In light of the growing widely utilized single-photon avalanche detectors, dead time is inevitable due to the presence of afterpulse. Dead time is a critical factor that influences the count rate and the secret key rate (SKR) of quantum key distribution (QKD). It is particularly significant in time-bin-encoding systems, while prior models fail to accurately estimate the gains. Of note, it introduces correlations between early bins and late bins, hence bits with strong correlation and security risks. Here, we put forward a realistic model that precisely characterizes the response of detectors for time-bin-encoding systems. Our model not only improves the accuracy of gain predictions, showing an excellent match with the Monte Carlo simulation results, but also provides theoretical support for data processing, helping to mitigate security risks. Finally, leveraging our model, we can get a much higher SKR than prior models and further enhance the performance of practical QKD systems by adjusting the detector parameters. Our simulation results demonstrate that our model is essential for the application and deployment of QKD systems in practice.

We present superconducting quantum circuits that exhibit atomic energy spectra and selection rules in ladder and lambda three-level configurations designed using genetic algorithms. These heuristic optimization techniques are employed to adapt the topology and parameters of a set of electrical circuits to find the suitable architecture matching the required energy levels and relevant transition matrix elements. We analyze the performance of the optimizer on one-dimensional single- and multiloop circuits to design ladder ($\mathrm{\Xi }$) and lambda ($\mathrm{\Lambda }$) three-level systems with specific transition matrix elements. As expected, attaining both the required energy spectrum and the needed selection rules is challenging for single-loop circuits, but they can be accurately obtained even with just two loops. Additionally, we show that our multiloop circuits are robust against random fluctuations in their circuit parameters, i.e., under eventual fabrication flaws. Developing an optimization algorithm for automated circuit quantization opens an avenue to engineering superconducting circuits with specific symmetry to be used as modules within large-scale setups. This may allow us to mitigate the well-known current errors observed in the first generation of quantum processors.

Reducing circuit depth and identifying an optimal trade-off between circuit depth and width is crucial for successful quantum computation. In this context, midcircuit measurement and feedforward have been shown to significantly reduce the depth of quantum circuits, particularly in implementing logical gates. By leveraging these techniques, we propose several parallelization strategies that reduce quantum circuit depth at the expense of increasing width in preparing various quantum states relevant to quantum simulation. With measurements and feedforward, we demonstrate that utilizing unary encoding as a bridge between two quantum states substantially reduces the circuit depth required for preparing quantum states, such as sparse quantum states and sums of Slater determinants within the first quantization framework, while maintaining an efficient circuit width. Additionally, we show that a Bethe wave function, characterized by its high degree of freedom in its phase, can be probabilistically prepared in a constant-depth quantum circuit using measurements and feedforward. We anticipate that our study will contribute to the reduction of circuit depth in initial state preparation, particularly for quantum simulation, which is a critical step toward achieving quantum advantage.

An analytical formulation of (optical-)phonon-mediated and momentum-resonant Franck-Condon emission of photoexcited electrons from polar semiconductors is shown to be very consistent with (i) the observed emission properties of a cesiated $\mathrm{Ga}\mathrm{As}(001)$ photocathode at 808 nm [J. Phys. D: Appl. Phys. 54, 205301 (2021)] and (ii) the measured spectral emission properties of a $\mathrm{Ga}\mathrm{N}(0001$) photocathode from just below its bandgap energy to 5 eV. The theoretical analysis in the parabolic band approximation predicts the form of both the quantum efficiency and mean transverse energy of photoemission as a function of the photocathode’s electron affinity and the electron temperature in the vicinity of its emission face. The good agreement between theory and experimental data also suggests that sub-10-nm rms surface roughness effects are not significant for polar semiconductor photocathodes.

Masers could transform medical sensing and boost qubit readout detection due to their superb low-noise amplification. The negatively charged nitrogen-vacancy diamond maser is the only continuous-wave solid-state maser operable at room temperature; however, it requires cumbersome magnets, which prevent its widespread use. We present a significant size reduction of the diamond maser oscillator using a much lighter electromagnet with a small footprint, reducing the weight from 2000 kg to a more portable 30 kg. We achieve a maximum oscillator output power near $-80$ dBm, ten times higher than the first implementation, and discover techniques to reduce the magnetic field strength required for masing by 30 mT through precise manipulation of spin orientation. With the diamond maser now shrunk to a size that can fit on a benchtop, we move continuous-wave room-temperature masers away from the confines of research laboratories and closer to transforming readouts in quantum computing, frequency standards, and quantum-limited medical sensing.

We study the single-photon purity of a resonantly driven single-photon source in the realistic scenario where the excitation laser pulse can leak into the detection path. We find that the duration of the excitation pulse strongly influences the quality of the single-photon source. We calculate the influence of this on the effective parameters describing the most relevant properties, including the two-photon component and Hong-Ou-Mandel visibility. Furthermore, we analyze how these properties can be strongly affected by frequency filtering of the outgoing field. Our results highlight that the relation between the two-photon component of the emission and the Hong-Ou-Mandel visibility is more complicated than typically assumed in the literature, and depends on the specific details of the source.

To identify optimal catalysts for efficient Z-scheme photocatalytic water splitting for hydrogen production, we explore 69 heterostructures of $M{A}_2{Z}_4/X{\mathrm{Se}}_2$ (M = $\mathrm{Zn}$, $\mathrm{Cd}$, $\mathrm{Hg}$; A = $\mathrm{Al},\mathrm{Ga},\mathrm{In}$; Z = $\mathrm{S},\mathrm{Se},\mathrm{Te}$; X = $\mathrm{Ti},\mathrm{Zr},\mathrm{Hf}$). After confirming the stability of these fully optimized structures, we identify six heterostructures with solar-to-hydrogen efficiency (${\eta }_{\mathrm{STH}}^{\mathrm{\prime }}$) ranging from 32.12% to a maximum of 40.63%. Nonadiabatic molecular dynamics simulations reveal that ${\mathrm{Zn}\mathrm{Ga}}_2{\mathrm{Se}}_4/{\mathrm{Zr}\mathrm{Se}}_2$ and ${\mathrm{Zn}\mathrm{Ga}}_2{\mathrm{Se}}_4/{\mathrm{Hf}\mathrm{Se}}_2$ exhibit slower electron transfer for the hydrogen evolution reaction (HER) and hole transfer for the oxygen evolution reaction (OER), suggesting enhanced stability in their reduction and oxidation processes. Additionally, ${\mathrm{Cd}\mathrm{Ga}}_2{\mathrm{Se}}_4/{\mathrm{Ti}\mathrm{Se}}_2$ shows the shortest interlayer electron-hole (e-h) recombination time, indicating superior photocatalytic efficiency. Notably, Gibbs free energy calculations confirm that both HER and OER in ${\mathrm{Cd}\mathrm{In}}_2{\mathrm{Se}}_4/{\mathrm{Ti}\mathrm{Se}}_2$ and ${\mathrm{Hg}\mathrm{Ga}}_2{\mathrm{Se}}_4/{\mathrm{Zr}\mathrm{Se}}_2$ can proceed spontaneously, highlighting their potential as efficient photocatalysts. The analysis indicates that the identified heterostructures, particularly ${\mathrm{Cd}\mathrm{Ga}}_2{\mathrm{Se}}_4/{\mathrm{Ti}\mathrm{Se}}_2$, hold significant promise for Z-scheme water splitting, offering a viable pathway for the development of $M{A}_2{Z}_4$-based materials in hydrogen production applications.

Dispersive mode coupling is a promising mechanism for the development of advanced micro- and nanoelectromechanical devices. However, strong coupling strength remains a key challenge limiting the practical applications of the dispersive mode coupling effect. Here, we experimentally demonstrate the significant tuning of the mode coupling coefficient of two flexural vibrational modes in a doubly clamped microelectromechanical system (MEMS) beam resonator using the thermally induced buckling effect, which enables variable coupling strengths to be implemented for practical applications. Furthermore, a theoretical model is developed to describe the mode coupling coefficient, showing that the tunability is due to the breakdown of the symmetric shape of the MEMS beam caused by buckling. Moreover, the theoretical model defines a simple relation between the coupling coefficient and the nonlinearity of the two coupled modes. These results provide valuable insight into physical mechanisms underlying the dispersive mode coupling effect and pave the way for the development of advanced MEMS devices with application-specific mode coupling strengths.

Color centers in diamond are promising systems for quantum technologies and nanophotonics as they are photostable emitters at room and elevated temperatures. The possibility of their electrical excitation has already been demonstrated within p-i-n diodes. However, this requires the growth of complex diamond structures. In contrast to these conventional approaches, we demonstrate the emission from color centers under electrical pumping in a Schottky diode configuration based on n-type diamond. Hydrogen passivation allows the modification of the Schottky barrier height and improves the injection of minority charge carriers needed for the electrical pumping, while electrons are provided by the n-type layer.

Since the 1990s, there has been a debate on whether the contact electrification (CE)-induced discharge mode is continuous or intermittent, and the understanding of the relationship between the electrode potential and Paschen’s curve remains controversial. In the present work, we demonstrate CE-induced microscale breakdown between two separating metal electrodes without an external voltage source and successfully reveal that continuous and intermittent discharge modes can both be realized by adjusting the electrode separation velocity. The CE-induced initial charge, electrode separation velocity, and breakdown time delay are identified as key factors, and their combined effects explicitly determine the breakdown mode. Further, it is found that in continuous breakdown mode with a sufficiently high initial charge, the electrode potential versus gap distance exactly follows the modified Paschen’s curve, which incorporates the field emission regime. This discovery suggests that a microscale breakdown curve can be determined based on a single characterization of the separating electrode potential without the need to perform redundant repeated experiments. The results of this study elucidate microscale breakdown physics with moving electrodes and inform better control and optimization of microelectromechanical devices for applications including energy harvesters, microsensors, and detectors.

Developed half a century ago, Thiele’s equation has played a crucial role in describing the motion of magnetic textures, ranging from simple magnetic domains to exotic magnetic solitons like skyrmions and hopfions. However, it remains a challenge to fully understand the collective dynamics of magnetic texture crystals, due to the complex many-body interactions. On the other hand, electrical circuits have recently been proved to be a powerful platform to realize rich physical phenomena in solids. In this work, we first construct a circuit unit to simulate Thiele’s equation, which enables us to experimentally observe the circuit-version “skyrmion Hall effect”. By coupling a pair of such circuit units, we find an emerging frequency comb under strong pumpings, a long-sought nonlinear effect in interacting skyrmion systems. By extending our strategy to circuit arrays, we observe the topological edge state, acting as the circuit counterpart of topological solitonic insulators. Our work builds a faithful connection between electric circuits and magnetic solitons, two seemingly unrelated areas, and opens the door for exploring collective magnetization dynamics in circuit networks.

Measurement is an essential component of robust and practical quantum computation. For superconducting qubits, the measurement process involves the effective manipulation of the joint qubit-resonator dynamics, and it should ideally provide the highest quality for qubit state discrimination with the shortest readout pulse and resonator reset time. Here, we harness model-free reinforcement learning (RL), together with a tailored training environment, to achieve this multifaceted optimization task. Using the IBM quantum device, we demonstrate that the pulse obtained by the RL agent not only successfully achieves state-of-the-art performance, with an assignment error of $(4.6\pm 0.4)\times {10}^{-3}$, but also executes the readout and the subsequent resonator reset almost three times faster than the system’s default process. Furthermore, the learned waveforms are robust against realistic parameter drifts and follow an intuitive form, making them readily implementable on existing hardware with little computational overhead. Our results provide an effective readout strategy to boost the performance of superconducting quantum processors and demonstrate the value of RL in providing optimal and practical solutions for complex quantum information processing tasks.

Metasurface designs are often based on the generalized Snell’s law, which controls wave refraction but lacks the capability for complex wavefront shaping. Combining it with advanced engineering techniques enables more complex wave trajectories, though achieving fully customized trajectories remains challenging and requires extensive design efforts. The holographic iterative angular spectrum approach (IASA) is based on the iterative computation of the forward and backward propagation of acoustic waves, allowing the realization of custom arbitrary patterns by considering numerous target planes. In this work, IASA is used as a unique metasurface inverse design approach, allowing for precise and conformal control over the phase of transmitted waves via designed unit cells. Various forms of wave manipulation are achieved, including self-bending and bottle beams. We demonstrate that the IASA, which involves reconstructing the complex field, enables successive beam steering. This capability is demonstrated by tailoring the unit cell profiles in the elastic host media, thereby creating thickness profiles that generate the desired phase gradients while allowing for near-perfect transmission of the wave energy. Experiments are conducted to validate the numerical predictions of the IASA for successive patterning.

We present a two-dimensional phononic crystal structure that exhibits exceptionally large complete band gaps, akin to those typically associated with bubble crystals. Our analysis begins with numerical simulations of a phononic crystal characterized by material parameters that are continuous functions of spatial coordinates, arriving at a sawtooth material parameter function, whose band structure can be replicated by a surprisingly simple configuration with inclusions touching at a single point, effectively functioning as a broadband filter. This allows us to perform acoustic frequency response measurements on a real system that confirm the sound isolation capabilities of our design, thereby underscoring its practical potential.

Predicting and analyzing global behavior of complex systems is challenging due to the intricate nature of their component interactions. Recent work has started modeling complex systems using networks endowed with multiway interactions among nodes, known as higher-order networks. Simplicial complexes are a class of higher-order networks that have received significant attention due to their topological structure and connections to Hodge theory. Topological signal processing (TSP) utilizes these connections to analyze and manipulate signals defined on non-Euclidean domains such as simplicial complexes. Such analysis of higher-order network data is important for many real-world problems, such as detecting failure or error in communication networks, sensor coverage analysis, statistical ranking problems, finding arbitrage currency markets, etc. However, as the dimension of higher-order networks increases, the complexity of TSP scales exponentially. In this work, we present a general quantum algorithm for implementing filtering processes in TSP and describe its application to extracting network data based on the Hodge decomposition. We leverage preexisting tools introduced in recent quantum algorithms for topological data analysis and combine them with spectral filtering techniques using the quantum singular value transformation framework. While this paper serves as a proof of concept, we obtain a superpolynomial improvement over the best-known classical algorithms for TSP filtering processes, modulo some important caveats about encoding and retrieving the data from a quantum state. The proposed algorithm generalizes the applicability of tools from quantum topological data analysis to novel applications in analyzing high-dimensional complex systems.