Resistive switching—the current- and voltage-induced change of electrical resistance—is at the core of memristive devices, which play an essential role in the emerging field of neuromorphic computing. This study is about resistive switching in a Mott insulator, which undergoes a thermally driven metal-to-insulator transition. Two distinct switching mechanisms are reported for such a system: electric-field-driven resistive switching and electrothermal resistive switching. The latter results from an instability caused by Joule heating. Here, we present the visualization of the reversible resistive switching in a planar $\mathrm{V}$${}_2$$\mathrm{O}$${}_3$ thin-film device using high-resolution wide-field microscopy in combination with electric transport measurements. We investigate the interaction of the electrothermal instability with the strain-induced spontaneous phase separation in the $\mathrm{V}$${}_2$$\mathrm{O}$${}_3$ thin film at the Mott transition. The photomicrographs show the formation of a narrow metallic filament with a minimum width $\lesssim 500$ nm. Although the filament formation and the overall shape of the current-voltage characteristics (IVCs) are typical of an electrothermal breakdown, we also observe atypical effects such as oblique filaments, filament splitting, and hysteretic IVCs with sawtoothlike jumps at high currents in the low-resistance regime. We are able to reproduce the experimental results in a numerical model based on a two-dimensional resistor network. This model demonstrates that resistive switching in this case is indeed electrothermal and that the intrinsic heterogeneity is responsible for the atypical effects. This heterogeneity is strongly influenced by strain, thereby establishing a link between switching dynamics and structural properties.

Physical unclonable function (PUF) has been proposed as a promising and trustworthy solution to a variety of cryptographic applications. Here we propose a nonimaging-based authentication scheme for optical PUFs materialized by random scattering media, in which the characteristic fingerprints of optical PUFs are extracted from stochastical fluctuations of the scattered light intensity with respect to laser challenges, which are detected by a single-pixel detector. The randomness, uniqueness, unpredictability, and robustness of the extracted fingerprints are validated to be qualified for real authentication applications. By increasing the key length and improving the signal-to-noise ratio, the false accept rate of a fake PUF can be dramatically lowered to the order of ${10}^{-28}$. In comparison to the conventional laser-speckle-imaging-based authentication with unique identity information obtained from textures of laser-speckle patterns, this nonimaging scheme can be implemented at small speckle size bellowing the Nyquist–Shannon sampling criterion of the commonly used CCD or CMOS cameras, offering benefits in system miniaturization and immunity against reverse engineering attacks simultaneously.

The rising need for hybrid physical platforms has triggered a renewed interest in the development of agile radio-frequency phononic circuits with complex functionalities. The combination of traveling waves with resonant mechanical elements appears as an appealing means of harnessing elastic vibration. In this work, we demonstrate that this combination can be further enriched by the occurrence of traveling surface acoustic waves (SAWs), induced by elastic nonlinearities, interacting with a pair of otherwise linear micron-scale mechanical resonators. Reduction of the resonator-gap distance and an increase in the SAW amplitude results in a frequency softening of the resonator-pair response that lies outside the usual picture of geometrical Duffing nonlinearities. The dynamics of the SAW excitation scheme allows further control of the resonator motion, notably leading to circular-polarization states. These results may pave the way toward versatile high-frequency phononic microelectromechanical-systems–nanoelectromechanical-systems circuits fitting both classical and quantum technologies.

Optimization is one of the key applications of quantum computing where a quantum speedup has been an eagerly anticipated outcome. A promising approach to optimization using quantum dynamics is to consider a linear combination $s(t)B+[1-s(t)]C$ of two noncommuting Hamiltonians $B$ and $C$, where $C$ encodes the solution to the optimization problem in its ground state, $B$ is a Hamiltonian whose ground state is easy to prepare, and $s(t)\in [0,1]$ is the bounded “switching schedule” or “path,” with $t\in [0,t_f]$. This approach encompasses two of the most widely studied quantum-optimization algorithms: quantum annealing [QA; continuous $s(t)$] and the quantum approximate optimization algorithm [QAOA; piecewise constant $s(t)$]. While it is notoriously difficult to prove a quantum advantage for either algorithm, it is possible to compare and contrast them by finding the optimal $s(t)$. Here we provide a rigorous analysis of this quantum optimal control problem, entirely within the geometric framework of Pontryagin’s maximum principle of optimal control theory. We extend earlier results, derived in a purely closed-system setting, to open systems. This is the natural setting for experimental realizations of QA and QAOA. In the closed-system setting it was shown that the optimal solution is a “bang-anneal-bang” schedule, with the bangs characterized by $s(t)=0$ and $s(t)=1$ in finite subintervals of $[0,t_f]$, in particular, $s(0)=0$ and $s(t_f)=1$, in contrast to the standard prescription $s(0)=1$ and $s(t_f)=0$ of QA. As an example, we prove that for a single spin-$1/2$, the optimal solution in the closed-system setting is the bang-bang schedule, switching midway from $s\equiv 0$ to $s\equiv 1$. For finite-dimensional environments and without any approximations we identify sufficient conditions ensuring that either the bang-anneal, anneal-bang, or bang-anneal-bang schedules are optimal, and recover the optimality of $s(0)=0$ and $s(t_f)=1$. However, for infinite-dimensional environments and a system described by an adiabatic Redfield master equation we do not recover the bang-type optimal solution. In fact we can only identify conditions under which $s(t_f)=1$, and even this result is not recovered in the fully Markovian limit, suggesting that the pure anneal-type schedule is optimal. Our open-system results have implications for the use of experimental quantum-information processors, which are by necessity noisy, and suggest that in this practical sense the optimal schedules for quantum optimization are likely to be continuous.

Networks of optical oscillators simulating coupled Ising spins have been recently proposed as a heuristic platform to solve hard optimization problems. These networks, called coherent Ising machines (CIMs), exploit the fact that the collective nonlinear dynamics of coupled oscillators can drive the system close to the global minimum of the classical Ising Hamiltonian, encoded in the coupling matrix of the network. To date, realizations of large-scale CIMs have been demonstrated using hybrid optical-electronic setups, where optical oscillators simulating different spins are subject to electronic feedback mechanisms emulating their mutual interaction. While the optical evolution ensures an ultrafast computation, the electronic coupling represents a bottleneck that causes the computational time to severely depend on the system size. Here, we propose an all-optical scalable CIM with fully programmable coupling. Our setup consists of an optical parametric amplifier with a spatial light modulator (SLM) within the parametric cavity. The spin variables are encoded in the binary phases of the optical wave front of the signal beam at different spatial points, defined by the pixels of the SLM. We first discuss how different coupling topologies can be achieved by different configurations of the SLM, and then benchmark our setup with a numerical simulation that mimics the dynamics of the proposed machine. In our proposal, both the spin dynamics and the coupling are fully performed in parallel, paving the way towards the realization of size-independent ultrafast optical hardware for large-scale computation purposes.

A multiple access channel (MAC) consists of multiple senders simultaneously transmitting their messages to a single receiver. For the classical-quantum case (CQ MAC), achievable rates are known assuming that all the messages are decoded, a common assumption in quantum network design. However, such a conventional design approach ignores the global network structure, i.e., the network topology. When a CQ MAC is given as a part of quantum network communication, this work shows that computation properties can be used to boost communication speeds with code design dependent on the network topology. We quantify achievable quantum communication rates of codes with the computation property for a two-sender CQ MAC. When the two-sender CQ MAC is a boson coherent channel with binary discrete modulation, we show that it achieves the maximum possible communication rate (the single-user capacity), which cannot be achieved with conventional design. Further, such a rate can be achieved by different detection methods: quantum (with and without quantum memory), on-off photon counting, and homodyne (each at different photon power). Finally, we describe two practical applications, one of which cryptographic.

High-fidelity two-qubit entangling gates are essential building blocks for fault-tolerant quantum computers. Over the past decade, tremendous efforts have been made to develop scalable high-fidelity two-qubit gates with superconducting quantum circuits. Recently, an easy-to-scale controlled-phase gate scheme that utilizes the tunable-coupling architecture with fixed-frequency qubits [Phys. Rev. Lett. 125, 240502 (2020); Phys. Rev. Lett. 125, 240503 (2020)] has been demonstrated with high fidelity and attracted broad interest. However, in-depth understanding of the underlying mechanism is still missing, preventing us from fully exploiting its potential. Here we present a comprehensive theoretical study, explaining the origin of the high-contrast $ZZ$ interaction. Based on improved understanding, we develop a general yet convenient method for shaping an adiabatic pulse in a multilevel system, and identify how to optimize the gate performance from design. Given state-of-the-art coherence properties, we expect the scheme to potentially achieve a two-qubit gate error rate near ${10}^{-5}$, which would drastically speed up the progress towards fault-tolerant quantum computation.

Due to the influence of dimension, two-dimensional (2D) all-inorganic double perovskites may be superior to their three-dimensional (3D) counterparts as environmentally friendly and efficient optoelectronic materials. Nevertheless, how the halogens affect the optoelectronic performance of 2D all-inorganic double perovskite is still unknown. Here, the photoelectric properties, including band structures, optical absorption spectra, carrier mobilities, and exciton binding energies, of 2D all-inorganic double perovskites ${\mathrm{Cs}}_4\mathrm{Ag}\mathrm{Bi}{X}_8$ ($X$ = $\mathrm{Cl}$, $\mathrm{Br}$, $\mathrm{I}$) with the Ruddlesden-Popper structure are studied via density-functional theory along with the spin-orbit coupling effect. Considering the influence of the exciton effect in low-dimensional materials, we also calculate light absorption by using the GW Bethe-Salpeter equation method. The obtained results show that the substitution of $\mathrm{Cl}$ with $\mathrm{Br}$ or $\mathrm{I}$ atoms decreases the band gap (from 2.706 eV to 2.221 and 1.715 eV) for ${\mathrm{Cs}}_4{\mathrm{Ag}\mathrm{Bi}\mathrm{Cl}}_8$, enhances the light-absorption performance, increases the mobility of carriers, and reduces the exciton binding energy (from 1529.90 meV to 1268.70 and 941.71 meV). Moreover, the outcomes demonstrate that 2D all-inorganic double perovskites ${\mathrm{Cs}}_4\mathrm{Ag}\mathrm{Bi}{X}_8$ ($X$ = $\mathrm{Cl}$, $\mathrm{Br}$, $\mathrm{I}$) might be better candidates for luminescent devices than photovoltaic materials, and the excellent performance of ${\mathrm{Cs}}_4{\mathrm{Ag}\mathrm{Bi}\mathrm{I}}_8$ makes it the optimal among the three. Our study will expand theoretical explorations for research into 2D all-inorganic double perovskite materials for potential luminescent or photovoltaic applications.

The advent of acoustic metamaterials has opened up an emerging frontier in the control of sound transmission. A key limitation, however, is that an acoustic metamaterial based on a single local resonator in the unit cell produces a restricted narrow-band attenuation peak. When multiple local resonators are used, the attenuation peaks that arise—while numerous—are each still narrow and separated by passbands. Here, we present an acoustic metamaterial concept that yields a subwavelength sound transmission loss through two antiresonances—in a single band gap—that are fully coupled and, hence, provide a broadband attenuation range; this is in addition to delivering a high isolation intensity for both peaks that exceeds 100 dB within the 3–5 kHz range or 60 dB around 1 kHz. The underlying coupled-resonance mechanism is triggered by ensuring that two local resonances appear between two coincident frequencies formed by the intersection of the incident acoustic waves sound line with Bragg dispersion curves governing in-plane wave motion orthogonal to the direction of transmission. This phenomenon is nominally realized in the form of a thin single-panel single-material pillared-plate structure with internal contiguous holes, a practical configuration that lends itself to design adjustments and optimization for a frequency range of interest, down to subkilohertz, and to mass fabrication.

A simple and universal design principle to realize the generalized Brewster effect (GBE) is proposed. Based on the generalized Kerker effect (GKE), we can realize the GBE for arbitrary polarizations, any incident angles, and any frequencies on the traditional dielectric interface by the destructive interference of various of multipoles. An adjustable metallic meta-surface has been designed to achieve this effect experimentally in the microwave band. By adjusting the suspension height of meta-surfaces above the interface, the incident angles and frequency corresponding to the GBE can be modulated easily. For the two different polarized incident waves, the GBE can be realized at the same frequency and same incident angle, which shows the validity of the GKE. This design principle opens the door for realizing tunable GBE devices that can be applied in many scenarios.

We generalize similarity theory based on the scaling and solution invariance of the Boltzmann equation, coupled with the Poisson equation, and demonstrate similarity laws for radio-frequency (rf) discharge plasmas across three nonlinear transitional regimes, namely, the alpha-gamma mode transition, the stochastic-Ohmic-heating mode transition, and the bounce-resonance-heating mode transition. Fundamental plasma parameters, e.g., the electron power absorption, under similar discharge conditions are examined via fully kinetic particle-in-cell simulations, and electron-kinetic invariance is exemplified in similar rf discharge plasmas. The results unambiguously confirm the applicability of similarity laws for rf plasmas in extended operating regimes, and strengthen the foundations and framework of similarity physics with universality.

We propose a scheme for the detection of microwave-induced photons through a current-biased Josephson junction, from the point of view of the statistical decision theory. Our analysis is based on the numerical study of the zero-voltage lifetime distribution in response to a periodic train of pulses, that mimics the absorption of photons. The statistical properties of the detection are retrieved comparing the thermally induced transitions with the distribution of the switchings to the finite voltage state due to the joint action of thermal noise and of the incident pulses. The capability to discriminate the photon arrival can be quantified through the Kumar-Caroll index, which is a good indicator of the signal-to-noise ratio. The index can be exploited to identify the system parameters best suited for the detection of weak microwave photons.

Nitrogen-vacancy color centers in diamond have attracted broad attention as quantum sensors for both static and dynamic magnetic, electrical, strain, and thermal fields and are particularly attractive for quantum sensing under pressure in diamond anvil cells. Optically based nuclear magnetic resonance may be possible at pressures greater than a few gigapascals and offers an attractive alternative to conventional Faraday-induction-based detection. Here, we present ac sensing results and demonstrate synchronized readout up to 6 GPa but find that the sensitivity is reduced due to inhomogeneities of the microwave field and pressure within the sample space. These experiments enable the possibility of all-optical high resolution magnetic resonance of nanoliter sample volumes at high pressures.

While operating a quantum random-number generator (QRNG), it is extremely useful to have a model of the physical entropy source to guarantee that the device is delivering randomness of genuine quantum origin. In this work we consider a QRNG based on a gain-switched laser diode and we develop a model to quantify its phase noise. This model is based on the laser rate equations and the state-of-the-art techniques for the characterization of laser diodes used in lightwave systems. These tools let us achieve a faithful modeling of the phase noise and we verify its accuracy through comparisons with experimental measurements. Furthermore, the model can be used to select optimal parameters to maximize the QRNG performance and monitor the device behavior to detect malfunctioning or malicious tampering of the device.

Spintronic terahertz (THz) emitters based on ferromagnetic (FM)-nonmagnetic (NM) heterostructures are the focus of many experimental endeavors for THz sources. However, existing spintronic THz sources control emission by changing the external magnetic field, which is inconvenient for compact integration. Therefore, various modulated mechanisms are in high demand. Here, we present an electric-field-controlled spintronic THz emitter based on the ferroelectric (FE)-FM/NM structure that can emit THz radiation with voltage-modulated amplitude and polarization angle. The modulation depth of the THz amplitude is up to 69%, and the maximum rotation of the THz polarization angle is 28°. In addition, after removing the electric field, two different THz-emission states can remain unchanged, demonstrating nonvolatile behavior. Modulation is attributed to magnetoelectric coupling between the FM and FE layers, which modifies magnetization of the FM layer by the electric-field-induced strain in the FE layer, and thus, modulates THz emission. Furthermore, the possibility of a programmable array-type THz source with varied bias voltage is theoretically demonstrated, allowing a spatially modulated THz light to be generated directly without the use of additional spatial light modulators. We believe that this electric-field-controlled THz emitter has the potential to be used in next-generation on-chip THz sources, THz memory devices, and THz ghost imaging.