The design and implementation of accelerators capable of providing high-quality bunches require precise and efficient online modeling tools. Current comprehensive beam dynamics studies are prohibitively costly and challenging to use for interactive system design. A precise high-speed method for beam dynamics analysis in accelerator components is presented and compared to the results of the conventional particle-in-cell codes. Using powerful mathematical techniques, the suggested method evaluates the temporal evolution of a bunch shape in six-dimensional (6D) phase space along the accelerators. The moment equations that govern the evolution of the bunch envelope in 6D phase space are introduced. The three-dimensional space-charge, external, and emittance forces are calculated to be fully analytically insensitive to different beam envelopes. Substituting the obtained forces into the beam-envelope equations establishes a set of six modified equations describing the beam dynamics using simple algebraic expressions. The whole solution considers the energy spread inherent to an electron beam. The model accuracy is demonstrated by studying beam transport through various components of an accelerator. Applying this analytical approach not only forms a style of physical thinking by indicating the factors that affect the behavior of the charged particle bunches but also has an ultrafast computational speed that is at least 3 orders of magnitude faster than that of particle tracking codes for designing today’s linear accelerators. Finally, the model’s feasibility is benchmarked for successfully designing a photoinjector for the advanced proton driven plasma wakefield acceleration experiment.

We develop a code to extract the signal-to-noise ratio (SNR) arising from the magnetic film in a recording medium. The approach allows us to separate the remanence and transition contributions from the global spatial noise. The results are in excellent agreement with the analysis performed on the same data sets by means of Seagate proprietary software based on ensemble wave-form analysis. We then apply this analytical approach to the results of heat-assisted magnetic recording (HAMR) dynamics simulations by means of the open-source multi-time-scale micromagnetic code mars and compare these with experimental spin-stand measurements of analogous systems. The proposed model could be used as the standard tool to understand the underlying physics of the noise components affecting HAMR operations and how to decrease the noise arising from the medium to improve the writing performance of HAMR.

We investigate targeted and localized material modifications produced by electropulsing of $\mathrm{Al}$-capped $\mathrm{Nb}$ microbridges with a multiterminal configuration. The affected regions of the $\mathrm{Nb}$/$\mathrm{Al}$ bilayer terminals are revealed by an in-lens secondary-electron detector in a scanning electron microscope and by Kelvin-probe force microscopy, both suggesting a decrease in the work function in the modified areas. In contrast, the affected areas are neither apparent through an Everhart-Thornley secondary-electron detector nor through atomic force microscopy, which indicates little morphological change in the microstructure. In addition, we demonstrate that the extent of the electroannealed regions is strongly influenced by the terminal geometry. These results are captured by complementary finite-element modeling, which permits us to estimate that a threshold temperature of $435\pm 35$ K is needed to induce material modifications. These findings provide further insights into the subtle modifications produced by gentle electroannealing of $\mathrm{Nb}$/$\mathrm{Al}$ microstructures and represent a step forward towards mastering this emerging nanofabrication technique.

A reconfigurable phononic crystal (PnC) is proposed where elastic properties can be modulated by rotation of asymmetric solid scatterers immersed in water. The scatterers are metallic rods with a cross section of ${120}^{\circ }$ circular sector. Orientation of each rod is independently controlled by an external electric motor that allows continuous variation of the local scattering parameters and dispersion of sound in the entire crystal. Due to asymmetry of the scatterers, the crystal band structure possesses highly anisotropic band gaps. Synchronous rotation of all the scatterers by a definite angle changes the regime of reflection to the regime of transmission and vice versa. The same mechanically tunable structure functions as a gradient index medium by incremental, angular reorientation of rods along both row and column, and, subsequently, can serve as a tunable acoustic lens, an acoustic beam splitter, and finally an acoustic beam steerer.

Scalable technologies for the production of biocompatible complex microfibers of controllable size and composition at competitively high throughput are urgently needed in order to meet the growing demand for such microstructures in pharmaceutical and biomedical applications. Here, we introduce an in-air microfluidic strategy with throughput greater than $1400$ ml $\mathrm{h}{}^{-}1$ (corresponding to more than $17000$ m of fiber per hour). The microfibers of uniform diameter have regular inclusions, which can potentially be used for encapsulating cells into a protecting and nutrient environment, or for finely tuning the release of various actives at individualized doses. With the help of a recently developed prototype, we test seven different liquid combinations and obtain seven types of fibers, whose average “dry” diameter ranges between $94\text{μ}\mathrm{m}$ and $170\text{μ}\mathrm{m}$. The principle of our approach is to solidify the complex liquid structures generated by the controlled collisions of a drop stream with a continuous liquid jet, in air, via ionic cross-linking. After the stream of water-based droplets, which constitute the inclusions, collides in-air with the alginate-based jet (jet 1), the generated drops-in-jet compound is brought into contact with a second jet (jet 2) containing divalent cations (${\mathrm{Sr}}^{2+}$ or ${\mathrm{Ca}}^{2+}$) to initiate the solidification. Finally, the fibers are collected “on the fly” via a horizontal spinning plate, allow to dry (i.e., to fully equilibrate under controlled conditions), and characterized by their elongation at break and Young’s modulus.

We report a theoretical study of the phase diffusion in a gain-switched single-mode semiconductor laser. We use stochastic rate equations for the electric field to analyze the phase statistics of the gain-switched laser. Their use avoids the instabilities obtained with rate equations for the photon number and optical phase when the photon number is small. However, we show that a new problem appears when the field equations are integrated: the variance of the optical phase becomes divergent. This divergence cannot be observed with the numerical integration of the commonly used equations for the photon number and optical phase because of the previous instabilities. The divergence of the phase variance means that this quantity does not reach a fixed value as the integration time step is decreased. We find that the phase variance increases as the integration time step decreases, with no sign of saturation behavior even for tiny steps. We explain the divergence by making the analogy of our problem with two-dimensional Brownian motion. The fact that the divergence appears is not surprising because in 1940 Paul Lèvy demonstrated that the variance of the polar angle in two-dimensional Brownian motion is a divergent quantity. Our results show that stochastic rate equations for the photon number and phase are not appropriate for describing the phase statistics when the photon number is small. Simulation of the stochastic rate equations for the electric field are consistent with Lèvy’s results but gives unphysical results since an infinite value is obtained for a quantity that can be measured.

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 $5\times {10}^{-4}$.