizations $<3{\mu }_B$. Transmission curves show that electronic resonances undergo a spin-splitting effect induced by $3d$ TM adatoms, and current versus bias-voltage curves show spin currents from which spin-polarized ratios (SPRs) $>75\mathrm{\%}$ for $\mathrm{Co}$ and approximately $100\mathrm{\%}$ for $\mathrm{Ni}$ adatoms are calculated. Finally, an oscillatory behavior is observed in SPR curves at low bias due to a combined effect of resonant tunneling and spin filtering. Therefore, these results suggest that the present antiferromagnetic graphene heterojunction with $\mathrm{Co}$ and $\mathrm{Ni}$ adatoms is suitable for future spin and low-power nanoelectronics as a spin-dependent resonant tunneling device.

We report a switchable time-varying mirror, composed of an indium-tin-oxide–gold bilayer, displaying a tenfold modulation of reflectivity ($\mathrm{\Delta }R\approx 0.6$), which saturates for a driving-pump intensity $I_{\mathrm{pump}}\approx 100\mathrm{GW}/{\mathrm{cm}}^2$. Upon interacting with the saturated time-varying mirror, the frequency content of a reflected pulse is extended up to 31 THz, well beyond the pump spectral content (2.8 THz). We interpret the spectral broadening as a progressive shortening of the mirror rise time from 110 fs to below 30 fs with increasing pump power, which is confirmed by four-wave-mixing experiments and partially captured by a linear time-varying model of the mirror. A temporal response unbounded by the pump bandwidth enables applications for spectral manipulation from time-varying systems with impact for communication networks, optical switching, and computing.

The discovery of very large piezo- and pyroelectric effects in ${\mathrm{Zr}\mathrm{O}}_2$ and ${\mathrm{Hf}\mathrm{O}}_2$-based thin films opens up opportunities to develop silicon-compatible sensor and actor devices. The effects are amplified close to the polar-orthorhombic to tetragonal phase-transition temperature. Molecular dynamics is the preferred technique to simulate such effects, though its application has to solve the dilemma between sufficient accuracy and sufficient efficiency of the interatomic force field. Here we present a deep neural-network-based interatomic force field of ${\mathrm{Zr}\mathrm{O}}_2$ learned from ab initio data using a systematic learning procedure in the deep potential framework. The model potential is verified to predict a variety of structural and dynamic properties with an accuracy comparable to density-functional-theory calculations. Then the deep potential model is used to reproduce the different thermal expansion and piezo- and pyroelectric phenomena in ${\mathrm{Zr}\mathrm{O}}_2$ with molecular dynamics calculations. At low temperature simulating the direct effect we find negative values for the piezo- and pyroelectric coefficients matching the ab initio calculations. Approaching the phase-transition temperature these values remain negative and become large. Simulating the field-induced effect above the phase-transition temperature we find positive, giant piezoelectric coefficients matching the observations. The model is able to explain the large values and the sign of the experimental observations in relation to the polar-orthorhombic to tetragonal phase transition. The model furthermore explains the recently observed giant dielectric constant in a similar system.

We provide a theoretical description of light scattering by a spherical particle the permittivity of which is modulated in time at twice the frequency of the incident light. Such a particle acts as a finite-sized photonic time crystal and, despite its subwavelength spatial extent, can host optical parametric amplification. Conditions of parametric Mie resonances in the sphere are derived. We show that time-modulated materials provide a route to tailor directional light amplification, qualitatively different from that in scatterers made from a gain media. We design two characteristic time-modulated spheres that simultaneously exhibit light amplification and desired radiation patterns, including those with zero backward and/or vanishing forward scattering. The latter sphere provides an opportunity for creating shadow-free detectors of incident light.

Some voltage sources are transient, lasting only for a moment of time, such as the voltage generated by converting a human motion into electricity. Such sources moreover tend to have a degree of randomness as well as internal resistance. We investigate how to put a number to how valuable a given transient source is. We derive several candidate measures via a systematic approach. We establish an interconvertibility hierarchy between such sources, where interconversion means adding passive interface circuits to the sources. Resistors at the ambient temperature are at the bottom of this hierarchy and sources with low internal resistance and high internal voltages are at the top. We provide three possible measures for a given source that assign a number to the source respecting this hierarchy. One measure captures how much “$\mathrm{unitdc}$” the source contains, meaning $1$ V dc with $1\mathrm{\Omega }$ internal resistance for $1$ s. Another measure relates to the signal-to-noise ratio of the voltage time series, whereas a third is based on the relative entropy between the voltage probability distribution and a thermal noise resistor. We argue that the $\mathrm{unitdc}$ measure is particularly useful by virtue of its operational interpretation in terms of the number of unit dc sources that one needs to combine to create the source or that can be distilled from the source.

Electromagnetic scattering bounds on subwavelength structures play an important role in estimating performance of antennas, radio frequency identification tags, and other wireless communication devices. An appealing approach to increase a scattering cross section is accommodating several spectrally overlapping resonances within a structure. However, numerous fundamental and practical restrictions have been found and led to the formulation of Chu-Harrington, Geyi, and other limits, which provide an upper bound to scattering efficiencies. Here we introduce a two-dimensional array of near-field coupled split-ring resonators and optimize its scattering performance with the aid of a genetic algorithm operating in 19-dimensional space. Experimental realization of the device is demonstrated to surpass the theoretical single-channel limit by a factor of >2, motivating the development of tighter bounds of scattering performance. A superradiant criterion is suggested to compare maximal scattering cross sections with the single-channel dipolar limit multiplied by the number of elements within the array. This empirical criterion, which aims to address performance of subwavelength arrays formed by near-field coupled elements, is found to be rather accurate in application to the superscatterer, reported here. Furthermore, the superradiant bound is empirically verified with a Monte Carlo simulation, collecting statistics on scattering cross sections of a large set of randomly distributed dipoles. The demonstrated flat superscatterer can find use as a passive electromagnetic beacon, making miniature airborne and terrestrial targets radar visible.

We discuss the effect of the dielectric environment (substrate/bottom oxide, gate insulator, and metal gates) on electronic transport in two-dimensional (2D) transition metal dichalcogenides (TMD) monolayers. We employ well-known ab initio methods to calculate the low-field carrier mobility in free-standing layers and use the dielectric continuum approximation to extend our study to layers in double-gate structures, including the effects of dielectric screening of the electron-phonon interaction caused by the bottom oxide and the gate insulator, and of scattering with hybrid interface optical-phonon/plasmon excitations (“remote phonon scattering”). We find that the presence of insulators with a high dielectric constant may improve significantly the carrier mobility. However, scattering with the interface hybrid excitations negates this gain and degrades the mobility significantly below its free-standing value. We find that this process is dominated by long-wavelength interactions that, for the carrier sheet density of interest, are strongly affected by the coupling with the 2D plasmons. Considering 2D layers in a double-gate geometry with ${\mathrm{Si}\mathrm{O}}_2$ as bottom oxide and various top-gate insulators, we find that the mobility decreases as the top-insulator dielectric constant increases (from hBN to ${\mathrm{Zr}\mathrm{O}}_2$), as expected. However, we observe two main deviations from this trend: high mobility is predicted in the case of the weakly polar hBN, and much lower than expected mobility is calculated in the case of gate-insulator/TMD/bottom-oxide stacks in which two or more polar materials have optical phonons with similar resonating frequencies. We also find that the effect of screening by metal gates is noticeable but not particularly strong. Finally, we discuss the effect of the TMD dielectric constant, of the free-carrier density, and of temperature on the transport properties of TMD monolayers.

Quantum transduction refers to the coherent conversion between microwave and optical states, which can be achieved by quantum teleportation if given high-fidelity microwave-optical entanglement, namely entanglement-based quantum transduction. Reliable microwave-optical entanglement can be generated using various platforms. In this paper we base the discussion on a piezo-optomechanical system and make the teleportation induced conversion scheme more concrete in the framework of quantum channel theory. By comparing the quantum capacity between the entanglement-based conversion channel and the traditional direct quantum transduction channel, we show that the entanglement-based scheme indeed admits a positive transduction rate when the direct quantum transduction has zero quantum capacity. Given two piezo-optomechanical systems, we also investigate the generation of microwave-microwave entanglement from entanglement swapping within continuous-variable and discrete-variable settings, showing the potentials of directly connecting microwave quantum processors by microwave-microwave quantum teleportation.

Ghost imaging with thermal light is a well-known imaging technique in which the light beam used for image reconstruction does not necessarily interact with the object. However, ghost imaging is generally time-consuming due to data acquisition and complex algorithm. Thus a fundamental question arises as to whether we can realize object identification without the need of reconstructing the full ghost images. Here we present a protocol of thermal-light-based object identification in a nonlocal manner, i.e., ghost identification, to answer this question positively. We employ a holography-type method to add an auxiliary planar phase together with the conjugate Fourier spectrum of the test object to the thermal light source, which revives the coherent transfer of complex spectrum from thermal light modulation to the second-order correlation function, regardless of no entanglement. In our experiment, we succeed in demonstrating the fairly good performance of ghost identification for both simple geometric objects and quick response codes, and particularly for fingerprints. Our protocol can find direct applications in secure biosensing and covert detection where very low light illumination levels are needed.

We propose a scheme to obtain highly sensitive oscillators in a coupled-resonator system with an exceptional point of degeneracy (EPD) and a small instability. The oscillator with the exceptional degeneracy is realized by using two coupled resonators with an almost balanced small-signal gain and loss, that saturates due to nonlinear effects of the active component, resulting in an oscillation frequency that is very sensitive to a perturbation of the circuit. Two cases are investigated, with two parallel $LC$ resonators with balanced small-signal gain and loss that are either coupled wirelessly by mutual inductance or coupled wired by a capacitor. This paper demonstrates theoretically and experimentally the conditions to obtain a second-order EPD oscillator and analyzes the ultrasensitivity of the oscillation frequency to components’ perturbation, including the case of asymmetric perturbation that breaks $PT$ symmetry. We discuss the effects of nonlinearity on the performance of the oscillator and how the proposed scheme improves the sensing’s sensitivity of perturbations. In contrast to previous methods, our proposed degenerate oscillator can sense either positive or negative changes of a circuit component. The degenerate oscillator circuit may find applications in various areas, such as ultrasensitive sensors, tunable oscillators, and modulators.