Motivated by our recent findings in [Phys. Rev. Lett. 128, 013902, 2022], which introduced a new class of electromagnetic bulk materials whose response is similar to conventional semiconductor transistors, here we propose a one-dimensional (1D) version of such materials based on transmission lines coupled with FET isolators. We demonstrate that the response of this 1D system is nonreciprocal and non-Hermitian, analogous to the idealized transistor-metamaterial, and is also characterized by a broken time-reversal symmetry. We analyze the wave propagation in the system and find that the interaction between the eigenmodes can either lead to gain or loss, depending on the propagation distance. Furthermore, it is also shown that the system may be operated at an exceptional point, wherein the response becomes singular, and the power gain is maximized. Finally, we demonstrate that the exceptional point coincides with the point of operation of typical microwave amplifiers, such as the distributed amplifier.

The research of magnetic tunnel junctions (MTJs) has scientific and technical significance for promoting the development of information storage technology, promoting spintronics and realizing higher-density integrated circuits. At present, mainly by improving the material and designing alternative structures, the performance and reliability of the MTJs can be enhanced. Recently, it has been reported that the monolayer $\mathrm{Cr}\mathrm{As}$ is expected to be a ferromagnetic half-metal with high Curie temperature. Here, using DFT+$U$, we investigate the magnetic and spin-dependent transport properties in a MTJ made of half-metallic $\mathrm{Cr}\mathrm{As}$ nanosheet. The magnetic anisotropy energy (MAE) along the $z$ axis is $637\text{μ}\mathrm{eV}$ per $\mathrm{Cr}$, and the ferromagnetism and out-of-plane easy magnetization are not affected by biaxial strain. In addition, the spin-dependent transport features of two MTJs with arsenene inserted in the $\mathrm{Cr}\mathrm{As}$ layer were theoretically developed and investigated. The findings reveal that tunneling magnetoresistance ratio peaks at approximately equal to ${10}^{10}$ and remains more than approximately equal to ${10}^6$ even when subjected to bias voltages as high as 1 V. Moreover, 100% spin-filtering effects can be observed in the devices. Our research offers viable MTJs possibilities for the next spintronic devices.

Reducing errors in quantum gates is critical to the development of quantum computers. To do so, any distortions in the control signals should be identified; however, conventional tools are not always applicable when part of the system is under high vacuum, cryogenic, or microscopic. Here, we demonstrate a method to detect and compensate for amplitude-dependent phase changes, using the qubit itself as a probe. The technique is implemented using a microwave-driven trapped-ion qubit, where correcting phase distortions leads to a threefold improvement in the error of single-qubit gates implemented with pulses of different amplitudes, to attain state-of-the-art performance benchmarked at $1.6(4)\times {10}^{-6}$ error per Clifford gate.

Efficient manipulation of spin waves (SWs) is considered as one of the promising means for encoding information with low power consumption in next-generation spintronic devices. The SW Doppler shift is a key phenomenon obtained by manipulating SW propagation. Here, we predict an efficient way to control the SW Doppler shift by voltage-control magnetic-anisotropy-boundary (MAB) movement in the FM-FE heterojunction. From the micromagnetic simulation, we verified that the SW Doppler shift aligns well with our theoretical predictions in $\mathrm{Fe}/{\mathrm{Ba}\mathrm{Ti}\mathrm{O}}_3$ heterostructure. The SW Doppler shift also shows an ultra-wide-band-shift (over 5 GHz) property with voltage. Such an efficient SW Doppler shift may provide one possible way to measure the Hawking radiation of an analogue black hole in the SW systems.

Optically pumped magnetometers (OPMs) based on the free-induction-decay (FID) configuration have recently attracted interest owing to their advantages, such as being calibration-free, easy to operate, and showing reduced light shifts induced by the pump light. Currently, alkali-metal vapor cells have been employed in most of the FID magnetometers. However, the relatively narrow linewidth of the magnetic resonance signal and nonlinear Zeeman (NLZ) effects generate bandwidth limitation and heading errors of these magnetometers in the Earth’s field, respectively. In this work, we propose a ${}^4\mathrm{He}$-based FID magnetometer, which not only has the advantages of being NLZ-free and showing a high bandwidth, but also maintains an enhanced sensitivity assisted by a multipass cell. This magnetometer demonstrates a magnetic field noise floor of $0.34\mathrm{pT}/{\mathrm{Hz}}^{1/2}$ with a Nyquist-limited bandwidth of $5$ kHz, which opens another route for atomic magnetometry using FID signals, and exhibits potential for applications in high-frequency magnetic field detection, such as biomagnetic measurements and magnetic communications in the geomagnetic field.

To derive the best oscillator phase noise when implementing a high-$Q$ resonator, the resonant spectral line shape must have high contrast and symmetry. Ideally, this line shape is second order and Lorentzian; however, in a high-mode-density spectral region, low-$Q$ background spurious modes interact and distort the resonance. For a sapphire-loaded cavity resonator operating with whispering-gallery modes confined within the sapphire crystal, we show that this high contrast and symmetry can be achieved by meticulously changing the dimensions of the surrounding metallic cavity shield to tune the background low-$Q$ structures into antiresonance. This works because the high-$Q$ resonances are primarily defined by the sapphire, while the background modes are defined by the cavity shield. Alternatively, it has been shown that a similar result can be achieved by exciting the high-$Q$ resonator with a balanced microwave dipole probe in a Mach-Zehnder interferometric configuration. The probe has been constructed from two separate coaxial electric field probes symmetrically inserted into a cylindrical-cavity resonator, from opposite sides with a small gap between them, so they can behave like an active wire-dipole antenna. The power into the two separate probes may be matched with an external variable attenuator in one of the arms of the interferometer. Conversely, the phase between the two electric field probes may be changed with an external variable phase shifter, which changes the nature of the field components to which the probe couples. The probe couples to the high-$Q$ resonant modes as well as low-$Q$ background modes, which can be made resonant or antiresonant with respect to the high-$Q$ modes by changing this external phase. When the background modes are in antiresonance, the line shape of the high-$Q$ mode can be made symmetric and with higher contrast. This technique has been applied to both whispering-gallery sapphire modes, as well as hollow-cavity resonators, without changing the dimensions of the cavity.

We present a design and implementation of a frequency-tunable superconducting resonator. The resonance frequency tunability is achieved by flux-coupling a superconducting LC loop to a current-biased feedline; the resulting screening current leads to a change of the kinetic inductance and shift in the resonance frequency. The thin-film aluminum resonator consists of an interdigitated capacitor and thin line inductors forming a closed superconducting loop. The magnetic flux from the nearby niobium current feedline induces Meissner shielding currents in the resonator loop leading to a change in the kinetic part of the total inductance of the resonator. We demonstrate continuous frequency tuning within 160 MHz around the resonant frequency of 2.7 GHz. We show that: (1) frequency up-conversion is achieved when a kilohertz ac modulation signal is superimposed onto the dc bias resulting in sidebands to the resonator tone; (2) three-wave mixing is attained by parametrically pumping the nonlinear kinetic inductance using a strong rf pump signal in the feedline. The simple architecture is amenable to large-array multiplexing and on-chip integration with other circuit components. The concept could be applied in flux magnetometers, up-converters, and parametric amplifiers operating above 4 K when alternative high-critical-temperature material with high kinetic inductance is used.

Much attention has focused on the transmon architecture for large-scale superconducting quantum devices; however, the fluxonium qubit has emerged as a possible successor. With a shunting inductor in parallel to a Josephson junction, the fluxonium offers larger anharmonicity and stronger protection against dielectric loss, leading to higher coherence times as compared to conventional transmon qubits. The interplay between the inductive and Josephson energy potentials of the fluxonium qubit leads to a rich dispersive-shift landscape when tuning the external flux. Here, we propose to exploit the features in the dispersive shift to improve qubit readout. Specifically, we report on theoretical simulations showing improved readout times and error rates by performing the readout at a flux-bias point with large dispersive shift. We expand the scheme to include different error channels and show that with an integration time of 155 ns, flux-pulse-assisted readout offers about a $5$-times improvement in the signal-to-noise ratio. Moreover, we show that the performance improvement persists in the presence of finite measurement efficiency combined with quasistatic flux noise and also when considering the increased Purcell rate at the flux-pulse-assisted readout point. We suggest a set of reasonable energy parameters for the fluxonium architecture that will allow for the implementation of our proposed flux-pulse-assisted readout scheme.

Piezoelectric materials produce a linear deformation in response to an applied electric field and are essential for precision-control devices. The conventional view is that polymers must be crystalline and possess remanent polarization after artificial poling to gain piezoelectricity. For the ferroelectric polymer poly(vinylidene difluoride), the required poling field is exceptionally high, up to 150–200 kV mm${}^{-1}$. Here, we circumvent this limitation by utilizing the elastic displacement of electric dipoles and creating a net polarization. We find that a subcoercive field of 49 kV mm${}^{-1}$ can induce a high piezoelectric coefficient, $d_{33}$, of −33 pm V${}^{-1}$ in an unpoled poly(vinylidene fluoride-trifluoroethylene) film. In this case, the dielectric acts as a piezoelectric, as long as a bias electric field is applied, with its piezoelectric coefficient increasing proportionally to the strength of the electric field until polarization saturation sets in. The proposed methodology is further extended to amorphous polymers, providing an opportunity to discover alternative piezoelectrics within the dielectric family.

Scattering-type scanning near-field optical microscope (s-SNOM) has become an essential tool to study polaritons—quasiparticles of light coupled to collective charge oscillations—via direct probing of their near field with a spatial resolution far beyond the diffraction limit. However, extraction of the polariton’s complex propagation constant from the near-field images requires subtle considerations that have not received the necessary attention so far. In this study, we discuss important yet overlooked aspects of the near-field analysis. First, we experimentally demonstrate that the sample orientation inside the s-SNOM may significantly affect the near-field interference pattern of mid-infrared polaritons, leading to an error in momentum measurement up to 7.7%, even for the modes with an effective index of 12.5. Second, we establish a methodology to correctly extract the polariton damping rate from the interference fringes depending on their origin—i.e., the s-SNOM nanotip or the material edge. Overall, our work provides a unified framework for the accurate extraction of the polariton momentum and damping from the near-field interference fringes.

Through nonlinear self-focusing, femtosecond pulses can propagate several kilometers beyond diffraction limits, forming an ionization channel in air known as a laser filaments. It has been demonstrated that in the wake of the filament, aerosols can be effectively cleared to improve the transmission of subsequent laser pulses or secondary light sources, pertinent to applications in atmospheric sensing. However, the current understanding of aerosol clearing is founded on interactions with droplets to simulate fogs and clouds and thus does not extend to solid particles or atmospheric debris. Using optical trapping, we isolate both graphite and silica microparticles and directly measure the subsequent displacement caused by the filament using time-resolved shadowgraphy. The shock wave from the filament is demonstrated to propel particles away from the filament, directly contributing to atmospheric debris clearing. Particles exposed to the laser light in either the intense filament core or the surrounding energy reservoir are axially displaced along the beam path. It is found that the optomechanical properties of the particle largely influence the axial displacement induced by laser exposure through mechanisms such as radiation pressure, mass ejection from ablation or optical damage, and particle deagglomeration.

Quantum neural networks (QNNs) and quantum kernels stand as prominent figures in the realm of quantum machine learning, poised to leverage the nascent capabilities of near-term quantum computers to surmount classical machine learning challenges. Nonetheless, the training-efficiency challenge poses a limitation on both QNNs and quantum kernels, curbing their efficacy when they are applied to extensive datasets. To confront this concern, we present a unified approach—coreset selection—aimed at expediting the training of QNNs and quantum kernels by distilling a judicious subset from the original training dataset. Furthermore, we analyze the generalization-error bounds of QNNs and quantum kernels when they are trained on such coresets, unveiling performance comparable with that of those trained on the complete original dataset. Through systematic numerical simulations, we illuminate the potential of coreset selection in expediting tasks encompassing synthetic data classification, identification of quantum correlations, and quantum compiling. Our work offers a useful way to improve diverse quantum machine learning models with a theoretical guarantee while reducing the training cost.

Mechanical metamaterials capable of large deformations are an emerging platform for functional devices and structures across scales. Bistable designs are particularly attractive since they endow a single object with two configurations that display distinct shapes, properties, and functionalities. We propose a strategy that takes a common (nonbistable) metamaterial design and transforms it into a bistable one; specifically, by allowing for irregular patterns through geometric perturbations of the unit cell and by leveraging the intercell constraints inherent to the large-deformation response of metamaterials. We exemplify this strategy by producing a design framework for bistable planar kirigami metamaterials, starting from the canonical rotating-squares pattern. The framework comprises explicit design formulas for cell-based kirigami with unprecedented control over the shape of the two stable states and an optimization methodology that allows for efficient tailoring of the geometric features of the designs to achieve target elastic properties as well as shape change. The versatility of this framework is illustrated through a wide variety of examples, including nonperiodic designs that achieve two arbitrarily shaped stable states. Quantitative and qualitative experiments, featuring prototypes with distinct engineering design details, complement the theory and shine light on the strengths and limitations of our design approach. These results show how to design bistable metamaterials from nonbistable templates, paving the way for further discovery of bistable systems and structures that are not simply arrangements of known bistable units.

We report on $\mathrm{Mo}$-$\mathrm{Si}$-based superconducting nanowire single-photon detectors on a gallium arsenide substrate. $\mathrm{Mo}$-$\mathrm{Si}$ deposited on a passivated $\mathrm{Ga}\mathrm{As}$ surface has the same critical temperature as $\mathrm{Mo}$-$\mathrm{Si}$ deposited on silicon. The critical temperature decreases slightly on depositing $\mathrm{Mo}$-$\mathrm{Si}$ directly on the native oxide of $\mathrm{Ga}\mathrm{As}$. Hence, $\mathrm{Mo}$-$\mathrm{Si}$ works well as a thin-film superconductor on $\mathrm{Ga}\mathrm{As}$. We propose that the amorphous structure of $\mathrm{Mo}$-$\mathrm{Si}$ ensures compatibility with the $\mathrm{Ga}\mathrm{As}$ matrix. Superconducting nanowire single-photon detectors (SNSPDs) are fabricated with $\mathrm{Mo}$-$\mathrm{Si}$ on $\mathrm{Ga}\mathrm{As}$ using a meander-wire design. The SNSPD metrics are very similar to those of devices fabricated with the same procedure on a silicon substrate. We observe a plateau in the response-versus-bias curve, signaling a saturated internal quantum efficiency. The plateau remains even at an elevated temperature, 2.2 K, at a wavelength of 980 nm. We achieve a timing jitter of 50 ps and a recovery time of 29 ns. These results point to the promise of integrating $\mathrm{Mo}$-$\mathrm{Si}$ SNSPDs with $\mathrm{Ga}\mathrm{As}$ photonic circuits.

Graphene-based Hall effect magnetic field sensors hold great promise for the development of ultrasensitive magnetometers with very low power consumption. Their performance is frequently analyzed using the so-called two-channel model where electron and hole conductivities are simply added. Unfortunately, this model is unable to capture all the sensor’s characteristics, particularly the bias current dependence of the magnetic field sensitivity. Here, we present an advanced model that provides an in-depth understanding of how graphene-based Hall sensors operate and demonstrate its ability to quantitatively assess their performance. First, we report on the fabrication of sensors based on different qualities of graphene, with the best devices achieving magnetic field sensitivities as high as 5000 Ω/T, outperforming the best silicon and narrow-gap semiconductor-based sensors. Then, we examine their performance in detail using the proposed numerical model, which combines Boltzmann formalism with distinct Fermi levels for electrons and holes, and a new method for the introduction of substrate-induced electron–hole puddles. Importantly, the dependences of magnetic field sensitivity on bias current, disorder, substrate, and Hall bar geometry are quantitatively reproduced for the first time. In addition, the model emphasizes that the performance of devices with widths of the order of the charge carrier diffusion length is significantly affected by the bias electric current due to the occurrence of charge carrier accumulation and depletion areas near the edges of the Hall bar, much larger than conventional Hall effect predictions. The formation of these areas induces a transverse diffusion charge carrier flux capable of counterbalancing the one induced by the Lorentz force when the Hall electric field cancels out in the ambipolar regime. Finally, we discuss how sensor performance can be enhanced by Fermi velocity engineering, paving the way for future ultrasensitive graphene-based Hall effect sensors.