In conventional transmon qubits, decoherence is dominated by a large number of parasitic two-level systems (TLS) residing at the edges of the large area coplanar shunt capacitor and junction leads. Avoiding these defects by improvements in design, fabrication, and materials proved to be a significant challenge that so far led to limited progress. The merged-element transmon qubit (“mergemon”), a recently proposed paradigm shift in transmon design, attempts to address these issues by engineering the Josephson junction to act as its own shunt capacitor. With its energy mostly confined within the junctions, efforts required to improve qubit coherence can be concentrated on the junction barrier, a potentially easier to control interface compared with exposed circuit areas. Incorporating an additional aluminum deposition and oxidation into the in situ bandaged Niemeyer-Dolan technique, we were able to fabricate flux-tunable mergemon qubits achieving mean $T_1$ relaxation times of up to $130\mathrm{\mu }\mathrm{s}$ ($Q\approx 3.3\times {10}^6$). TLS spectroscopy under applied strain and electric fields, together with systematic design variations, revealed that even for mergemon qubits—despite their significantly reduced footprint and increased junction barrier volume—careful design considerations are still essential to avoid $T_1$ limitations due to surface loss.

Remote magnetic sensing can be used to monitor the position of objects in real time, enabling ground transport monitoring, underground infrastructure mapping, and hazardous detection. However, magnetic signals are typically weak and complex, requiring sophisticated physical models to analyze them and a detailed knowledge of the system under study, factors that are frequently unavailable. In this work, we provide a solution to these limitations by demonstrating a machine-learning (ML) method that can be trained exclusively on experimental data, without the need of any physical model, to predict the position of a magnetic target in real time. The target can be any object with a magnetic signal above the floor noise, and in this case, we use a quantum diamond magnetometer to track variations of a few hundred nanoteslas produced by an elevator moving along a single axis. The one-dimensional movement is a simple yet challenging scenario, resembling realistic environments such as high buildings, tunnels, or train circuits, and is the first step toward building broader applications. Our ML algorithm can be trained in approximately 40 min, achieving over 80% accuracy in predicting the target’s position at a rate of 10 Hz, for a positional error tolerance of 30 cm, which is well within the the 4-m spacing between parking levels. Our results open up the possibility to apply this ML method more generally for real-time monitoring of magnetic objects, which will broaden the scope of magnetic detection applications.

Studying the electrical signals generated by living cells is key to understanding numerous biological phenomena. Electrochromic optical recording (ECORE) uses the electrochromism exhibited by certain materials to noninvasively measure these signals in real time. In this work, we report on the development of ECORE based on a high-numerical-aperture microscope objective. We demonstrate the recording of extracellular action potentials from cardiomyocytes with single-cell resolution and a high sensitivity of $6.3\mathrm{\mu }\mathrm{V}$. Combining ECORE with microscopy simplifies the optical setup, allows for the simultaneous imaging of specimens, and makes ECORE accessible to a broader community of researchers, thereby enabling a better understanding of the biological processes that are integral to life.

Nitrogen-vacancy (NV) centers in nanodiamonds have emerged as a promising quantum sensing platform for biomedical imaging applications, yet random orientations of individual particles present significant challenges in large-scale sensor calibration. In this study, we demonstrate a novel approach to simultaneously determine each particle’s crystallographic axes and the surrounding local vector magnetic field. Specifically, a minimum of four distinct bias fields is required to unambiguously extract both the orientation and the local field. We validate our method experimentally using NV centers in two scenarios: (i) in a bulk diamond with known crystal orientation as a proof of concept and (ii) on various single nanodiamonds to mimic real-world applications. Our work represents a crucial step toward unlocking the full potential of nanodiamonds for advanced applications such as in situ biomedical imaging and nanoscale sensing in complex environments.

Spatiotemporal (ST) vortices carrying transverse orbital angular momentum have attracted growing interest in both optics and acoustics. However, despite diverse generation schemes, a fundamental limitation persists: the vortex radius increases with the topological charge, leading to a size mismatch that complicates efficient multiplexing and detection. Here, we introduce the concept of perfect ST acoustic vortices to decouple the vortex size from the topological charge. Unlike conventional counterparts, these vortices maintain a nearly constant radius regardless of the topological charge. We theoretically construct this new class of vortices via the Fourier transform of an ST Bessel-Gaussian spectrum in the wave-vector-frequency domain. Experimentally, we implement the synthesis using a one-dimensional acoustic phased array. We then characterize the generated fields, confirming their size invariance and investigating their propagation and superposition properties. By enabling a nearly charge-independent vortex radius, our work overcomes a critical constraint of ST acoustic vortices, paving the way for high-capacity acoustic communications, sensing, and particle manipulation.

Understanding the mechanisms that limit high-fidelity readout in circuit quantum electrodynamics is essential for its optimization. Multiphoton resonances are understood to be a limiting factor, causing population transfer from the computational states to higher-energy states under drive. This effect, known as measurement-induced state transitions, has been extensively studied for the transmon qubit. While this exploration has begun for the fluxonium qubit, a systematic study of this effect is lacking. Here, we bridge this gap by theoretically studying measurement-induced state transitions in the fluxonium qubit over a wide range of parameters, comprising essentially all experimentally explored ranges. We find that lighter fluxoniums are less susceptible to these state transitions when compared with their heavier counterparts. We attribute this effect to the combination of lower density of multiphoton resonances, a smaller requisite coupling for a given dispersive shift, and a more harmoniclike structure of the charge operator. We confirm the validity of our analysis by performing time-dependent readout simulations. Finally, we consider the impact of the superinductor’s array modes on measurement-induced state transitions over a large range of parameters.

Solid-state spin ensembles addressed via superconducting circuits are promising candidates for quantum memory applications, offering multimodal storage capability and second-long coherence times at their clock transition. Implementing practical memory schemes requires dynamic control over both the resonator frequency and bandwidth, and most crucially, deterministic access to the clock transitions. In this article, we report measurements of a superconducting resonator whose frequency can be tuned by passing a DC current through the high-kinetic-inductance thin film, and whose bandwidth can be tuned by parametric coupling to a low-Q buffer resonator. Using this resonator, we deterministically address an ensemble of bismuth donors at their clock transition, measuring a Hahn-echo coherence time of 450 ms. We demonstrate RF driving of the bismuth donor hyperfine transitions, as well as dynamic bandwidth control of the resonator.

Accurate magnetic field direction sensing in compact platforms is critical in applications spanning magnetic navigation, space science, and biomedical imaging. We demonstrate a single-optical-axis vector optically pumped magnetometer based on Rabi oscillations between Zeeman sublevels driven by a series of resonant radiofrequency (rf) polarization ellipses (PEs). A calibration protocol based on controlled rotations of the dc magnetic field determines the spatial orientation of each PE. We develop a detailed theoretical model describing the angular dependence of the Rabi frequencies, incorporating key systematics including rf Stark shifts and Bloch-Siegert shifts. We also account for an rf-based heading-error systematic affecting Rabi-frequency measurements arising from the nonlinear Zeeman effect. Simultaneous Larmor measurements yield the magnitude of the magnetic field, enabling integrated vector-scalar measurements. The magnetometer achieves deadzone-free vector operation with $80\mathrm{\mu }\text{rad}$ mean angular accuracy and angular noise densities as low as $8\mathrm{\mu }\text{rad}/\sqrt{\mathrm{Hz}}$, offering a pathway toward miniaturized sensors without requiring 3D optical access or sensor rotations.

Crosstalk refers to unwanted qubit addressing. This is particularly detrimental when scaling up quantum information systems because unintended interactions limit their overall performance. For superconducting qubits, tunable couplings and frequency tunability achieved through externally applied magnetic fluxes enable high-fidelity entangling gates; however, they also introduce crosstalk through unintended flux coupling. In this work, we investigate the impact of time-dependent external magnetic fluxes in quantized circuits on superconducting qubit couplings. We find that nontrivial cross-voltage driving emerges between capacitively linked qubits when the magnetic flux threading the superconducting loop of a qubit varies in time, in a manner analogous to Faraday’s law of induction. Crucially, we show that this effect enables fast single-qubit control through the coupler element in standard tunable-coupler architectures, potentially eliminating the need for individual microwave $XY$ control lines.

We theoretically investigate the emergence of quantum nonlinearities in the optical response of periodic lattices of two-level quantum emitters coherently driven by a laser. For subwavelength lattice periods, where the system behaves as a quantum metasurface, we find that a resonant incident plane wave can populate excitonic Bloch states with parallel wavevectors different from the incident field, including those lying outside the light cone. Closely related to resonance fluorescence, the far-field emission from the system in the strong-driving regime is dominated by a broadband background of photons spanning a wide range of frequencies and wavevectors. Moreover, we show that, within a mean-field description and for lattice periods approaching the driving wavelength, the emitter lattice enters a bistable regime due to the renormalization of the driving rate, in striking contrast with its classical (bosonic) analog. This bistable behavior enables the selective activation and deactivation of the optical quantum nonlinearities of the system.

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.