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.

We propose silicon nitride ($\mathrm{S}{\mathrm{i}}_3{\mathrm{N}}_4$) nanowaveguide-based CMOS-compatible integrated sources of spectrally pure visible-telecom photon pairs. Our designs are intrinsically devoid of spectral correlations, eradicating the need for narrow-band filtering that reduces the source brightness. Further, the signal photons lie in the visible–near-infrared (vis-NIR) band, which can provide increased heralding efficiency of the idler photon in the telecom band using high-efficiency single-photon detectors. For example, a rectangular-cross-section silica-clad $\mathrm{S}{\mathrm{i}}_3{\mathrm{N}}_4$ nanowaveguide with dimensions $680\text{~nm}\times 840\text{~nm}$ and length 7 cm provides a heralded single-photon purity of approximately $91\mathrm{\%}$ for a pump wavelength of 956.8 nm and a pump bandwidth of 1200 GHz, with the signal and idler lying in the visible and telecom bands at approximately $701$ nm and approximately $1505$ nm, respectively, demonstrating a peak spectral brightness of approximately $1.2\times {10}^4$ photon $\mathrm{pairs}/\mathrm{s}/\mathrm{nm}/{\mathrm{mW}}^2$ for a linear propagation loss of 0.5 dB/cm. The effect of propagation losses on the purity is also studied for such source designs. We also propose some designs capable of generating high-purity photon pairs to address various specific vis-NIR quantum memory transitions with telecom-band interfacing for long-distance hybrid quantum networks. Such compact on-chip sources could have potential applications in various quantum technologies, such as sensing, simulation, computing, communication, and networking.

Coherent acoustic phonon generation and detection assisted by optical resonances are at the core of efficient optophononic transduction processes. However, when one is dealing with a single optical resonance, the optimum generation and detection conditions occur at different laser wavelengths, i.e., different detunings from the cavity mode. In this work, we theoretically propose and experimentally demonstrate the use of elliptical micropillars to reach these conditions simultaneously at a single wavelength. Elliptical micropillar optophononic resonators present two optical modes with orthogonal polarizations at different wavelengths. By using a cross-polarization-scheme pump-probe experiment, we exploit the mode splitting and couple the pump beam to one mode while the probe is detuned from the other mode. In this way, at a particular micropillar ellipticity, the phonon-generation and phonon-detection processes are both enhanced. We report an enhancement of the coherent-phonon-generation-detection process by a factor of approximately 3.1 when comparing the highest achievable signals from elliptical and circular micropillars. Our findings constitute a step forward in tailoring light-matter interaction for more-efficient ultrahigh-frequency optophononic devices.

Quantum entanglement is a fundamental property commonly used in various quantum information protocols and algorithms. Nonetheless, the problem of identifying entanglement has still not reached a general solution for systems larger than $2\times 3$. In this study, we use deep convolutional NNs, a type of supervised machine learning, to identify quantum entanglement for any bipartition in a three-qubit system. We demonstrate that training the model on synthetically generated datasets of random density matrices excluding challenging positive-under-partial-transposition entangled states (PPTES), which cannot be identified (and correctly labeled) in general, leads to good model accuracy even for PPTES states, that were outside the training data. Our aim is to enhance the model’s generalization on PPTES. By applying entanglement-preserving symmetry operations through a triple Siamese network trained in a semisupervised manner, we improve the model’s accuracy and ability to recognize PPTES. Moreover, by constructing an ensemble of Siamese models, even better generalization is observed, in analogy with the idea of finding separate types of entanglement witnesses for different classes of states.

We investigate the electrical properties of nominally undoped, 10–40-nm-thick $\mathrm{Sc}\mathrm{N}$(111) layers grown on nearly lattice-matched $\mathrm{Ga}\mathrm{N}$:$\mathrm{Fe}$/${\mathrm{Al}}_2{\mathrm{O}}_3$(0001) templates by plasma-assisted molecular beam epitaxy. Hall-effect measurements yield electron concentrations of $0.7\text{--}3.1\times {10}^{19}{\text{cm}}^{-3}$ and mobilities of 50–160 ${\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ at room temperature. The temperature-dependent (4–360 K) conductivity exhibits two distinct regimes, suggesting two-band conduction in an impurity band and the conduction band. Assuming a single shallow donor in $\mathrm{Sc}\mathrm{N}$ and employing the standard two-band conduction model, we extract the carrier density and mobility in these bands. The results reveal nondegenerate characteristics for a 40-nm-thick layer, while thinner layers are weakly degenerate. For the nondegenerate layer, the donor ionization energy amounts to approximately 12 meV. The electron mobility of the layers is limited by ionized-impurity scattering and phonon scattering at low and high temperatures, respectively. Fits with an expression for optical phonon scattering developed for weakly degenerate semiconductors return an effective phonon energy of $(61\pm 5)\mathrm{meV}$, a value in between the energies of the longitudinal optical ($\approx 84\mathrm{meV}$) and transverse optical ($\approx 45\mathrm{meV}$) phonon modes in $\mathrm{Sc}\mathrm{N}$.

We observe strongly anisotropic third-harmonic generation mediated by resonant sum-frequency driving of Raman-active phonons with terahertz light, extending light-induced dual control of structural and optical properties in solids. Either strong enhancement or strong suppression of the third harmonic can be achieved, covering six orders of magnitude, a result of interference between purely electronic and phonon-mediated contributions to the polarization field. These findings enrich capabilities for tailoring nonlinear optics via phononics and for the spectroscopy of crystalline structural dynamics.

Phase engineering plays a pivotal role in various scientific research and industrial applications including microwaves, optics, and acoustics. However, traditional phase engineering methods lack the flexibility for arbitrary programming due to the absence of digitization. Conversely, while broadband digitization technology holds promise, its utilization in wave engineering has been constrained by narrowband transmission capabilities. In this study, we introduce a novel approach to broadband digital phase encoding spanning frequencies from 50 to 500 kHz. Our methodology is exemplified through the development of an acoustic soft metacollimator with simulations and experiments conducted underwater across different frequency ranges. Our findings demonstrate that the soft metacollimator significantly enhances transmission intensity by 7.7 dB and improves spatial resolution by a factor of 10. Moreover, the soft metacollimator enables novel functionalities such as long-distance energy enhancement and broadband coding properties in underwater acoustics. This pioneering broadband digital phase encoding technique holds promise for advancing next-generation phase engineering and offers versatile applications in biomedical ultrasonics, acoustic communications, and the development of underwater broadband acoustic antennas.

Two-dimensional (2D) metal halide perovskites have garnered significant attention in the field of light-emitting diodes due to their high photoluminescence quantum yield and tunability. In comparison with the widely studied 2D Ruddlesden-Popper (RP) perovskites, the 2D Dion-Jacobson (DJ) double perovskites, which offer higher stability and nontoxicity, have received relatively less attention. In addition, the regulatory effects of halogens on the luminescence mechanisms and performance of 2D DJ double perovskites remain unknown. Utilizing density functional theory, the stability and luminescence properties of 2D DJ double-perovskite $({\mathrm{C}}_6{\mathrm{H}}_{16}{\mathrm{N}}_2{)}_2\mathrm{Ag}\mathrm{Bi}{X}_8\cdot {\mathrm{H}}_2\mathrm{O}$ ($X$ = $\mathrm{Cl},\mathrm{Br},\mathrm{I}$) are effectively modulated through halogen substitution. The findings indicate that the dissociation energy incrementally increases as the halogen changes from $\mathrm{I}$ to $\mathrm{Br}$ to $\mathrm{Cl}$, suggesting that the incorporation of lighter halogens enhances structural stability. The minimal self-trapping formation energy (i.e., 0.08 eV) of $({\mathrm{C}}_6{\mathrm{H}}_{16}{\mathrm{N}}_2{)}_2{\mathrm{Ag}\mathrm{Bi}\mathrm{I}}_8\cdot {\mathrm{H}}_2\mathrm{O}$ facilitates easy detrapment of self-trapped excitons, leading to a tendency toward free exciton luminescence. The self-trapping formation energies of $({\mathrm{C}}_6{\mathrm{H}}_{16}{\mathrm{N}}_2{)}_2{\mathrm{Ag}\mathrm{Bi}\mathrm{Br}}_8\cdot {\mathrm{H}}_2\mathrm{O}$ and $({\mathrm{C}}_6{\mathrm{H}}_{16}{\mathrm{N}}_2{)}_2{\mathrm{Ag}\mathrm{Bi}\mathrm{Cl}}_8\cdot {\mathrm{H}}_2\mathrm{O}$ are 0.77 eV and 0.96 eV, respectively, indicating substantial self-trapping depths; thus, favoring self-trapped exciton luminescence. In addition, the transition dipole moments of $({\mathrm{C}}_6{\mathrm{H}}_{16}{\mathrm{N}}_2{)}_2{\mathrm{Ag}\mathrm{Bi}\mathrm{Br}}_8\cdot {\mathrm{H}}_2\mathrm{O}$ and $({\mathrm{C}}_6{\mathrm{H}}_{16}{\mathrm{N}}_2{)}_2{\mathrm{Ag}\mathrm{Bi}\mathrm{Cl}}_8\cdot {\mathrm{H}}_2\mathrm{O}$ are substantially higher than those of $({\mathrm{C}}_6{\mathrm{H}}_{16}{\mathrm{N}}_2{)}_2{\mathrm{Ag}\mathrm{Bi}\mathrm{I}}_8\cdot {\mathrm{H}}_2\mathrm{O}$, suggesting that substituting I with $\mathrm{Cl}$ and $\mathrm{Br}$ enhances the luminous efficiency of 2D DJ double-perovskite $({\mathrm{C}}_6{\mathrm{H}}_{16}{\mathrm{N}}_2{)}_2\mathrm{Ag}\mathrm{Bi}{X}_8\cdot {\mathrm{H}}_2\mathrm{O}$ ($X$ = $\mathrm{Cl},\mathrm{Br},\mathrm{I}$). These results indicate that halogen substitution can not only affect the stability of 2D DJ perovskites but also modulate their luminescent properties. Our research provides theoretical insights for the experimental design of superior luminescent materials.

Two-level system (TLS) loss typically limits the coherence of superconducting quantum circuits. The loss induced by TLS defects is nonlinear, resulting in quality factors with a strong dependence on the circulating microwave power. We observe frequency mixing due to this nonlinearity by applying a two-tone drive to a coplanar waveguide resonator and measuring the intermodulation products using a multifrequency lock-in technique. This intermodulation spectroscopy method provides an efficient approach to characterizing TLS loss in superconducting circuits. Using harmonic balance reconstruction, we recover the nonlinear parameters of the device-TLS interaction, which are in good agreement with the standard tunneling model for TLSs.

Overdamped Josephson junctions usually do not exhibit chaotic behavior in their phase dynamics, either because the phase-space dimension is less than $3$ (as in the case of a single overdamped ac-driven junction) or due to the general tendency of systems to become less chaotic with increasing dissipation (as in the case of coupled overdamped junctions). Here we consider the so-called ${\phi }_0$ superconductor/ferromagnet/superconductor Josephson junction in which the current flowing through the junction may induce magnetization dynamics in the ferromagnetic interlayer. We find that due to the induced magnetization dynamics, even in the overdamped limit (i.e., for a junction without capacitance), the junction may exhibit chaos and hysteresis that in some cases leads to multiple branches in its current-voltage characteristics. We also show that pulsed current signals can be used to switch between the different voltage states, even in the presence of added thermal noise. Such switching could be used in cryogenic memory components.

Recent progress shows that a surface-acoustic-wave (SAW) cavity can not only induce quantum acoustic dynamics, but can also form optomechanical-like systems. Its operating frequencies in the microwave band make it resistant to the thermal noise of surrounding environments, while its radiation-pressure couplings make it susceptible to weak forces. Based on these advantages, we propose a gyroscope comprising coupled microwave-SAW cavities. In this paper we systematically consider the range, signal-to-noise ratio, and sensitivity, which are the three most important indices to gyroscopes, but only partially considered in existing works. Additionally, we establish the fundamental limits of sensitivity when the quantum input is in the vacuum state and the squeezed vacuum state. We find that squeezing improves sensitivity and can surpass the standard quantum limit. However, this improvement can only reach up to $\sqrt{2}/2$ even as the squeezed parameter approaches infinity, which is rarely noted in recent works. Finally, we also offer analytical constraints for cooperativity and squeezed parameters. These constraints can be utilized to design gyroscopes based on coupled cavities in experiments.

Universal robust quantum control is essential for large-scale quantum computation. The geometric phase, as a key element with intrinsic error-resilient features, can be well integrated into a quantum control process to enhance control robustness. However, due to the inability to optionally avoid some trajectory segments seriously affected by systematic errors, the robust universality of current geometric quantum control based on some special geometric evolution trajectories is still lacking, resulting in unsatisfactory performance of arbitrary geometric gate. Here, we propose another scheme for universal robust geometric control based on geometric trajectory correction, where enough available evolution parameters are introduced, without more experimental demands, to ensure the effective reduction of sensitivity to systematic errors. Numerical simulation shows that our implemented arbitrary geometric gates have error-resilient features better than conventional ones. In addition, we also verify the feasibility of the high-fidelity physical implementation of our scheme on superconducting quantum circuits. Therefore, our theoretical work is expected to offer an attractive avenue for realizing practical quantum computation in existing experimental platforms.

We analyze the experimental error budget of parametric resonance gates in a tunable coupler architecture. We identify and characterize various sources of errors, including incoherent, leakage, amplitude, and phase errors. By varying the two-qubit gate time, we explore the dynamics of these errors and their impact on the gate fidelity. To accurately capture the impact of incoherent errors on gate fidelity, we measure the coherence times of qubits under gate operating conditions. Our findings reveal that the incoherent errors, mainly arising from qubit relaxation and dephasing due to white noise, limit the fidelity of the two-qubit gates. Moreover, we demonstrate that leakage to noncomputational states is the second largest contributor to the two-qubit gates infidelity, as characterized using leakage-randomized benchmarking. The error budgeting methodology we develop here can be effectively applied to other types of gate implementations.

In this work, we investigate optical properties of inorganic and organic metal halide perovskites (${\mathrm{Cs}\mathrm{Pb}\mathrm{X}}_3$ and ${\mathrm{MAPb}\mathrm{X}}_3$ with $X=\{\mathrm{Cl},\mathrm{Br},\mathrm{I}\}$). We use the eight-band $k\cdot p$ model to calculate the electronic band structures for perovskite materials of cubic and pseudocubic phase. In order to find respective parameters, the band structures of considered materials were obtained within state-of-the-art density-functional theory and used next as targets to adjust the $k\cdot p$ bands and determine the values of $k\cdot p$ parameters. The calculated band structures were used to obtain the material gain for bulk crystals (${\mathrm{Cs}\mathrm{Pb}\mathrm{Cl}}_3$, ${\mathrm{Cs}\mathrm{Pb}\mathrm{Br}}_3$, ${\mathrm{Cs}\mathrm{Pb}\mathrm{I}}_3$, ${\mathrm{MAPb}\mathrm{Cl}}_3$, ${\mathrm{MAPb}\mathrm{Br}}_3$, and ${\mathrm{MAPb}\mathrm{I}}_3$), which is compared with the material gain in well-established III-V semiconductors. It was found that for these perovskites a positive material gain appears at lower carrier density than for the reference materials ($\mathrm{Ga}\mathrm{As}$ and $\mathrm{In}\mathrm{P}$). We demonstrate that from the point of view of the electronic band structure, the studied perovskites are very promising gain medium for lasers.