One-way sound-wave propagation has been a research hotspot in acoustics during the past decade, with the production of acoustic diode, acoustic topological insulator, and parity-time-symmetric acoustic metamaterial. Here, we investigate topological pumping for sound waves in a coupled cavity chain along selective paths driven by dynamic modulation. We first showcase the adiabatic passage of sound energy in a heterostructured Su-Schrieffer-Heeger cavity chain. By solving the Dirac-like governing equation, we calculate the eigenspectrum of this model and prove the existence of a topologically protected interface state and end states. From the eigenspectrum, if no level crossing exists, energy transfer of the sound wave (or acoustic topological pumping) is reversed under adiabatic modulation, leading to the release and return of sound energy for a specific topological state. However, when the level crossing occurs, we find that topological pumping between the interface state and end states can be selectively excited, rendering complete transfer of the sound energy between two topological states. Moreover, topological pumping of sound waves can exist along the asymmetric paths under driven modulation. Our work has promising applications for robust, selective, and efficient sound energy transfer in acoustic network systems.

When spin-orbit torques are measured using spin-torque ferromagnetic resonance, two alternative ways of analyzing the results to extract the torque efficiencies—lineshape analysis and analysis of the change in linewidth versus direct current—often give inconsistent results. We identify a source for these inconsistencies. We show that fits of spin-torque ferromagnetic resonance data to the standard analysis framework leave significant residuals that we identify as due to (i) current-induced excitations of a small volume of magnetic material with magnetic damping much larger than that of the bulk of the magnetic layer, which we speculate is associated with the heavy-metal–magnet interface and (ii) oscillations of the sample magnetization at the modulation frequency due to heating. The dependence of the residual signals on direct current can interfere with an accurate extraction of spin-torque efficiencies by the linewidth method. We show that the discrepancies between the two types of analysis can be largely eliminated by extrapolation of the window of magnetic fields used in the linewidth fits to small values so as to minimize the influence of the residual signals.

Reference-frame-independent (RFI) measurement-device-independent (MDI) quantum key distribution (QKD) is immune to all detector side-channel attacks and is insensitive to slow drifts of reference frames; thus, it is a promising candidate for the realization of long-distance quantum secure communications. However, its practical performance is predominantly limited by the finite-data-size effect compared with standard MDI QKD. By combining the collective constraint and joint-study strategy with our parameter estimation processes for the RFI-MDI-QKD protocol, we reduce the influence of the finite-data-size effect and substantially improve the protocol’s performance under realistic experimental conditions. Furthermore, a proof-of-principle experimental demonstration is conducted, and a transmission distance of up to 200 km is achieved. Therefore, this study represents a further step toward long-distance quantum secure communications.

In this study, the influence of generalized-gradient-approximation (GGA) deficiencies, $d$-band occupancy, and itinerancy in the magnetoelectric (ME) effect is investigated from first principles by changing the interface composition of the ${\mathrm{Fe}}_3\mathrm{Si/}M{/\mathrm{Ba}\mathrm{Ti}\mathrm{O}}_3$ heterostructure, where $M$ stands for an atomic layer of pure $3d$ or $4d$ metals (from Sc to $\mathrm{Cd}$). The lattice overestimation of the ferroelectric phase leads to the overestimation of the magnetoelectricity, which can be partially corrected by using the Perdew-Burke-Ernzerhof functional revised for solids. The inclusion of Hubbard-like corrections generally predicts larger changes of interface magnetization upon the reversal of polarization direction, although preserving the trends of plain GGA calculations. The itinerancy of $4d$ states does not favor the interface ME coupling due to the lower density of states near the Fermi level. Here, it is shown how the control of the $d$-state energy levels through their electronic occupancy has the potential to substantially enhance the interface ME effect induced by bonding effects. A substantial increase of magnetoelectricity in the ${\mathrm{Fe}}_3\mathrm{Si/}{\mathrm{Ba}\mathrm{Ti}\mathrm{O}}_3$ heterostructure can be achieved by replacing interface $\mathrm{Fe}$ atoms with V or $\mathrm{Mn}$.

We propose a design to realize integrated broadband nonreciprocal microwave isolators and circulators using superconducting circuit elements without any magnetic materials. To obtain a broadband response, we develop a waveguide-based design by temporal modulations. The corresponding compact traveling-wave structure is implemented with integrated superconducting composite right-/left-handed transmission lines. The calculations show that the bandwidth of 580 MHz can be realized over a nonreciprocal isolation of 20 dB in reflections. Such on-chip isolators and circulators are useful for cryogenic integrated microwave connections and measurements, such as protecting qubits from the amplified reflected signal in multiplexed readout.

We derive an explicit partial-wave (Mie) series for the image of a dielectric microsphere collected by a typical infinity-corrected microscope. We model the propagation of the illumination and scattered vector fields through the optical components of the microscope by using the angular-spectrum theorem with the help of Wigner rotation matrix elements, allowing us to identify the contribution from spin-orbit helicity reversal. We consider a high numerical aperture objective well beyond the validity range of the paraxial approximation. The spherical aberration introduced by refraction at the planar interface between the sample and the glass slide is fully taken into account. By comparing our theoretical model with images of colloidal particles placed at different positions with respect to the objective focal plane, we characterize their radii and refractive index. We employ polystyrene microspheres with a known refractive index in order to fit the transverse attenuation length describing the transmission loss of the scattered field. As an application, we measure the radius and refractive index of individual silica beads. We compare the result for the radius with an independent measurement using high-resolution scanning electron microscopy. To validate the result for the refractive index, we develop a second method, independent of the theoretical model, based on the image contrast in glycerin-water solutions. In all cases we find very good agreement between our method and the validation procedures. In addition, the nonparaxial theory provides a reliable description of the images found for all focal-plane positions and for both polystyrene and silica microspheres. Our approach allows a common optical microscope to be used to measure the refractive index and radius of spherical particles covering the entire size range from the Rayleigh regime to the ray optics one.

Metasurfaces are ultrathin structures which are constituted by an array of subwavelength scatterers with designable scattering responses. They have opened up unprecedented exciting opportunities for extraordinary wave engineering processes. On the other hand, frequency converters have drawn wide attention due to their vital applications in telecommunication systems, healthcare devices, radio astronomy, military radars, and biological sensing systems. Here, we show that a spurious-free and linear frequency-converter metasurface can be realized by leveraging unique properties of engineered transmissive temporal supercells. Such a metasurface is formed by time-modulated supercells, which are composed of temporal and static patch resonators and phase shifters. This represents a frequency-converter metasurface possessing large frequency-conversion ratio with controllable frequency bands and transmission magnitude. In contrast to conventional nonlinear mixers, the proposed temporal frequency converter offers a linear response. In addition, by taking advantage of the proposed surface-interconnector-phaser-surface (SIPS) architecture, a spurious-free and linear frequency conversion is achievable, where all undesired mixing products are strongly suppressed. The proposed metasurface may be digitally controlled and programmed through a field programmable gate array. This makes the spurious-free and linear frequency-converter metasurface a prominent solution for wireless and satellite telecommunication systems, as well as invisibility cloaks and radars. This study opens a way to realize more complicated and enhanced-efficiency spectrum-changing metasurfaces.

Curved focal-plane arrays offer significant advantages compared with their flat counterparts. However, the curving of the material induces strains which alter its optoelectronic properties. A comprehensive framework is presented for the computational investigation of the band gaps of ${\mathrm{InAs}}_{1-x}{\mathrm{Sb}}_x$ alloys under various strain conditions that are relevant for manufacturing curved focal-plane arrays. The framework consists of both standard and hybrid functional density functional theory (DFT) calculations, finite-element analysis (FEA) calculations, and Gaussian process (GP) regression. The DFT calculations are used for investigating the change of the band gap of the material under various strain conditions. This dataset is then used for training a GP model which is utilized to assess the effects of strain across the device, based on the FEA simulations. The results show excellent predictive capabilities for the machine-learning model at a significantly reduced computational cost and are directly transferable to other device designs.

The interaction of traveling electrons with impurities or other inclusions in the crystal lattices of materials is a generic operation present in practically any quantum setup. That quantum scattering is hindered when the propagation medium changes abruptly from one region to another and, thus, a thin film in-between can be used to enhance the effect by smoothening the textural contrast. If the dimension and the material of this mediator layer is optimized, the scattering probability from the same dopant is increased by several orders of magnitude, across a wide band of impinging energies. The optimization is assisted by a semianalytical formulation, based on the small size of the lattice disorders, and provides fast and reliable results for an arbitrarily extensive assortment of quantum media. The proposed configurations exhibit significant robustness to engineering defects and may inspire experimental efforts towards designing integrated systems for sensing and quantum detection.

Hybrid magnetic shields with both active field generating components and high-permeability magnetic shielding are increasingly needed for various technologies and experiments that require precision-controlled magnetic field environments. However, the fields generated by the active components interact with the passive magnetic shield, distorting the desired field profiles. Consequently, optimization of the active components needed to generate user-specified target fields must include coupling to the high-permeability passive components. Here, we consider the optimization of planar active systems, on which an arbitrary static current flows, coupled to a closed high-permeability cylindrical shield. We modify the Green’s function for the magnetic vector potential to match boundary conditions on the shield’s interior surface, enabling us to construct an inverse optimization problem to design planar coils that generate user-specified magnetic fields inside high-permeability shields. We validate our methodology by designing two biplanar hybrid active-passive systems, which generate a constant transverse field, $\mathbf{B}=\hat{\mathbf{x}}$, and a linear field gradient, $\mathbf{B}=(-x\hat{\mathbf{x}}-y\hat{\mathbf{y}}+2z\hat{\mathbf{z}})$, respectively. For both systems, the inverse-optimized magnetic field profiles agree well with forward numerical simulations. Our design methodology is accurate and flexible, facilitating the miniaturization of high-performance hybrid magnetic field generating technologies with strict design constraints and spatial limitations.

The experimental optimization of a two-qubit controlled-$Z$ (cz) gate is realized following two different data-driven gradient-ascent pulse engineering (GRAPE) protocols with the aim of optimizing the gate operator and the output quantum state, respectively. For both GRAPE protocols, the key computation of gradients utilizes mixed information of the input $Z$-control pulse and the experimental measurement. With an imperfect initial pulse in a flattop waveform, our experimental implementation shows that the cz gate is quickly improved and the gate fidelities subject to the two optimized pulses are around 99%. Our experimental study confirms the applicability of the data-driven GRAPE protocols in the gate optimization problem.

The thermal spin torque induced by the spin-dependent Seebeck effect in double-barrier tunnel junctions is derived considering free-electron and tight-binding calculations. We show that in systems comprising ferromagnetic electrodes and nonmagnetic barriers, the in-plane component of the thermal spin torque is the dominant term, whereas in junctions comprising nonmagnetic electrodes and ferromagnetic barriers, both components, the in-plane and the out-of-plane, are comparable in magnitude. Moreover, larger torque amplitudes up to 3 orders of magnitude are obtained in the second system as a result of the spin-filtering effect; consequently, double-barrier tunnel junctions in the presence of magnetic insulators offer an enhanced thermal spin-torque mechanism for reliable applications. We propose taking advantage of quantum resonant tunneling through resonance states below the Fermi level in these structures that can pave a route toward achieving larger spin-torque efficiencies, even when considering smaller values of the exchange splitting. Furthermore, we identify the parameters needed to tune efficiently these resonant states.

Nonreciprocal signal operation is highly desired for various acoustic applications, where protection from unwanted backscattering can be realized, so that transmitting and receiving signals are processed in full-duplex mode. Here, we present the realization of a class of nonreciprocal circulators based on simply structured acoustic metagratings, which consist only of a few solid cylinders and a steady fluid flow with low velocity. These nonreciprocal metagratings are intelligently designed via a diffraction analysis of the linearized potential flow equation and a genetic-algorithm-based optimization process. Unitary reflection efficiency between desired ports of the circulators is demonstrated through full-wave numerical simulations, confirming nonreciprocal and robust circulation of the acoustic signal over a broad range of flow velocity magnitudes and profiles. Our design provides a feasible degree of tunability, including switching from reciprocal to nonreciprocal operation and reversing the handedness of the circulator, presenting a convenient and efficient approach for the realization of nonreciprocal acoustic devices from wavelength-thick metagratings. It may find applications in various scenarios, including underwater communication, energy harvesting, and acoustic sensing.

Electron paramagnetic resonance (EPR) is a versatile tool to probe spin physics in organic semiconductor materials. A common method used to detect the spin-½ paramagnetic resonance in organic light-emitting diodes (OLEDs) is to measure the device resistance under EPR conditions, i.e., to record electrically detected magnetic resonance (EDMR). Here, we present ultralow-frequency EDMR experiments on OLEDs that exhibit a qualitatively new line shape because of a quasistatic magnetic field effect: the modulation of the static ultrasmall field-effect magnetoresistance arising from the magnetic field amplitude $B_1$ of the radio frequency (rf) radiation. The disappearance of spin-½ Zeeman resonances of individual charge carriers in the OLED, i.e., the resonances at magnetic fields where the Zeeman splitting matches the photon energy of the incident rf radiation, coincides with the emergence of the quasistatic effect. We discuss the origin of this quasistatic magnetic field effect, its characteristic line shape in terms of the magnetic field dependence, the influence of experimental parameters, and the application potential with regards to EDMR experiments. The EDMR line shape can be inferred numerically from the magnetoresistance measurements. This approach enables a unique means of determining the drive-field strength $B_1$ in EDMR under driving conditions where alternative methods employing an analysis of the Zeeman resonance—such as power broadening and Rabi flopping—are not applicable.

This is a comment on the assumptions, model, and calculations in the paper by Farhat et al. [Phys. Rev. Appl. 11, 044089 (2019)] on cloaking of thermal waves in solids. The differences between the two thermal flux laws considered in the latter paper are also critically discussed, specifically showing that the chosen model does not correspond to the Maxwell-Cattaneo hyperbolic (wave) theory of heat transfer.