Using a high quality factor 3 GHz bulk acoustic wave resonator device, we demonstrate the acoustically driven single quantum spin transition ($|m_s=0⟩\leftrightarrow |\pm 1⟩$) for diamond nitrogen-vacancy (N-$V$) centers and characterize the corresponding stress susceptibility. A key challenge is to disentangle the unintentional magnetic driving field generated by the device current from the intentional stress driving within the device. We quantify these driving fields independently using Rabi spectroscopy before studying the more complicated case in which both are resonant with the single quantum spin transition. By building an equivalent circuit model to describe the device’s current and mechanical dynamics, we quantitatively model the experiment to establish their relative contributions and compare with our results. We find that the stress susceptibility of the N-$V$ center spin single quantum transition is around $\sqrt{2}(0.5\pm 0.2)$ times that for double quantum transition ($|+1⟩\leftrightarrow |-1⟩$). Although acoustic driving in the double quantum basis is valuable for quantum-enhanced sensing applications, double quantum driving lacks the ability to manipulate N-$V$ center spins out of the $|m_s=0⟩$ initialization state. Our results demonstrate that efficient all-acoustic quantum control over N-$V$ centers is possible, and is especially promising for sensing applications that benefit from the compact footprint and location selectivity of acoustic devices.

We experimentally, numerically, and analytically investigate transition waves in a mechanical metamaterial comprising a chain of tristable elements. Transition waves can be initiated from different stable phases within the highly tunable tristable energy landscape. These waves propagate via coupled translational and rotational motion that leads to formation of stationary domain walls if transition waves of incompatible type collide. Since domain formation and propagation is fully reversible, the unique nonlinear behavior could be exploited in new classes of reconfigurable metamaterials.

In the Gaussian-modulated coherent state quantum key distribution (QKD) protocol, the sender first generates Gaussian-distributed random numbers and then encodes them on weak laser pulses actively by performing amplitude and phase modulations. Recently, an equivalent passive QKD scheme has been proposed by exploring the intrinsic field fluctuations of a thermal source [B. Qi, P. G. Evans, and W. P. Grice, Phys. Rev. A 97, 012317 (2018)]. This passive QKD scheme is especially appealing for chip-scale implementation since no active modulation is required. In this paper, we conduct an experimental study of the passively encoded QKD scheme using an off-the-shelf amplified spontaneous emission source operated in continuous-wave mode. Our results show that the excess noise introduced by the passive state preparation scheme can be effectively suppressed by applying optical attenuation and a secure key can be generated over metro-area distances.

The growing need for classical as well as quantum optical sensing places increasingly stringent requirements upon the desired characteristics of the engendered fields. Specifically, achieving superior field enhancement plays a critical role in applications ranging from chem-bio sensing, Raman and infrared spectroscopies to ion trapping and qubit control in emerging quantum-information science. Due to their low optical losses and ability to exhibit resonant field enhancements, all dielectric multilayers are emerging as an optical material system not only useful to classical photonics and sensing but also of potential to be integrated with quantum materials and quantum sensing. The recently introduced concept of zero-admittance layers [1] within dielectric multilayer materials, enables the creation and control of resonant fields orders of magnitude larger than the exciting field. Here, invoking the zero-admittance concept, we design, fabricate, and characterize an all-dielectric nonabsorbing stack and demonstrate the engendered huge field enhancement. Describing the fields in terms of Bloch surface waves, we connect the surface field to the semiperiodicity in the dielectric domains of the stack. As a specific application of the resonant field, we propose and demonstrate refractive-index sensing for the detection of trace amounts of an analyte. The results include a quantification of the sensitivity of the device with respect to the profile of the exciting field. The experimental results are shown to be in good agreement with theoretical calculations.

A cavity quantum electrodynamical (QED) system beyond the strong-coupling regime is expected to exhibit intriguing quantum phenomena. Here we report a direct measurement of the photon-dressed qubit transition frequencies up to four photons by harnessing the same type of state transitions in an ultrastrongly coupled circuit QED system realized by inductively coupling a superconducting flux qubit to a coplanar-waveguide resonator. This demonstrates a convincing observation of the photon-dressed Bloch-Siegert shift in the ultrastrongly coupled quantum system. Moreover, our results show that the photon-dressed Bloch-Siegert shift becomes more pronounced as the photon number increases, which is a characteristic of the quantum Rabi model.

The application in cryptography of quantum algorithms for prime factorization fostered the interest in quantum computing. However, quantum computers, and particularly quantum annealers, can also be helpful to construct secure cryptographic keys. Indeed, finding robust Boolean functions for cryptography is an important problem in sequence ciphers, block ciphers, and hash functions, among others. Due to the superexponential size $\mathcal{O}(2^{2^n})$ of the associated space, finding $n$-variable Boolean functions with global cryptographic constraints is computationally hard. This problem has already been addressed employing generic low-connected incoherent D-Wave quantum annealers. However, the limited connectivity of the Chimera graph, together with the exponential growth in the complexity of the Boolean-function design problem, limit the problem scalability. Here, we propose a special-purpose coherent quantum-annealing architecture with three couplers per qubit, designed to optimally encode the bent-function design problem. A coherent quantum annealer with this tree-type architecture has the potential to solve the eight-variable bent-function design problem, which is classically unsolved, with only $127$ physical qubits and $126$ couplers. This paves the way to reach useful quantum supremacy within the framework of quantum annealing for cryptographic purposes.

Engineering the surface morphology is an effective way to obtain specific functionalities in applications of smart surfaces. Developing two-dimensional (2D) smart materials, which can undergo morphology changes under appropriate external stimuli, is a promising route to pursue for smart-surface design. In the work presented here, using density-functional-theory calculations, we systematically study the thermodynamic stability, crystal structure, and piezoelectric properties of black phosphorene (black $\mathrm{P}$) doped with metallic atoms on one side with different concentrations. Particularly, $\mathrm{Li}$-doped black $\mathrm{P}$ (${\mathrm{P}}_4{\mathrm{Li}}_2$) has a $d_{31}$ value of 6.28 pm/V, which is at least 4 times larger than that of any other 2D piezoelectric material. Via finite-element-method simulations, we show a ${\mathrm{P}}_4{\mathrm{Li}}_2$-based prototype design for obtaining a surface-morphology change under a vertical electric field. The surface swelling pattern can be detected by human fingers with an appropriate design of the geometry. This demonstrates the promise of 2D piezoelectric materials for use in smart-surface applications.

Most quantum processors requires pulse sequences for controlling quantum states. Here we present an alternative algorithm for computing an optimal pulse sequence so as to perform a specific task, being an implementation of a quantum gate or a quantum-state preparation. In our method, we reduce greatly the number of parameters to be fitted by using a limited number of functions as the modulations for the amplitude and phase of the radio-frequency pulses. Unlike other algorithms, ours uses some approximations to make it fast and scalable, and uses some efficient methods to obtain pulses that are robust to common types of experimental errors. We demonstrate the success of the proposed algorithm by performing several real experiments for 4-, 7-, and 12-quantum-bit systems using NMR. In addition, we show the efficiency of the algorithm, finding pulses for controlling with good fidelity the quantum states of spins in fictional square two-dimensional lattices containing 16, 36, and 100 qubits.

We report a coherent Ising machine (CIM) that can simulate the Ising model with external magnetic fields. To represent Ising spins, the CIM employs degenerate optical parametric oscillator (DOPO) pulses, which are coupled by a measurement-feedback scheme. By adding to a feedback signal a term that is proportional to a given magnetic field and an absolute value of the measured amplitude of a DOPO pulse, the networked DOPO pulses behave like Ising spins in a magnetic field. Using the CIM, we implement antiferromagnetically coupled one- and two-dimensional networks of the Ising spins in uniform magnetic fields. At the field amplitudes that agree with a theoretical prediction, we experimentally observe a transition from an antiferromagnetic state governed by spin-spin interactions to a ferromagnetic state induced by the external magnetic field. In addition, staggered magnetization measurement reveals that the CIM behaves differently depending on the network dimensionality, which is consistent with the results obtained with the analytical solution and Monte Carlo simulations, but cannot be reproduced with a simple mean-field theory. These results suggest that the CIM clearly simulates the behavior of Ising spin networks better than a mere mean-field solver and will thus become a useful tool for simulating various statistical physics problems based on the Ising model.

Nitrogen-vacancy centers in diamond allow measurement of environment properties such as temperature, magnetic and electric fields at the nanoscale level, of utmost relevance for several research fields, ranging from nanotechnologies to biosensing. The working principle is based on the measurement of the resonance frequency shift of a single nitrogen-vacancy center (or an ensemble of them), usually detected by monitoring the center photoluminescence emission intensity. Albeit several schemes have already been proposed, the search for the simplest and most effective one is of key relevance for real applications. Here we present a continuous-wave lock-in-based technique able to reach high sensitivity in temperature measurement at microscale or nanoscale volumes (4.8 ${\mathrm{mK}/\mathrm{Hz}}^{1/2}$ in $\mu {\mathrm{m}}^3$). Furthermore, the present method has the advantage of being insensitive to environmental magnetic noise that in general introduces a bias in the temperature measurement.

The breaking of the Lorentz reciprocity law is a nontrivial task, since it usually requires bulky magnets or complicated time-modulation dynamic techniques to be accomplished. In this work, we present a simple and compact design of a nonlinear bifacial dielectric metasurface to achieve strong self-induced passive nonreciprocal transmission without the use of external biases. The proposed design is ideal for free-space-optics applications, can operate under both incident polarizations, and requires very low input excitation power to reach the nonreciprocal regime. It is composed of two passive silicon-based metasurfaces exhibiting Fano and Lorentzian resonances embedded in an ultrathin glass substrate. Highly asymmetric field enhancement is achieved with the proposed design that leads to strong nonreciprocity at low excitation intensities due to the large Kerr nonlinearity of silicon. Moreover, cascade designs are presented to further reduce the insertion loss, broaden the nonreciprocal intensity range, and increase the isolation ratio by enhancement of the transmission contrast. Finally, it is demonstrated that the proposed nonlinear metasurface is robust with regard to fabrication imperfections and can achieve large isolation for a relatively broad input power range even in the case of two incident waves impinging at the same time from both directions. The current work is expected to lead to several compact nonreciprocal nanophotonic devices, such as all-optical diodes, isolators, circulators, and ultrathin protective layers for sensitive optical components.

Nanoscale spintronic oscillators have attracted research interest in recent years because of their potential application in neuromorphic computing. Sustainable magnetization precession or gyrotropic motion of a skyrmion triggered by spin-polarized current or spin current gives rise to spintronic oscillation. However, the amplitude of skyrmion oscillation is exceedingly small. In this work, we report a skyrmion oscillation with a much larger amplitude based on periodical deformation of the skyrmion in a region with geometric confinement. This periodical deformation is induced by spatial-dependent spin current and is relevant to the skyrmion Hall effect. The amplitude and period of the oscillation can be well manipulated by modifying the damping coefficient, the slope of spin-current density, and the dimension of medium. The characteristic frequency of skyrmion oscillation determined by fast Fourier transformation is split under an external magnetic field that breaks the symmetry for the oscillation in the left and right parts of the skyrmion. This work paves the way to develop nanoscale skyrmion oscillators with tunable frequency and amplitude for neuromorphic computing.

The ability to dynamically control the radiative transfer of heat at the nanoscale holds the key to the development of a diverse number of technologies, ranging from nanoscale thermal-management systems to improved thermophotovoltaic devices. Recently, graphene has emerged as an ideal material to achieve this goal, since it can be electrically doped to support surface plasmons, collective oscillations of the conduction electrons. These resonances produce large and spectrally narrow optical cross sections, which dictate the emission and absorption properties of the graphene nanostructure and, thus, the heat that it radiatively exchanges with other objects and the environment. For attainable levels of doping, the plasmons supported by graphene nanostructures naturally lie in the midinfrared part of the spectrum, which is the most relevant wavelength range for radiative heat transfer under realistic temperatures. Furthermore, these resonances are actively tunable, thus providing full dynamic control over the heat transfer. Motivated by this great potential, we present a comprehensive analysis of the temporal evolution of the radiative heat transfer between arrangements of graphene nanodisks, showing that it is possible to exploit the tunability of these structures to obtain actively controlled heat transfer scenarios not possible with conventional passive nanostructures. The results of this work provide a framework for achieving fully dynamical control over nanoscale radiative heat transfer and thus provide fundamental insights into this process.

Atom interferometers offer excellent sensitivity to gravitational and inertial signals but have limited dynamic range. We introduce a scheme that improves this trade-off by a factor of 50 using composite fringes, obtained from sets of measurements with slightly varying interrogation times, as in a moiré effect. We analyze analytically the performance gain in this approach and the trade-offs it entails between sensitivity, dynamic range, and bandwidth, and we experimentally validate the analysis over a wide range of parameters. Combining composite-fringe measurements with a particle-filter estimation protocol, we demonstrate continuous tracking of a rapidly varying signal over a span 2 orders of magnitude larger than the dynamic range of a traditional atom interferometer.

We design superinductors made of strongly disordered superconductors for implementation in “hybrid” superconducting quantum circuits. The superinductors are fabricated as meandered nanowires made from granular aluminum films. Optimization of the device geometry enables the realization of superinductors with an inductance of approximately 1 $\mu \mathrm{H}$ and a self-resonance frequency of over 3 GHz. These compact superinductors are attractive for a wide range of applications, from superconducting circuits for quantum computing to microwave elements of cryogenic parametric amplifiers and kinetic inductance photon detectors.

We analyze the dynamics of passively mode-locked integrated external-cavity surface-emitting lasers (MIXSELs) using a first-principles dynamical model based on delay algebraic equations. We show that the third-order dispersion stemming from the lasing microcavity induces a train of decaying satellites on the leading edge of the pulse. Due to the nonlinear interaction with carriers, these satellites may get amplified, thereby destabilizing the mode-locked states. In the long-cavity regime, the localized structures that exist below the lasing threshold are found to be deeply affected by this instability. As it originates from a global bifurcation of the saddle-node infinite-period type, we explain why the pulses exhibit forms of behavior characteristic of excitable systems. Using the multiple-time-scale and the functional-mapping methods, we derive rigorously a master equation for MIXSELs in which third-order dispersion is an essential ingredient. We compare the bifurcation diagrams of the two models and assess their good agreement.

Magnetic skyrmions, which are a topologically nontrivial spin texture, have been considered as promising information carriers in future electronic devices because of their nanoscale size, low depinning current density, and high motion velocity. Despite the broad interest in skyrmion racetrack memory, researchers have recently been exploiting logic functions enabled by using the particlelike behavior of skyrmions. These functions can be applied to unconventional computing, such as stochastic computing (SC), which treats data as probabilities and is superior to binary computing due to its simplicity of logic operation. In this paper, we demonstrate SC implemented by the use of skyrmionic logic devices. We propose a skyrmionic and-or logic device as a multiplier in the stochastic domain and two skyrmionic multiplexer (MUX) logic devices as stochastic adders. With the assistance of voltage-controlled magnetic anisotropy (VCMA), precise control of skyrmion collisions is not required in the skyrmionic and-or logic device, thus improving the operational robustness. In the two MUX logic devices, skyrmions can be driven by the Zhang-Li torque or the spin-orbit torque (SOT). In particular, we can regulate the skyrmion motion flexibly by using VCMA or voltage-controlled Dzyaloshinskii-Moriya interaction in the SOT case. Furthermore, a 3-bit stochastic multiplier and adder are demonstrated by micromagnetic simulations. Simulations in synthetic antiferromagnets show that the performance of our skyrmionic logic gates can be optimized through the use of advanced materials. Our work opens up possibilities for implementing SC using skyrmionic logic devices.