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.

We theoretically clarify the mechanism of the strain-tuning effect for control of the mechanical nonlinearity in doubly clamped MEMS beam resonators and experimentally demonstrate that nonlinearity can be effectively suppressed with a preloaded lattice-mismatch strain in the MEMS beam. Mechanical nonlinearity arises from the hardening- and softening-nonlinearity terms in the Duffing motion equation of the MEMS beam. By approaching the buckling condition of the MEMS beam, the substantially increased softening nonlinearity greatly compensates for the hardening nonlinearity, resulting in suppression of the total nonlinearity. Utilizing this knowledge, we fabricate ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ MEMS beams with a preloaded lattice-mismatch strain, which is achieved by adding a small amount (x = ∼0.4%) of indium to the $\mathrm{Ga}\mathrm{As}$ MEMS beam during wafer growth. The buckling condition in the experiment is achieved by carefully modulating the length of the ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ MEMS beams. As a result, mechanical nonlinearity is largely modulated from hardening to softening and reaches a quasizero value near the buckling condition, demonstrating the effectiveness of using lattice mismatch for controlling the mechanical nonlinearity of MEMS resonators.

Two-dimensional α-${\mathrm{In}}_2{\mathrm{Se}}_3$ has drawn broad attention due to its high photoresponse and unique room-temperature interlocked in-plane and out-of-plane ferroelectricity with an ultralow switching electric field. Here, we investigate the photoresponse in a lateral monolayer (ML) α-${\mathrm{In}}_2{\mathrm{Se}}_3$ p-i-n junction by using ab initio quantum transport simulations. The maximum photoresponses of the lateral α-${\mathrm{In}}_2{\mathrm{Se}}_3$ p-i-n junction are up to 69.2 and 31.6 mA/W for the ferroelectric wurtzite and zincblende phases (shortly named WZ’ and ZB’) α-${\mathrm{In}}_2{\mathrm{Se}}_3$, respectively, which are 8–17 times higher than that of the extensively researched graphene photodetector (4 mA/W). Remarkably, the ferroelectric photoresponses, defined as the photoresponse change ratio between the two ferroelectric states, of the lateral ML WZ’ and ZB’-${\mathrm{In}}_2{\mathrm{Se}}_3$ photodetectors have average values of 127% and 121% with surprising maximum values of $2\times {10}^6\mathrm{\%}$ and $1\times {10}^7\mathrm{\%}$, respectively. The physical mechanism comes from the electron density redistribution altered by the atomic displacements due to the polarization switch, rather than the built-in potential change induced by the surface polarization charges. Such ferroelectric-tunable photoresponses in the α-${\mathrm{In}}_2{\mathrm{Se}}_3$ photodetector suggest a potential in the fabrication of future optical detection and storage integrated devices.

Square-root topology offers a distinctive scheme towards topological phases with unconventional spectral properties. In many cases, the formation of square-root topology is governed by the inherent lattice symmetry, with honeycomb lattices being prominent examples. Here, we report on the experimental discovery of square-root higher-order topological insulator phases in an acoustic metamaterial with decorated rectangular lattice. Through acoustic pump-probe techniques, we explore the boundary-dependent topological edge and corner states in both the parent and square-root acoustic metamaterials. We further validate their higher-order topologies as well as their spectral connections via various simulations. Our work not only substantiates the square-root higher-order topology in rectangular-lattice systems from the experimental aspect, but also serves as a step towards versatile higher-order topological phenomena in photonic, acoustic, and mechanical metamaterials.

As a powerful optical tool, spectrometers are widely used in materials analysis and light-source characterization. Although commercial spectrometers based on dispersive diffraction or Fourier transform offer high resolution and broad dynamic range, spatial information of the sample is lost, limiting its application in snapshot hyperspectral imaging. Herein, we propose a computational spectrometer based on the tailored point spread function of the imaging system that uses disordered microstructures fabricated by low-cost photolithography. The spectrometer achieves a high quantum efficiency ($Q_{\mathrm{avg}}\sim 60\mathrm{\%}$) and ultracompact footprint as low as $24.5\times 24.5\mu {\mathrm{m}}^2$, exhibits outstanding performance in weak-light hyperspectral imaging (e.g., Raman or fluorescence spectroscopy) in the microscopic system. Apart from the advancements mentioned above, our spectrometer offers good compatibility with commercial microscopes, implying it is a promising candidate for spectral imaging in chemistry, biology, and others.

Comagnetometers have been widely used in fundamental research, such as tests of spin-gravity coupling and searches for permanent electric dipole moments (EDMs). Here, we report a comagnetometer that consists of simultaneous closed-loop oscillation of the $F=1$ and $F=2$ hyperfine Zeeman transitions of optically aligned ${}^{87}$$\mathrm{Rb}$ atoms. The Allan deviation of the $F=2$ and $F=1$ oscillation-frequency ratio reaches $1\times {10}^{-9}$ after running continuously for 2 days, corresponding to a sensitivity of approximately $4\times {10}^{-19}\mathrm{eV}$ for the measurement of the hypothetical spin-dependent gravitational energy of the proton. The long-term stability of the comagnetometer can be further improved by system-level thermal control and optimization of the experimental parameters.

We study the magnetic and electrical transport properties of magnetic tunnel junctions (MTJs) consisting of a ${\mathrm{Co}}_{75}{\mathrm{Mn}}_{25}{/\mathrm{Mo}/\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$ multilayer prepared using a mass-production-compatible magnetron sputtering system. The ${\mathrm{Co}}_{75}{\mathrm{Mn}}_{25}{/\mathrm{Mo}/\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$ multilayer sandwiched between two MgO layers exhibits remarkable perpendicular magnetic anisotropy, and a uniaxial magnetic anisotropy constant as large as $0.2{\mathrm{MJ}/\mathrm{m}}^3$ is achieved by optimizing the ${\mathrm{Co}}_{75}{\mathrm{Mn}}_{25}$ layer thickness as well as the annealing temperature. The current-in-plane tunneling measurement reveals a large tunneling magnetoresistance of over 100% in perpendicularly magnetized MTJs. These experimental results indicate the applicability of ${\mathrm{Co}}_{75}{\mathrm{Mn}}_{25}$ alloy for magnetic random access memory devices.

Near-field radiative heat transfer between closely placed objects may exceed the far-field black-body radiation limit by orders of magnitude, especially between polaritonic materials. Great efforts have been made to experimentally measure this fundamental effect in various systems. In this work, we manage to experimentally characterize the near-field thermal emission between less explored plasmon-phonon hybrid material systems made of a graphene-$\mathrm{Si}\mathrm{C}$ heterostructure. The experiment is carried out using a custom-designed all-optical measurement setup. A heat flux enhancement factor of 26 over the black-body radiation limit is obtained at a 150-nm vacuum gap. Three fundamental modes, i.e., surface plasmon and phonon polaritons with frustrated modes, are identified to contribute the near-field heat flux in this hybrid system. The measured results are well reproduced from the analytical calculations, indicating the robustness of the measurement. Deeper experimental explorations for near-field heat transfer between more complicated material systems could be encouraged in the future.

Band-stop filters play an important role in wireless communications by blocking signals within specified frequency bands and allowing useful frequencies to pass through. As a special type of higher symmetry, glide symmetry achieves the reconfiguration of band gaps by manipulating the dispersion relationships of periodic structures, which is of great value for filter designs. However, existing band-stop filters based on glide symmetry are mainly realized with three-dimensional transmission lines, and only one band gap is reconfigured. Here, we explore glide symmetry in planar substrate-integrated coaxial lines (SICL) to reconfigure multiple band gaps and, in the meantime, customize their bandwidths and locations. For practical applications, three types of SICL band-stop filters with different glide symmetries are designed, fabricated, and measured. By changing the geometry of the glide-symmetric inner conductor, two distinct stopbands can be reconfigured and their bandwidths can be customized. The measurement results agree well with the numerical results, demonstrating high controllability and good filtering performance. The proposed glide-symmetric SICL provides a facile method to implement controllable band-stop filters and may facilitate more planar implementations with glide symmetry for dispersion manipulations.

Pumping at low Reynolds number is a ubiquitously encountered feature, both in biological organisms and engineered devices. Generating net flow requires the presence of an asymmetry in the system, which traditionally comes from geometric flow rectifiers. Here, we study a valveless system of $N$ oscillating pumps in series, where the asymmetry comes not from the geometry but from time, that is, the phase shifts between the pumps. Experimental and theoretical results are in very good agreement. We provide the optimal phase shifts leading to the maximal net flow in the continuous $N\to \mathrm{\infty }$ limit. The maximal flow rate is larger by 25% than that of a traditional peristaltic sinusoidal wave, for the same amplitude of actuation. Our results hopefully pave the way for the design of more efficient microfluidic pumps.

A finite-temperature micromagnetic study of magnetization switching and write-error rates in a perpendicular magnetic tunnel junction with and without synthetic antiferromagnetic layer (SAF) is presented. In the absence of SAF, magnetization switching is induced by domain-wall nucleation and propagation. Although the various modes of domain-wall propagation are observed to delay switching, it does not show an appreciable impact on the overall write-error-rate slopes. In the presence of the nonuniform stray field from the SAF assembly, the domain-wall-based switching modes turn on more complex magnetization dynamics that impedes the switching process. In cases where the SAF layers fail to balance each other contributing to a stronger stray field, incoherent switching modes give rise to metastable states with significantly longer lifetimes, and a dramatic change in the write-error slopes is observed. Simulation results are compared to recent experimental findings from time-domain measurements of spin-transfer-torque switching and measurements of anomalous write-error rates. These results directly prove the long-predicted relation of the SAF stray field to write-error anomaly in perpendicular spin-transfer-torque magnetic random-access memory and could be useful to solve the anomalous write-error problems.

Bilayer graphene is a nanomaterial that allows for well-defined, separated quantum states to be defined by electrostatic gating and, therefore, provides an attractive platform to construct tunable quantum dots. When a magnetic field perpendicular to the graphene layers is applied, the graphene valley degeneracy is lifted, and splitting of the energy levels of the dot is observed. Although bilayer graphene quantum dots have recently been realized in experiments, it is critically important to devise robust methods that can identify the observed quantum states from accessible measurement data. Here, we develop an efficient algorithm for extracting the model parameters needed to characterize the states of a bilayer graphene quantum dot. Specifically, we put forward a Hamiltonian-guided random search method and demonstrate robust identification of quantum states on both simulated and experimental data.

Quantum error correction will be an essential ingredient in realizing fault-tolerant quantum computing. However, most correction schemes rely on the assumption that errors are sufficiently uncorrelated in space and time. In superconducting qubits, this assumption is drastically violated in the presence of ionizing radiation, which creates bursts of high-energy phonons in the substrate. These phonons can break Cooper pairs in the superconductor and, thus, create quasiparticles over large areas, consequently reducing qubit coherence across the quantum device in a correlated fashion. A potential mitigation technique is to place large volumes of normal or superconducting metal on the device, capable of reducing the phonon energy to below the superconducting gap of the qubits. To investigate the effectiveness of this method, we fabricate a quantum device with four nominally identical nanowire-based transmon qubits. On the device, half of the niobium-titanium-nitride ground plane is replaced with aluminum ($\mathrm{Al}$), which has a significantly lower superconducting gap. We deterministically inject high-energy phonons into the substrate by voltage biasing a galvanically isolated Josephson junction. In the presence of the small-gap material, we find a factor of 2–5 less degradation in the injection-dependent qubit lifetimes and observe that the undesired excited qubit state population is mitigated by a similar factor. We furthermore turn the $\mathrm{Al}$ normal with a magnetic field, finding no change in the phonon protection. This suggests that the efficacy of the protection in our device is not limited by the size of the superconducting gap in the $\mathrm{Al}$ ground plane. Our results provide a promising foundation for protecting superconducting-qubit processors against correlated errors from ionizing radiation.

Dielectric loss is one of the major decoherence sources of superconducting qubits. Contemporary high-coherence superconducting qubits are formed by material systems mostly consisting of superconducting films on substrate with low dielectric loss, where the loss mainly originates from the surfaces and interfaces. Among the multiple candidates for material systems, a combination of titanium nitride ($\mathrm{Ti}\mathrm{N}$) film and sapphire substrate has good potential because of its chemical stability against oxidization and its high quality at interfaces. In this work, we report a $\mathrm{Ti}\mathrm{N}$ film deposited onto sapphire substrate achieving low dielectric loss at the material interface. Through the systematic characterizations of a series of transmon qubits fabricated with identical batches of $\mathrm{Ti}\mathrm{N}$ base layers but different geometries of qubit shunting capacitors with various participation ratios of the material interface, we quantitatively extract the loss-tangent value at the substrate-metal interface, which is smaller than $8.9\times {10}^{-4}$ in a 1-nm disordered layer, and a limiting quality factor of about 7.3 million. By optimizing the interface participation ratio of the two-dimensional (2D) transmon qubit, we reproducibly achieve a quality factor of 5.7 million on average. The best 2D qubits show lifetimes of up to $300\mu \mathrm{s}$ and quality factors achieving 8.1 million. We demonstrate that $\mathrm{Ti}\mathrm{N}$ film on sapphire substrate is an ideal material system for high-coherence superconducting qubits. Our analyses further suggest that the interface dielectric loss around the Josephson-junction part of the circuit could be the dominant limitation of lifetimes for state-of-the-art transmon qubits.