An inertial sensor design is proposed in this paper to achieve high sensitivity and large dynamic range in the subhertz-frequency regime. High acceleration sensitivity is obtained by combining optical cavity readout systems with monolithically fabricated mechanical resonators. A high-sensitivity heterodyne interferometer simultaneously monitors the test mass with an extensive dynamic range for low-stiffness resonators. The bandwidth is tuned by optical feedback cooling to the test mass via radiation pressure interaction using an intensity-modulated laser. The transfer gain of the feedback system is analyzed to optimize system parameters towards the minimum cooling temperature that can be achieved. To practically implement the inertial sensor, we propose a dynamic cooling mechanism to improve cooling efficiency while operating at low optical power levels. The overall system layout presents an integrated design that is compact and lightweight.

Metagratings offer promising opportunities for highly efficient yet anomalous wave-diffraction control characterized by scattering matrices. In this work, those degenerated states of scattering matrices, arising from elaborately induced non-Hermiticity and encoding unique scattering behaviors, are of interest. In particular, we theoretically and experimentally investigate the exotic degeneracies, exceptional points (EPs) and diabolic points (DPs) in such scattering systems. We show the distinct response strength induced by EP and DP. The extremely asymmetrical reflection occurs on the EP state in a metagrating, which exhibits a stronger reaction to disturbances and offers opportunities for microvariable detection and external perturbation monitoring. As a contrast, a stable dual-angle absorber is proposed on the DP state, which is almost unaffected by external perturbations. Our work may pave the way for extreme wave manipulation via non-Hermitian metagratings on degenerated states.

Programmable unitary photonic devices are emerging as promising tools to implement unitary transformation for quantum information processing, machine learning, and optical communication. These devices typically use a rectangular mesh of Mach-Zehnder interferometers, which has a clear mathematical structure and can be configured deterministically. However, this mesh architecture is sensitive to fabrication errors, and the correction techniques are still under investigation. In contrast, the multiplane light-conversion (MPLC) architecture is more robust against fabrication errors, but a deterministic method for configuring the converter has not yet been developed due to its complex mathematical structure. In this work, we propose a fast iterative configuration method for MPLC, following the mathematical review of the matrix distance and proposal of an alternative norm. We show through numerical simulations that adding a few redundant layers significantly improves the convergence of the MPLC architecture, making it a practical and attractive option. We also consider the effects of finite resolution and cross-talk in phase shifters in our simulations. In addition, we propose a phase-insensitive distance suited for applications using only intensity detections. Our method demonstrates orders of magnitude better accuracy and a 20-fold speedup compared to previous approaches.

Bound states in the continuum (BICs) have been demonstrated as a powerful tool for trapping acoustic fields in an acoustic resonator. It has been widely recognized that symmetry-protected (SP) BICs result from symmetry incompatibility of some eigenmodes of a resonator with propagating modes of waveguides. The most typical example of SP BIC is the odd eigenmode of the resonator with the eigenfrequency embedded into the propagating band of even propagating eigenmodes of the waveguide. In this work, we consider a more sophisticated case of an acoustic cuboid resonator that is opened by the attachment of two cylindrical waveguides. We show that BICs can be sustained in an open acoustic resonator with reduced symmetry. For symmetrical positions of waveguides, the eigenmodes of the cuboid can also be classified as SP BICs and show different stability against the shifts of waveguides from the positions of symmetry of the cuboid. We fabricate a series of coupled waveguide resonators and experimentally verify the existence of these BICs by identifying the vanished linewidth of Fano resonance in transmission spectra. Besides, we also show that evanescent modes of waveguides play a role in the formation of BICs in a nonaxisymmetric waveguide-resonator system by tuning the angle θ between two waveguides. Consequently, the eigenmodes remain SP BICs for θ = 0° and θ = 180° but convert into accidental BICs at θ ≈ 85°or θ ≈ 275°. Such accidental BICs are also experimentally verified. Our results enrich the understanding of SP BICs and accidental BICs, and provide alternative methods of routing acoustic waves and designing acoustic devices requiring fine spectrum features, such as filters and sensors.

Combining the capabilities of gate-defined quantum transport devices in $\mathrm{Ga}\mathrm{As}$-based heterostructures and of optically addressed self-assembled quantum dots could open up broad perspectives in quantum technologies. For example, interfacing stationary solid-state qubits with photonic quantum states would open up a pathway towards the realization of a quantum network with extended quantum processing capacity in each node. While gated devices allow very flexible confinement of electrons or holes, the confinement of excitons without some element of self-assembly is much harder. To address this limitation, we introduce a technique to realize exciton traps in quantum wells via local electric fields by thinning a heterostructure down to a 220-nm-thick membrane. We show that mobilities over $1\times {10}^6{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ can be retained and that quantum point contacts and Coulomb oscillations can be observed on this structure, which implies that the thinning does not compromise the heterostructure quality. Furthermore, the local lowering of the exciton energy via the quantum-confined Stark effect is confirmed, thus forming exciton traps. These results lay the technological foundations for devices like single-photon sources, spin-photon interfaces and eventually quantum network nodes in $\mathrm{Ga}\mathrm{As}$ quantum wells, realized entirely with a top-down fabrication process.

High-fidelity entangling gates are essential for quantum computation. Currently, most approaches to designing such gates are based either on simple, analytical pulse waveforms or on ones obtained from numerical optimization techniques. In both cases, it is typically not possible to obtain a global understanding of the space of waveforms that generate a target gate operation, which can make it challenging to identify globally time-optimal waveforms that respect amplitude and bandwidth constraints. Here, we show that in the case of weakly coupled qubits, it is possible to find all pulses that implement a target entangling gate in near-minimal time. We do this by mapping quantum evolution onto geometric space curves. We derive the minimal conditions these curves must satisfy in order to guarantee a gate with a desired entangling power is implemented. Pulse waveforms are extracted from the curvatures of these curves. We illustrate our method by designing fast, cnot-equivalent entangling gates for silicon quantum dot spin qubits with fidelities exceeding 99 %. We show that fidelities can be further improved while maintaining low bandwidth requirements by using geometrically derived pulses as initial guesses in numerical optimization routines.

Self-compensated comagnetometers, employing overlapping samples of spin-polarized alkali and noble gases (for example K-${}^3\mathrm{He}$) are promising sensors for exotic beyond-the-standard-model fields and high-precision metrology such as rotation sensing. When the comagnetometer operates in the so-called self-compensated regime, the effective field, originating from contact interactions between the alkali valence electrons and the noble-gas nuclei, is compensated with an applied magnetic field. When the comagnetometer begins operation in a given magnetic field, spin-exchange optical pumping establishes an equilibrium between the alkali electron-spin polarization and the nuclear-spin polarization. Subsequently, when the magnetic field is tuned to the compensation point, the spin polarization is brought out of the equilibrium conditions. This causes a practical issue for long measurement times. We report on a method for closed-loop control of the compensation field. This method allows optimization of the operating parameters, especially magnetic field gradients, in spite of the inherently slow (hours to days) dynamics of the system. With the optimization, higher stable nuclear polarization, longer relaxation times, and stronger electron-nuclear coupling are achieved which is useful for nuclear-spin-based quantum memory, spin amplifiers, and gyroscopes. The optimized sensor demonstrates a sensitivity comparable to the best previous comagnetometer but with four times lower noble-gas density. This paves the way for applications in both fundamental and applied science.

High-pressure experiments are crucial in modern interdisciplinary research fields such as engineering quantum materials, yet local probing techniques remain restricted due to the tight confinement of the pressure chamber in certain pressure devices. Recently, the negatively charged nitrogen-vacancy (N-$V$) center has emerged as a robust and versatile quantum sensor in pressurized environments. There are two popular ways to implement N-$V$ sensing in a diamond anvil cell (DAC), which is a conventional workhorse in the high-pressure community: create implanted N-$V$ centers (IN-$V\mathrm{s}$) at the diamond anvil tip or immerse N-$V$-enriched nanodiamonds (NDs) in the pressure medium. Nonetheless, there are limited studies on comparing the local stress environments experienced by these sensor types as well as their performances as pressure gauges. In this work, by probing the N-$V$ energy levels with the optically detected magnetic resonance (ODMR) method, we experimentally reveal a dramatic difference in the partially reconstructed stress tensors of IN-$V\mathrm{s}$ and NDs incorporated in the same DAC. Our measurement results agree with computational simulations, concluding that IN-$V\mathrm{s}$ perceive a more nonhydrostatic environment dominated by a uniaxial stress along the DAC axis. This provides insights on the suitable choice of N-$V$ sensors for specific purposes and the stress distribution in a DAC. We further propose some possible methods, such as using NDs and diamond nanopillars, to extend the maximum working pressure of quantum sensing based on ODMR spectroscopy, since the maximum working pressure could be restricted by nonhydrostaticity of the pressure environment. Moreover, we explore more sensing applications of the N-$V$ center by studying how pressure modifies different aspects of the N-$V$ system. We perform a PL study using both IN-$V\mathrm{s}$ and NDs to determine the pressure dependence of the zero-phonon line, which helps developing an all-optical pressure sensing protocol with the N-$V$ center. We also characterize the spin-lattice relaxation ($T_1$) time of IN-$V\mathrm{s}$ under pressure to lay a foundation for robust pulsed measurements with N-$V$ centers in pressurized environments.

We present our experimental and theoretical framework, which combines a broadband superluminescent diode with fast learning algorithms to provide speed and accuracy improvements for the optimization of on-dimensional optical dipole potentials, here generated with a digital micromirror device. To characterize the setup and potential speckle patterns arising from coherence, we compare the superluminescent diode to a single-mode laser by investigating interference properties. We employ machine-learning tools to train a physics-inspired model acting as a digital twin of the optical system predicting the behavior of the optical apparatus including all its imperfections. Implementing an iterative algorithm based on iterative learning control we optimize optical potentials an order of magnitude faster than heuristic optimization methods. We compare iterative model-based “offline” optimization and experimental feedback-based “online” optimization. Our methods provide a route to fast optimization of optical potentials, which is relevant for the dynamical manipulation of ultracold gases.

This study demonstrates imaging of the magnetic response to external pressure using a hybrid structure of magnetostrictive (MS) layers and nitrogen-vacancy ($\mathrm{N}$-V) centers in diamonds. The MS layer facilitates pressure-to-magnetic field conversion, detected by $\mathrm{N}$-V centers. We use hybrid materials comprising in-plane magnetized ${\mathrm{Sm}\mathrm{Fe}}_2$ as a MS layer and diamond with $\mathrm{N}$-V centers perfectly aligned in the vertical [111] orientation to effectively detect the pressure-to-magnetic field conversion. The pressure coefficient, defined as the change in the resonant frequency of the optically detected magnetic resonance in response to pressure, is imaged by widefield imaging, while varying the pressure applied to the MS–$\mathrm{N}$-V structure. We observe a pressure-dependent change in the resonant frequency. Through widefield imaging, the pressure coefficients are found to be correlated with the multidomain structure of the MS layer, which must be considered in widefield pressure imaging. The highest pressure coefficient is 8.2 kHz ${\mathrm{kPa}}^{-1}$ in a domain—550 times greater than that achieved by a single structure of the $\mathrm{N}$-V center. We propose and discuss the approach of using MS disk arrays consisting of a single domain to improve the sensitivity and controllability and to enable accurate calibration of pressure imaging.

The accuracy of single-electron currents produced in hybrid turnstiles at high operation frequencies is, among other errors, limited by electrons tunneling in the wrong direction. Increasing the barrier transparency between the island and the leads, together with the source-drain bias, helps to suppress these events in a larger frequency range, although they lead to some additional errors. We experimentally demonstrate a driving scheme that suppresses tunneling in the wrong direction, thus extending the range of frequencies for generating accurate single-electron currents. The main feature of this approach is an additional ac signal applied to the bias with frequency twice that applied to the gate electrode. This allows additional modulation of the island chemical potential. By using this approach under certain parameters, we improve the single-electron current accuracy by one order of magnitude. Finally, we show through experimentally contrasted calculations that our method can improve accuracy even in devices for which the usual gate driving gives errors of the order of ${10}^{-3}$ at high frequencies and can bring them under $5\times {10}^{-4}$.

Nanomagnets are the building blocks of many existing and emergent spintronic technologies. The magnetization dynamics of nanomagnets is often dominated by nonlinear processes, which have been recently shown to have many surprising features and far-reaching implications for applications. Here we develop a theoretical framework uncovering the selection rules for multimagnon processes and discuss their underlying mechanisms. For its technological relevance, we focus on the degenerate three-magnon process in thin elliptical nanodisks to illustrate our findings. We parameterize the selection rules through a set of magnon interaction coefficients which we calculate using micromagnetic simulations. We postulate the selection rules and investigate how they are altered by perturbations that break the symmetry of static magnetization configuration and spatial spin-wave profiles and that can be realized by applying off-symmetry-axis or nonuniform magnetic fields. Our work provides the phenomenological understanding of the mechanics of magnon interaction as well as the formalism for determining the interaction coefficients from simulations and experimental data. Our results serve as a guide to analyze the magnon processes inherently present in spin-torque devices in order to boost their performance or to engineer a specific nonlinear response in a nanomagnet used in a neuromorphic or quantum magnonic application.

Point defects in solid-state materials are promising systems in the fields of quantum information, communications, and sensing. Particular to sensing, defects with coherent spin states are used for high-sensitivity room-temperature sensing under ambient conditions. Applications involving magnetic sensing with point defects are mostly dominated by the nitrogen-vacancy center in diamond, which possesses an attractive combination of spin coherence at room temperature as well as spin initialization and read-out. However, spin defects in other materials are explored as alternatives, especially in industrially mature materials, such as silicon carbide. Here, we report on the improved sensitivity of a magnetic sensor system utilizing an ensemble of silicon vacancies in silicon carbide due to isotopic purification of the host crystal. A maximum sensitivity of 4.0 nT/√Hz is reported, representing an order of magnitude improvement from the best previously reported sensitivity. Additional sensing modalities, such as angle-resolved magnetic imaging and highly broadband ac field sensing, are also demonstrated.

Our ability to efficiently detect and generate far-infrared (i.e., terahertz) radiation is vital in areas spanning from biomedical imaging to interstellar spectroscopy. Despite decades of intense research, bridging the terahertz gap between electronics and optics remains a major challenge due to the lack of robust materials that can efficiently operate in this frequency range, and two-dimensional (2D) type-II heterostructures may be ideal candidates to fill this gap. Herein, using highly accurate many-body perturbation theory within the GW plus Bethe-Salpeter equation approach, we predict that a type-II heterostructure consisting of an electron-rich ${\mathrm{C}}_3\mathrm{N}$ and an electron deficient ${\mathrm{C}}_3\mathrm{B}$ monolayers can give rise to extraordinary optical activities in the mid- to far-infrared range. ${\mathrm{C}}_3\mathrm{N}$ and ${\mathrm{C}}_3\mathrm{B}$ are two graphene-derived 2D materials that have attracted increasing research attention. Although both ${\mathrm{C}}_3\mathrm{N}$ and ${\mathrm{C}}_3\mathrm{B}$ monolayers are moderate-gap 2D materials, and they couple only through the rather weak van der Waals interactions, the bilayer heterostructure surprisingly supports extremely bright, low-energy interlayer excitons with large binding energies of 0.2–0.4 eV, offering an ideal material with interlayer excitonic states for mid- to far-infrared applications at room temperature. We also investigate in detail the properties and formation mechanism of the inter- and intralayer excitons.

Valence-change memory (VCM) cells are promising candidates for future nonvolatile memory devices. A special setup of VCM devices consists of bilayer cells where two thin oxide layers are placed in between two metal electrodes. One oxide layer serves as a tunnel barrier, whereas the second oxide layer is a highly doped conductive semiconductor. Experiments show that an exchange of oxygen between the two layers changes the resistance of the cell. However, the exchange process and how it influences the resistance is not well understood yet. With a drift-diffusion model for electrons and oxygen vacancies, we investigate the movement and exchange of oxygen vacancies and their influence on the band structure as well as on the shape of the tunnel barrier. The simulation results show that a high oxygen-vacancy concentration lowers the height of the tunnel barrier; thus it increases the conductivity of the bilayer cell. The effect of the band lowering is stronger in materials with low permittivity. Hence, two different resistance states evolve if there is an exchange of oxygen between the two oxide layers with different permittivities. Thereby, the switching polarity depends on the relation of the permittivities of the two oxide layers. Furthermore, it is revealed that resistance switching can be induced by the movement of vacancies only inside the conductive oxide, without any oxygen exchange between the layers.

In this work, stable ${\mathrm{Cs}}_2\mathrm{Ag}M{\mathrm{Cl}}_6(M$ = $\mathrm{Bi},\mathrm{In},\mathrm{Sb}$) double perovskite materials are successfully employed as photoanodes for solar water oxidation. These materials show an extraordinary oxidative stability, where ${\mathrm{Cs}}_2{\mathrm{Ag}\mathrm{In}\mathrm{Cl}}_6$ and ${\mathrm{Cs}}_2{\mathrm{Ag}\mathrm{Bi}\mathrm{Cl}}_6$ are stable between 0 and 1.2 V (vs $\mathrm{Ag}\text{−}\mathrm{Ag}\mathrm{Cl}$) and ${\mathrm{Cs}}_2{\mathrm{Ag}\mathrm{Sb}\mathrm{Cl}}_6$ between 0 and 0.75 V (vs $\mathrm{Ag}\text{−}\mathrm{Ag}\mathrm{Cl}$) over 100 cycles of electrochemical cycling. This enables us to employ these materials for photoelectrochemical (PEC) water oxidation in ${\mathrm{CH}}_3\mathrm{CN}$ and ${\mathrm{H}}_2\mathrm{O}$, with and without ${\mathrm{Ir}\mathrm{O}}_x$ cocatalyst. All three materials show PEC activity, with ${\mathrm{Cs}}_2{\mathrm{Ag}\mathrm{In}\mathrm{Cl}}_6$ showing the highest performance. At 1.23 V (vs a reversible hydrogen electrode), a photocurrent density of 0.5 mA ${\mathrm{cm}}^{-2}$ is observed, and with an applied overpotential of 600 mV, the photocurrent increases to about 0.75 mA ${\mathrm{cm}}^{-2}$. From the band gap of ${\mathrm{Cs}}_2{\mathrm{Ag}\mathrm{In}\mathrm{Cl}}_6$, the estimated theoretical maximum current is around 0.82 mA ${\mathrm{cm}}^{-2}$. So, ${\mathrm{Cs}}_2{\mathrm{Ag}\mathrm{In}\mathrm{Cl}}_6$-coated ${\mathrm{Ir}\mathrm{O}}_x$ gives nearly 60% and about 90% of the theoretical maximum photocurrent at zero bias and at an applied overpotential, respectively. The device is shown to be stable and reproducibility is confirmed.

The electroviscous effect deals with the change in the viscosity of fluids due to an external electric field. Here, we report experimental studies on the electroviscous effects in a ferroelectric nematic liquid crystal. It is synthesized by accomplishing an optimized synthetic route, which provides higher yield than the conventional one. We measure electric-field-dependent viscosity under a steady shear at different temperatures. In the low-field range, the increase in viscosity [$\mathrm{\Delta }\eta =\eta (E)-{\eta }_0$] is proportional to $E^2$ and the corresponding viscoelectric coefficient [$f\approx {10}^{-9}{\mathrm{m}}^2/{\mathrm{V}}^2$] of the ferroelectric nematic is 2 orders of magnitude larger than the apolar nematic liquid crystals and the largest ever measured for a fluid. The apparent viscosity measured under a high electric field shows a power-law divergence $\eta \sim (T-T_c{)}^{-0.7\pm 0.05}$, followed by nearly an order of magnitude drop below the $N$-$N_F$ phase transition. Experimental results within the dynamical scaling approximation demonstrate rapid growth of polar domains under a strong electric field as the $N$-$N_F$ phase transition is approached. The gigantic electroviscous effects demonstrated here are useful for emerging applications and understanding striking electrohydromechanical effects in ferroelectric nematic liquid crystals.