Antiferromagnets are emerging as promising competitors for advanced multistate memories because of their ignorable stray fields, ultrafast resonance dynamics and, especially, the rich variety of spin-order responses. Motivated by the determination of stripy intralayer antiferromagnetic order and the exceptional ambient stability of two-dimensional (2D) $\mathrm{Cr}\mathrm{O}\mathrm{Cl}$, we here propose a single-basement magnetic tunnel junctions (MTJs) composed of 2D intralayer antiferromagnets and predict the spin-resolved transport properties represented by $\mathrm{Ag}/\mathrm{Cr}\mathrm{O}\mathrm{Cl}/\mathrm{Ag}$ MTJs. Using ab initio quantum-transport simulations, four different resistance states are demonstrated in 2D $\mathrm{Cr}\mathrm{O}\mathrm{Cl}$-based MTJs with the magnetic order of $\mathrm{Cr}\mathrm{O}\mathrm{Cl}$ evolving from antiferromagnetism, followed by different metamagnetic arrangements and, finally, to ferromagnetism. The resistance area is remarkably low (only 0.02–0.2 Ω µ${\mathrm{m}}^2$) in the $\mathrm{Ag}/$monolayer $\mathrm{Cr}\mathrm{O}\mathrm{Cl}/\mathrm{Ag}$ MTJ, indicating high energy efficiency. The calculated tunneling magnetoresistance generated between any magnetic order and the ground antiferromagnetic order reaches an impressive range of 110% to 900% and 410% to 48 000% for the monolayer and bilayer $\mathrm{Cr}\mathrm{O}\mathrm{Cl}$-based MTJ, respectively, at zero bias and decreases as the bias increases. Our work also provides an insightful idea for using metamagnetic transitions and the related multiple conductance effect in 2D intralayer antiferromagnets, which paves a new way for future nonvolatile energy-efficient multiple-state memory devices.

We report on the three-dimensional (3D) structure determination of a twisted hexagonal boron nitride (h-BN) heterointerface from a single-view dataset using multislice ptychography. We identify the buried heterointerface between two twisted h-BN flakes with a lateral resolution of 0.57 Å and a depth resolution of 2.5 nm. The latter represents a significant improvement (∼2.7 times) over the aperture-limited depth resolution of incoherent imaging modes, such as annular-dark-field scanning transmission electron microscopy. This improvement is attributed to the diffraction signal extending beyond the aperture edge, with the depth resolution set by the curvature of the Ewald sphere. Future advancements in this approach could enhance the depth resolution to the subnanometer level and enable the identification of individual dopants, defects, and color centers in twisted heterointerfaces and other materials.

Quantum diamond microscopy using nitrogen-vacancy ($\mathrm{N}$-V) defects in diamond crystals has enabled the magnetic field imaging of a wide variety of nanoscale current profiles. Intimately linked with the imaging process is the problem of reconstructing the current density, which provides critical insight into the structure under study. This manifests as a nontrivial inverse problem of current reconstruction from noisy data, typically conducted via Fourier-based approaches. Learning algorithms and Bayesian methods have been proposed as novel alternatives for inference-based reconstructions. We study the applicability of Fourier-based and Bayesian methods for reconstructing two-dimensional current density maps from magnetic field images obtained from $\mathrm{N}$-V imaging. We discuss extensive numerical simulations to elucidate the performance of the reconstruction algorithms in various parameter regimes, and further validate our analysis by performing reconstructions on experimental data. Finally, we examine parameter regimes that favor specific reconstruction algorithms and provide an empirical approach for selecting regularization in Bayesian methods.

We show that we can harness two recent experimental developments to build a compact hardware emulator for the classical Heisenberg model in statistical physics. The first is the demonstration of spin-diffusion lengths in excess of microns in graphene even at room temperature. The second is the demonstration of low-barrier magnets (LBMs) whose magnetization can fluctuate rapidly even at subnanosecond rates. Using experimentally benchmarked circuit models, we show that an array of LBMs driven by an external current source has a steady-state distribution corresponding to a classical system with an energy function of the form $E=-(1/2)\sum _{i,j}{J}_{ij}({\hat{m}}_i\cdot {\hat{m}}_j$). This may seem surprising for a nonequilibrium system, but we show that it can be justified by a Lyapunov function corresponding to a system of coupled Landau–Lifshitz–Gilbert (LLG) equations. The Lyapunov function we construct describes LBMs interacting through the spin currents they inject into the spin-neutral substrate. We suggest ways to tune the coupling coefficient $J_{ij}$ so that it can be used as a hardware solver for optimization problems involving continuous variables represented by vector magnetizations, similar to the role of the Ising model in solving optimization problems with binary variables. Finally, we train a Heisenberg xor gate based on a network of four coupled stochastic LLG equations, illustrating the concept of probabilistic computing with a programmable Heisenberg model.

The rf radiation spectrum is central to wireless and radar systems among numerous high-frequency device technologies. Here, we demonstrate sensing of rf signals in the technologically relevant 2.4-GHz range utilizing vanadium dioxide (${\mathrm{VO}}_2$), a quantum material that has garnered significant interest for its insulator-to-metal transition. We find that the electrical resistance of both stoichiometric as well as off-stoichiometric vanadium oxide films can be modulated with rf wave exposures from a distance. The response of the materials to the rf waves can be enhanced by either increasing the power received by the sample or reducing channel separation. We report a significant $\sim 73\mathrm{\%}$ drop in resistance with a $5\text{μ}\mathrm{m}$ channel gap of the ${\mathrm{VO}}_2$ film at a characteristic response time of $16\text{μ}\mathrm{s}$. The peak sensitivity is proximal to the phase-transition-temperature boundary that can be engineered via doping and crystal chemistry. Dynamic sensing measurements highlight the films’ rapid response and broad-spectrum sensitivity. Engineering electronic phase boundaries in correlated electron systems could offer new capabilities in emerging communication technologies.

The time-dependent forcing and work per cycle required to drive sinusoidal spinning of a taut-line buzzer is analytically derived, both on and off resonance, from the nonlinear equation of motion. To test predictions, a model experimental system is constructed and characterized in terms of contraction versus twist angle and damped oscillations. The predicted force profile is then approximately implemented by hand. Nearly sinusoidal motion is observed and the energy injection per cycle needed to maintain steady-state oscillations is found to agree with theory. Additional force profiles are implemented, one in an attempt to maximize nonsinusoidal response and one to minimize operator effort. With the latter, an aluminum disk of radius 5 cm and height 0.95 cm was spun at a peak speed of over 11 000 revolutions per minute for 15 min. The corresponding hand-powered centrifuge system would require $1.5$ times more force, twice the power, and triple the time in order to run $90$ times more samples than the prior state of the art.

Equations of motion are developed for the oscillatory rotation of a disk suspended between twisted strings kept under tension by a hanging mass, to which additional forces may be applied. In the absence of forcing, damped harmonic oscillations are observed to decay with an exponential time envelope for two different string types. This is consistent with damping caused by string viscosity, rather than air turbulence, and may be quantified in terms of a quality factor. To test the proposed equation of motion and model for viscous damping within the string, we measure both the natural oscillation frequency and the quality factor for widely varied values of string length, string radius, disk moment of inertia, and hanging mass. The data are found to scale in good accord with predictions. A variation where rotational kinetic energy is converted back and forth to spring potential energy is also discussed.

Bound states in the continuum (BICs) defy the conventional wisdom that assumes a spectral separation between propagating waves, that carry energy away, and spatially localized waves corresponding to discrete frequencies. They can be described as resonance states with infinite lifetime, i.e., leaky modes with zero leakage. The advent of metamaterials and nanophotonics allowed the creation of BICs in a variety of systems. Mainly, BICs have been realized by destructive interference between outgoing resonant modes or by exploiting engineered global symmetries that enforce the decoupling of a symmetry-incompatible bound mode from the surrounding radiation modes. Here, we study BICs relying on a different mechanism, namely local symmetries that enforce a field concentration on a part of a complex system without implying any global symmetry. We experimentally implement these BICs using microwaves in a compact one-dimensional photonic network. We demonstrate that such BICs form an infinite ladder in $k$-space and emerge from the annihilation of two topological singularities, a zero and a pole, of the measured scattering matrix. This alternative for achieving BICs in complex wave systems may be useful for applications such as sensing, lasing, and enhancement of nonlinear interactions that require high-$Q$ modes.

Optics and photonics have recently captured interest as a platform to accelerate linear matrix processing, otherwise a bottleneck in traditional digital electronics. In this paper we propose an all-photonic computational accelerator wherein information is encoded in the amplitudes of frequency modes stored in a single ring resonator. Interaction among these modes is enabled by nonlinear optical processes. Both the matrix multiplication and elementwise activation functions on these modes (the artificial neurons) are performed through coherent processes, enabling the direct representation of negative and complex numbers without having to pass through digital electronics, a common limitation in today’s photonic architectures. This design also has a drastically lower hardware footprint compared with today’s electronic and optical accelerators, as the entirety of the matrix multiplication happens in a single multimode resonator on chip. Our architecture is unique in providing a completely unitary, reversible mode of computation, enabling on-chip analog Hamiltonian-echo backpropagation for gradient descent and other self-learning tasks. Moreover, the computational speed increases with the power of the pumps to arbitrarily high rates, as long as the circuitry can sustain the higher optical power. Lastly, the design presented here is a less demanding version of a future room-temperature quantum computational device. Therefore, while this architecture is already viable today, direct reinvestments in it would be enabling its evolution into quantum computational hardware.

Existing charge-coupled devices (CCDs) operate by detecting either the electrons or holes created in an ionization event. We propose an imager, the dual-sided CCD, which collects and measures both charge carriers on opposite sides of the device via a dual-buried channel architecture. We show that this dual detection strategy provides exceptional dark-count rejection and enhanced timing capabilities. These advancements have wide-ranging implications for dark-matter searches, near-infrared/optical spectroscopy, and time-domain x-ray astrophysics.

Near-term quantum algorithms require high-performance entangling gates in a scalable quantum processor, and Mølmer-Sørensen (MS) gates on trapped ions have achieved some of the highest entanglement fidelities to date. However, technical noise that induces small changes in the motional frequencies of the ion chain is a dominant error source in these gates that limits scalability. Here, we report an improved MS-gate design that mitigates the impact of motional-frequency changes by using a class of laser waveforms with three salient features: a spectrally compact Fourier transform, a constant frequency that balances contributions to the MS-gate rotation angle from all motional modes, and a simple parameterization. Together, these features suppress residual coherent displacement of the motional modes, generate robustness to rotation-angle error, and provide low technical complexity and computational overhead. As a proof of concept, we implement this gate design on a four-ion chain, achieving a $<1\mathrm{\%}$ fidelity reduction over a $\pm 10$ kHz range of frequency offset from the calibrated detuning. We also analyze the applicability of our gate design to longer ion chains through numerical simulations of our gate design on chains of up to 33 ions. This robustness to motional-frequency changes represents an order-of-magnitude improvement over the more complicated waveform-shaping techniques that currently dominate the robust-gate literature, and our simulations predict that a high level of robustness can be maintained in these systems for longer ion chains. In addition, the robustness and simplicity of our approach promises to accelerate the development of quantum architectures that invoke long algorithmic runtimes and frequent calibration.

Universal quantum computers require a large network of qubits robust against errors. Recent theoretical and experimental studies on donor nuclear spins in silicon, engineered on semiconductor platforms compatible with industrial fabrication, show their coherent behavior and potential for scalability. Here we present a hardware-efficient quantum protocol that corrects phase flips of a nuclear spin using explicit experimentally feasible operations. We introduce moment angular system encoding that uses the large Hilbert space provided by the nuclear spin of the donor to encode the information and employ the electron spin of the donor as an ancilla for error correction. Simulations using present-day experimental manipulation fidelities predict significant improvement in logical qubit fidelity over existing spin quantum-error-correction protocols. These results provide a realizable blueprint for a corrected spin-based qubit.

We demonstrate how to measure in situ for heading errors of optically pumped magnetometers (OPMs) in the challenging parameter regime of compact vapor cells with imperfect optical pumping and high buffer gas pressure. For this, we utilize microwave-driven Ramsey and Rabi frequency spectroscopy (FS) to independently characterize scalar heading errors in free-induction-decay (FID) signals. Both of these approaches suppress 5-nT inaccuracies in geomagnetic fields caused by nonlinear Zeeman shifts in FID measurements to below 0.6 nT. For Ramsey FS, we implement short periods of microwave interrogation within a $\pi /2\text{−}{t}_R\text{−}3\pi /2$ Ramsey interferometry sequence, effectively circumventing systematic errors from off-resonant driving. Conversely, Rabi FS leverages an atom-microwave Hamiltonian for accurate modeling of Rabi oscillation frequencies, achieving a measurement precision down to $80\mathrm{pT}/\sqrt{\text{Hz}}$ that is limited primarily by technical microwave noise. We show that the fundamental sensitivity of Rabi FS is $30\mathrm{pT}/\sqrt{\text{Hz}}$ with our vapor cell parameters through a Cramér-Rao lower-bound analysis. This work paves the way for future investigations into the accuracy of hyperfine structure magnetometry and contributes to the broader applicability of OPMs in fields ranging from navigation and geophysics to space exploration and unexploded ordinance detection, where heading error mitigation is essential.

Cold metals have recently gained attention as a promising platform for innovative devices, such as tunnel diodes with negative differential resistance (NDR) and field-effect transistors with subthreshold swings below the thermionic limit. Recently discovered two-dimensional (2D) ${\mathit{\text{MA}}}_2{Z}_4$ ($M$ = $\mathrm{Ti}$, $\mathrm{Zr}$, $\mathrm{Hf}$, $\mathrm{Nb}$, $\mathrm{Ta}$; $A$ = $\mathrm{Si}$, $\mathrm{Ge}$; $Z$ = $\mathrm{N}$, $\mathrm{P}$) compounds exhibit both cold-metallic and semiconducting behavior. In this work, we present a computational study of lateral heterojunction tunnel diodes based on 2D ${\mathrm{Nb}\mathrm{Si}}_2{\mathrm{N}}_4$ and ${\mathrm{Hf}\mathrm{Si}}_2{\mathrm{N}}_4$ compounds. Employing density-functional theory combined with a nonequilibrium Green-function method, we investigate the current-voltage ($I$-$V$) characteristics of lateral tunnel diodes with varying barrier thicknesses in both zigzag and armchair orientations. We find that tunnel diodes in the zigzag orientation exhibit significantly higher peak current densities, while those in the armchair orientation display larger peak-to-valley current ratios (PVCRs) compared to the zigzag orientation. Our findings suggest that ${\mathit{\text{MA}}}_2{Z}_4$ materials are promising candidates for realizing NDR tunnel diodes with ultrahigh PVCR values, which could have potential applications in memory, logic circuits, and other electronic devices.

We investigate a scheme to enhance the sensitivity in detecting weak variations in a parameter of an optoelectromechanical system by detecting the system response at radio frequencies. We consider a setup where either one or two mechanical modes mediate the interaction between an optical cavity and an rf resonator. This system can be operated in a regime of impedance matching where thermal fluctuations are redistributed among the system elements and, in particular, rf output noise can be reduced to the quantum vacuum noise level. We show that this effect can be used to boost the sensitivity in detecting parameter variations also in regimes of high thermal noise. We characterize the performance of this protocol in detecting variations in the capacitance of the rf resonator.

We explore properties of the radio-frequency atomic magnetometer, specifically its sensitivity to the polarization of an oscillating magnetic field. This aspect can be particularly relevant to configurations where the sensor monitors fields created by more than one source. The discussion, illustrated by theoretical and experimental studies, is done in the context of the signals produced by electrically conductive and magnetically permeable plates in magnetic induction tomography measurements. We show that different components of the secondary magnetic fields create the object response depending on the properties of the material, with the polarization of the radio-frequency field varying across the object’s surface. We argue that the ability of the sensor to simultaneously detect different field components enables the optimization of measurement strategies for different object compositions.

Topological pumping, as an efficient form of energy transfer protected by nontrivial topology, has been intensively studied in condensed matter physics and optics. Conventional topological pumping schemes are mainly related to edge states. In this work, we realize a topological pumping based on Fock lattices for acoustic waves. The acoustic Fock lattice supports defect states that are protected by nearly constant band gaps throughout the modulation process, which enables a high pumping efficiency. We experimentally verify the existence of the topological pumping in acoustic Fock lattices and, moreover, demonstrate an acoustic beam focusing as a potential application. Our results pave the way for more efficient adiabatic topological propagations, which can be extended to other systems.

Measurement-device-independent quantum key distribution (MDIQKD) can resist all attacks on the detection devices, but there are still some security issues related to the source side. One possible solution is to use the passive protocol to eliminate the side channels introduced by active modulators at the source. Recently, a fully passive QKD protocol was proposed that could simultaneously achieve passive encoding and passive decoy-state modulation using linear optics. In this work, we propose a fully passive MDIQKD scheme that can protect the system from both side channels of source modulators and attacks on the measurement devices, which can significantly improve the implementation security of the QKD systems. We provide a specific passive encoding strategy and a method for decoy-state analysis, followed by simulation results for the secure key rate in the asymptotic scenario. Our work offers a feasible way to improve the implementation security of QKD systems and serves as a reference for achieving passive QKD schemes using realistic devices.

The radio-frequency (rf) single-electron box is becoming an attractive charge sensor for semiconductor-based quantum computing devices due to its high sensitivity and small footprint, which facilitate the design of highly connected qubit architectures. However, an understanding of its ultimate sensitivity is missing due to the lack of a noise model. Here, we quantify the intrinsic noise of the single-electron box arising from stochastic cyclic electron tunneling between a quantum dot and a reservoir driven by a periodic gate voltage. We use both a master-equation formalism and Markov Monte Carlo simulations to calculate the gate-noise current and find that the noise mechanism can be represented as a cyclostationary process. We consider the implications of this cyclostationary noise on the ultimate sensitivity of single-electron box sensors for fast high-fidelity readout of spin qubits, in particular evaluating results for rf reflectometry implementations and the back action of the sensor on a qubit. Furthermore, we determine the conditions under which the intrinsic noise limit could be measured experimentally and techniques by which the noise can be suppressed to enhance qubit-readout fidelity.