This paper is a contribution to the Physical Review Applied collection titled Physics-Inspired Computing.

We demonstrate that thermally stable perpendicular magnetic tunnel junctions, widely used in spin-transfer torque magnetic random-access memory, can be actuated with nanosecond pulses to exhibit tunable stochastic behavior. This actuated-stochastic tunnel junction (A-sMTJ) concept produces random telegraph noise, with control over fluctuation rate and probability bias. The device response is shown to be consistent with a Poisson process, with fluctuation rates tunable over two orders of magnitude, with average state dwell times ranging from approximately 30 ns to $2\text{μ}\mathrm{s}$. These results establish A-sMTJ as a versatile platform for integrating deterministic, stochastic, and in-memory functionality on a single chip—advancing the development of probabilistic, neuromorphic, and unconventional computing systems.

Hole-spin qubits in $\mathrm{Ge}$/$\mathrm{Ge}\mathrm{Si}$ heterostructures benefit from the clean environment of epitaxial interfaces and from the intrinsic spin-orbit coupling that enables efficient electrical control, which makes them promising candidates for quantum computation. However, spin-orbit coupling also enhances the sensitivity to electrical disorder, potentially increasing variability. In this work, we perform numerical simulations in a realistic device geometry to quantify the variability of the charge and spin properties of $\mathrm{Ge}$ qubits induced by charge traps at the $\mathrm{Si}\mathrm{Ge}$/oxide interfaces. We show that while the variability of charge properties remains moderate, spin properties ($g$ factors and Rabi frequencies) show significant dispersion. We explore the implications of this variability for large-scale architectures, and provide guidelines to minimize variability both in terms of interface quality requirements and optimal operation strategies.

The development of deterministic single-photon sources emitting in the telecommunication bands is a key challenge for quantum communication and photonic quantum computing. Here, we investigate the optical properties and single-photon emission of molecular beam epitaxy (MBE) grown semiconductor quantum dots (QDs) emitting in the telecom O- and C-bands. The QDs are embedded in an $\mathrm{In}\mathrm{Ga}\mathrm{As}$ matrix with fixed indium content grown on top of a compositionally graded $\mathrm{In}\mathrm{Ga}\mathrm{As}$ buffer. This structure allows for the future implementation of electrically contacted nanocavities to enable high-quality and bright QD emission. In detailed optical characterizations we observe linewidths as low as $50\text{μ}\mathrm{eV}$, close to the spectrometer resolution limit, low fine structure splittings close to $10\text{μ}\mathrm{eV}$, and $g^{(2)}(0)$ values as low as $0.08$. These results advance the current performance metrics for MBE-grown QDs on $\mathrm{Ga}\mathrm{As}$ substrates emitting in the telecom bands and showcase the potential of the heterostructures presented for further integration into photonic devices.

Modern Hopfield neural networks (HNNs), also known as dense associative memories (DAMs), enhance simple recurrent neural networks leveraging nonlinearities in their energy functions. They have broad applications in combinatorial optimization, memory storage, deep-learning transformers, and correlated pattern recognition. To date, research on DAMs has been primarily theoretical, with implementations limited to CPUs and GPUs. Here, we propose and experimentally demonstrate a nonlinear-optical Hopfield neural network (NOHNN) for realizing DAMs using correlated patterns. The NOHNN incorporates both two-body and four-body interactions. Notably, the inclusion of four-body interactions scores a minimum tenfold improvement in the storage of uncorrelated patterns, surpassing the traditional capacity limit of HNNs. For correlated patterns, the storage capacity increases by up to a factor of 50. Benchmark testing on Modified National Institute of Standards and Technology (MNIST) handwritten digits shows a 5.5 times improvement in pattern storage, with significantly cleaner and less noisy retrieval. These results highlight the potential of nonlinear-optical DAMs for large-scale data optimization, computer vision, and graph-network tasks.

The generation of magnon polarons, which are hybridized states resulting from strong magnon-phonon coupling, is key to enabling coherent manipulation in acoustic and spintronic devices. However, the conventional device configuration, a magnetic thin film on a thick piezoelectric layer, often has difficulty achieving large magnon-phonon coupling due to very small spatial-mode overlap. Here, we demonstrate the generation of magnon polarons by using a mode-matched on-chip magnomechanical system. A configuration with a thin piezoelectric film on a magnetic layer with a thickness of several micrometers has been found to sustain deeply distributed magnon modes that enable magnetoelastic coupling to phonons over almost the entire mode volume. The enhanced spatial-mode overlap has generated magnon polarons the spectra of which have shown distinct avoided crossings. This magnomechanical system will facilitate the utilization of coherent magnon-phonon conversion and their hybrid states in functional phononic devices.

Quantum optical coherence tomography (QOCT) offers a simple way to cancel dispersion broadening in a sample while also providing twice the resolution compared to classical OCT. However, to achieve these advantages, a bright and broadband source of entangled photon pairs is required. A simple implementation uses collinear spontaneous parametric down-conversion in a Michelson interferometer (MI), yet this autocorrelation scheme suffers from parasitic terms and sensitivity to phase noise. Here, we introduce a cross-correlation MI-based QOCT that overcomes these drawbacks, significantly advancing QOCT toward practical applications.

Acousto-optic deflectors (AODs) enable spatiotemporal control of laser beams through diffraction at an ultrasonic grating that is controllable by radio-frequency (rf) waveforms. These devices are a widely used tool for high-bandwidth random-access scanning applications, such as optical tweezers in quantum technology. A single AOD can generate multiple optical tweezers by multitone rf input in one dimension. Two-dimensional (2D) patterns can be realized with two perpendicularly oriented AODs. As the acousto-optical response depends nonlinearly on the applied frequency components, phases, and amplitudes, and in addition experiences dimensional coupling in 2D setups, intensity regulation becomes a unique challenge. Guided by coupled-wave theory and experimental observations, we derive a compute-efficient model which we implement on a graphics processing unit. Only one-time sampling of single-tone laser-power calibration is needed for model parameter determination, allowing for straightforward integration into optical instruments. We implement and experimentally validate an open-loop diffraction efficiency control system that enables programmable 2D multibeam trajectories with intensity control applied at every time step during digital signal generation, overcoming the limited flexibility, pattern-size constraints, and bandwidth limitations of methods using precalculation and precalibration of a predefined pattern set or closed-loop feedback. The system is capable of stable real-time waveform streaming of arrays with up to $50\times 50$ tweezers with minimal time resolution of 1.4 ns (700 MS/s) and a peak latency below 257 µs for execution of newly requested patterns. Reactive, real-time 2D multibeam laser patterning and scanning with strict intensity matching will substantially benefit parallelization and increasing data rates in materials processing, microscopy, and applications of optical tweezers, where the work presented is of immediate relevance for time-critical conditional operations with minimal latency.

We introduce a new method, rooted in estimation theory, to detect individual atoms in site-resolved images of microtrap arrays, such as optical lattices or optical tweezer arrays. Using labeled test images, we demonstrate drastic improvement of the detection accuracy, compared with the popular method based on Wiener deconvolution, when the intersite distance is comparable with the radius of the point spread function. The runtime of our method scales approximately linearly with the number of sites, and remains well below 100 ms for an array of $100\times 100$ sites on a desktop computer. It is therefore fully compatible with a real-time usage. Finally, we propose a rigorous definition for the signal-to-noise ratio of the problem and show that it can be used as a predictor for the detection error rate. Our work opens the prospect for future experiments with increased array sizes or reduced intersite distances.

Understanding intricate magnetic structures in materials is essential for advancing materials science, spintronics, and geology. Recent developments of quantum-enabled magnetometers, such as those based on nitrogen-vacancy centers in diamond, have enabled direct imaging of magnetic field distributions across a wide range of magnetic profiles. However, reconstructing the magnetization from an experimentally measured magnetic field map is a complex inverse problem, further complicated by measurement noise, finite spatial resolution, and variations in sample-to-sensor distance. In this work, we present an efficient graphics-processing-unit-accelerated method for reconstructing spatially varying magnetization density from measured magnetic fields with minimal prior assumptions. We validate our method by simulating diverse magnetic structures under realistic experimental conditions, including multidomain ferromagnetism and magnetic spin textures such as a skyrmion, an antiskyrmion, and a meron. Experimentally, we reconstruct the magnetization of a micrometer-scale Apollo lunar mare basalt (sample 10003,184) and a nanometer-scale twisted double-trilayer ${\mathrm{Cr}\mathrm{I}}_3$ sample. The basalt exhibits soft ferromagnetic domains consistent with previous paleomagnetic studies, whereas the ${\mathrm{Cr}\mathrm{I}}_3$ system reveals a well-defined hexagonal magnetic moiré superlattice. Our approach provides a versatile and universal tool for investigating complex magnetization profiles, paving the way for future quantum sensing experiments.

Smith-Purcell radiation (SPR) provides a powerful platform for manipulating light at the nanoscale across a broad spectral range. In this study, we demonstrate SPR modulation using a two-dimensional metallic chirped metagrating (CMG) composed of nanowires with a gradually varying separation along the groove direction. By simply shifting the electron beam along this direction, without altering the grating structure, we achieve multiple functionalities, including tunable emission angles, adjustable focal positions, dual-focus control, and dual-wavelength emission. Experimental validation through CMG fabrication and angle-resolved cathodoluminescence measurements confirms effective control of SPR dispersion via electron beam positioning. We further extend this concept to electron sheet excitation and demonstrate the generation of focused vortex SPR. This work offers a robust and reconfigurable strategy for developing tunable nanoscale light sources.

Nuclear magnetic resonance (NMR) spectroscopy is a powerful technique for the study of local magnetism in a variety of materials. However, the inherently low sensitivity of conventional inductively detected solid-state NMR typically requires a large number of spins, reducing its applicability to two-dimensional materials and nanoscale thin films. To overcome this experimental challenge, we introduce a probe based on a superconducting meander-line surface coil that significantly enhances the NMR sensitivity for thin samples. Using an $\mathrm{Nb}\mathrm{N}$ meander with an optimized geometry, we demonstrate the sensitivity of this technique by detecting the NMR signal of a 150-nm-thick boron film containing only approximately $2\times {10}^{16}{}^{11}$$\mathrm{B}$ nuclear spins. Spin-echo measurements and theoretical modeling offer insight into the parameters limiting the performance of the coil. This work hopefully lays the foundation for developing highly sensitive NMR probes, potentially unlocking new opportunities for studying atomically thin materials.

Magnetic resonance spectroscopy (MRS) offers significant diagnostic potential but is inherently constrained by a low signal-to-noise ratio (SNR). While increasing the main magnetic field strength $B_0$ is theoretically linked to increased SNR, practically obtained gains in SNR from $B_0^{7/4}$ to $B_0$, depending on the domination of thermal noise at high $B_0$, are not always realized. Especially in clinical settings, the maximum reachable SNR is further constrained by the total available acquisition time (TA) and the regulatory limits on maximum tolerable specific absorption rate (SAR). This work attempts to derive mathematical expressions that enable systematic analysis of the theoretically achievable SNR gain. One important notion in this context is the SNR gain per unit of measurement time as a function of the main magnetic field $B_0$ strengths in the case of unlocalized X-nuclei and localized (1H and X-nuclei) single-voxel spectroscopy (SVS) pulse sequences. Our findings indicate that, under given fixed total amount of (patient acceptable) measurement time TA and maximum tolerable SAR limitation, together with conditions that ensure the adiabaticity of specific sequences, there exists an optimal magnetic field strength $B_0$ that maximizes SNR per unit of measurement time (${\mathrm{SNR}}_t$). Beyond this optimal $B_0$, further increases in field strength do not yield proportional improvements in ${\mathrm{SNR}}_t$. Key factors are identified, including rf-pulse bandwidth scaling with $B_0$ and longitudinal relaxation time ($T_1$) dependencies, that impact the net gain as well. Our theoretical analysis emphasizes critical considerations for optimizing SNR per unit time in clinical MRS, even challenging the presumption that higher magnetic fields $B_0$ always yield improved SNR per unit of measurement time performance.

Superconducting quantum interference devices (SQUIDs) are exceptionally sensitive magnetometers, capable of detecting weak magnetic fields. Miniaturizing these devices and integrating them onto scanning probes enables high-resolution imaging at low temperatures. Here, we fabricate nanometer-scale niobium SQUIDs with inner-loop sizes down to $10\mathrm{nm}$ at the apex of individual planar silicon cantilevers via a combination of wafer-scale optical lithography and focused ion beam (FIB) milling. These robust SQUID-on-lever probes overcome many of the limitations of existing devices, achieving spatial resolution better than 100 nm, magnetic flux sensitivity of 0.3 $\mathrm{\mu }{\mathrm{\Phi }}_0$/$\sqrt{\mathrm{Hz}}$, and operation in magnetic fields up to about 0.5 T at 4.2 K. Nanopatterning via $\mathrm{Ne}$- or $\mathrm{He}$-FIB milling allows for the incorporation of a modulation line for coupling magnetic flux into the SQUID or a third Josephson junction, for shifting its phase. Such advanced functionality, combined with high spatial resolution, large magnetic field range, and the ease of use of a cantilever-based scanning probe, extends the applicability of scanning SQUID microscopy to a wide range of magnetic, normally conducting, superconducting, and quantum Hall systems. We demonstrate magnetic imaging of skyrmions at the surface of bulk ${\mathrm{Cu}}_2{\mathrm{OSe}\mathrm{O}}_3$. Analysis of the point spread function determined from imaging a single skyrmion yields a full width at half maximum of 71 nm. Moreover, we image modulated magnetization patterns with a period of 65 nm.

This paper proposes a method for generating tunable, low-frequency signals in the ionosphere and propagating them back to the ground. The concept is based on a ground-based, mobile, modulated rf source, which generates signals in the ionosphere at frequencies in the megahertz range. These low-frequency signals propagate to the observation point on the ground. The low-frequency signal is parametrically generated by a ground-based, modulated high-frequency rf source. By tuning the modulation frequency to the local plasma frequency in the ionosphere, a resonant condition generates a low-frequency signal that propagates to the observation point on the ground. The ionospheric electrons are subjected to the modulated rf source and experience a nonlinear force—a ponderomotive force—which resonantly drives a current. The frequency of the driving current is at the modulation frequency, that is, the local plasma frequency. The driving current, driven by a resonance, increases in time, and generates a low-frequency signal. In an example, we find that a 1-MW source at 94 GHz and modulated at 9 MHz can generate a 9-MHz signal 500 km from the source at an intensity greater than the minimum detectable level. The low-frequency signal from the ionosphere can propagate back to the ground-based detector by means of surface waves. This concept may have applications in compact, mobile, and low-cost over-the-horizon radar and as a method to monitor ionospheric parameters.

Altermagnets, a recently discovered class of magnetic materials exhibiting ferromagneticlike spin-split bands and antiferromagneticlike compensated magnetic order, have attracted significant interest for next-generation spintronic applications. Ruthenium dioxide (${\mathrm{Ru}\mathrm{O}}_2$) has emerged as a promising altermagnetic candidate due to its compensated antiparallel magnetic order and strong spin-split electronic bands. However, recent experimental and theoretical reports also suggest that ${\mathrm{Ru}\mathrm{O}}_2$ may be nonmagnetic in its ground state, underscoring the need for deeper investigations into its magnetic character. Specifically, (100)-oriented ${\mathrm{Ru}\mathrm{O}}_2$ films are expected to generate spin currents with transverse spin polarization parallel to the Néel vector. Here, we investigate magnetotransport in epitaxial ${\mathrm{Ru}\mathrm{O}}_2/$permalloy (Py) heterostructures to examine spin Hall magnetoresistance and interfacial effects generated in such systems. Our measurements reveal a pronounced negative angular-dependent magnetoresistance for variation of magnetic field direction perpendicular to the charge current direction. Detailed measurements dependent on temperature, magnetic field, and crystallographic orientation indicate that interface-generated spin current (IGSC) at the ${\mathrm{Ru}\mathrm{O}}_2/$Py interface predominantly governs the observed magnetoresistance. In addition, the role of the IGSC contribution to the observed magnetoresistance was demonstrated through drift-diffusion calculations. This shows that strong interface effects predominate over possible altermagnetic contributions from ${\mathrm{Ru}\mathrm{O}}_2$. Our results show that the role of IGSCs is crucial and should not be overlooked in studies of altermagnetic systems. A critical step in this direction is disentangling interfacial from altermagnetic contributions. The insight into interfacial contributions from altermagnetic influences is essential for the advancement of ${\mathrm{Ru}\mathrm{O}}_2$-based spintronic memory and sensing applications.

We report the fabrication and optical characterization of an air-suspended photonic crystal nanobeam cavity in yttrium iron garnet (YIG) realized by focused-ion-beam milling. YIG’s combination of low optical loss and ferrimagnetism makes it highly attractive for quantum technologies, yet prior work has largely been focused on millimeter-scale spheres and simple microstructures, hindering true on-chip integration. Demonstrating nanometer-scale patterning in a suspended geometry therefore represents an important advance. Finite-element simulations predict that the same structure supports a flapping-type mechanical mode at ${\mathrm{\Omega }}_m/2\pi \approx 1.52\mathrm{GHz}$ and a backward-volume spin-wave mode at ${\omega }_{\mathrm{mag}}/2\pi =11.59\mathrm{GHz}$ under an in-plane bias field. Although we measured only the photonic resonance (intrinsic $Q\approx 2\times {10}^3$) in this study, the device lays the groundwork for future exploration of coupled photon-phonon-magnon dynamics once higher optical quality factors are achieved.

We demonstrate a small-footprint electromechanical phase shifter with a compact active length of $10\text{μ}\mathrm{m}$ fabricated on a suspended $\mathrm{Ga}\mathrm{As}$ membrane, offering versatile integration with quantum-dot single-photon sources. The phase shifter is based on a slot waveguide, whose slot width can be controlled by electrostatic forces, enabling a large effective refractive index change $\mathrm{\Delta }{n}_{\mathrm{eff}}>0.1$. We observe up to $3\pi$ phase modulation with 10.6 V applied bias, and a figure of merit $V_{\pi }L=5.7\times {10}^{-3}\mathrm{V}\mathrm{cm}$. Integration with a Mach-Zehnder interferometer further allows routing of single photons with an extinction ratio of up to 24 dB at cryogenic temperatures. This device enables advanced manipulation of quantum emitters and the realization of reconfigurable quantum photonic integrated circuits.

A compact multiangle x-ray FLASH radiotherapy (MAX-FLASH) system is presented. This system is designed to combine the FLASH effect triggered by ultrahigh dose rate (UHDR) radiation and precise radiotherapy delivered from multiple angles in the subsecond period. Taking practical needs of clinical practice into consideration, a key emphasis in design lies in the miniaturization of dimension. A compact high-current accelerator has been developed as an UHDR x-ray source, featuring a total length of 65 cm, which serves as the foundation for system compactness. A rapid rf distribution network is utilized to switch the supply of the accelerator array installed at various angles, thereby reducing the requirement for the number of high-power sources. As a result, the MAX-FLASH system is 3 m in diameter and 2.5 m in length, which can be installed in most hospital radiotherapy treatment rooms. Comprehensive breakthroughs have been achieved in the core components of the system, and currently, we are in the process of constructing the prototype machine. The MAX-FLASH system represents a solution for the clinical translation of FLASH radiotherapy with advantages of compactness, economy, clinical efficiency, and therapeutic efficacy.

Noisy voltage sources can be a limiting factor for both fundamental physics experiments and device applications in quantum information, mesoscopic circuits, magnetometry, and other fields. The best commercial dc voltage sources can be programmed to approximately six digits and have intrinsic noise in the microvolt range. On the other hand, the noise levels in metrological Josephson-junction-based voltage standards is subfemtovolt. Although such voltage standards can be considered “noiseless,” they are generally not designed for continuous tuning of the output voltage nor for supplying current to a load at cryogenic temperatures. We propose a Josephson-effect-based voltage source, as opposed to a voltage standard, operating in the $30-160\text{−}\text{μ}\mathrm{V}$ range, which can supply over 100 nA of current to loads at mK temperatures. We describe the operating principle, the sample design, and the calibration procedure to obtain continuous tunability. We show the current-voltage characteristics of the device, demonstrate how the voltage can be adjusted without dc control connections to room-temperature electronics, and showcase an experiment coupling the source to a mesoscopic load: a small Josephson junction. Finally, we characterize the performance of our source by measuring the voltage noise at the load, 50 pV rms, which is attributed to parasitic resistances in the cabling. This work establishes the use of the Josephson effect for voltage biasing of extremely sensitive quantum devices.

Partial tomography, which focuses on reconstructing reduced density matrices (RDMs), has emerged as a promising approach for characterizing complex quantum systems, particularly when full state tomography is impractical. Recently, overlapping tomography has been proposed as an efficient method for determining all $k$-qubit RDMs using logarithmic polynomial measurements, though it has not yet reached the ultimate limit. Here, we introduce a unified framework for optimal quantum overlapping tomography by mapping the problem to the clique cover model. This framework provides the most efficient and experimentally feasible measurement schemes to date, significantly reducing the measurement costs. Our approach is also applicable to determining RDMs with different topological structures. Moreover, we experimentally validate the feasibility of our schemes on a practical nuclear-spin processor using average measurements, and further apply our method to noisy data from superconducting quantum processors employing projection measurements. The results validate the utility of overlapping tomography, paving the way for the characterization of advanced quantum systems and learning about state properties in the future.