The research on the properties of spin waves (SWs) in three-dimensional nanosystems is an innovative idea in the field of magnonics. Mastering and understanding the nature of magnetization dynamics and binding of SWs at surfaces, edges, and in-volume parts of three-dimensional magnetic systems enables the discovery of alternative phenomena and suggests other possibilities for their use in magnonic and spintronic devices. In this work, we use numerical methods to study the effect of geometry and external magnetic field manipulations on the localization and dynamics of SWs in crescent-shaped (CS) waveguides. It is shown that changing the magnetic field direction in these waveguides breaks the symmetry and affects the localization of eigenmodes with respect to the static demagnetizing field. This, in turn, has a direct effect on their frequency. Furthermore, CS structures are found to be characterized by significant saturation at certain field orientations, resulting in a cylindrical magnetization distribution. Thus, we present chirality-based nonreciprocal dispersion relations for high-frequency SWs, which can be controlled by the field direction (shape symmetry) and its amplitude (saturation).

Imaging in a broad light-intensity regime with a high signal-to-noise ratio is a key capability in fields as diverse as quantum metrology and astronomy. Achieving high signal-to-noise ratios in quantum imaging leads to surpassing the classical limit in parameter estimation. In astronomical detection, the search for habitable exoplanets demands imaging of atmospheres in the infrared to look for biosignatures. These optical applications are hampered by detection noise, which critically limits their potential, and thus demands photon-number and spatial-resolution detectors. Here we report an imaging device in the infrared wavelength range able to arbitrarily reduce the readout noise. We built a measured exposure skipper-CCD sensor instrument equipped with a thick back-illuminated sensor, with photon-number-resolving capability in a wide dynamic range, spatial resolution, high quantum efficiency in the near-infrared and ultralow dark counts. This device allows us to image objects in a broad range of intensities within the same frame and, by reducing the readout noise to less than 0.2e${}^{-}$, to distinguish even those shapes with less than two photons per pixel, unveiling what was previously hidden in the noise. These results pave the way for building high-standard infrared imagers based on skipper CCDs.

The development of modular and versatile quantum interconnect hardware is a key next step in the scaling of quantum information platforms to larger size and greater functionality. For superconducting quantum systems, fast and well-controlled tunable circuit couplers will be paramount for achieving high-fidelity and resource-efficient connectivity, whether for performing two-qubit gate operations, encoding or decoding a quantum data bus, or interfacing across modalities. Here we propose a versatile and internally tunable double-transmon coupler (DTC) architecture that implements tunable coupling via flux-controlled interference in a three-junction dc superconducting quantum interference device. Crucially, the DTC possesses an internally defined zero-coupling state that is independent of the coupled data qubits or circuit resonators. This makes it particularly attractive as a modular and versatile design element for realizing fast and robust linear coupling in several applications such as high-fidelity two-qubit gate operations, qubit readout, and quantum bus interfacing.

A remarkable phenomenon in contemporary physics is quantum scarring in systems whose classical dynamics are chaotic, where certain wave functions tend to concentrate on classical periodic orbits of low periods. Quantum scarring has been studied for more than four decades, but detecting quantum scars still mostly relies on human visualization of the wave-function patterns. The widespread and successful applications of machine learning in many branches of physics suggest the possibility of using artificial neural networks for automated detection of quantum scars. Conventional machine learning often requires substantial training data, but, for quantum scars, this poses a significant challenge: in typical systems the available distinct quantum scarring states are rare. We develop a meta machine-learning approach to accurately detect quantum scars in a fully automated and highly efficient fashion. In particular, taking advantage of some standard large datasets such as Omniglot from the field of image classification, we train a “preliminary” version of the neural network that has the ability to distinguish different classes of noisy images. We then perform few-shot classification to further train the neural network but with a small number of quantum scars. We demonstrate that the meta-learning scheme can find the correct quantum scars from thousands of images of wave functions without any human intervention, regardless of the symmetry of the underlying system. From a general applied point of view, our success opens the door to exploiting meta learning for solving challenging image detection and classification problems in other fields of science and engineering. For example, in microlasing systems, identifying scarring states is critical as these states are desired for directional emission. The task is also important for quantum-dot devices where the scarring states can lead to resonances in the conductance.

Photon-number-resolving (PNR) single-photon detectors are an enabling technology in many areas, such as photonic quantum computing, nonclassical light-source characterization, and quantum imaging. Here, we demonstrate high-efficiency PNR detectors using a parallel superconducting nanowire single-photon-detector (P-SNSPD) architecture that does not suffer from crosstalk between the pixels and that is free of latching. The behavior of the detector is modeled and used to predict the possible outcomes given a certain number of incoming photons. We apply our model to a four-pixel P-SNSPD with a system detection efficiency of 92.5%. We also demonstrate how this detector allows reconstructing the photon-number statistics of a coherent source of light, which paves the way towards the characterization of the photon statistics of other types of light source using a single detector.

We show that terahertz (THz) emission spectroscopy is a method to probe the in-plane magnetic state in spintronic heterostructures with exchange bias between the constituent antiferromagnetic and ferromagnetic layers. We demonstrate that the intense laser excitation can switch and reversibly reset the direction of the pinning axis arising from the exchange bias in technologically relevant tunnel magnetoresistance elements. By applying the double-pump THz emission spectroscopy, we reveal that this switching exhibits a threshold character in the pump intensity.

Vertical plasmonic coupling in double-layer graphene leads to two hybridized plasmonic modes: the optical and the acoustic plasmon with symmetric and antisymmetric charge distributions across the interlayer gap, respectively. However, in most experiments based on far-field excitation, only the optical plasmon are dominantly excited in the double-layer graphene systems. Here, we propose strategies to selectively and efficiently excite the acoustic plasmon with single or multiple nanoemitters. The analytical model developed here elucidates the role of the position and arrangement of the emitters on the symmetry of the resulting graphene plasmons. In addition, we present an optimal device structure to enable an experimental observation of the acoustic plasmon in double-layer graphene toward the ultimate level of plasmonic confinement defined by a monoatomic spacer, which is inaccessible with a graphene-on-a-mirror architecture.

High-fidelity quantum gate operations are essential for achieving scalable quantum circuits. In spin qubit quantum computing systems, metallic gates and antennas that are necessary for qubit operation, initialization, and readout, also cause detrimental effects by enhancing fluctuations of electromagnetic fields. Therefore, evanescent wave Johnson noise (EWJN) caused by near-field thermal and vacuum fluctuations becomes an important unmitigated noise, which induces the decoherence of spin qubits and limits the quantum gate operation fidelity. Here, we first develop a macroscopic quantum electrodynamics theory of EWJN to account for the dynamics of two spin qubits interacting with metallic circuitry. Then we propose a numerical technique based on volume integral equations to quantify EWJN strength in the vicinity of nanofabricated metallic gates with arbitrary geometry. We study the limits to two-spin-qubit gate fidelity from EWJN-induced relaxation processes in two experimentally relevant quantum computing platforms: (a) the silicon quantum dot system and (b) nitrogen-vacancy centers in diamond. Finally, we introduce a Lindbladian engineering method to optimize the control pulse sequence design and show its enhanced performance over Hamiltonian engineering in mitigating the influence of thermal and vacuum fluctuations. Our work leverages advances in computational electromagnetics, fluctuational electrodynamics, and open quantum systems to suppress the effects of near-field thermal and vacuum fluctuations and reach the limits of two-spin-qubit gate fidelity.

$\mathrm{Ga}\mathrm{N}$/$(\mathrm{Al},\mathrm{Ga})\mathrm{N}$ heterojunctions are at the heart of high-electron-mobility transistors that are being adopted for high-power and high-frequency applications. The strong polarization fields present at this interface significantly enhance the density of the two-dimensional electron gas (2DEG) that is confined on the $\mathrm{Ga}\mathrm{N}$ side of the junction. The microscopic origin of these electrons has been debated over the years: after excluding that they would be contributed by bulk donors, a model that identifies surface states on the $(\mathrm{Al},\mathrm{Ga})\mathrm{N}$ surface as the source of electrons has become widely adopted. Recently, it has become clear, however, that the measured density of surface states is insufficient to account for the high electron density in the 2DEG. Here we demonstrate, based on state-of-the-art first-principles calculations, that the source of electrons is intrinsic to the overall structure and that the negative charge in the 2DEG is balanced by fixed charge on the surface. We perform a rigorous study of polarization, using our recently developed methodology for quantifying polarization fields within the finite-sized systems that can be addressed with density-functional calculations. The results show that the electrons that appear in the 2DEG originate locally at the interface, and that the net charge at the interface is predominantly compensated by fixed charge on the surface, rather than surface states. We elucidate the source of this fixed charge and associate it with surface reconstructions or the presence of heterovalent impurities (such as oxygen). Our results force a reassessment of the impact of surface states on the density of the 2DEG: rather than serving as a supply of electrons, the surface states mainly act to pin the Fermi level. Our conclusions allow a fresh interpretation of experimental observations and allow devising guidelines for optimizing carrier densities in the 2DEG.

Pure disulfide ${\mathrm{Cu}\mathrm{In}\mathrm{S}}_2$ ($\mathrm{CIS}$) with a proper band gap (∼1.53 eV) is a promising photovoltaic material, yet it exhibits a relatively low power-conversion efficiency among the $\mathrm{Cu}$-III-${\mathrm{VI}}_2$ (III = $\mathrm{In}$, $\mathrm{Ga}$; VI = $\mathrm{S}$, $\mathrm{Se}$) chalcopyrite solar cells. First-principles calculations show that the inferior photovoltaic performance of $\mathrm{CIS}$ solar cells can be attributed to several factors, including a strong compensation effect, great band-tail states, and defect levels within the band gap, which result in a limited carrier concentration and severe nonradiative recombination of carriers. The strong compensation effect between the ${\mathrm{In}}_{\mathrm{Cu}}$ donor and ${\mathrm{Cu}}_{\mathrm{In}}$ or V${}_{\mathrm{Cu}}$ acceptor defects makes it difficult to maintain a desired p-type conductivity as the elemental chemical potential changes. Besides, ${\mathrm{In}}_{\mathrm{Cu}}$- and ${\mathrm{Cu}}_{\mathrm{In}}$-related defects induce a downward shift of the conduction-band minimum and an upward shift of the valence-band maximum, respectively. The presence of the ${\mathrm{Cu}}_{\mathrm{In}}$-defect level within the band gap is inevitable under the $\mathrm{In}$-poor and $\mathrm{S}$-rich conditions required for the p-type characteristic of $\mathrm{CIS}$. To address these issues, we find that phosphorus ($\mathrm{P}$) incorporation, forming an isoelectronic $2{\mathrm{P}}_{\mathrm{S}}+{\mathrm{In}}_{\mathrm{Cu}}$ defect complex, can enhance the p-type characteristic by consuming the residual electrons of ${\mathrm{In}}_{\mathrm{Cu}}$-donor defects and alleviate their band-tail states by introducing substantial atomic orbital coupling of ${\mathrm{P}}_{\mathrm{S}}$ and ${\mathrm{In}}_{\mathrm{Cu}}$ defects. Moreover, the required $\mathrm{In}$-rich, $\mathrm{S}$-poor, and $\mathrm{P}$-rich conditions, to facilitate the formation of the beneficial $2{\mathrm{P}}_{\mathrm{S}}+{\mathrm{In}}_{\mathrm{Cu}}$ defect complex, can also suppress the formation of ${\mathrm{Cu}}_{\mathrm{In}}$ defects, accordingly, reducing the associated band-tail state and defect state. This study reveals the origins of the stagnated efficiency of $\mathrm{CIS}$ and that defect-complex engineering by anion doping is a feasible way to improve the electronic properties of multicomponent semiconductors.

We propose to use quantum computers to simulate infection spreading in networks. We first show the analogy between the infection distribution and spin-lattice configurations with Ising-type interactions. Then, since the spreading process can be modeled as a classical Markovian process, we show that the spreading process can be simulated using the evolution of a quantum thermal dynamic model with a parameterized Hamiltonian. In particular, we analytically and numerically analyze the evolution behavior of the Hamiltonian, and prove that the evolution simulates a classical Markovian process, which describes the well-known epidemiological stochastic susceptible and infectious (SI) model. A practical method to determine the parameters of the thermal dynamic Hamiltonian from epidemiological inputs is exhibited. As an example, we simulate the infection spreading process of the SARS-Cov-2 variant Omicron in a small-world network.

Spectroscopic probing of electron de-excitation is a matter of debate in many materials that exhibit slow photoresponse. Sequential photoluminescence spectra with varying delay times between scans (SPSVDT) is explored as a strategy to investigate the dynamics of electron de-excitations in such systems with zinc oxide ($\mathrm{Zn}\mathrm{O}$) thin films, a prototypical material exhibiting slow photoresponse, as a model. The selection of model system is based on its wide use in various optoelectronic applications as an electron transport layer as well as an active layer. The peak intensities of PL spectra of $\mathrm{Zn}\mathrm{O}$ thin film exhibited an exponential decrease while performing continuous PL measurements and is attributed to the prolonged lifetime of electrons in the conduction band. The time constant (${\tau }_{\mathrm{cap}}$) associated with the slow electron relaxations to the trap states is estimated to be 1.95 minutes by performing SPSVDT. The very slow electron relaxations to the trap states in $\mathrm{Zn}\mathrm{O}$ after UV exposure is found to be affecting the Fermi level of $\mathrm{Zn}\mathrm{O}$ thin film, reverse saturation current in ${\text{ITO/ZnO/MoO}}_3/\mathrm{Ag}$ heterojunction and photocurrent in ITO/$\mathrm{Zn}\mathrm{O}$/$\mathrm{Ag}$ device. These modest variations are successfully simulated using the time constant ${\tau }_{\mathrm{cap}}$ obtained from the analysis. The proposed strategy can be adopted to quantitatively interpret the electron trapping dynamics in materials exhibiting slow photoresponse and can be used to tune their photoresponse.

We analyze the operation of terahertz (THz) bolometric detectors based on field-effect transistor (FET) structures with graphene channels (GCs) and black-phosphorus and black-arsenic gate barrier layers (BLs). Such GC FETs use two-dimensional electron gas (2DEG) heating by the incident THz radiation leading to the thermionic emission of hot electrons from the GC via the BL into the gate. Due to the excitation of plasmonic oscillations in the GC by THz signals, the GC FET detector response can be pronouncedly resonant, leading to elevated values of the detector responsivity. The lateral thermal conductivity of the 2DEG and its Peltier cooling can markedly affect the GC FET responsivity, in particular its spectral characteristics. These effects should be considered for the optimization of GC FET detectors.

We build a parametric amplifier with a Josephson field-effect transistor (JoFET) as the active element. The resonant frequency of the device is field-effect tunable over a range of $2\mathrm{GHz}$. The JoFET amplifier has $20\mathrm{dB}$ of gain, $4\mathrm{MHz}$ of instantaneous bandwidth, and a 1-dB compression point of $-125.5\mathrm{dBm}$ when operated at a fixed resonance frequency.

In this work, we experimentally demonstrate the concept of space-time wave localization using programmable defects. The dynamic properties of the local resonators of an electromechanical metamaterial, comprising piezoelectric elements connected to synthetic impedance circuits, are digitally controlled to modulate a trivial point defect in space and time. The experimental results show that the vibration energy is gradually transferred and localized over subsequent unit cells according to the defect position. The practical realization of space-time wave localization using programmable defects in elastic metamaterials may enable innovative solutions for information transmission, multiplexing and demultiplexing, sensing, and coding.

Fast and efficient switching of nanomagnets is one of the main challenges in the development of future magnetic memories. We numerically investigate the evolution of static and dynamic spin-wave (SW) magnetization in short (50–400 nm in length and 120 nm in diameter) cylindrical ferromagnetic nanowires, where competing magnetization configurations of a single vortex (SV) and a Bloch-point vortex domain wall (BP-DW) can be formed. For a limited nanowire length range (between 150 and 300 nm), we demonstrate reversible transitions induced by a microwave field (forwards) and by opposite spin currents (backwards) between topologically different SV and BP-DW states. By tuning the nanowire length, the excitation frequency, the microwave pulse duration, and the spin-current value, we show that the optimum (low-power) manipulation of the BP-DW can be achieved with a microwave excitation tuned to the main SW mode for nanowire lengths around 230–250 nm, where single-vortex domain-wall magnetization reversal via nucleation and propagation of a SV-DW transition takes place. An analytical model of the dynamics of the Bloch point provides an estimate of the gyrotropic mode frequency close to that obtained via micromagnetic simulations. A practical implementation of the method in a device is proposed, involving microwave excitation and the generation of opposite spin currents via the spin-orbit torque. Our findings open up an alternative pathway for the creation of topological magnetic memories.

The efficacy of a physical reservoir computer model based on traveling spin waves in a spin-wave delay-line active-ring resonator was demonstrated recently. In the present work, we investigate how this neuromorphic device can be adapted for sensing applications. In this “reservoir computing for sensing” framework, we exploit strong coupling of the physical reservoir to its environment to utilize the reservoir as a sensing element. The dynamics of traveling spin waves in delay-line active rings are strongly dependent on the magnetic field and carrier frequency of those spin waves. Treating the spin-wave frequency as an environmental variable, we excite the active ring into different dynamical states by modulating the carrier frequency of a drive signal of microwave pulses injected into the ring. Training a linear regression on the time-multiplexed output from the ring allows the periodic amplitude patterns of the spin waves to be mapped reproducibly onto two-dimensional trajectories, representing periodic “behavioral” targets. Our work demonstrates the versatility of a magnonic resonator as a multipurpose computing and sensing device.

Realizing a quantum network will require long-lived quantum memories with optical interfaces incorporated into a scalable architecture. Color-center quantum emitters in diamond have emerged as a promising memory modality due to their optical properties and compatibility with scalable integration. However, developing a scalable color-center emitter module requires significant advances in the areas of heterogeneous integration and cryogenically compatible packaging. Here we report on a cryogenically stable and network compatible quantum emitter module for memory use. This quantum emitter module is a significant development towards advanced quantum networking applications such as distributed sensing and processing.

We assess the continuous-wave and dynamic performance of a photonic laser cavity consisting of a silicon-rich-nitride-on-insulator disk resonator overlaid with a transition-metal dichalcogenide (TMD) bilayer heterostructure (${\mathrm{WSe}}_2/{\mathrm{Mo}\mathrm{S}}_2$) acting as the gain medium. The optically pumped TMD heterostructure fosters an interlayer exciton with long radiative recombination lifetime, providing light emission in the near-infrared ($\sim 1130$ nm). Following a meticulous design process, we propose a silicon-on-insulator-compatible, monolithically integrated optical source capable of emitting milliwatt power inside an integrated waveguide, featuring a low pump-power threshold of $\sim 16{\mathrm{kW}/\mathrm{cm}}^2$, and exhibiting an estimated total quantum efficiency of approximately 1.7%. The proposed laser cavity is analyzed and designed using a rigorous theoretical framework based on perturbation theory and temporal coupled-mode theory, capable of treating nanophotonic cavities of any geometry and material composition comprising both bulk and/or sheet gain media. The framework is built upon fundamental electromagnetic and semiclassical gain equations, rendering it general and adaptable to different cavity configurations and gain-media descriptions. It constitutes a powerful tool for the efficient analysis of contemporary micro- and nanophotonic semiconductor lasers, since it is capable of predicting fundamental laser characteristics, providing design directives, deriving continuous-wave design metrics, and evaluating the dynamic response of realistic laser cavities.

Quantum optical microcombs in integrated ring resonators generate entangled photon pairs over many spectral modes, and allow the preparation of high-dimensional qudit states. Ideally, those sources should be programmable and have a high generation rate, with comb lines tightly spaced for the implementation of efficient qudit gates based on electro-optic frequency mixing. While these requirements cannot all be satisfied by a single resonator device, for which there is a trade-off between the high generation rate and tight bin spacing, a promising strategy is the use of multiple resonators, each generating photon pairs in specific frequency bins via spontaneous four-wave mixing. Based on this approach we present a programmable silicon photonics device for the generation of frequency-bin-entangled qudits, in which bin spacing, qudit dimension, and the bipartite quantum state can be reconfigured on chip. Using resonators with a radius of $22\text{μ}\mathrm{m}$, we achieve a high brightness [about $\mathrm{MHz}/(\mathrm{mW}{)}^2$] per comb line with a bin spacing of 15 GHz, and fidelities above 85% with maximally entangled Bell states up to a Hilbert space dimension of 16. By individually addressing each spectral mode, we realize states that cannot be generated on chip using a single resonator. We measure the correlation matrices of maximally entangled two-qubit and two-qutrit states on a set of mutually unbiased bases, finding fidelities exceeding 98%, and indicating that the source can find application in high-dimensional secure communication protocols.