Spin-wave (SW) amplification utilizing the spin-transfer torques induced by spin-polarized current is numerically investigated. The current drives the magnetic domain wall and creates an effective moving media for SWs. When the polarized current density is sufficiently large the speed of magnetic domain-wall gets faster than the velocity of SW. Combined with a spatially inhomogeneous current density profile prepared with a bottleneck magnetic structure, a peculiar SW cavity is formed. As the cavity length is changed, the transmission and reflection signal of SWs show a resonant amplification feature. We present numerical results for transient amplification of SW pulses in the time domain. Dispersion relation, arising from magnetostatic interactions, which describes the SW propagation in the low wave-number region is included. The SW amplification effects due to competition of damping and nonadiabatic spin-transfer effect in the Zhang-Li spin-transfer torque is discussed. These results benefit both the theoretical understanding and experimental realization of SW lasers utilizing the analogue black-hole horizons.

One of the most promising applications of quantum networks is entanglement-assisted sensing. The field of quantum metrology exploits quantum correlations to improve the precision bound for applications such as precision timekeeping, field sensing, and biological imaging. When measuring multiple spatially distributed parameters, current literature focuses on quantum entanglement in the discrete-variable case and quantum squeezing in the continuous-variable case, distributed amongst all of the sensors in a given network. However, it can be difficult to ensure that all sensors preshare entanglement of sufficiently high fidelity. This work probes the space between fully entangled and fully classical sensing networks by modeling a star network with probabilistic entanglement generation that is attempting to estimate the average of local parameters. The quantum Fisher information is used to determine which protocols best utilize entanglement as a resource for different network conditions. It is shown that without entanglement distillation there is a threshold fidelity below which classical sensing is preferable. For a network with a given number of sensors and links characterized by a certain initial fidelity and probability of success, this work outlines when and how to use entanglement, when to store it, and when it needs to be distilled.

Superconducting qubits are one of the most mature platforms for achieving practical quantum computers, but significant performance improvements are still needed. To improve the engineering of these systems, three-dimensional (3D) full-wave computational electromagnetics analyses are increasingly being turned to. Unfortunately, existing analysis approaches often rely on full-wave simulations using eigenmode solvers that are typically cumbersome, not robust, and computationally prohibitive if devices with more than a few qubits are to be analyzed. To improve the characterization of superconducting circuits while circumventing these drawbacks, this work begins the development of an alternative modeling framework that we illustrate in the context of evaluating the qubit-qubit exchange coupling rate between transmon qubits. This quantity is a key design parameter that determines the entanglement rate for fast multiqubit gate performance and also affects decoherence sources like qubit crosstalk. Our modeling framework uses a field-based formalism in the context of macroscopic quantum electrodynamics, which we use to show that the qubit-qubit exchange coupling rate can be related to the electromagnetic dyadic Green’s function linking the qubits together. We further show how the needed quantity involving the dyadic Green’s function can be related to the impedance response of the system that can be easily and efficiently computed with classical computational electromagnetics tools. We demonstrate the validity and efficacy of this approach by simulating four practical multiqubit superconducting circuits and evaluating their qubit-qubit exchange coupling rates. We validate our results against a 3D numerical diagonalization method and against experimental data where available. We also demonstrate the impact of the qubit-qubit exchange coupling rate on qubit crosstalk by simulating a multicoupler device and identifying operating points where the qubit crosstalk becomes zero.

Nonvolatile ferroelectric memory based on hafnium oxide thin films combines low power consumption, high speed, and record endurance. However, the commercialization of such memory is constrained due to the limited information retention time caused by the change in coercive voltage over time. Many efforts are under way to slow down this effect by material and interface engineering, and their results are being evaluated by comparison of the switchable polarization and coercive voltages at the time of writing information and reading it after a certain storage time. However, the real-life scenarios of a memory cell are much-more complex than a simple write-store-read cycle. Coercive voltages evolve over the whole lifetime of the memory cell, and read failure can occur in subsequent stages. In this work, we propose a predictive model for calculating the evolution of coercive voltages under any scenario of memory-cell lifetime. In particular, it takes into account multiple write-store-read-rewrite (both to the same state and to the opposite state) cycles, temperature fluctuations to which the memory chip is inevitably subjected, the influence of the selected operating frequency, and the procedure for recovery of the memory cell after its aging by a series of bipolar pulses, which is considered by memory designers as one of the options for increasing the retention time. The model is based on the polarization-switching dynamics described by the Landau-Khalatnikov equation and the phenomenon of charge injection into interfacial traps under the action of both the applied field and the spontaneous polarization field. The injected charge induces an internal built-in electric field in the ferroelectric layer, and leads to a change in the total field and an apparent change in the coercive voltage. The calculation results are in agreement with the results of experiments simulating different real-life scenarios of a memory cell based on a 10-nm-thick ${\mathrm{Hf}}_{0.5}{\mathrm{Zr}}_{0.5}{\mathrm{O}}_2$ film. The proposed model could serve as an intelligent tool for both material engineering and memory-chip design, which entails a new step in the development of ferroelectric memory.

Exceptional points (EPs) are non-Hermitian degeneracies, where eigenvalues coalesce and their corresponding eigenvectors collapse. Consequently, non-Hermitian systems biased at EPs can react much more drastically to external perturbations. Here, we establish and experimentally demonstrate a scheme using EPs to enhance the sensitivity of piezoelectric resonance sensors. By connecting an external inductance-capacitor circuit with a nonlinear negative resistance to the piezoelectric resonance sensors, they are transformed into a parity-time symmetric system featuring the linear loss and nonlinear gain, which supports a second-order EP. The nonlinear gain enables the system to achieve a steady-state oscillation, and the resonance-frequency shift owing to the adsorbed analytes follows cubic-root behavior rather than square-root behavior. As a proof of concept, a piezoelectric sensor covered with graphene oxide for humidity sensing has been fabricated and characterized. At relatively low humidity levels, a sensitivity enhancement of more than an order of magnitude is observed in comparison to that of a standard arrangement. Our results pave the way for piezoelectric sensors with unprecedented sensitivity.

The use of photon active interrogation to detect special nuclear material has held significant theoretical promise, as the interrogating source particles, photons, are fundamentally different from one of the main signatures of special nuclear material: neutrons produced in nuclear fission. However, neutrons produced by photonuclear reactions in the accelerator target, collimator, and environment can obscure the fission neutron signal. These ($\gamma ,n$) neutrons could be discriminated from fission neutrons by their energy spectrum, but common detectors sensitive to the neutron spectrum, like organic scintillators, are typically hampered by the intense photon background characteristic of photon-based active interrogation. In contrast, high-pressure ${}^4\mathrm{He}$-based scintillation detectors are well -suited to photon active interrogation, as they are similarly sensitive to fast neutrons and can measure their spectrum, but show little response to gamma rays. In this work, a photon active interrogation system utilizing a ${}^4\mathrm{He}$ scintillation detector and a 9 MeV linac-bremsstrahlung x-ray source was experimentally evaluated. The detector was shown to be capable of operating in intense gamma-ray environments and detecting photofission neutrons from ${}^{238}\mathrm{U}$ when interrogated by this x-ray source. The photofission neutrons show clear spectral separation from ($\gamma$,n) neutrons produced in lead, a common shielding material.

We report low-loss multiscan waveguides fabricated in fused silica using femtosecond-laser-writing technology. The multiscan principle allows the writing regime to be tailored to excel at key features of any integrated photonic platform: coupling losses and propagation losses. We optimized the writing parameters for different sizes of square-shaped waveguides and reached the mode overlap value with a standard single-mode optical fiber of above 98.8% and demonstrated very low coupling losses of 0.2 dB/facet on average. Propagation losses in the fabricated waveguides amounted to 0.07 dB/cm. We applied the developed recipe to the fabrication of a fiber-coupled 25-channel interferometer with total insertion losses below 1 dB. The findings of this work are of interest for broad range of applications and in particular for optical information processing and quantum photonics.

Two grapes irradiated inside a microwave (MW) oven typically produce a series of sparks and can ignite a violent plasma. The underlying cause of the plasma has been attributed to the formation of morphological-dependent resonances (MDRs) in the aqueous dielectric dimers that lead to the generation of a strong evanescent MW hotspot between them. Previous experiments have focused on the electric field component of the field as the driving force behind the plasma ignition. Here we couple an ensemble of nitrogen-vacancy (N-$V$) spins in nanodiamonds (NDs) to the magnetic field component of the dimer MW field. We demonstrate the efficient coupling of the N-$V$ spins to the MW magnetic field hotspot formed between the grape dimers using optically detected magnetic resonance (ODMR). The ODMR measurements are performed by coupling N-$V$ spins in NDs to the evanescent MW fields of a copper wire. When placing a pair of grapes around the NDs and matching the ND position with the expected magnetic field hotspot, we see an enhancement in the ODMR contrast by more than a factor of 2 compared to the measurements without grapes. Using finite-element modeling, we attribute our experimental observation of the field enhancement to the MW hotspot formation between the grape dimers. The present study not only validates previous work on understanding grape-dimer resonator geometries, but it also opens up another avenue for exploring alternative MW resonator designs for quantum technologies.

Selective control of individual spin qubits is needed for scalable quantum computing based on spin states. Achieving high-fidelity in both single-qubit and two-qubit gates, essential components of universal quantum computers, necessitates highly localized control fields. These fields must be capable of addressing specific spin qubits while minimizing gate errors and crosstalk in adjacent qubits. Overcoming the challenge of generating a localized radio-frequency magnetic field, in the absence of elementary magnetic monopoles, we introduce a technique that combines divergent and convergent nanoscale magnetic skyrmions. This approach produces a precise control field that manipulates spin qubits with high fidelity. We propose the use of 2D skyrmions, which are much easier to fabricate than 3D hedgehog structures. The latter are considered emergent magnetic monopoles, but their fabrication is challenging. In contrast, 2D skyrmions can be created with use of standard semiconductor-foundry processes and offer effective magnetic field confinement in two dimensions. Our comparative analysis of the density-matrix evolution and gate fidelities in scenarios involving proximal skyrmions and nanomagnets indicates potential gate fidelities surpassing 99.95% for $\pi /2$ gates and 99.90% for $\pi$ gates. The average field induced at the nearest neighboring qubit by skyrmions is 0.0332 mT along the $x$ axis (200 nm), while it is 0.507 mT for the nanomagnet case. Notably, the skyrmion configuration generates a significantly lower field on neighboring spin qubits, i.e., 15 times smaller field or approximately $23.5$ dB compared to nanomagnets that produce the same field at the target qubit, making it a more-suitable candidate for scalable quantum control architectures by reducing disturbances in adjacent qubits.

Two-dimensional photodetectors have become the preferred choice for future photodetectors with their advantages of being lightweight, portable, and high performance. However, the intrinsic properties of two-dimensional semiconductors make them susceptible to external disturbances, rendering them unsuitable for complex usage scenarios. Additionally, the power supply issue of small-scale devices relies on traditional van der Waals heterostructures with type-II band alignment for self-powering. Nonetheless, lattice and momentum mismatch issues for heterostructures have consistently hindered design efforts. Through first-principle calculations and quantum transport simulations, we introduce a groundbreaking self-powered near-infrared (NIR) photodetector based on 2D homobilayer BAs, achieved through tailored stacking techniques that induce spontaneous electrical polarization. The resulting ab-stacked BAs/BAs vdW homostructure forms a robust p-n homojunction endowed with a potent built-in polarization electric field, courtesy of the inherent spontaneous polarization along the out-of-plane direction. Leveraging broad momentum-space compatibility, high carrier mobility, and robust NIR light absorption, our 2D homobilayer BAs photodetector exhibits a wide NIR response range (780 nm–1600 nm) and an impressive responsivity of 122 mA/W even at zero bias. Under varying strains, homostructures of bilayer BAs exhibit remarkable robustness, maintaining a direct band gap and type-II band alignment across the entire strain spectrum. Moreover, under compressive strain, notable enhancements are observed in optical absorption coefficient and responsivity. Notably, the polarization induced by the ab-stacked interface is tunable, with polarization intensity increasing with the number of interfaces, presenting a straightforward method to amplify system polarization. Our study delineates a viable strategy for developing high-performance self-powered NIR photodetectors through strategic stacking engineering.

This paper develops strategies based on the properties of the minimum-phase reflection coefficient to design a passive absorber with minimum volume. It can be demonstrated that, of all the designs achieving the same target absorption spectrum in the one-dimensional reflection problem, the design with minimum-phase reflection requires the smallest total volume. This property is particularly useful for guiding the optimization of acoustic absorbers. We define a cost function involving the complex-valued minimum-phase reflection coefficient, which is uniquely determined from the prescribed absorption spectrum. We also introduce a volume-optimality factor to validate whether a given design achieves minimum-phase reflection. This factor is the volume ratio of a designed system to the corresponding minimum-phase system, thus measuring the volume efficiency of the design. Applying these strategies to absorbers composed of Helmholtz resonators, we show theoretically and numerically that using $N$ resonances yields up to $2^N$ distinct designs with identical absorption spectra. Specifically, we present designs with single and dual resonators in parallel and series configurations. The minimum-phase design is consistently more compact than the non-minimum-phase designs. These findings validate our design strategies and highlight their importance, as traditional optimization methods for a target absorption spectrum may easily lead to non-minimum-volume designs.

Self-assembled swarms of interactive active units, which are adaptive and dynamically reconfigurable to accommodate different functionalities, represent a promising platform for the development of next-generation robotics. Here, we utilize the emergent collective behavior of active magnetic colloids confined in quasi-two-dimensional arrays of overlapping wells to demonstrate the self-organization of a colloidal swarm into a dynamic pump architecture capable of controlled transport of passive cargo particles. This dynamic architecture provides a global unidirectional looping flow pattern along the entire length of the system. We show that the flow direction of the dynamic swarm-based pump can be externally controlled by a phase shift of a driving magnetic field energizing the swarm. The experimental observations are supported by computational modeling based on phenomenological coarse-grained particle dynamics coupled to shallow-water Navier-Stokes hydrodynamics. Our findings demonstrate how the emergent collective behavior of a swarm can be orchestrated into a desired functionality by exploiting the interplay between activity and confinement potentials.

Nonreciprocity can not only generate quantum resources but can also shield noise and reverse interference from driving signals. We investigate the advantages of nonreciprocal coupling in sensing a driving signal. In general, we find that nonreciprocal coupling performs better than the corresponding reciprocal coupling. And we show that homodyne measurement is the optimal measurement. A single nonreciprocal coupling can increase measurement precision by up to 2 times. With the use of $N$ nonreciprocal couplings in parallel, the measurement precision can be increased by $N^2$ times compared with the corresponding reciprocal coupling. In a nonzero-temperature dissipative environment, we demonstrate that nonreciprocal quantum sensing has greater robustness regarding thermal noise than reciprocal quantum sensing.

Our investigation reveals a groundbreaking discovery of a negative inverse orbital Hall effect (IOHE) in $\mathrm{Ge}$ thin films. We employed the spin pumping technique where the spin-orbital coupled current is injected into $\mathrm{Ge}$ films using $\mathrm{YIG}/\mathrm{Pt}(2)/\mathrm{Ge}(t_{\mathrm{Ge}})$ and $\mathrm{YIG}/\mathrm{W}(2)/\mathrm{Ge}(t_{\mathrm{Ge}})$ heterostructures. Through comprehensive analysis, we observe significant reductions in the signals generated by coherent (rf-driven) and incoherent (thermal-driven) spin-orbital pumping techniques. These reductions are attributed to the presence of a remarkably strong negative IOHE in $\mathrm{Ge}$, showing its magnitude comparable with the spin-to-charge signal in $\mathrm{Pt}$. Our findings reveal that, although the spin-to-charge conversion in $\mathrm{Ge}$ is negligible, the orbital-to-charge conversion exhibits a large magnitude. Our results are innovative and pioneering in the investigation of negative IOHE by the injection of spin-orbital currents.

In sensing technologies, the efforts for increasing sensitivity are usually at the cost of restraining the dynamic range. Here, we propose a closed-loop-control-based weak-value amplification (WVA) method to address this trade-off problem. Different from the previous adaptive WVA schemes, our method can significantly increase the dynamic range without sacrificing sensitivity, and hence improve the sensing dynamic factor by several orders of magnitude, in principle. Moreover, by employing the small weak value in the feedback process, the phase-modulation uncertainty can be effectively suppressed and provide much higher SNR in practice. Additionally, our approach achieves high-sensitivity detection and high-precision closed-loop control on two distinct dimensions within the same weak-measurement system. Our method can find broad applications on sensing technologies that require both high sensitivity and wide dynamic range, such as the sensing of temperature, magnetic fields, pressure, angular velocity, and so on.