The dependence of the velocity of surface acoustic waves (SAWs) as a function of an external applied magnetic field is investigated in a $\mathrm{Fe}$ thin film epitaxially grown on a piezoelectric $\mathrm{Ga}\mathrm{As}$ substrate. The SAW velocity is observed to strongly depend on both the amplitude and direction of the magnetic field. To interpret the experimental data a phenomenological approach to the relative change in SAW velocity is implemented. We find that the experimental velocity variation can be well reproduced provided that the spin-wave dispersion is taken into account. The validity of this phenomenological model is attested by the comparison with a fully magnetoelastic one. Nonreciprocity of SAW velocity is also addressed both experimentally and theoretically.

Understanding the profile of a qubit’s wave function is key to its quantum applications. Unlike conducting systems, where a scanning tunneling microscope can be used to probe the electron distribution, there is no direct method for solid-state-defect-based qubits in wide-band-gap semiconductors. In this work, we use pressure as a tuning method and a nuclear spin as an atomic scale probe to monitor the hyperfine structure of negatively charged nitrogen-vacancy (N-$V$) centers in diamonds under pressure. We present a detailed study on the nearest-neighbor ${}^{13}\mathrm{C}$ hyperfine splitting in the optically detected magnetic resonance spectrum of N-$V$ centers at different pressures. By examining the ${}^{13}\mathrm{C}$ hyperfine interaction upon pressurizing, we show that the N-$V$ hyperfine parameters have prominent changes, resulting in an increase in the N-$V$ electron spin density and rehybridization from $s{p}^3$ to $s{p}^2$ bonds. The ab initio calculations of strain dependence of the N-$V$ center’s hyperfine levels are done independently. The theoretical results qualitatively agree well with experimental data without introducing any fitting parameters. Furthermore, this method can be adopted to probe the evolution of wave function in other defect systems. This potential capability could play a role in developing magnetometry and quantum information processing using the defect centers.

Archetypal motors produce work when two slowly varying degrees of freedom (DOF) move around a closed loop of finite area in the parameter space. Here, instead, we propose a simple autonomous monoparametric optomechanical engine that utilizes nonlinearities to turn a constant energy current into a nonconservative mechanical force. The latter self-sustains the periodic motion of a mechanical DOF whose frequency is orders of magnitude smaller than the photonic DOF. We identify conditions under which the maximum extracted mechanical power is invariant and show self-induced robustness of the power production against imperfections and driving noise.

Quantum communication between remote superconducting systems is being studied intensively to increase the number of integrated superconducting qubits and to realize a distributed quantum computer. Since optical photons must be used for communication outside a dilution refrigerator, the direct conversion of microwave photons to optical photons has been widely investigated. However, the direct conversion approach suffers from added photon noise, heating due to a strong optical pump, and the requirement for large cooperativity. Instead, for quantum communication between superconducting qubits, we propose an entanglement distribution scheme using a solid-state spin quantum memory that works as an interface for both microwave and optical photons. The quantum memory enables quantum communication without significant heating inside the refrigerator, in contrast to schemes using high-power optical pumps. Moreover, introducing the quantum memory naturally makes it possible to herald entanglement and parallelization using multiple memories.

Fast, sensitive, and compact devices that implement nonlinear activation functions are needed to form fully connected photonic neural networks (PNNs). However, even in highly nonlinear media, optical nonlinearities are relatively weak. We propose here a scheme for implementing nonlinear activation functions that relies on band-structure-engineered nanostructures. This scheme realizes the smallest possible hybrid optoelectronic approach, relying on fast electronic processes to implement nonlinearity instead of a true optical nonlinearity. Using well-established simplified density-matrix models, we demonstrate architectures that exhibit a low-intensity threshold of $3.5\mu \mathrm{W}$ along with a fast optical response of 10 ps in a relatively small linear footprint of $4\mu \mathrm{m}$. We also show that PNN training performance is improved in handwritten pattern recognition when applying our simulated nonlinear activation function, indicating potential for creating deep fully connected PNNs.

Topological insulators (TIs) open up many avenues for designing future electronic devices. Using the Bardeen transfer Hamiltonian method, we calculate the current density of electron tunneling between two slabs of $\mathrm{Bi}$${}_2$$\mathrm{Se}$${}_3$. We calculate the three-dimensional TI tunnel-diode current-voltage (J-V) characteristics for different doping concentrations, tunnel-barrier heights and thicknesses, and 3D-TI band gaps. The difference in the Fermi levels of the slabs determines the peak and valley voltages. The tunnel-barrier width and height affect the magnitude of the current without affecting the shape of the $J$-$V$ characteristics. The band gap of the 3D TI determines the magnitude of the tunnel current, albeit at a lesser rate than the tunnel-barrier potential; thus the device characteristics are robust under changing TI material. The high peak-to-valley ratio of 3D-TI tunnel diodes, the controllability of the valley-current location, and the simple construction provide advantages over other negative-differential resistance devices.

A recent major technological breakthrough in superconducting circuits is the realization of more than 50 qubits arranged on two-dimensional (2D) lattices with tunable nearest-neighbor couplings. We propose a protocol to generate Greenberger-Horne-Zeilinger (GHZ) states on 2D superconducting-qubit lattices by applying multiqubit controlled-$i$swap gates in parallel. The multiqubit gate can be naturally implemented based on an effective three-body interaction, which can be synthesized with appropriate detunings and coupling strengths between qubits. We simulate the preparation process of GHZ states with realistic parameters, and show a $37$-qubit GHZ state can be generated with a controlled-$i$swap depth of $3$. Our proposal provides a more promising method of generating GHZ states on the latest 2D superconducting-qubit architectures and will stimulate the preparation of multiqubit entangled states based on multiqubit gates.

We show how a pulse-level implementation of certain multiqubit gates allows for the simultaneous execution of single- and multiqubit gates acting on overlapping sets of qubits, in a mechanism we name absorption. With absorption as a foundation, we present an algorithm to schedule the execution of a quantum circuit as a pulse sequence on a neutral-atom quantum processor with a single channel for single- and multiqubit gate execution. For any quantum circuit of practical relevance, we observe that the algorithm results in an optimal utilization of the available resources that cannot be surpassed by a different scheduling strategy. Our benchmarks against a custom scheduler attempting to maximize parallelization at the gate level show the time gained by the pulse-level scheduler is proportional to the depth and is most pronounced for quantum circuits with fewer qubits. We finally illustrate how the concept of absorption can also be used in other architectures.

Back hopping or telegraphic switching describes stochastic bistate oscillation in magnetic tunnel junctions (MTJ) at high bias, which significantly increases the write error rate of memory storage. Nevertheless, this unfavorable stochastic switching could be utilized to construct extremely compact spiking neuron circuits, where both the spike frequency and duty cycle are proportional to the applied bias voltage. This MTJ neuron is the fundamental building block of future all-spin neural networks. This paper analyzes the mechanism of stochastic switching in the MTJ neuron in detail. The self-heating effect at high bias is identified to induce thermal perturbation through a reduced energy barrier of a weakened perpendicular magnetic anisotropy. The high spike frequency is measured up to 10 MHz, which is mainly limited by the transition time between states and the multistate switching when approaching the Curie temperature of the ferromagnetic phase. Finally, to quantitatively describe the stochastic switching phenomenon, we establish a bias-dependent Néel-Arrhenius compact model, which calibrates well with experimental data. Based on this model, the potential and design guideline for realizing MTJ neurons with a spike frequency toward the gigahertz range are also discussed.

We successfully demonstrate low-frequency noise suppression in the resonant-frequency fluctuations of a cavity-embedded Cooper-pair transistor (cCPT) driven at single-photon occupancy. In particular, we report a reduction in the resonant-frequency fluctuations caused by the internal charge noise over a bandwidth of approximately $1.4\text{kHz}$ when the cavity is driven at an average photon number $n=10$ and a bandwidth of 11 Hz for an average $n=1$. The gate-dependent tunability of the cCPT allows us to implement a feedback scheme derived from the Pound-Drever-Hall locking technique. This reduces fluctuations due to intrinsic charge noise, which otherwise interfere with the operation of the cCPT as a near quantum-limited electrometer. Our technique can be generalized to achieve frequency stabilization in tunable microwave resonators that play a vital role in today’s quantum computing architectures, thereby moderating the limitations in detection caused by the intrinsic $1/f$ noise on such circuit devices. The work discusses the various aspects relating to the operation of a fully functional feedback loop down to the single-photon level.

The application of graphene–photonic crystal microcavity structures with phase-change-material layers in spectral analysis is studied. It is found that selective absorption of different wavelengths of light can be realized by changing the crystallinity of the phase-change material, so that different wavelengths of light can be separated and measured in the time domain to perform spectral analysis. Since the phase-transition time of phase-change materials is very fast, this spectral analysis can be done with submicrosecond time resolution. This kind of spectral-analysis device has a simple structure and can be integrated with a chip. It has important prospects for application in the fields of biological molecular recognition, biological sensing, and chemical analysis.

Enabling scalable and energy-efficient control of spin defects in solid-state media is desirable for realizing transformative quantum information technologies. Exploiting voltage-controlled magnetic anisotropy, we report the coherent manipulation of nitrogen-vacancy ($\mathrm{N}$-V) centers by spatially confined magnetic stray fields produced by a proximate resonant magnetic tunnel junction (MTJ). Remarkably, the coherent coupling between $\mathrm{N}$-V centers and the MTJ can be systematically controlled by a dc bias voltage, allowing for appreciable electrical tunability in the presented hybrid system. In comparison with current state-of-the-art techniques, the demonstrated $\mathrm{N}$-V-based quantum operational platform exhibits significant advantages in scalability, device compatibility, and energy efficiency, further expanding the role of $\mathrm{N}$-V centers in a broad range of quantum computing, sensing, and communications applications.

Cation disorder is increasingly being studied as a means of tuning properties in electronic materials. Through Monte Carlo simulations and first-principles calculations, we determine the ranges of elastic moduli and polarization constants in $\mathrm{Zn}\mathrm{Ge}\mathrm{N}$${}_2$ as a function of ordering. We use heterostructure calculations to demonstrate a disorder-dependent type-I or type-II band offset between relaxed $\mathrm{Ga}\mathrm{N}$ and strained $\mathrm{Zn}\mathrm{Ge}\mathrm{N}$${}_2$ in the polar [001] direction. Modeling polarization and band offsets, we then use these calculations to comment on which values are desirable for device design, which parameters are obtainable through targeted growth techniques, and the challenges that remain in realizing a device and understanding cation disorder in heterostructures. Although the elastic and polarization properties change little with the order parameter, the band characteristics undergo dramatic shifts, creating a type-I band offset in partially disordered material.

Conventional acoustic metamaterial barriers are usually used to realize sound insulation for a continuous broad bandwidth. In some practical scenes, however, the noise signal from complex noise sources, such as gearboxes and motors, are usually complicated spectra with discrete peaks or valleys, instead of perfect continuous ones. In this case, the broadband design is unnecessary and may increase the thickness of metamaterials if too many frequency components are included. We need only a customized spectrum to achieve sound insulation at desired discrete frequencies, while, at other ones, insulation can be unwanted to preserve an effective signal other than noise. Here, we propose a customizable acoustic metamaterial barrier (CAMB) with intelligent and selective sound insulation. The CAMB is a composite structure consisting of a Helmholtz resonator (HR) and a microperforated panel (MPP) destined to control sound insulation at low frequencies, within 200–1000 Hz, and high-frequencies, within 1000–3000 Hz, respectively. An inverse process is proposed for customizable design. We use analytical and numerical methods to obtain the design libraries for the parameters of the HRs and MPPs. We numerically and experimentally demonstrate an example of a CAMB customized for a practical complex noise source. The sample has a thickness of 48 mm, approximately λ/11 for the lowest targeted frequency. The proposed customizable concept may pave the way for acoustic insulating metamaterials for complex sound sources with different frequency components.

Quantum systems must be prepared, controlled, and measured with high fidelity in order to perform complex quantum algorithms. Control fidelities have greatly improved in silicon spin qubits but state-preparation and readout fidelities have generally been poor. By operating with low electron temperatures and employing high-bandwidth cryogenic amplifiers, we demonstrate single-qubit readout visibilities $>99\mathrm{\%}$, exceeding the threshold for quantum error correction. In the same device, we achieve average single-qubit control fidelities $>99.95\mathrm{\%}$. Our results show that silicon spin qubits can be operated with high overall operation fidelity.

This work theoretically investigates the possibility of generating amplitude-squeezed light with high-quantum-efficiency interband cascade lasers. Based on a stochastic approach, we show that, by employing the suppressed-pump-noise configuration, this kind of midinfrared source enables operation with considerable amplitude squeezing over a large bandwidth of several GHz. Our results facilitate future midinfrared quantum photonic applications such as free-space secure communications.

Efficient manipulation of magnons for information processing is a central topic in spintronics and magnonics. An outstanding challenge for long-distance spin transport with minimal dissipation is to overcome the relaxation of magnons and to amplify the spin current they carry. Here, we propose to amplify magnon currents based on the realization of the bosonic Klein paradox in magnetic nanostructures. This paradox involves the antimagnon, carrying opposite spin and energy, the existence of which is usually precluded by ferromagnetic instabilities, as it is an excitation at negative energy. We show that by appropriately tuning the effective dissipation through spin-orbit torques, both magnons and antimagnons are dynamically stabilized. As a result, we find that the reflection coefficient of incident magnons at an interface between two coupled magnets can become larger than one, thereby amplifying the reflected magnon current. Our findings can lead to magnon amplifier devices for spintronic applications. Furthermore, our findings yield a solid-state platform to study the relativistic behavior of bosonic particles, which is an outstanding challenge with fundamental particles.

Topological systems are inherently robust to disorder and continuous perturbations, resulting in dissipation-free edge transport of electrons in quantum solids, or reflectionless guiding of photons and phonons in classical wave systems characterized by topological invariants. Realization of robust, lossless topological insulators in the technologically important terahertz (THz) frequency range has drawn immense attention. In this paper, we propose a Kagome lattice structure that supports topological edge states with valley-dependent transport. Optical topological insulators are subsequently fabricated, in which mirror symmetry is broken and degenerate states are lifted at $K(K^{\prime })$ valleys in the band structure. Theoretical and numerical analyses show that four types of edge states can be obtained. The performance of the fabricated topological waveguides is characterized with THz time-domain spectroscopy (TDS). Nearly identical THz wave transmission in the 0.437–0.453-THz range is verified with a straight-line topological waveguide and Z-shaped waveguides with multiple sharp turning corners, which quantitatively illustrate the strong backscattering suppression effects from the nontrivial topology edge states of the structures. Particularly, the THz TDS measurements provide important extra time-domain information about the topological waveguiding mode, which is generally unavailable from previous topological insulator investigations solely replying on power spectrum measurements.