Extracting the current-phase relationship (CPR) of a single superconducting junction is challenging in practice and traditionally involves embedding the junction in a larger superconducting circuit containing SQUIDs and/or resonators. Applying ac driving to the junction has proven to be a viable and less invasive way to extract information about the few lowest harmonics of the CPR, by locating the integer and fractional Shapiro steps in the $IV$-curve of the driven junction. Here, we present an alternative driving-based method that allows to extract the full harmonic content of a CPR in a relatively noninvasive way, by fitting the measured critical currents of the driven junction as a function of driving power. We test our method, both using numerical simulations and in experiments, and we show that it works accurately.

We report several technical approaches that significantly improve the performance of a vapor-cell atomic electrometer operating in the quasi-dc frequency domain ($\ll 1\mathrm{kHz}$). With a very small active volume of approximately $11{\text{mm}}^3$ inside a $2\times 2\times 2{\text{cm}}^3$ vapor cell, we demonstrated a noise floor for electric field (E-field) sensitivity ranging from $0.2\mathrm{to}7.7(\mathrm{mV}/\mathrm{m})/\sqrt{\mathrm{Hz}}$ for a frequency band of 1–100 Hz. Our work utilizes only a bare vapor cell for electrometry, without any metal parts or electrodes, to ensure minimal distortion of the measured E-field and to minimize the effective sensing volume for high spatial resolution. The E-field-sensitive atomic state (Rydberg state) is excited and read out optically, maximizing the simplicity of the system design and enabling the miniaturization of quasi-dc E-field sensors for potential applications, such as diagnostics of electronics without physical contact, communications in and below the super-low frequency (SLF) band, proximity detection, remote activity surveillance, tracing charge signatures, and research in bioscience and geoscience.

Small and device-dependent valley splittings remain a key challenge for electron spin qubits in silicon (Si), directly limiting qubit fidelity, device uniformity, and the scalability of Si-based quantum processors. In silicon-germanium (Si-Ge) heterostructures, this problem can be addressed through engineering of the epitaxial layer stack. Several heuristic strategies have been proposed to enhance the energy gap between the two nearly degenerate valley states in strained $\mathrm{Si}/\mathrm{Si}$-Ge quantum wells (QWs), e.g., sharp $\mathrm{Si}/\mathrm{Si}$-Ge interfaces, Ge spikes, or oscillating Ge concentrations within the QW. Here, we develop a systematic variational optimization approach to compute optimal Ge concentration profiles that enhance selected properties of the intervalley coupling matrix element. Our free-shape optimization framework is augmented by realistic technological constraints to ensure feasibility of the resulting epitaxial profiles and is based on an effective-mass envelope-function theory accounting for strain and compositional alloy disorder. Previously proposed heterostructure designs are recovered as special cases of the constrained optimization problem. Our main result is a novel heterostructure design, which we refer to as the modulated wiggle well, providing both a large deterministic enhancement of the valley splitting and a strong suppression of disorder-induced variability. In addition, this design enables wide electrical tunability of the valley splitting—from approximately $200\mathrm{\mu }\mathrm{eV}$ to above 1 meV—offering opportunities for engineering robust and switchable silicon qubits.

Nonradiative carrier recombination at point defects (PDs) can severely limit the efficiency of semiconductor optoelectronic devices. However, such nonradiative PDs are difficult to study directly because they are atomic scale, do not emit localized luminescence, and are typically buried deep in a semiconductor heterostructure. As a result, their properties are usually inferred from macroscale optical measurements interpreted using the $ABC$ recombination model. Here, we map carrier diffusion and nonradiative recombination at individual PDs in a single InGaN/GaN quantum well on the nanometer and nanosecond scale using time-resolved cathodoluminescence. Applying a diffusion-recombination model, we extract the room-temperature intrinsic quantum well carrier lifetime (approximately 4.0 ns) and diffusivity ($0.83\pm 0.05{\mathrm{cm}}^2{\mathrm{s}}^{-1}$), together with the capture cross section of a single isolated PD in the well at $6.8\pm 0.9{\mathrm{nm}}^2$. We show that nonradiative recombination at this PD takes a minimum time of $16\pm 2\mathrm{ps}$. This limitation predicts that the macroscale defect-related nonradiative recombination rate will saturate above carrier densities routinely reached during device operation, a behavior unaccounted for by the standard $ABC$ model.

We demonstrate all-optical manipulation of magnetic domains in $\mathrm{NiO}/\mathrm{Pt}$ and $\mathrm{CoO}/\mathrm{Pt}$ thin films with insulating antiferromagnetic layers. Using magneto-optical birefringence imaging, we show that even a single laser pulse can thermally demagnetize the antiferromagnet, leading to a random redistribution of domains. By sweeping the laser beam, domain-wall motion is induced, enabling partial switching of the antiferromagnetic order. The behavior is captured by an analytical model in which temperature gradients generated by the moving beam exert a thermal pressure on domain walls in the form of a ponderomotive force. Of note, the 90° domains can be reversibly toggled solely by reversing the direction of the thermal gradient, demonstrating all-optical switching without the need for electric currents. These findings establish a route toward ultrafast optical manipulation of fully compensated antiferromagnets, with potential impact on nonvolatile memory technologies and antiferromagnetic spintronics.

This work presents an industrially scalable, power-efficient quantum magnetometer chip based on 4H-silicon carbide (SiC) technology, leveraging wafer-scale fabrication techniques to optimize V2 silicon-vacancy color centers for reproducible fabrication. The integration of these color center ensembles into a planar silicon carbide waveguide enables efficient excitation of a large ensemble and simplifies fluorescence extraction compared with standard confocal methods. We report continuous wave (cw) optically detected magnetic resonance measurements, complemented by Rabi, Ramsey, and Hahn-echo sequences, which demonstrate the coherent capabilities of the large embedded ensemble of V2 centers. Based on the data, our device exhibits sensor shot-noise-limited sensitivities of around $30\mathrm{nT}/\sqrt{\mathrm{Hz}}$, 2–3 orders of magnitude lower compared with more complex confocal techniques. This concept simplifies the quantum sensor architecture, enhances sensitivity, and streamlines optical excitation and collection, thereby paving the way for the development of next-generation SiC-quantum sensing technologies.

Hard x-ray experiments, such as high-resolution imaging and scattering of complex materials, increasingly demand pulses from x-ray free-electron lasers (XFELs) with high photon energy, high spectral density, and narrow bandwidth. Here, we report the demonstration of hard x-ray self-seeding at 18 keV at the European XFEL. This demonstration narrows the bandwidth to 0.8 eV, compared with 20.5 eV in self-amplified spontaneous emission operation, and increases the average spectral density by a factor of 6, reaching $170\mathrm{\mu }\mathrm{J}/\mathrm{eV}$ within the same experimental campaign. To advance narrow band operation toward even higher photon energies, we explore the harmonic generation mechanism in self-seeding mode, utilizing the second-harmonic generation self-seeding (SHGSS) scheme, where an initial seed is established at half the target photon energy and the harmonic is coherently amplified downstream. Start-to-end simulations at the 30 keV frontier revealed that while direct self-seeding yielded negligible amplification under typical experimental beam conditions, the SHGSS scheme could deliver an average spectral density of approximately $50\mathrm{\mu }\mathrm{J}/\mathrm{eV}$ with a 0.25 eV bandwidth using the beamline supplemented with superconducting undulators. With these goals in mind, we experimentally investigated SHGSS at 15 and 18 keV, providing the first proof-of-principle for this scheme. Our results are expected to support cutting-edge experiments in materials science, structural biology, and beyond.

Resonant laser excitation of a two-level system with subsequent single-photon emission can be used to generate single photons with high indistinguishability or Hong-Ou-Mandel (HOM) visibility. However, spectral overlap between the excitation laser and emitted photons generally poses significant challenges. Furthermore, emitter reexcitation intrinsically limits achievable single-photon purity. Established solutions mitigate these issues at significant cost to source efficiency and with increased source complexity. This motivates the use of few-level systems with spectral separation of excitation and emission pathways. One option is a three-level cascade. However, without targeted lifetime engineering of emitting states, the cascade naturally limits achievable photon indistinguishability. Here we study a semiconductor quantum dot with resonant and selective cavity enhancement of the biexciton-to-exciton transition. Following resonant two-photon excitation of the biexciton state, we collect the emitted single photon with the cavity. This approach circumvents emitter reexcitation and naturally introduces spectral separation between the excitation laser and emitted single photon. Supported by experimental results, we theoretically demonstrate that with selective Purcell enhancement, the observed quality quantifiers of single-photon emission (purity, equivalently $g^{(2)}(0)$, and HOM visibility $\mathcal{V}$, equivalently indistinguishability) are competitive with respect to high-quality deterministic quantum-dot single-photon sources. This is already achieved without systematic optimization or targeted system engineering, which firmly places the reported approach as a viable route to the next generation of highest-quality quantum-dot-based deterministic single-photon sources.

We investigate the interplay between quadratic and biquadratic exchange coupling in an exchange-coupled $\mathrm{Co}\text{−}\mathrm{Fe}/\text{Ru}/\text{Ni}\text{−}\mathrm{Fe}$ synthetic ferrimagnetic structure and compare its behavior to that of the same trilayer, patterned into an artificial spin ice (ASI). The static and dynamic properties are investigated by vibrating-sample magnetometry and Brillouin light-scattering spectroscopy and interpreted using MuMax3 micromagnetic simulations. Following this combined experimental and computational approach, we quantify the quadratic and biquadratic interfacial coupling terms and reveal their impact on magnetization reversal and layer-resolved spin-wave dynamics in the CoFe and NiFe layers. Furthermore, simulations of the evolution of the static configurations of the ASI system as a function of the applied magnetic field reveal the significant role played by the quadratic and biquadratic coupling terms in determining the equilibrium configurations of the nanoislands, in competition with the magnetostatic interactions imposed by the shape anisotropy of the nanoelements. This competition promotes mutually orthogonal layer magnetizations and may give rise to frustrated equilibrium states in which the magnetization aligns along the hard axis of the nanoelements. Our results suggest artificial spin lattices as promising reconfigurable magnonic devices where magnetic ground-state tunability may enable field-controlled magnonic crystals, spin-wave filters, and artificial spin-ice metamaterials with switchable dynamic properties.

Quantum error correction (QEC) requires ancilla qubits to extract error syndromes from data qubits, which store quantum information. However, ancilla errors can propagate back to the data qubits, introducing additional errors and limiting fault tolerance. In superconducting quantum circuits, Kerr-cat qubits (KCQs), which exhibit strongly biased noise, have been proposed as ancillas to suppress this back-action and enhance QEC performance. Here, we experimentally demonstrate a beam-splitter interaction between a KCQ and a transmon, realizing an effective ${\widehat{Z}}_{\mathrm{cat}}{\widehat{X}}_q$ coupling that can be employed for parity measurements in QEC protocols. We characterize the interaction across a range of cat sizes and drive amplitudes, confirming the expected scaling of the interaction rate. These results establish a step toward hybrid architectures that combine transmons as data qubits with noise-biased bosonic ancillas, enabling hardware-efficient syndrome extraction and advancing the development of fault-tolerant quantum processors.

Photonic and microwave devices are foundational elements of both precision physics experiments and modern telecommunications and sensing technology. Electromagnetic inverse design intelligently searches for devices that optimally fulfill a particular task. However, most realistic design tasks are highly nonconvex problems. Thus, when treated as generic optimization tasks, they require extensive initialization sweeps to identify high-performing topologies. Here, we introduce convex preoptimization, a data-free, physics-informed alternative for intelligent design search. By lifting the electromagnetic fields to an outer-product representation and relaxing the local Maxwell constraints, we convert the original nonconvex problem to a convex one that can be solved to global optimality. The solution provides a physics-informed prior for the original task: a material choice at every pixel, together with a spatial map of the ambiguity of each choice, which guides subsequent refinement. We show that, for a wide range of photonic and microwave devices, convex preoptimization consistently dominates conventional initialization sweeps in both best and average performance while using a fraction of the compute. Convex preoptimization thus provides a physics-informed, interpretable, and training-free front end for inverse design.

Spin manipulation by hopping has recently emerged as a promising strategy to control hole spins in quantum dots (QDs) using exclusively baseband control, thereby mitigating power dissipation and high-frequency management constraints in large-scale architectures. Unlike conventional approaches such as electric-dipole spin resonance, this mechanism exploits dot-to-dot variations of the spin precession axes to enable spin rotations. However, it is intrinsically disorder-dependent: in the absence of sufficient variability, the precession axes remain aligned, and spin manipulation becomes ineffective. This fundamental reliance on disorder raises concerns regarding its compatibility with the long-term evolution of spin qubit platforms toward improved material quality, cleaner interfaces, and enhanced device reproducibility. Here, we numerically assess the viability of spin manipulation by hopping as a function of disorder strength and demonstrate that its implementation is indeed increasingly constrained as disorder is reduced. To overcome this limitation, we propose an alternative strategy based on hopping between intentionally squeezed quantum dots. This approach retains the advantages of baseband control while being independent of disorder and robust against moderate variability, thereby offering improved prospects for scalable hole-spin quantum computing architectures.

Structured light in the quantum regime has garnered considerable attention due to the opportunities it offers when mixing light’s internal degrees of freedom, for high-dimensional and multidimensional quantum states of light. A popular example is to harness polarization and spatial entangled photons with a shared topological invariant that is robust against numerous families of noisy quantum channels. Yet, producing such states with high purity and adaptability remains challenging. Here, we introduce a compact, self-locking Mach-Zehnder interferometer that integrates digital spatial light modulators with static beam displacers to map spatial-mode entanglement from a parametric down-conversion source onto topological entanglement with high fidelity. The device also mimics the action of a reprogrammable controlled-unitary gate, digitally driven by the spatial light modulator with the desired target state preparation achieved through postselection. This approach is an enabling platform and provides a practical route to generating reliable, high-purity quantum structured light with topological features, both at the single-photon level and as entangled states, a direction of growing topical interest.

Nucleation of vortex-antivortex pairs (VAPs) is believed to play a central role in the photon detection mechanism of superconducting detectors; however, their direct dynamic observation has remained challenging. Here, we report the direct observation of photon-induced VAP dynamics in a current-carrying superconductor as quantized voltage signals following photon absorption. The observed signals are interpreted as discrete phase-slip events, where each vortex traversal induces a $2\pi$ phase change of the superconducting order parameter, resulting in a quantized voltage pulse whose time integral is given by the magnetic flux quantum ${\mathrm{\Phi }}_0$. We analyze the resulting quantized signals as a function of bias current, base temperature, and input photon-number states and find that the number of VAPs generated per absorbed photon becomes effectively stabilized under specific conditions. Under these conditions, we demonstrate photon-number-resolving capability by directly counting phase-slip-induced voltage quanta. Our results reveal a detection mechanism governed by phase dynamics rather than conventional resistive transitions and represent the first demonstration of photon-number resolution based on VAP counting, enabling fast multi-photon-resolving superconducting detectors for quantum optics.

Atom interferometric inertial sensors offer exceptional sensitivity but are fundamentally constrained by the periodic phase response of matter-wave interference, which imposes an intrinsic half-fringe dynamic-range limit and prevents continuous inertial tracking. In multiaxis configurations, additional cross-coupling between acceleration and rotation further complicates closed-loop operation. Here we demonstrate dual-channel closed-loop operation of an atomic beam interferometer, realizing decoupled feedback control of acceleration- and rotation-induced phases and overcoming the half-fringe limitation. Using continuous, transversely cooled ${}^{87}\mathrm{Rb}$ atomic beams, the interferometric phases associated with rotation and acceleration are independently extracted, tracked across multiple fringes, and actively compensated through Raman frequency modulation. This closed-loop scheme enables unambiguous measurements up to $\pm 1°/\mathrm{s}$ (i.e., $\pm 0.017\mathrm{r}\mathrm{a}\mathrm{d}/\mathrm{s}$) in rotation and $\pm 0.17\mathrm{g}$ (i.e., $\pm 1.67\mathrm{m}/{\mathrm{s}}^2$) in acceleration while maintaining high fringe contrast, corresponding to nearly two orders-of-magnitude extension beyond the conventional half-fringe limit. The sensor achieves a long-term stability of $4\times {10}^{-4}°/\mathrm{h}$ for rotation and $4\mathrm{\mu }\mathrm{g}$ for acceleration at an averaging time of $1000\mathrm{s}$. By converting the intrinsically periodic interferometric response into stabilized phase-encoded inertial channels, this work demonstrates a dual-channel closed-loop operation of atomic beam interferometry and represents a step toward practical quantum inertial navigation under dynamic conditions.

Battery-free and wireless sensing presents a crucial challenge for large-scale and inaccessible infrastructure, where the difficulty of power supply severely limits continuous monitoring. Here, we develop a quartz-crystal-microbalance (QCM) technique that enables fully passive sensing at distances exceeding 50 m, far beyond the operating range of conventional battery-free sensors. This capability is enabled by a few-millimeter-sized MEMS-QCM sensor incorporating integrated antennas for efficient coupling between mechanical resonance and free-space electromagnetic waves. On the sensor side, an extended antenna of approximately 100 mm in length is connected to the MEMS-QCM to enhance wireless signal transmission. On the instrumentation side, Yagi–Uda antennas are employed for efficient signal reception and transmission. By exploiting controlled bending of an ultrathin quartz resonator, we achieve battery-free strain sensing at distances of over 50 m. We further introduce a new sensing mechanism for displacement measurement based on polarization relaxation on the electrodeless quartz surface, which also operates wirelessly and without an onboard power source. In addition, the platform is shown to be applicable to the wireless, battery-free hydrogen-gas sensing. These results establish a general framework for long-range, battery-free mechanical and chemical sensing, opening a path toward continuous, nondestructive monitoring of social and industrial infrastructure.

Controlling how waves slow, stop, and localize lies at the heart of modern condensed-matter and metamaterial physics. Flexoelectricity, the coupling of strain gradients and polarization that is universally present in all dielectrics, scales with structure size, thus offering a route to size-dependent electromechanics, especially in materials with nanoscale feature size. We show that a suitably designed flexoelectric medium permits a tunable landscape of band extrema where elastic and electric fields strictly co-localize, thus leading to localized vibrations, and zero-group-velocity and stationary-inflection modes that trap energy in place. The resulting defect-cavity modes exhibit dual amplification of open-circuit voltage and mass responsivity, both scaling inversely with the localization length that is determined by the curvature of the dispersion curve and frequency shift from the minimum. This framework unifies nonlocal lattice physics with continuum electromechanics, establishing a continuum route to engineer and electrically read out band-edge phenomena for ultrasensitive detection and reconfigurable energy harvesting.

Superconducting nanowire single-photon detectors (SNSPDs) have demonstrated timing jitter in the few-picosecond regime, yet their timing resolution deteriorates substantially under high-count-rate operation. Existing interpretations mainly attribute this degradation to deterministic waveform distortions, such as multiphoton responses and pulse pile-up; however, the experimentally observed jitter broadening at high count rates cannot be fully accounted for within this picture. Here, we show that stochastic baseline fluctuations arising from finite-memory readout dynamics constitute an intrinsic source of the count-rate-dependent timing jitter in SNSPD systems. For stochastically arriving photons, overlapping recovery responses accumulate in the readout chain and generate statistically fluctuating baselines, which are converted into timing uncertainty through threshold-based timing extraction. We develop a stochastic-process framework that quantitatively connects photon statistics, readout dynamics, and timing jitter. The framework predicts characteristic scaling behaviors, including a nonmonotonic dependence of baseline fluctuations under pulsed excitation with a maximum near half of the repetition frequency. These predictions are quantitatively verified through systematic variations of count rate, circuit time constant, and detector dynamical properties. Our results identify stochastic baseline dynamics as a fundamental mechanism limiting timing resolution in high-count-rate SNSPD operation and provide a general framework for optimizing finite-memory high-speed photon-counting systems.

High-performance numerical methods are essential not only for advancing quantum many-body physics but also for enabling integration with emerging quantum computing platforms. We present a scalable, general-purpose parallel algorithm for quantum simulations based on or-represented quantum algebra (ORQA). This framework applies to arbitrary spin systems and naturally integrates with quantum circuit simulation in the Heisenberg picture, making it particularly relevant to recent large-scale experiments on superconducting qubit processors [Y. Kim et al. Nature (London) 618, 500 (2023)]. As a benchmark, we simulate the kicked Ising model on a 127-qubit heavy-hexagon lattice, tracking the time evolution of local magnetization using up to $1\times {10}^{12}$ Pauli strings. Executed on the supercomputer Fugaku, our simulations exhibit strong scaling up to $2^{17}$ parallel processes with near-linear communication overhead. These results establish ORQA as a practical and high-performance tool for quantum many-body dynamics and highlight its potential for integration into hybrid quantum-classical computational frameworks, complementing recent advances in tensor network and surrogate simulation techniques.

Volatile resistive switching in correlated-electron systems, characterized by an abrupt resistance decrease under applied current, is crucial for developing next-generation electronics. Despite its technological significance, the underlying physics remains elusive. Inorganic thin films on substrates—the widely studied platform for resistive switching—usually exhibit broad temperature-induced metal-insulator transitions (MITs) and substantial heat dissipation. These factors complicate the nonlinear thermal effect induced by Joule heating, a key contributor to resistive switching, rendering it excessively complex and difficult to decipher. Here, we investigate a resistive-switched state in the bulk organic conductor $($$d7\text{−}\text{DMe}\text{−}\text{DCNQI}{)}_2\text{Cu}$, which undergoes an extremely sharp first-order MIT and exhibits weak heat dissipation, using resistance and ${}^1\mathrm{H}\text{−}\text{NMR}$ measurements. These extreme conditions make the Joule heating effect vivid, allowing us to observe peculiar phenomena, including temperature locking to the MIT and “inverse Ohm’s law”—an inverse proportionality between voltage and current. These findings provide fundamental insights into the nonlinear thermal effect in resistive switching, offering a pathway to efficient resistive switching technologies.