We present a hybrid quantum system composed of a magnetic vortex and a nanomechanical resonator. We show that the gyrotropic mode of the vortex can coherently couple to the quantized mechanical motion of the resonator through magnetic interaction. Benefiting from the topologically protected properties and the low damping of vortices, as well as the excellent coherent features of nanomechanical resonators, the proposed system can achieve strong coupling and even the ultrastrong-coupling regime by choosing appropriate parameters. In combination with other quantum systems, such as a nitrogen-vacancy (N-V) center, coherent state transfer between the vortex excitation and the spin can be realized. This setup provides a potential platform for quantum information processing and investigations into the ultrastrong-coupling regime and macroscopic quantum physics.

We study the twist-induced control of the near-field radiative heat transfer between two hyperbolic antiferromagnetic insulators under external magnetic fields. We show that the near-field heat flux can be affected by both the twist angle $\theta$ and the magnitude of the applied magnetic field with different broken symmetries. Irrespective of the twist angle, the external magnetic field causes the radiative heat flux to change nonmonotonically, and the minimum heat flux can be found with magnetic fields of approximately 1.5 T. Such nonmonotonic behavior is because the magnetic field can radically change the nature of the magnon polaritons with time-reversal symmetry breaking. The field not only affects the topological structure of surface magnon polaritons but also induces volume magnon polaritons that progressively dominate the heat transfer as the field increases. We further propose a twist-induced thermal switch device with inversion symmetry breaking, which can strongly regulate radiative heat flux through different magnetic fields. Our findings account for a characteristic modulation of radiative heat transfer with implications for applications in efficient thermal management.

We introduce a simple widely applicable formalism for designing “error-divisible” two-qubit gates: a quantum gate set where fractional rotations have proportionally reduced error compared to the full entangling gate. In current noisy intermediate-scale quantum (NISQ) algorithms, performance is largely constrained by error proliferation at high circuit depths, of which two-qubit gate error is generally the dominant contribution. Further, in many hardware implementations, arbitrary two-qubit rotations must be composed from multiple two-qubit stock gates, further increasing error. This work introduces a set of criteria, and example wave forms and protocols to satisfy them, using superconducting qubits with tunable couplers for constructing continuous gate sets with significantly reduced leakage and dynamic $ZZ$ errors for small-angle rotations. If implemented at scale, NISQ-algorithm performance could be significantly improved by our error-divisible gate protocols.

The ability to control and modify infrared excitations in condensed matter is of both fundamental and applied interest. Here we explore a system supporting low-energy excitations, in particular, mid-infrared localized plasmon modes and phonon polaritons that are tuned to be strongly coupled. We study the coupled modes by using far-field infrared spectroscopy, state-of-the-art monochromated electron energy-loss spectroscopy, numerical simulations, and analytical modeling. We demonstrate that the electron probe facilitates a precise characterization of polaritons constituting the coupled system, and enables an active control over the coupling and the resulting sample response both in frequency and space. Although far-field optical spectra can be substantially different from near-field electron energy-loss spectra, we show that a direct comparison is possible via postprocessing and right positioning of the electron beam. The resulting spectra allow us to evaluate the key parameters of the coupled system, such as the coupling strength, which we demonstrate to be probe independent. Our work establishes a rigorous description of the spectral features observed in light- and localized electron-based spectroscopies, which can be extended to the analysis of analogous optical systems with applications in heat management and electromagnetic field concentration or nanofocusing.

We study using three-dimensional kinetic Monte Carlo (KMC) simulations to what extent the formation of Schottky contacts between a metal electrode and a molecularly doped disordered organic semiconductor can be understood from the theory for crystalline inorganic semiconductors, adapted to include the effects of the localized nature of the states in which the charge carriers reside and the hopping transport in between these states. The thickness of the Schottky-contact depletion region is found to be significantly smaller than as expected when the energetical disorder is neglected. The presence of energetic disorder is also found to influence the voltage dependence of the width of the depletion regions near the contacts of single-layer double-Schottky-contact devices. The voltage drop over the two depletion regions and the remaining charge-neutral bulk layer is shown to be described successfully by a semianalytical model, based on an accurately parameterized bulk mobility function of the dopant concentration, energetic disorder, and the electric field. We furthermore find that the mobility in the depletion regions is drastically reduced. As a result, the depletion-region formation process can be ultraslow, with a characteristic time scale ranging from microseconds to beyond milliseconds.

High-precision and wide-band microthrust testing and evaluation on the ground is key to achieving precision, drag-free control, which is extremely demanding for spaceborne gravitational-wave detection missions. However, unlike a stable space environment, complex ground vibrations inevitably affect the evaluation of microthrust. To reduce the interference of ground vibrations, this paper proposes a suppression solution based on a hanging pendulum with an adjustable mass center. The theoretical analysis shows that the response of the pendulum to ground vibration can be effectively reduced with a matched mass center. The dual-frequency modulation experiment verified the linear relationship between vibration noise and mass-center-to-swing-point distance. By setting the mass-center distance from the swing point to 0.2 mm, the noise of the sub-micro-Newton balance is better than $0.1\text{μ}\mathrm{N}{\mathrm{Hz}}^{-1/2}$ from 6 mHz to 20 Hz. This method effectively suppresses the interference of ground vibration, which helps us to obtain an accurate microthrust model for space gravitational-wave missions, while providing a solution for weak-force, ground testing over wide-band frequencies.

We have developed a method of determining the effective mass of charge carriers in a two-dimensional electron system (2DES) based on the phase analysis of the radiation transmitted through the 2DES. This measurement technique is demonstrated in the experiments with a high-quality 2DES in $\mathrm{In}\mathrm{As}$/$\mathrm{Al}\mathrm{Sb}$ and $\mathrm{Ga}\mathrm{As}$/$(\mathrm{Al},\mathrm{Ga})\mathrm{As}$ heterostructures. Furthermore, we find that the proposed approach gives compatible results with the cyclotron resonance spectroscopy method.

Surface acoustic waves at frequencies beyond a few GHz are promising components for quantum technology applications. Applying scanning x-ray diffraction microscopy we directly map with a high lateral spatial resolution the complete strain field generated near the $\mathrm{Ga}\mathrm{As}$ surface by a standing surface acoustic wave with wavelength $\lambda \simeq 500$ nm corresponding to frequencies near 6 GHz. We find that the lattice distortions perpendicular to the surface are phase shifted compared to those in propagation direction. Model calculations based on Rayleigh waves confirm our measurements. Our results represent a break through in providing a full characterization of a radio-frequency surface acoustic wave beyond plain imaging.

Recently synthesized two-dimensional (2D) van der Waals (vdW) ferromagnets, ${\mathrm{Fe}}_x{\mathrm{Ge}\mathrm{Te}}_2$ (x = 4 and 5), have attracted great attention due to their room-temperature Curie temperature. By using ab initio noncollinear-spin quantum transport simulations, we predict a monotonic increasing tendency of the tunneling magnetoresistance (TMR) with increasing θ (the angle between the spins of the two electrodes) in ${\mathrm{Fe}}_x{\mathrm{Ge}\mathrm{Te}}_2$/graphene/${\mathrm{Fe}}_x{\mathrm{Ge}\mathrm{Te}}_2$ vdW magnetic tunnel junctions (MTJs). The calculated maximal TMR of the two MTJs is up to 7000% and 1100% for x = 4 and 5, respectively, and the former is even nearly 6 times larger than that of the commonly used $\mathrm{Mg}\mathrm{O}$-based MTJ (1000% at 5 K) owing to nearly perfect spin polarization. The calculated maximal spin-transfer torque per voltage is 1–2 orders of magnitude larger than that of the $\mathrm{Mg}\mathrm{O}$-based one, resulting in a reduction in the critical current for magnetization reversal by a factor of 4–5. Such high-performance 2D ${\mathrm{Fe}}_x{\mathrm{Ge}\mathrm{Te}}_2$-based (x = 4 and 5) MTJs are promising for next-generation room-temperature nonvolatile memories.

This paper mainly studies the problem of metasurfaces between different mediums. Despite recent advances on metasurfaces, there has been a critical limitation that wave manipulation between two different mediums is theoretically impossible with general approaches since full transmission and desired phase shift cannot be achieved simultaneously. In order to get over this theoretical issue, we propose an asymmetric metasurface that can manipulate waves between two different mediums. By introducing the asymmetry, achieving both full transmission and desired phase shift simultaneously is theoretically enabled. From theoretical approaches, an actual continuum metasurface is designed and applied to various wave manipulations between two different mediums. We expect that the proposed metasurface can be applied to various applications in the area of ultrasonics and vibrations.

True random number generators are of great interest in many computing applications, such as cryptography, neuromorphic systems, and Monte Carlo simulations. Here, we investigate perpendicular magnetic-tunnel-junction nanopillars (pMTJs) activated by short-duration (nanosecond) pulses in the ballistic limit for such applications. In this limit, a pulse can transform the Boltzmann distribution of initial free-layer magnetization states into randomly magnetized down or up states, i.e., a bit that is 0 or 1, easily determined by measurement of the tunnel resistance of the junction. It is demonstrated that bit streams with millions of events: (1) are very well approximated by a normal distribution; (2) pass multiple statistical tests for true randomness, including all the National Institute of Standards and Technology tests for random number generators with only one xor operation; (3) can be used to create a uniform distribution of 8-bit random numbers; and (4) can have no drift in the bit probability with time. The results presented here show that pMTJs operated in the ballistic regime can generate true random numbers at around 50-MHz bit rates, while being more robust to environmental changes, such as their operating temperature, compared to other stochastic nanomagnetic devices.

$L{1}_0$-phase $\mathrm{Fe}\mathrm{Pt}$ is well known for its unusually robust perpendicular magnetic anisotropy (PMA) properties arising from strong conduction-electron spin-orbit coupling (SOC) with the $\mathrm{Fe}$ orbital moment. The strong PMA enables stable magnetic storage and memory devices with ultrahigh capacity. Meanwhile, SOC is also the premise of the recently discovered spin-orbit-torque (SOT) effect, which opens avenues for possible electrical manipulation of magnetization for $L{1}_0$-$\mathrm{Fe}\mathrm{Pt}$. The bulk SOT of the $L{1}_0$-$\mathrm{Fe}\mathrm{Pt}$ single layer was discovered recently; this leads to the magnetization of $L{1}_0$-$\mathrm{Fe}\mathrm{Pt}$ reversibly switching on itself. However, deterministic SOT switching of bulk perpendicularly magnetized $\mathrm{Fe}\mathrm{Pt}$ magnets relies on an external magnetic field to break the symmetry. Here, the symmetry-breaking issue is resolved by interlayer exchange coupling, where the $\mathrm{Fe}\mathrm{Pt}$ layer is coupled with an in-plane magnetized $\mathrm{Ni}\mathrm{Fe}$ layer through a $\mathrm{Ti}\mathrm{N}$ spacer layer. Furthermore, our device also presents memristive or gradual switching behaviors, even without an external field, offering the potential for constructing spin synapses and spin neurons for neuromorphic computing. An artificial neural network with high accuracy (∼91.17%) is realized based on the constructed synapses and neurons. Our work paves the way for field-free SOT switching of single bulk PMA magnets and their potential applications in neuromorphic computing.

We report on the observation of large and tunable perpendicular magnetic anisotropy (PMA) in $\mathrm{Co}$-based ferromagnetic (FM) films induced by an antiferromagnetic (AFM) $\delta \text{−}\mathrm{Mn}$ layer. The perpendicular anisotropic energy of the bilayers can be manipulated by varying the thickness of the $\delta \text{−}\mathrm{Mn}$ layer and the maximum reaches 2.41 × ${10}^7$ erg/cm${}^3$. The coercivity of bilayers is also effectively regulated over a broad range from 0.02 to 7.09 T. We demonstrate that this large PMA originates from strong interfacial exchange coupling and the AFM anisotropic energy. In addition, the antisymmetric longitudinal magnetoresistance occurs in the bilayers in the absence of asymmetric geometry or magnetic field. These results enrich our understanding of AFM-induced large PMA in an AFM/FM bilayer system and provide a promising bilayer structure for exploring high-density magnetic memory devices.

A label-free approach to improving the performances of confocal scanning imaging is proposed. We experimentally demonstrate its feasibility using acoustic waves. It relies on a way to encode spatial information using the temporal dimension. By moving an emitter, used to insonify an object, along a circular path, we create a temporally modulated wavefield. Because of the symmetries of the problem, the spatiotemporal input field can be decomposed into harmonics corresponding to different spatial vortices. Acquiring the back-reflected waves with receivers that are also rotating, multiple images of the same object with different point spread functions are obtained. Not only is the resolution improved compared to a standard confocal configuration, but the accumulation of information also allows the building of images that beat the diffraction limit.

Here, we show that space-time modulations provide an exciting route to realize complex nonreciprocal couplings, and, in particular, the elusive axion response. We develop an analytical formalism to homogenize anisotropic space-time crystals in the long wavelength limit. It is found that space-time crystals with suitable glide-rotation symmetry can have a giant axion-type response, several orders of magnitude larger than in natural materials. The nonreciprocal axion response may have interesting applications in optics, for example, in electromagnetic isolation, and in addition may enable exciting forms of light-wave interactions.

We investigate the anisotropy of Gilbert damping in $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}(001)$ films with body-centered-cubic crystalline structure using the ferromagnetic resonance method. The $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}(001)$ films are epitaxied on $\mathrm{Mg}\mathrm{O}(001)$ by means of pulsed-laser deposition using a ${\mathrm{Co}}_{0.4}{\mathrm{Fe}}_{0.4}{\mathrm{B}}_{0.2}$ target. The measured damping constant shows a clear four-fold symmetry with respect to the in-plane field orientation with a maximum-minimum ratio larger than 650%, and maximum damping exists for the field along $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}⟨100⟩$. Such a large damping anisotropy can trigger the strong field-orientation dependence of microwave-excited magnetization precession. The anisotropic magnetoresistance (AMR) in $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}(001)$ films shows little current-orientation dependence, indicating that AMR has weak correlation with the origin of damping anisotropy. Our experimental results provide an effective way to control intrinsic damping with magnetization orientation for designing and optimizing the performance of spintronics devices based on $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$.

Magnetic skyrmions have promising applications as nonvolatile memory for their particlelike and topological-protected features. The recently reported artificial skyrmion has shown a better prospect due to its controllable size and site. Here, we propose and experimentally demonstrate multiple nonvolatile skyrmion states in nanostructured synthetic antiferromagnetic $[\mathrm{Pt}/\mathrm{Co}{]}_4/\mathrm{Ru}/[\mathrm{Co}/\mathrm{Pt}{]}_4$ multilayers at room temperature. Our magneto-optical Kerr effect measurements of major and minor reversal curves reveal that the skyrmion can exist at zero field with memristive behavior. Using the observed multiple skyrmion states, nonvolatile memory behavior is demonstrated. Our results provide evidence for the concept of skyrmion-based nonvolatile memory, i.e., skyrmion-based memristors and artificial synapses or neurons.

Making an object invisible means that an incident electromagnetic wave is not scattered with the object being present, which has been widely studied over the past few decades. Nevertheless, rendering omnidirectional invisibility for a large object in free space is still challenging. Here, we present a dual-band antiscattering coating using a double-layered metamaterial. By putting such coatings on both sides of a thin dielectric slab, omnidirectional invisibility is observed. It is shown that the peculiar double-layered metamaterial can provide electric polarization neutralization to the original thin slab, and the Maxwell Garnett mixing rule is safely invoked to guarantee the full elimination of scattering regardless of the incident angle. Full-wave simulations and experimental measurements exhibit that a composite made of a dielectric slab sandwiched by two antiscattering coatings can avoid any scattering in both the C and Ku bands with arbitrary incident angles. The proposed approach is quite simple and nondestructive to the original objects, which can be conveniently scaled to other scenarios including terahertz and optical applications, for boosting various internet of things devices, radar radomes, noninterference isolators in optics, and invisibility cloaks.

The design of fast algorithms for combinatorial optimization greatly contributes to a plethora of domains such as logistics, finance, and chemistry. Quantum approximate optimization algorithms (QAOAs), which utilize the power of quantum machines and inherit the spirit of adiabatic evolution, are approaches to tackle combinatorial problems with potential runtime speedups. However, hurdled by the limited quantum resources nowadays, QAOAs are infeasible to manipulate large-scale problems. To address this issue, here we revisit the MaxCut problem via the divide-and-conquer heuristic: seek the solutions of subgraphs in parallel and then merge these solutions to obtain the global solution. Because of the ${\mathbb{Z}}_2$ symmetry in MaxCut, we prove that the merging process can be further cast into a new MaxCut problem and thus be addressed by QAOAs or other MaxCut solvers. In view of this, we propose QAOA-in-QAOA (${\mathrm{QAOA}}^2$) to solve arbitrary large-scale MaxCut problems using small quantum machines. We also prove that the performance of ${\mathrm{QAOA}}^2$ is lower bounded with respect to the divide-and-conquer process. Experiment results illustrate that, under different graph settings, ${\mathrm{QAOA}}^2$ attains a competitive or even better performance over classical algorithms when the node count is around $2000$. Our method can be seamlessly embedded into other advanced strategies to enhance the capability of QAOAs in large-scale combinatorial optimization problems.

Metalenses with high-efficiency focusing functionality and a water-flow-permeable structure are desired in various acoustic applications, such as medical imaging and underwater navigation. Here, we propose a design paradigm for a metalens for waterborne sound with a compact and simple configuration. The metalens is composed of an open central region reserved as a steady water-flow channel, and a metagrating-based peripheral region consisting of a grating of meta-atoms. Each meta-atom containing two elliptical iron cylinders is smartly designed according to the grating diffraction theory and intelligent optimization algorithm, so that it can deflect a normally incident wave along the desired direction toward the focal spot. In this way, subdiffraction focusing with a high energy concentration ratio is achieved, which breaks the conventional Rayleigh-Abbe diffraction limit in the focal plane. Here the subdiffraction focusing is due to the coherent interference in the far field of the ±first-order diffracted waves from each meta-atom, and is attributed to the superoscillation phenomenon. Interestingly, the focal depth of the metalens can be conveniently tuned by applying a background water flow with different velocities and directions, and the superresolution focusing effect is sustained regardless of whether there is a water flow or not. Since the magnitude and direction of the water-flow velocity can be electrically controlled, the compact and open configuration of the metalens not only provides a flexible and practical solution for sharp and controllable sound focusing, but also has potential applications in metagrating-based planar acoustic devices.