We theoretically study the electromagnetic wave radiation of magnons driven by acoustic phonons in systems with strong magnon-phonon interaction. We evaluate the field dependence of radiation intensity spectra that exhibits the avoided crossing, a characteristic of strongly coupled systems. At the crossover where the magnon and phonon eigenstates are hybridized, we demonstrate the existence of two resonant radiation frequencies with circular polarization and the enhancement of antenna radiation efficiency by over 100 times. We can also reconfigure the ellipticity of the antenna polarization by changing the applied out-of-plane magnetic field. Our results open up possibilities of developing ultracompact antennas by using the hybridized magnon-phonon mode.

Generation of circularly polarized high harmonics with the same helicity to all orders is indispensable for chiral-sensitive spectroscopy with attosecond temporal resolution. Solid-state samples have added a valuable asset in controlling the polarization of emitted harmonics. However, maintaining the identical helicity of the emitted harmonics to all orders is a daunting task. In this work, we demonstrate a robust recipe for efficient generation of circularly polarized harmonics with the same helicity. For this purpose, a nontrivial tailored driving field, consisting of two co-rotating laser pulses with frequencies $\omega$ and $2\omega$, is utilized to generate harmonics from graphene. The Lissajous figure of the total driving pulse exhibits an absence of rotational symmetry, which imposes no constraint on the helicity of the emitted harmonics. Our approach to generating circularly polarized harmonics with the same helicity is robust against various perturbations in the setup, such as variation in the subcycle phase difference or the intensity ratio of the $\omega$ and $2\omega$ pulses, as rotational symmetry of the total driving pulse remains absent. Our approach is expected to be equally applicable to other two-dimensional materials, among others, transition-metal dichalcogenides and hexagonal boron nitride, as our approach is based on the absence of rotational symmetry of the driving pulse. Our work paves the way for establishing compact solid-state chiral extreme ultraviolet sources, opening a realm for chiral light-matter interaction on its intrinsic timescale.

The hot-carrier solar cell (HCSC) has the potential for converting solar energy into electrochemical energy with an efficiency of 85.4%. For this, in addition to an idealized light absorber, the HCSC has to be connected to the external load by means of the so-called monoenergetic energy-selective contacts (ESCs). However, the thermodynamic properties that these types of contact have to exhibit, such as their electric, thermal conductivity, and Seebeck coefficient, have not been explored. This paper aims to fill this gap. In this respect, we model electron transport in nonideal ESCs using the transport theory proposed by Datta and Landauer, which has allowed us to calculate the value of these parameters as a function of the temperature and electrochemical potential of operation. Our findings also reveal that, to preserve the HCSC efficiency above 82%, the ESCs could require in the order of $3\times {10}^{19}{\mathrm{cm}}^{-3}$ electron states. As the ESCs depart from ideality, the temperature of the hot carriers at which optimum efficiency is obtained increases to above 2540 K. The monoenenergetic selective contact characterized by the highest energy demands an electric, thermal conductivity, and Seebeck coefficient that, when combined, are characterized by a high thermoelectric figure of merit $(ZT\approx 8)$. We are not aware of any material exhibiting this figure of merit, which illustrates the difficulty in putting the HCSC concept into practice. Conversely, our work supports the idea that pursuing materials capable of transporting electrons ballistically through monoenergetic electron channels can provide the key for achieving materials characterized by high $ZT$.

The emergence and development of acoustic artificial materials stimulate a large number of applications for the manipulation of ultrasonic and elastic waves. Most of the reported metasurfaces and metamaterials only work effectively at a given frequency and target a fixed functionality. Reconfigurable designs may solve this problem, to a certain extent, but require the introduction of complex active control components. Here, by integrating multiple functionalities on a conformally mapped Mikaelian lens with a hyperbolic secant refractive-index distribution, a broadband passive lens is presented and accomplished on an elastic plate. Just by adjusting the location and type of source, different regulations of flexural waves can be switched from one sort to another. Through projecting the refractive-index profile onto the thickness of the plates, the proposed Mikaelian lens is designed and fabricated. Theoretical prediction and experimental results demonstrate beam scanning at an angle of up to 120° from 30 to 180 kHz and achromatic subwavelength focusing with a full width at half maximum of around 0.3λ from 30 to 120 kHz. Without using locally resonant materials or active control elements, this work provides a feasible way to construct multifunctional flexural-wave devices.

This paper proposes a scheme and design of a cryogenic multiplexing control chip (MCC), which enables one microwave cable to control many superconducting qubits. Designed based on an adjustable inductor, the MCC is a nonreciprocal device that generates microwave signals between 4 and 8 GHz and constructs a $XY$ control circuit to operate qubits. The effects of filters, isolators, power splitters, qubits, and load impedance on the MCC are analyzed in the frequency-domain simulation. And in the time-domain simulation, the response time, Gaussian pulses, single sideband, amplitude, and frequency compensation of the MCC are examined. The MCC can be integrated with the superconducting qubits on the same package at the 20-mK temperature stage with insertion loss of $-6$ dB and isolation of $-26$ dB. The time-domain simulation has revealed that the output specific microwave pulses of the MCC can be used for driving the superconducting qubits with crosstalk of about $-80$ dB. This is much less than $-40$ dB of the physical requirements on crosstalk, which can meet fidelity on single-qubit (99.92%) and two-qubit gates (99.40%). The power dissipation for each qubit control is less than 1 pW. The MCC can greatly reduce the number of microwave control lines and is beneficial to the development of a large-scale superconducting quantum processor.

Hybrid superconducting-photonic microresonators are a promising platform for realizing microwave-to-optical transduction. However, the absorption of scattered photons by the superconductors leads to unintended microwave resonance-frequency variation and line-width broadening. Here, we experimentally study the dynamics of this effect and its impact on microwave-to-optics conversion in an integrated lithium niobate–superconductor hybrid-resonator platform. We unveil an adiabatic frequency shifting of the intracavity microwave photons induced by the fast photoresponses of the thin-film superconducting resonator. As a result, the temporal and spectral responses of electro-optics transduction are modified and well described by our theoretical model. This work provides insights into the light-induced conversion dynamics that must be considered in future designs of hybrid superconducting-photonic system.

Driven quantum nonlinear oscillators, while essential for quantum technologies, are generally prone to complex chaotic dynamics that fall beyond the reach of perturbative analysis. By focusing on subharmonic bifurcations of a harmonically driven oscillator, we provide a recipe for the choice of the oscillator’s parameters that ensures a regular dynamical behavior independently of the driving strength. We show that this suppression of chaotic phenomena is compatible with a strong quantum nonlinear effect reflected by the confinement rate in the degenerate manifold spanned by stable subharmonic orbits.

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.