Electromagnetic wave scattering in electrically large, irregularly shaped, environments is a common phenomenon. The deterministic, or first-principles, study of this process is usually computationally expensive and the results exhibit extreme sensitivity to scattering details. For this reason, the deterministic approach is often dropped in favor of a statistical one. The random coupling model (RCM) Hemmady et al. [Phys. Rev. Lett. 94, 014102 (2005).] is one such approach that has found great success in providing a statistical characterization for wave chaotic systems in the frequency domain. Here we aim to transform the RCM into the time domain and generalize it to alternative situations. The proposed time-domain RCM (TDRCM) method can treat a wave chaotic system with multiple ports and modes. Two features are now possible with the time-domain approach for chaotic resonators: the incorporation of early time short-orbit transmission path effects between the ports, and the inclusion of arbitrary nonlinear or time-varying port load impedances. We conduct short-pulse time-domain experiments in wave chaotic enclosures, and use the TDRCM to simulate the corresponding experimental setup. We also examine a diode-loaded port and compare experimental results with a numerical TDRCM treatment and find agreement.

We introduce a hybrid tripartite quantum system for strong coupling between a semiconductor spin, a mechanical phonon, and a microwave excitation of a superconducting circuit. Consisting of a piezoelectric resonator with an integrated diamond strain concentrator, this system achieves microwave-acoustic and spin-acoustic coupling rates of approximately megahertz or greater, allowing simultaneous ultrahigh cooperativities (approximately ${10}^3$ and approximately ${10}^2$, respectively). From finite-element modeling and master-equation simulations, we estimate superconducting-circuit-to-spin quantum state transfer fidelities exceeding 0.95 on the basis of separately demonstrated device parameters. We anticipate that this device will enable hybrid quantum architectures that leverage the advantages of both superconducting circuits and solid-state spins for information processing, memory, and networking.

Dynamically encircling exceptional points (EPs) in the parameter space of a non-Hermitian system has drawn widespread attention for realizing asymmetric mode switching and related applications. While previous works have been restricted to two-level systems, the switching between edge states in a multistate system that contains more complex dynamics remains to be explored. Herein, asymmetric switching of edge modes in multistate systems is demonstrated by encircling multiple EPs. A multistate non-Hermitian system containing multiple EPs is constructed based on a one-dimensional lattice, which can support four edge-localized modes. Dynamically encircling the EPs in parameter space leads to a specific edge mode surviving to the end, determined by the encircling direction. Correspondingly, optical waveguide arrays are designed to investigate mode evolution through refractive index modulation. Simulated results show that clockwise (counterclockwise) encirclement leads to an output mode localized at the left (right) side of the waveguide array with a phase difference of 0 (π), regardless of the input modes; this excellently illustrates the asymmetric switching of edge modes in a multistate system. Moreover, such an effect can be extended to the multistate system with an arbitrary number of EPs. This work enables investigation of interesting effects in non-Hermitian physics by engineering the EPs and topological properties, especially for complex multistate systems, which will be useful for developing functional nanophotonic devices.

Electron-phonon (e-ph) coupling and transport are ubiquitous in modern electronic devices. The coupled electron and phonon Boltzmann transport equations (BTEs) hold great potential for the simulation of thermal transport in metal and semiconductor systems. However, solving the BTEs is often computationally challenging owing to their high dimensional complexity and a wide span of heat carrier properties, which hinder large-scale thermal modeling at the device level. In this work, we present a physics-informed neural network framework for solving the coupled electron and phonon BTEs. Instead of relying on labeled data, the proposed framework directly learns the spatiotemporal solutions (i.e., the electron and phonon distribution functions) within a parameterized space by enforcing physical laws. The efficacy of this framework is demonstrated through its ability to accurately resolve temperature profiles in low-dimensional thermal transport problems and visualize the ultrafast electron and phonon dynamics in laser heating experiments on thin metal films. The results indicate that our approach can accurately describe nonequilibrium e-ph energy transfer with improved efficiency, opening alternative avenues for the predictive design and optimization of micro- and nanostructures.

Nonlinear effects and dynamics are found in a wide range of research fields. In magnetic materials, nonlinear spin dynamics enables ultrafast manipulation of spin, which promises high-speed nonvolatile information processing and storage for future spintronic applications. However, a nonlinear spin response is not yet demonstrated in a nonmagnetic material that lacks strong magnetic interactions. Dilute nitride III-V materials, e.g., ($\mathrm{Ga},\mathrm{N})\mathrm{As}$, have the ability to amplify the conduction-electron-spin polarization by filtering out minority spins via spin-polarized defect states at room temperature. Here, by employing coupled rate equations, we theoretically demonstrate the emergence of a nonlinear spin response in such a defect-enabled room-temperature spin amplifier. Furthermore, we showcase the proposed spin nonlinearity in a ($\mathrm{Ga},\mathrm{N})\mathrm{As}$-$\mathrm{In}\mathrm{As}$ quantum dot (QD) coupled all-semiconductor nanostructure, by measuring the higher-harmonic generation, which converts the modulation of excitation polarization into the second-, third-, and fourth-order harmonic oscillations of the QD’s photoluminescence intensity and polarization. The observed spin nonlinearity originates from defect-mediated spin-dependent recombination, which can be conveniently tuned with an external magnetic field and can potentially operate at a speed exceeding 1 GHz. The demonstrated spin nonlinearity could pave the way for nonlinear spintronic and optospintronic device applications based on nonmagnetic semiconductors with simultaneously achievable high operation speed and nonlinear response.

Alignment-based optically pumped magnetometers (OPMs) are capable of measuring oscillating magnetic fields with high sensitivity in the $\mathrm{fT}/\sqrt{\mathrm{Hz}}$ range. Until now, alignment-based magnetometers have only used paraffin-coated vapour cells to extend the spin relaxation lifetimes of the alkali vapour. The drawback of these cells is that they are hand blown and are therefore time intensive, and somewhat unreliable, to produce. Buffer-gas cells, on the other hand, can be manufactured on a mass scale using microfabrication techniques. In this work we use hand-blown cells, but our methods also apply to microfabricated buffer-gas cells. We present the first demonstration of an alignment-based magnetometer using a buffer-gas vapour cell containing caesium (Cs) alkali vapour and nitrogen (${\mathrm{N}}_2$) buffer gas. The OPM is operated at $55{}^{\circ }\mathrm{C}$ and we achieve a $325\mathrm{fT}/\sqrt{\mathrm{Hz}}$ sensitivity to 10 kHz oscillating magnetic fields with an 800 Hz bandwidth. The alignment-based magnetometer uses a single low-power laser beam for optical pumping and probing and could potentially allow for more rapid commercialization of radio-frequency OPMs, due to the robustness of the one-beam geometry.

Paramagnetic defects and nuclear spins are often the major sources of decoherence and spin relaxation in solid-state qubits realized by optically addressable point defect spins in semiconductors. It is commonly accepted that a high degree of depletion of nuclear spins can enhance the coherence time by reducing magnetic noise. Here we show that the isotope purification beyond a certain optimal level can become contraproductive when both electron and nuclear spins are present in the vicinity of the qubits, particularly for half-spin systems. Using state-of-the-art numerical tools and considering the silicon-vacancy qubit in various spin environments, we demonstrate that the coupling of the spin-3/2 qubit to a spin bath of spin-1/2 point defects in the lattice can be significantly enhanced by isotope purification. The enhanced coupling shortens the spin-relaxation time that in turn may limit the coherence time of spin qubits. Our results can be generalized to triplet point defect qubits, such as the nitrogen-vacancy center in diamond and the divacancy in silicon carbide.

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.