Physical Review Applied

Nanodiamond sensors can measure magnetic fields at the nanoscale, but their random orientations create a calibration paradox: sensing the field requires knowing the orientation, while finding the orientation assumes the field is known. This is particularly prominent where one needs to separate the local field from the sample from the applied external bias field. This study shows that applying four non-coplanar bias fields breaks this deadlock, revealing both quantities at once. This self-calibration could enable nanodiamond sensing in complex environments such as living cells and aqueous solutions, where precalibrating the sensor is not an option.

Fluxonium qubits are promising candidates for fault-tolerant quantum processors, but their dispersive readout remains challenging due to measurement-induced state transitions (MIST) that populate higher-excited states. To address this, the authors systematically explore MIST across a broad range of fluxonium parameters using branch analysis. They find that lighter fluxoniums are less susceptible to these transitions than heavier counterparts, and trace this to three key mechanisms. These results provide a path toward optimizing fluxonium design for fast, high-fidelity readout.

Spatiotemporal acoustic vortices provide an intriguing route to encode transverse orbital angular momentum (OAM) in sound, but they have a fundamental constraint: Normally, their size grows with topological charge. This work overcomes that constraint by experimentally realizing perfect spatiotemporal acoustic vortices whose size remains nearly unchanged across different topological charges. The common vortex size also allows distinct OAM states to overlap and interfere on the same ring, producing programmable petal-like pressure patterns. These capabilities could facilitate acoustic multiplexing and detection, and structured-wave sensing and particle manipulation.

Spins in solids could provide long-lived memories for superconducting quantum computers, but practical devices need a microwave interface that remains tunable in a magnetic field. This study develops a superconducting resonator with adjustable frequency and bandwidth to address bismuth spins in silicon at a clock transition, where they are less sensitive to magnetic noise. The authors measure quantum coherence lasting nearly half a second and show that tuning the resonator can suppress the spins’ microwave echoes. These capabilities provide building blocks for controlling the storage and retrieval of quantum information in spin-based memories.

Accurate measurements of magnetic field direction are essential for applications ranging from magnetic navigation to space science, yet achieving high angular accuracy with compact optically pumped magnetometers (OPMs) remains challenging. The authors demonstrate a vector OPM based on radio-frequency-driven Zeeman Rabi oscillations in a microfabricated rubidium vapor cell. By combining calibrated radio-frequency fields with a Floquet-based atomic model, they achieve dead-zone-free vector measurements with a mean angular accuracy of 80 μrad using a single optical axis. This is a promising route toward compact, high-accuracy OPMs that can be integrated into existing architectures.

Chaotic magnetization dynamics in magnetic tunnel junctions (MTJs) could provide a compact physical platform for brain-inspired computing, but practical use requires electrical control and robustness against unavoidable thermal fluctuations. The authors use theory to show that spin-transfer torque generated by ac and dc currents enables both the excitation and control of chaos in a perpendicularly magnetized MTJ, with the dc current acting as an effective control parameter. Surprisingly, thermal fluctuations do not simply wash out the nonlinear dynamics; instead, they can promote noise-induced chaos.

Superconducting qubits are a leading platform for quantum computing, but scaling up their circuitry is limited by crosstalk between qubits, and by the growing density of control wiring. This theoretical study uncovers a particular kind of crosstalk: When the magnetic flux through one qubit varies in time, that qubit’s neighbors see this as a voltage source woven into the circuit itself, in a circuit version of Faraday’s law of induction. Rather than a bane, though, this crosstalk mechanism could be turned into a boon, enabling quantum gates with less wiring per qubit.

Periodic lattices of quantum emitters are promising platforms for quantum information processing, single-photon storage, and precision sensing. However, their performance is limited by an incomplete understanding of how intrinsic quantum nonlinearities shape their behavior beyond the linear (single-excitation) regime. The authors theoretically investigate coherently driven emitter lattices and show that quantum anharmonicity populates a continuum of excitonic Bloch states, including modes outside the light cone (subradiant), giving rise to broadband, incoherent emission that dominates under strong driving, and revealing an unexpected bistable regime controlled by effective driving renormalization.

The Dzyaloshinskii-Moriya (DM) interaction underlies chiral magnetic phenomena such as skyrmions, but its vectors are generally fixed by crystalline symmetry and difficult to manipulate. In this Letter the authors demonstrate control of DM-vector ordering in magnetic superlattices by mechanically breaking rotational and translational symmetries, using a strain technique. This fresh approach to engineering the DM interaction through control of structural symmetry seems to carry broad implications for chiral magnetism and spintronic applications.

Ultranarrow-linewidth lasers with suppressed high-frequency phase noise are critical for quantum control and precision metrology, but standard optical phase locking (OPL) is limited at high frequencies by feedback latency. This Letter presents a feedforward architecture that recycles the existing beat signal to drive a single electro-optic modulator, achieving robust phase-noise suppression that exceeds 30 dB from 10 kHz to 10 MHz. This approach is hardware-efficient, compatible with standard OPL setups, and readily extendable to gigahertz-level frequency offsets and optical frequency combs, offering a scalable route toward high-fidelity coherent control.

Although high-spin nuclear qudits in silicon offer a hardware-efficient route to quantum computation and error correction, scale-up is hindered by the weak, short-range interactions between nuclei. This Letter shows that electron spins can serve as a tunable quantum bus, mediating strong interactions between distant nuclear qudits and enabling high-fidelity two-qudit gates. The resulting coupling is robust against electron decoherence and charge noise while reaching a strength of about 100 kHz over distances exceeding 20 nm. This approach is a practical path toward scalable, electrically controlled qudit processors compatible with silicon-based quantum architectures.

Plasma photonic crystals (PPCs) are promising for controlling electromagnetic waves, but conventional systems with water electrodes suffer from strong microwave absorption. This study employs an electrode based on dimethyl silicone oil for dielectric barrier discharge, yielding a number of advantages that include significantly improved microwave signals. The system produces tunable triangular elements in a honeycomb superlattice that hosts robust valley-locked topological edge states. Here is a fresh approach to high-performance PPCs for integrated microwave devices and topological applications.

Extracting the current-phase relationship (CPR) of superconducting junctions in a noninvasive manner has proven to be difficult in the lab. The authors present an innovative method that overcomes the main limitations of conventional approaches, allowing the full harmonic content of the CPR to be extracted by subjecting the junction to a high-frequency ac drive. This method is demonstrated experimentally and found to be robust against experimental noise.

For electric field (E-field) sensing applications that require high sensitivity with very small sensing volume or high spatial resolution at low frequencies near dc, conventional electronic approaches suffer from low efficiency (due to small antenna size) and higher intrinsic noise. Using a simple vapor cell, innovative technical schemes, and an unexpected phenomenon reminiscent of magnetoresistance, the authors demonstrate a state-of-the-art quasi-dc atomic E-field sensor using “antenna-less” atoms. This approach is expected to enable electronic diagnostics, communication, proximity detection, charge-signature tracing, bioscience and geoscience research, and more.

Silicon spin qubits offer a promising platform for scalable quantum processors, but small and highly variable valley splittings can compromise qubit operation across a device. This work uses constrained variational optimization with epitaxial-growth-motivated constraints to design feasible germanium concentration profiles in Si/Si-Ge quantum wells, to enhance deterministic valley splitting while suppressing variability induced by alloy disorder. The principal result is a “modulated wiggle well”, which provides a large, robust, and electrically tunable valley splitting. The approach provides a systematic route for designing reproducible and epitaxially feasible silicon-qubit devices.

Nonradiative point defects in semiconductors are often a major hurdle for efficiency and reliability in optoelectronics, yet directly probing these defects remains difficult. This study uses time-resolved cathodoluminescence mapping plus a diffusion-recombination model to isolate and quantify carrier dynamics at a single point defect in an (In,Ga)N/GaN quantum well. Recombination at the defect is rate-limited by a 16-ps delay, setting a hard bottleneck on defect activity. This insight predicts that defect-related nonradiative recombination saturates at carrier densities routinely reached in LEDs, revealing a fundamental limit not captured by traditional macroscale recombination models.

Antiferromagnets are promising for fast, energy-efficient, high-density memory technologies, but their compensated spins make their domains difficult to image and control optically. The authors use magneto-optical birefringence imaging to show that a Pt capping layer enables ultrafast laser pulses to modify domains in insulating NiO and CoO films. Sweeping the beam generates a thermal gradient that drives domain walls toward colder regions, producing partial switching that can be reversed by changing the sweep direction. These findings provide a path toward optically controlled antiferromagnetic memory devices.

SiC-based quantum magnetometry is vital for advanced sensing fields because it offers a path to volume-compatible chips, but it is held back because its sensitivity is lower than other platforms like N-V diamond color centers. This study uses a planar 4H-SiC waveguide platform and wafer-scale fabrication to seamlessly integrate and excite large V2 ensembles. Notably, this streamlined layout achieves a shot-noise limit of 30 nT/√Hz, boosting sensitivity by 2–3 orders of magnitude over that of standard, confocal SiC methods while preserving coherent control.

Semiconductor quantum dots are excellent deterministic sources of single photons. One of the best routes to generate pure single photons is through the biexciton-exciton cascade, but achieving high Hong-Ou-Mandel visibility (or photon indistinguishability) is fundamentally hindered by the finite ratio of radiative lifetimes of the two excited electronic states in that cascade. This study reduces the biexciton lifetime by selective cavity enhancement, and uses the single photon from the biexciton-to-exciton transition. This approach is a powerful path to achieving excellent single-photon indistinguishability and purity, plus high brightness.

Producing narrow-band, high-spectral-density pulses from an x-ray free-electron laser (XFEL) is important for demanding applications at extreme photon energies, such as high-resolution spectroscopy and studies of structural dynamics, but conventional hard-x-ray self-seeding becomes increasingly limited as photon energy increases. This study extends hard-x-ray self-seeding at the European XFEL to 18 keV, and explores second-harmonic-generation self-seeding as a route toward even higher photon energies. Experiments demonstrate coherent amplification of the second harmonic at 15 and 18 keV, while simulations show the potential for narrowband operation at 30 keV.

Artificial spin ices are promising for reconfigurable magnonics and unconventional computing because their magnetic states and spin-wave spectra can be tailored, but available control mechanisms are limited. The authors pattern an exchange-coupled Co-Fe/Ru/Ni-Fe synthetic ferrimagnet into an artificial spin lattice and study its static and dynamic behavior. They show that quadratic and biquadratic interlayer exchange coupling can stabilize orthogonal layer magnetizations, drive hard-axis alignment, and reshape the spin-wave spectrum. This additional degree of freedom may enable field-reconfigurable magnonic crystals, spin-wave filters, and artificial-spin-ice metamaterials.

Fault-tolerant quantum computing depends on ancilla qubits that can extract error syndromes from data qubits, but ancilla errors can propagate back to the data qubits, contaminating the very information they are meant to protect. The Kerr-cat qubit has been proposed as a better ancilla, but a suitable interaction between Kerr-cat and transmon qubits needs to be experimentally verified. This study finds a beam-splitter interaction between a Kerr-cat and a transmon, producing an effective ẐX̂ coupling suitable for parity readout, and confirms expected behavior across different cat sizes and drive strengths.

Narrow superconducting strips are key elements of photodetectors, resonators, and quantum circuits, in which magnetic vortices can degrade device performance. The lower critical field (above which a vortex-containing state is favorable) for such strips is usually estimated using Pearl-London theory, which cannot yield the vortex core’s temperature dependence microscopically. By solving the two-dimensional Usadel equations self-consistently, this study provides a microscopic calculation of the lower critical field at any temperature below the superconducting critical temperature, giving a quantitative basis for predicting the vortex-free field tolerance of superconducting thin-film devices.

$XY$ (planar-spin) Hamiltonians arise in phase synchronization and retrieval and analog formulations of hard optimization, which motivates fast, low-power physical solvers. However, gain-based $XY$ systems that encode each spin with a single complex field can become trapped in metastable states. The authors introduce an annealer that represents each spin with two coupled complex components, and uses a graph-independent locking term that exploits the extra degree of freedom to bypass barriers. For challenging graph families, this higher-dimensional annealing improves ground-state recovery compared to one-component approaches, supporting more reliable photonic and analog $XY$ optimization.

Materials that remain ventilating while providing airborne sound insulation are highly desirable for everyday noise controllers. This review discusses lattice metamaterials as an innovative class of advanced structures capable of fulfilling both functions. The authors survey sound-insulation performance and mechanisms, and identify the most effective types of architectures among distinct categories of lattice metamaterials. In addition, they propose numerical strategies to enable accelerated design exploration.