Nonvolatile memory devices have received a lot of interest in both industry and academia in the last decade. Transition metal oxide-based memories offer potential applications as universal memory and artificial synapses. Here we focus on the one-time conditioning of metal/oxide/metal structures leading to the formation of a conducting filament in $\mathrm{Ti}\mathrm{N}/{\mathrm{Ta}}_x{\mathrm{O}}_{1-x}/\mathrm{Ti}\mathrm{N}$ structures and develop a finite-element model of this process. The process considered here consists of two steps. First the thermal runaway increases the temperature of the device and sets up large temperature gradients. In the second step, the lateral temperature gradient drives the ion motion forming a $\mathrm{Ta}$-rich and $\mathrm{O}$-poor filament. The process comes to steady state when the ion flux due to concentration and stress gradients balances the thermophoretic fluxes. The model replicated the structure of the filament including the size of the $\mathrm{Ta}$-rich filament core (20 nm diameter), the surrounding $\mathrm{Ta}$-depleted ring (50 nm), and the compositions of both regions. In addition, the model reproduced characteristic dynamics of the electroformation with slow changes of conductance during the incubation period, rapid increase of conductance during compositional runaway, and saturation. The range of critical material parameters, namely transport heats for $\mathrm{Ta}$ and $\mathrm{O}$, is discussed.

We study the coherence in time and space of electromagnetic fields propagated through complex media. Whether for localization, imaging, or telecommunication, the development of dedicated numerical techniques is generally based on the exploitation of simplified models considering either coherent or diffuse fields. The optimization of such applications in conditions of partial coherence can therefore be particularly challenging, requiring the development of hybrid algorithms adaptable to prior knowledge on the processed fields. The objective of this work is to provide numerical techniques for decomposing an electromagnetic field into subspaces that can then be filtered according to their level of spatial and temporal coherence. In contrast to the studies carried out on space-space transfer matrices notably used for the calculation of Wigner-Smith operators, these decompositions are carried out on space-time matrices in order to facilitate the study of temporal dispersion. The theory is developed for illustrative purposes using experimental results from a leaky resonant system, but seem to be applicable to any scattering and reverberating media capable of transforming localized and coherent excitations into complex and diffuse distributions. To conclude this work, the proposed technique is exploited to improve image reconstruction in a millimeter-wave computational imaging demonstration. In the studied context and from a more general perspective, we propose a technique to select the most suitable subspaces for each application operating under conditions of partial coherence, whether these correspond in the most extreme cases to ballistic paths or to diffuse fields.

We propose a realistic setup, inspired by already existing experiments, within which we develop a general formalism for the implementation of distributed quantum gates. Mediated by a quantum link that establishes a bidirectional quantum channel between distant nodes, our proposal works both for inter- and intranode communication and handles scenarios ranging from the few to the many modes limit of the quantum link. We are able to design fast and reliable state transfer protocols in every regime of operation, which, together with a detailed description of the scattering process, allows us to engineer two sets of deterministic universal distributed quantum gates: gates whose implementation in quantum networks does not need entanglement distribution nor measurements. By employing a realistic description of the physical setup we identify the most relevant imperfections in the quantum links as well as optimal points of operation with resulting infidelities of $1-F\approx {10}^{-2}$–${10}^{-3}$.

We demonstrate that time-delayed nonlinear effects in exciton polaritons can be used to construct neural networks where information is coded in optical pulses arriving consecutively on the sample. The highly nonlinear effects are induced by time-dependent interactions with the excitonic reservoir. These nonlinearities allow the creation of a nonlinear XOR logic gate that can perform operations on the picosecond timescale. An optoelectronic neural network based on the constructed logic gate performs the classification of spoken digits with a high accuracy rate.

The recently discovered antisymmetric interlayer exchange coupling (AIEC) plays pivotal roles in the magnetization switching of a synthetic antiferromagnet by inducing magnetization canting. Large AIEC is reported for perpendicularly magnetized $\mathrm{Pt}/\mathrm{Co}/\mathrm{Ir}/\mathrm{Co}/\mathrm{Pt}$ with wedge-shaped layers. The effective field of AIEC is related with symmetric interlayer exchange coupling, providing a guide for enhancing AIEC. We develop an extended Stoner–Wohlfarth model for a synthetic antiferromagnet, revealing key factors in its magnetization switching. Combining the theoretical knowledge and the experimental results, perpendicular magnetization switching is achieved solely by an in-plane magnetic field.

Memristors are among the most promising elements for modern microelectronics, having unique properties such as quasicontinuous change of conductance and long-term storage of resistive states. However, identifying the physical mechanisms of resistive switching and of the evolution of conductive filaments in such structures still remains a major challenge. In this work, aiming at a better understanding of these phenomena, we experimentally investigate an unusual effect of enhanced conductive filament stability in memristors with copper filaments under the applied voltage and present a simplified theoretical model of the effect of a quantum current through a filament on its shape. Our semiquantitative, continuous model indeed predicts that, for a thin filament, the “quantum pressure” exerted on its walls by the recoil of charge carriers can well compete with the surface tension and crucially affect the evolution of the filament profile at voltages of around 1 V. At lower voltages, the quantum pressure is expected to provide extra stability to the filaments supporting quantized conductance, which we also reveal experimentally using a methodology focusing on retention statistics. Our results indicate that the recoil effects could be potentially important for resistive switching in memristive devices with metallic filaments and that taking them into account in the rational design of memristors could help achieve their better retention and plasticity characteristics.

A dead layer without superconductivity, i.e., zero resistance and Meissner effect, commonly exists at the interface between substrate and superconducting film. It is demonstrated that the dead layer can significantly degrade the performance of superconducting devices at microwave frequencies. Knowing the thickness of the dead layer, $d_{\mathrm{dead}}$, is necessary when using a two-coil mutual-inductance (TCMI) technique to measure the absolute value of the magnetic penetration depth, $\lambda$, in superconducting films. However, there is still no established method to determine $d_{\mathrm{dead}}$ in superconducting films. Here, we propose employing the TCMI technique to determine $d_{\mathrm{dead}}$, based on a multilayered model in which the dead layer is sandwiched between superconducting layer and substrate. Using this method, we find that $d_{\mathrm{dead}}=43\pm 10\mathrm{nm}$ for our superconducting $\mathrm{FeSe}$ films, in agreement with the absence of diamagnetism for the 42-nm-thick film. With the $d_{\mathrm{dead}}$ determined, we accurately determine the absolute value of $\lambda$ in superconducting $\mathrm{FeSe}$ films.

Reverse annealing is a variant of quantum annealing, in which the system is prepared in a classical state, reverse annealed to an inversion point, and then forward annealed. We report on reverse annealing experiments using the D-Wave 2000Q device, with a focus on the $p=2p$-spin problem, which undergoes a second-order quantum phase transition with a gap that closes polynomially in the number of spins. We concentrate on the total and partial success probabilities, with the latter being the probabilities of finding each of two degenerate ground states of all spins up or all spins down, and the former being their sum. The empirical partial success probabilities exhibit a strong asymmetry between the two degenerate ground states, depending on the initial state of the reverse anneal. To explain these results, we perform open-system simulations using master equations in the limits of weak and strong coupling to the bath. The former, known as the adiabatic master equation, with decoherence in the instantaneous energy eigenbasis, predicts perfect symmetry between the two degenerate ground states, thus failing to agree with the experiment. In contrast, the latter, known as the polaron transformed Redfield equation, is in close agreement with experiment. Thus, our results present a challenge to the sufficiency of the weak system-bath coupling limit in describing the dynamics of current experimental quantum annealers, at least for reverse annealing on timescales of a microsecond or longer.

The all-d-metal $\mathrm{Ni}\text{−}(\mathrm{Co})\text{−}\mathrm{Mn}\text{−}\mathrm{Ti}$-based Heusler alloys are found to show a giant magnetocaloric effect near room temperature and are thereby potential materials for solid-state refrigeration. However, the relative large thermal hysteresis and the moderate ferromagnetic magnetization provides limitations for real applications. In the present study, we demonstrate that introducing interstitial B atoms within ${\mathrm{Ni}}_{36.5}{\mathrm{Co}}_{13.5}{\mathrm{Mn}}_{35}{\mathrm{Ti}}_{15}$ alloys can effectively decrease the thermal hysteresis Δ$T_{\mathrm{hys}}$ (down to 4.4 K), and simultaneously improve the saturation magnetization (maximum 40% enhancement) for low concentrations of B doping (up to 0.4 at. %). In comparison to the undoped reference material, the maximum magnetic entropy change (Δ$S_m$) for the ${\mathrm{Ni}}_{36.5}{\mathrm{Co}}_{13.5}{\mathrm{Mn}}_{35}{\mathrm{Ti}}_{15}{\mathrm{B}}_{0.4}$ alloy shows a remarkable improvement from 9.7 to 24.3 J ${\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ for an applied magnetic field change (Δ${\mu }_{0}H$) of 5 T (30.2 J ${\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ for Δ${\mu }_{0}H$ = 7 T). Additionally, due to the obtained low thermal hysteresis Δ$T_{\mathrm{hys}}$, the maximum reversible Δ$S_m^{\mathrm{rev}}$ amounts to 18.9 J ${\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ at 283 K for Δ${\mu }_{0}H$ = 5 T (22.0 J ${\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ at 281 K for Δ${\mu }_{0}H$ = 7 T), which is competitive to the traditional $\mathrm{Ni}\text{−}\mathrm{Mn}$-$X$-based Heusler alloys (X = $\mathrm{Ga}$, $\mathrm{In}$, $\mathrm{Sn}$, $\mathrm{Sb}$). The enhancement of the magnetic moments by B doping is also observed in first-principles calculations. These calculations clarify the atomic occupancy of B and the changes in the electronic configuration. Our current study indicates that interstitial doping with a light element (boron) is an effective method to improve the magnetocaloric effect in these all-d-metal $\mathrm{Ni}\text{−}\mathrm{Co}\text{−}\mathrm{Mn}\text{−}\mathrm{Ti}$-based magnetic Heusler compounds.

Lasers with well-controlled relative frequencies are indispensable for many applications in science and technology. We present a frequency-offset locking method for lasers based on beat-frequency discrimination utilizing hybrid electronic $LC$ filters. The method is specifically designed for decoupling the tightness of the lock from the broadness of its capture range. The presented demonstration locks two free-running diode lasers at 780 nm with a 5.5-GHz offset. It displays an offset frequency instability below 55 Hz for time scales in excess of 1000 s and a minimum of 12 Hz at 10-s averaging. The performance is complemented with a 190-MHz lock-capture range, a tuning range of up to 1 GHz, and a frequency ramp agility of $200\mathrm{kHz}/\mu \mathrm{s}$.

The resolution of far-field imaging systems suffers from the diffraction limit. In this work, a blind structured illumination microscopy (blind-SIM) system that achieves subwavelength resolution in acoustic far-field imaging is developed. In blind-SIM, precise knowledge of the structured illumination (SI) patterns is not required. Instead, the acoustic SIs are generated by a randomly located acoustic diffraction grating with a subwavelength grating period. The spatial frequency mixing between the object and the SIs converts evanescent waves to propagating waves that can reach sensors in the far field. The image of the object is then reconstructed from multiple far-field measurements utilizing a joint-sparsity compressive sensing algorithm. Numerical simulations and experiments in kilohertz airborne sound are performed to validate the proposed system. A subwavelength imaging resolution of around 1/3 wavelength is achieved in experiments, and around 1/5 wavelength in simulations. The blind-SIM system largely reduces the imaging hardware complexity compared with existing acoustic subwavelength technologies that mostly rely on metamaterial lenses, time-reversal mirrors, or the precise localization of time-varying contrast agents. The combination of blind-SIM and the joint-sparsity compressive sensing algorithm for image reconstruction reduces the requirements for an object’s sparsity level and enhances the system’s robustness against noise. Our work has potential in improving the practicality of acoustic far-field subwavelength imaging in medical diagnoses, nondestructive testing, and underwater acoustic imaging and communications.

Due to challenging efficiency limits facing conventional and unconventional electronic architectures, information processors based on photonics have attracted renewed interest. Research communities have yet to settle on definitive techniques to describe the performance of this class of information processors. Photonic systems are different from electronic ones, and the existing concepts of computer performance measurement cannot necessarily apply. In this paper, we quantify the power use of photonic neural networks with state-of-the-art and future hardware. We derive scaling laws, physical limits, and platform platform performance metrics. We find that overall performance takes on different dominant scaling laws depending on scale, bandwidth, and resolution, which means that energy efficiency characteristics of a photonic processor can be completely described by no less than seven performance metrics over the range of relevant operating domains. The introduction of these analytical strategies provides a much needed foundation and reference for quantitative roadmapping and commercial value assignment for silicon photonic neural networks.

Lasers, light-emitting diodes, and other optoelectronic devices employing $\mathrm{In}\mathrm{As}$/$\mathrm{In}\mathrm{P}$ quantum dots (QDs) instead of quantum wells (QWs) as their active parts benefit from the quasi-zero-dimensional (0D) density of states while maintaining the emission at the communication-relevant range of $1.55\mu \mathrm{m}$. However, for certain application purposes, the substitution of QWs with QDs is advantageous only if QDs can either be treated as isolated objects or exhibit quantum-mechanical coupling between deeply confined states. Here, we compare two material systems of $\mathrm{In}\mathrm{As}$/($\mathrm{In}$,$\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{As}$/$\mathrm{In}\mathrm{P}$ and $\mathrm{In}\mathrm{As}$/($\mathrm{In}$,$\mathrm{Al}$)$\mathrm{As}$/$\mathrm{In}\mathrm{P}$ elongated QDs (quantum dashes, QDashes) of comparable surface densities and interdash distances. We investigate the presence and type of coupling between QDashes, focusing on the direct tunnel coupling for both types of carriers manifested already at low temperature. In the time-resolved photoluminescence (PL) experiment, we observe a significant dispersion of the PL decay time for $\mathrm{In}\mathrm{As}$/($\mathrm{In}$,$\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{As}$ QDashes, resulting from the migration of carriers from high- to low-energy QDashes due to substantial tunnel coupling. In the case of $\mathrm{In}\mathrm{As}$/($\mathrm{In}$,$\mathrm{Al}$)$\mathrm{As}$ QDashes, the dispersionless time dependence points toward the absence of such coupling. We confirm this interpretation with multiband $\mathit{k}\cdot \mathit{p}$ calculations. We check that the proposed coupling scenario is possible and show its much higher probability for $\mathrm{In}\mathrm{As}$/($\mathrm{In}$,$\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{As}$ than for $\mathrm{In}\mathrm{As}$/($\mathrm{In}$,$\mathrm{Al}$)$\mathrm{As}$ QDashes. Finally, in temperature-dependent PL, we observe the redistribution of holes among QDashes via thermal excitation to wetting-layer states for both systems, constituting an additional interdash coupling channel at elevated temperatures. Our results indicate that the $\mathrm{In}\mathrm{As}$/($\mathrm{In}$,$\mathrm{Al}$)$\mathrm{As}$ QDash system is preferential for optoelectronic applications where QD isolation is highly sought after, whereas $\mathrm{In}\mathrm{As}$/($\mathrm{In}$,$\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{As}$ QDashes exhibit quantum-mechanical coupling between deeply confined states.

We report an artificial acoustic compressibility-near-zero medium made of a phononic crystal composed of epoxy blocks arranged in a square lattice. Its anisotropic effective density leads to a linear cross in its isofrequency contour in the vicinity of the Brillouin zone center, as its effective compressibility approaches zero. When a Gaussian beam is normally incident on the phononic crystal, a splitting effect is achieved at the frequency of the crossing point. Based on such a beam-splitting effect, an acoustic cloaking of an irregular-shaped object embedded in the phononic crystal is demonstrated both theoretically and experimentally. Such an anisotropic zero-index material offers a potential method to control acoustic waves.

Multiple reflections between transducer and imaged object can naturally occur in ultrasound imaging and other acoustic sensing applications such as sonar. The repeated interaction of the emitted wave front with the imaged object is traditionally regarded as an undesired reverberation artifact, often misinterpreted as fictitious acoustic boundaries. We introduce high-order reflection pulse-echo (HOPE) ultrasound, a method that leverages high-order reflections to improve on several aspects of conventional ultrasound imaging. HOPE is experimentally demonstrated to resolve submicrometer features by breaking through the sampling limit. The major contrast enhancement of the high reflection orders allowed defects within materials invisible to conventional scanning acoustic microscopy to be revealed. The technique is further shown to improve accuracy of frequency-dependent ultrasound attenuation measurements from biological tissues. HOPE ultrasound requires no additional hardware and is easy to implement, underscoring its potential to boost imaging performance in biomedical imaging, nondestructive testing, and other acoustic sensing applications.

Quantum networks with the ability to coherently transfer quantum states between different nodes open up possibilities for upscaling quantum technologies. In this work, we propose a theoretical protocol to implement the multinode state transfer and nonlocal state preparation in a one-dimensional network consisting of many quantum nodes coupled to a common unidirectional channel. By time-dependently modulating the coupling strength between the nodes and the channel, a quantum state stored in one node can be distributed to the other nodes through the channel, while a series of dark-state conditions ensure a desired final state. This protocol can also be used to prepare nonlocal entanglement states of these nodes, such as the W -type state. Importantly, it can be realized in various quantum platforms with currently available technologies.