Slow light in topological valley photonic crystal structures offers possibilities to enhance light-matter interaction. We report a topological cavity based on slow-light topological edge modes for broadband Purcell enhancement. The topological edge modes with large group indices over 100 can be realized with a bearded interface between two topologically distinct valley photonic crystals, featuring the greatly enhanced Purcell factor because of the increased local density of states. In the slow-light regime, the topological cavity supports much more cavity modes with a higher quality factor than that in the fast-light regime, which is both demonstrated theoretically and experimentally. We demonstrate the cavity enables the broadband Purcell enhancement together with substantial Purcell factor, benefiting from dense cavity modes with a high quality factor in a wide spectral range. It has great benefit for the realization of high-efficiency quantum-dot-based single-photon sources and entangled-photon sources with less restriction on spectral match. Such a topological cavity could serve as a significant building block toward the development of photonic integrated circuits with embedded quantum emitters.

Quantum phase estimation (QPE) is the key subroutine of several quantum computing algorithms as well as a central ingredient in quantum computational chemistry and quantum simulation. While QPE strategies have focused on the estimation of a single phase, applications to the simultaneous estimation of several phases may bring substantial advantages; for instance, in the presence of spatial or temporal constraints. In this work, we study a Bayesian algorithm for the parallel (simultaneous) estimation of multiple arbitrary phases. The protocol gives access to correlations in the Bayesian multiphase distribution resulting in covariance matrix elements scaling as $O(N_T^{-2})$, with respect to the total number of quantum resources $N_T$. The parallel estimation allows to surpass the sensitivity of sequential single-phase estimation strategies for optimal linear combinations of phases. Furthermore, the algorithm proves a certain noise resilience and can be implemented using single photons and standard optical elements in currently accessible experiments.

An understanding of the spin-transport properties in semiconductor barriers is essential to improve the performance of spin-polarized light-emitting diodes (spin LEDs) for future optospintronics integration in information processing. Here, we report on the temperature and bias-voltage dependence of spin-transport properties in an ${\mathrm{In}}_{0.5}{\mathrm{Ga}}_{0.5}\mathrm{As}$ quantum dot (QD) spin LED using a combination of spin-dependent electroluminescence (EL) and time-resolved photoluminescence. The QD EL spin polarization increases with an increase in temperature above 125 K; this is attributed to the improved conversion efficiency from spin polarization of electrons to circular polarization of photons of the QDs. We find that both the electric field and temperature can enhance spin relaxation in the undoped $\mathrm{Ga}\mathrm{As}$ barrier above 200 K. At 298 K, the QD EL spin polarization decreases beyond 2.5 V; this is attributed to the enhanced D’yakonov Perel’ spin relaxation in the undoped $\mathrm{Ga}\mathrm{As}$ barrier caused by the increase in electron temperature. This study provides valuable insights into the spin-relaxation mechanism in the semiconductor barrier during the room-temperature operation of the QD spin LED.

In materials science, intentional doping has been widely used to improve the properties of a variety of materials. However, such an approach is not yet exploited in the fast-growing field of electrocaloric materials, which represent a serious alternative for next-generation cooling systems. Here we demonstrate with ${\mathrm{Ba}}_{0.9}{\mathrm{Sr}}_{0.1}{\mathrm{Hf}}_{0.1}{\mathrm{Ti}}_{0.9}{\mathrm{O}}_3$, an ecofriendly ferroelectric material, that doping with 2% of $\mathrm{Cu}$ introduces defect dipoles into the ferroelectric matrix and results in (i) enhancement of the adiabatic temperature change ΔT by up to 54% while maintaining performance after a large number (up to ${10}^4$) of electric field cycles, (ii) suppression of the parasitic irreversibility of ΔT between on-field and off-field states, and (iii) an alternative design of refrigeration cycle with a prepoled sample, allowing a two-field-step process showing both conventional (ΔT > 0) and inverse (ΔT < 0) responses when the field is sequentially varied. We also demonstrate that doping significantly increases the energy storage density (by up to 72%). The defect engineering approach therefore offers a path for designing ferroelectrics with improved electrocaloric performances. Beyond ferroelectrics, this strategy could also be promising in other solid-state caloric materials (magnetocalorics, elastocalorics, etc.).

We propose a spin valve that is based on van der Waals antiferromagnetism and is fully electrically controlled. The device is composed of two antiferromagnetic terminals that allow for vertical bias control and a linked central scattering potential region. The magnetoresistance varies significantly when the bias orientations in two terminals are switched from parallel to antiparallel, because this switch induces a mismatch of the bands for the same spin projection in different parts of the system. It is shown from density-functional calculations that bilayer graphene encapsulated by two atomic layers of ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ provides a material platform to realize the antiferromagnetism, which is robust against the required vertical electric fields.

Thermal diodes, which allow heat flow in one direction but prohibit it in the reverse direction, are fundamental for thermal manipulation. Here, we report the construction of a thermal diode consisting of ${\mathrm{Ni}}_{0.85}{\mathrm{Fe}}_{0.15}\mathrm{S}$ (bonded with 10 wt % silver) and alumina. A high rectification factor γ of 1.51 is achieved with a relatively small temperature bias of 97 K near room temperature, indicating great potential applications in the field of nonlinear thermal control. The excellent rectification effect is attributed to the high contrast change in thermal conductivity of ${\mathrm{Ni}}_{0.85}{\mathrm{Fe}}_{0.15}\mathrm{S}$. In addition, we demonstrate that the maximum thermal rectification ratio is strongly dependent on the cold-terminal temperature of the thermal diode, the length ratio between the two segments, and the steepness of the first-order phase transition of ${\mathrm{Ni}}_{0.85}{\mathrm{Fe}}_{0.15}\mathrm{S}$.

Magnetic skyrmions are topological spin textures with nanoscale size, which have great potential for spintronics applications. However, they are very sensitive to film thicknesses and interfaces in ultrathin films or multilayer heterostructures, and methods to generate and tune skyrmions are needed in order to use them in real-world applications. Electric field gating has been shown to modify the magnetic characteristics of thin films; however, these changes are limited by the low electric fields achievable using solid gate electrodes. In this work, we use ionic liquid gating to modify the magnetic characteristics of perpendicularly magnetized $\mathrm{Mg}\mathrm{O}$/$\mathrm{Mn}$${}_2$$\mathrm{Co}\mathrm{Al}$/$\mathrm{Pd}$ ultrathin films, applying a range of voltage sequences to generate skyrmions through both nonvolatile and volatile changes to these films. We achieve a giant anisotropy field tunability of 109.8 mT ${\mathrm{V}}^{-1}$ that is nonreversible, which can be ascribed to magneto-ionic effects. Reversible changes to the anisotropy and volatile skyrmion formation are achieved via electrostatic charge accumulation, which could induce an in-plane Rashba field. Our results strongly demonstrate that ionic liquid gating is a versatile method to engineer both nonvolatile and volatile skyrmions by tuning the magnetic characteristics of films to the regimes where they can exist.

We study the spin-dependent transport properties of a spin valve based on a double quantum dot. Each quantum dot is assumed to be strongly coupled to its own ferromagnetic lead, while the coupling between the dots is relatively weak. The current flowing through the system is determined within perturbation theory in the hopping between the dots, whereas the spectrum of a quantum-dot–ferromagnetic-lead subsystem is determined by means of the numerical renormalization group method. The spin-dependent charge fluctuations between ferromagnets and quantum dots generate an effective exchange field, which splits the double-dot levels. Such a field can be controlled, separately for each quantum dot, by the gate voltages or by changing the magnetic configuration of the external leads. We demonstrate that the considered double-quantum-dot spin-valve setup exhibits enhanced magnetoresistive properties, including both normal and inverse tunnel magnetoresistance. We also show that this system allows for the generation of highly spin-polarized currents, which can be controlled by purely electrical means. The considered double quantum dot with ferromagnetic contacts can thus serve as an efficient voltage-tunable spin valve characterized by high output parameters.

The surface spin flop, observed in synthetic antiferromagnets (SAFs) with uniaxial anisotropy and strong antiferromagnetic (AF) interlayer exchange coupling, can be considered as a laterally homogeneous, vertical AF domain wall pushed into the SAF from either the top or the bottom in the presence of a strong external vertical magnetic field. As a result, the AF domain wall can be described as a one-dimensional entity. In this work, we present a concept to stabilize laterally homogeneous vertical AF domain walls by local variation of the perpendicular magnetic anisotropy in $\mathrm{Co}/\mathrm{Pt}$-based SAFs. Our approach not only allows the stabilization of the vertical AF domain wall in the absence of any external magnetic field, but furthermore enables a deterministic selection among four different remanent states, each one stable within a broad external magnetic field range of almost one tesla. We also demonstrate an extension to our concept by stabilizing two coexisting vertical AF domain walls, thus yielding a system with a total of six different selectable (and reprogrammable) remanent states. The controlled stabilization of noncollinear AF textures in the form of vertical AF domain walls at remanence could be used as an infrastructure for propagating spin waves within the AF domain wall itself, as well as for tuning the dynamic behavior of perpendicular standing spin wave modes existing vertically across the SAF.

Photoinduced second-harmonic generation (SHG) in centrosymmetric materials like silica and silicon nitride has been commonly explained as an effective second-order (${\chi }^{(2)}$) process mediated by a dc electric field and the medium’s third-order (${\chi }^{(3)}$) nonlinearity. In this explanation, the coherent photogalvanic effect is the source of a dc electric field whose spatial periodicity naturally enables quasi-phase-matching. While successful in explaining many observations from experiment, the behavior at low input powers, and in particular, the apparent existence of a threshold for efficient photoinduced SHG observed in some experiments has largely been overlooked theoretically. In this Paper, we reconsider photoinduced SHG within the framework of four-wave mixing (FWM), involving degenerate pump, second-harmonic signal, and dc electric field. We propose a hypothesis that photoinduced SHG is a FWM-mediated dc Kerr optical parametric oscillation or amplification process. This hypothesis can explain the threshold behavior, and moreover, predicts unconventional light amplification, both of which we verify by experiments in silicon-nitride microresonators. Finally, we discuss the physical implications of our work in various platforms and future directions.

We use a semiclassical formalism to optimize a microwave single-photon detector based on switching events of a current-biased Josephson junction coupled to a resonator. To detect very rare events, the average time between dark counts ${\tau }_{\mathrm{dark}}$ should be maximized taking into account that the switching time ${\tau }_{\mathrm{sw}}$ should be sufficiently small. We demonstrate that these times can be tuned in a wide range by changing the junction parameters, and ${\tau }_{\mathrm{dark}}/{\tau }_{\mathrm{sw}}\sim {10}^9$ can be achieved. Therefore, a junction-resonator arrangement can be used to detect extremely low photon fluxes (e.g., for searching for galactic axions).

We discuss the realization of a universal set of ultrafast single- and two-qubit operations with superconducting quantum circuits and investigate the most relevant physical and technical limitations that arise when pushing for faster and faster gates. With the help of numerical optimization techniques, we establish a fundamental bound on the minimal gate time, which is determined independently of the qubit design solely by its nonlinearity. In addition, important practical restrictions arise from the finite qubit transition frequency and the limited bandwidth of the control pulses. We show that, for highly anharmonic flux qubits and commercially available control electronics, elementary single- and two-qubit operations can be implemented in about 100 ps with residual gate errors below ${10}^{-4}$. Under the same conditions, we simulate the complete execution of a compressed version of Shor’s algorithm for factoring the number 15 in about 1 ns. These results demonstrate that, compared to state-of-the-art implementations with transmon qubits, a hundredfold increase in the speed of gate operations with superconducting circuits is still feasible.

We apply inverse-design methods to produce two-dimensional plasma metamaterial (PMM) devices. Backpropagated finite-difference frequency-domain (FDFD) simulations are used to design waveguides and demultiplexers operating under both TE and TM modes. Demultiplexing and waveguiding are demonstrated for devices composed of plasma elements with reasonable plasma densities of approximately $7$ GHz, allowing for future in situ training and experimental realization of these designs. We also explore the possible applicability of PMMs to nonlinear boolean operations for use in optical computing. Functionally complete logical connectives (or and and) are achieved in the TM mode.

This paper proposes cavity resonators under parity-time- ($PT$) symmetric conditions for robust wireless power transfer (WPT) with minimal exposure to electromagnetic (EM) fields. Transceiver cavities with the same resonant frequencies as that of the fundamental mode are coupled through very small evanescent waves leaking out of the lattices on the top surface of the cavities. This reduces the magnitude of the EM fields near the power-delivery system by $10$ to $10000$ times compared with conventional coil systems. Regardless of the small coupling factors between the cavities, the power-transfer efficiency is maintained at a high level due to the high quality factors of the cavities. According to coupled-mode theory, the resonant frequency of the coupled system changes sensitively with respect to the receiver position, making power delivery unstable. In this work, a feedback loop at the transmit cavity along with the saturated gain of the power amplifier achieves the $PT$-symmetric condition. The power-transfer efficiency is robust because the operating frequency is automatically locked to the resonant frequency, regardless of the operation conditions. The proposed cavity system is promising for supplementing the conventional coil system for WPT applications in which minimal field exposure is crucial. The paper is accompanied by videos demonstrating experiments.

The crucial ability to suppress low-frequency waterborne sound efficiently has far-reaching implications for underwater noise-control engineering. Here, we propose an ultrathin composite metasurface in deep subwavelength thickness based on pentamode metamaterials (PMs) backed by rubber-metal resonators (RMRs). As a demonstration, average absorptance $87.8\mathrm{\%}$ at subkilohertz frequencies ranging from 760 to 920 Hz (wavelength $\lambda$ from 30.9 to 25.5 times of absorber thickness) is achieved. In the composite system, PM layer functions as an energy converter, which converts incident waterborne sound into mechanical vibrations, suggesting that sound waves could be absorbed only if the converted vibrations are eliminated by the backing RMRs. Beyond as a converter, PM itself also exhibits an additional stringlike resonant mode originating from the fixed-boundary constraints, which gives rise to intense vibrations of RMRs that facilitate energy dissipation. Further investigation demonstrates the unique property of robust high-efficiency absorption for extended frequency region and wide oblique incidence angles. The proposed methodology paves the way for a class of low-frequency underwater absorber design platforms, allowing devices for versatile applications to be envisioned.

We propose that the nonlinear radio-frequency dynamics and nanoscale size of topological magnetic structures associated with their well-defined internal modes advocate their use as in materio scalable frequency multipliers for spintronic systems. Frequency multipliers allow for frequency conversion between input and output frequencies, and thereby significantly increase the range of controllably accessible frequencies. In particular, we explore the excitation of eigenmodes of topological magnetic textures by fractions of the corresponding eigenfrequencies. We show via micromagnetic simulations that low-frequency perturbations to the system can efficiently excite bound modes with a higher amplitude. For example, we excite the eigenmodes of isolated ferromagnetic skyrmions by applying half, a third, and a quarter of the corresponding eigenfrequency. We predict that frequency multiplication via magnetic structures is a general phenomenon that is independent of the particular properties of the magnetic texture and works for magnetic vortices, droplets, and other topological textures.