Topological half vortices in momentum space have recently attracted great interest in photonic research. Characterized by winding polarization distributions, topological half vortices carry half-integer topological charges, underlying many exotic optical phenomena. To date, in systems with continuous translational symmetry, the study of momentum-space topological vortices is based mostly on the band degeneracies. Here we theoretically propose that by optical cavities that contain uniaxial materials, two types of momentum-space topological half vortex can emerge. With both a Hamiltonian model and eigenmode simulations, we reveal that paired degenerate and nondegenerate half vortices can be spawned from zero topological charge with different energy detuning of bands. The winding configurations of topological half vortices are closely associated with the distinctive characteristics of Berry curvature. Our work broadens methods to generate and manipulate topological half vortices in momentum space, and also offers a perspective of polarization vortices in the investigation of cavity photons in anisotropic media.

We demonstrate a Josephson parametric amplifier design with a band-pass impedance-matching network based on a third-order Chebyshev prototype. We measured eight amplifiers operating at 4.6 GHz that exhibit gains of 20 dB with less than 1-dB gain ripple and bandwidth up to 500 MHz. The amplifiers further achieve high-output-saturation powers around $-73\mathrm{dBm}$ based on the use of rf superconducting quantum interference device arrays as their nonlinear element. We characterize the system readout efficiency and its signal-to-noise ratio near saturation using a Sycamore processor, finding the data consistent with near quantum limited noise performance of the amplifiers. In addition, we measure the amplifiers’ intermodulation distortion in two-tone experiments as a function of input power and intertone detuning, and observe excess distortion at small detuning with a pronounced dip as a function of signal power, which we interpret in terms of power-dependent dielectric losses.

Radio-frequency reflectometry techniques enable high-bandwidth readout of semiconductor quantum dots. Careful impedance matching of the resonant circuit is required to achieve high sensitivity, which, however, proves challenging at cryogenic temperatures. Gallium arsenide–based voltage-tunable capacitors, so-called varactor diodes, can be used for in situ tuning of the circuit impedance but deteriorate and fail at temperatures below 10 K and in magnetic fields. Here, we investigate a varactor based on strontium titanate with a hyperabrupt capacitance-voltage characteristic, i.e., a capacitance tunability similar to the best gallium arsenide–based devices. The varactor design introduced here is compact, scalable, and easy to wire bond, with an accessible capacitance range from 45 pF to 3.2 pF. We tune a resonant inductor-capacitor circuit to perfect impedance matching and observe robust temperature- and field-independent matching down to 11 mK and up to 2 T in-plane field. Finally, we perform gate-dispersive charge sensing on a germanium-silicon core-shell nanowire hole double quantum dot, paving the way toward gate-based single-shot spin readout. Our results bring small magnetic field–resilient highly tunable varactors to mK temperatures, expanding the toolbox of cryo-radio-frequency applications.

Superresolution metalenses that can realize a far-field needle focusing effect are highly desired in various applications, such as medical diagnosing, ultrasonic detection, and underwater communication. Here, we propose a design paradigm of a metalens with superresolution for waterborne sound, which is based on metagrating and has a compact and simple configuration. The central region of the metalens is open, surrounded by a metagrating-based peripheral region. In the radial cross section, each meta-atom contains two elliptical iron cylinders that are intelligently designed to deflect the impinging wave along the desired direction towards the intended focal spot. In this way, a needle focusing effect with high intensity and superresolution is achieved in the far-field region, where the conventional Rayleigh-Abbe diffraction limit is broken due to the coherent interference of the ±1 order diffraction channels. Since the design principle does not rely on sharp resonances, the foci of waves carrying different frequencies partially overlap so that an achromatic focusing functionality is realized where incident waves in a continuous frequency range can be simultaneously focused around the same spots. The compact, planar, and center-open configuration of the metalens not only provides a flexible and practical solution to acoustic needle focusing functionality, but also has potential applications as a powerful planar component that can be easily integrated.

Ferroelectric ${\mathrm{Hf}\mathrm{O}}_2$ is a promising material for ferroelectric memory applications due to its compatibility with the complementary metal-oxide-semiconductor technology. However, its practical application is hindered by the reliability issues associated with oxygen vacancies ($V_{\mathrm{O}}$), such as endurance failure and wake-up effect. $\mathrm{La}$ doping is a highly effective approach to improving the endurance and reducing the wake-up effect. In this study, we investigate the origin of these performance improvements in ferroelectric ${\mathrm{Hf}\mathrm{O}}_2$ through density functional theory calculations. Our results reveal that the undoped ferroelectric ${\mathrm{Hf}\mathrm{O}}_2$ requires a certain amount of $V_{\mathrm{O}}$ to stabilize its phase, which can only be achieved under a relatively oxygen-poor condition. However, the neutral charge state of $V_{\mathrm{O}}$, stabilized by the Fermi level above the (0/2+) transition level, may cause a dielectric breakdown. $\mathrm{La}$ doping at the $\mathrm{Hf}$ sites significantly reduces the formation enthalpy of $V_{\mathrm{O}}$ by pushing down the Fermi level, leading to a moderate concentration of the doubly positive $V_{\mathrm{O}}$ $(V_{\mathrm{O}}^{2+})$ in most of the chemical potential regions. This prevents the dielectric breakdown and improves the device endurance. Furthermore, the presence of $\mathrm{La}$ dopants in ferroelectric ${\mathrm{Hf}\mathrm{O}}_2$ significantly increases the diffusion barrier of $V_{\mathrm{O}}^{2+}$, reducing the wake-up effect. Our findings provide insights into the design and optimization of ferroelectric films for practical applications.

We study the frequency nonreciprocity of the spin waves in symmetric ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}/\mathrm{Ru}/{\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$ synthetic antiferromagnet stacks set in the scissors state by in-plane applied fields. Using a combination of Brillouin light scattering and propagating spin-wave spectroscopy experiments, we show that the acoustical spin waves in synthetic antiferromagnets possess a unique feature if their wave vector is parallel to the applied field: the frequency nonreciprocity due to layer-to-layer dipolar interactions can be so large that the acoustical spin waves transfer energy in a unidirectional manner for a wide and bipolar interval of wave vectors. Analytical modeling and full micromagnetic calculations are conducted to account for the dispersion relations of the optical and acoustical spin waves for arbitrary field orientations. Our formalism provides a simple and direct method to understand and design devices harnessing unidirectional propagation of spin waves in synthetic antiferromagnets.

Terahertz (THz) technology offers a variety of applications in medical spectroscopy and imaging. In such applications, tissues are commonly assumed to be optically homogeneous at the THz-wavelength scale, while the THz-wave–tissue interactions are described (in a simplified manner) within the effective medium theory (EMT). Meanwhile, recent studies of tissues from human, animal, and plants involving emerging modalities of superresolution (beyond the Abbe diffraction limit) THz microscopy, have found tissue inhomogeneities with dimensions comparable to the THz wavelength (approximately $\lambda$), which can lead to THz-wave scattering effects. This poses a problem of studying an interplay between the THz-wave absorption and the scattering phenomena in soft turbid tissues. To mitigate this challenge, in this paper, a tissue-mimicking phantom is developed that has the form of a gelatin slab, as a highly absorbing hydrated matrix, into which silicon dioxide ($\mathrm{Si}\mathrm{O}$${}_2$) microparticles are embedded, with their lower refractive index and loss and subwavelength or mesoscale diameters. The analytical methods of Lorenz-Mie scattering theory have predicted a nonisotropic differential extinction cross section for such scatterers, which results in the non-Rayleigh scattering regime and casts doubt on the applicability of EMT for such tissues. Surprisingly, we have found theoretically, using the radiative transfer theory (RTT), and confirmed experimentally, using THz pulsed spectroscopy, that the effective optical properties of the proposed phantom are still determined by EMT over wide ranges of the diameters ($d\le 0.47\lambda$) and volume fractions ($f_v\le 0.2$) of the scatterers. This effect was attributed to the strong THz-wave loss in a host medium and, therefore, should be general for a variety of soft tissues in the THz range. Thus, our findings hopefully broaden the applicability of EMT for describing the interactions between THz radiation and soft turbid tissues.

We report on a ballistic and fully tunable Josephson-junction system consisting of two parallel ribbons of graphene in contact with superconducting molybdenum-rhenium. By electrostatic gating of the two individual graphene ribbons, we gain control over the real-space distribution of the superconducting current density, which can be continuously tuned between the two ribbons. We extract the respective gate-dependent spatial distributions of the real-space current density by employing Fourier and Hilbert transformations of the magnetic-field-induced modulation of the critical current. This approach is fast and does not rely on a symmetric current profile. It is therefore a universally applicable tool, potentially useful for carefully adjusting Josephson junctions.

High-repetition-rate gas targets constitute an essential component in intense laser matter interaction studies. The technology becomes challenging as the repetition rate approaches the kilohertz regime. In this regime, cantilever-based gas valves are employed, which can open and close in tens of microseconds, resulting in a unique kind of gas characteristics in both the spatial and temporal domain. Here we characterize piezo cantilever-based kilohertz pulsed gas valves in the low density regime, where it provides sufficient peak gas density for high-harmonic generation while releasing a significantly smaller amount of gas reducing the vacuum load within the interaction chamber, suitable for high-vacuum applications. In order to obtain reliable information of the gas density in the target jet, space-time resolved characterization is performed. The gas-jet system is validated by conducting interferometric gas density estimations and high-harmonic generation measurements at the Extreme Light Infrastructure Attosecond Light Pulse Source facility. Our results demonstrate that while employing such targets for optimal high-harmonic generation, the high intensity interaction should be confined to a suitable time window, after the cantilever opening. The measured gas density evolution correlates well with the integrated high-harmonic flux and state-of-the-art three-dimensional simulation results, establishing the importance of such metrology.

Quantum gates based on geometric phases possess intrinsic noise-resilience features and attract much attention. However, the implementations of previous geometric quantum computation typically require a long pulse time of gates. As a result, their experimental control inevitably suffers from the cumulative disturbances of systematic errors due to excessive time consumption. Here, we experimentally implement noncyclic and nonadiabatic geometric quantum gates in a superconducting circuit, significantly shortening the gate time. Moreover, we experimentally verify that our universal single-qubit geometric gates are more robust to both the Rabi frequency and the qubit frequency shift-induced error, compared with the conventional dynamical gates, using the randomized benchmarking method. This scheme can also be utilized to construct two-qubit geometric operations while the generation of maximally entangled Bell states is demonstrated. Therefore, our results provide a promising routine to achieve fast, high-fidelity, and error-resilient quantum gates in superconducting quantum circuits.

In the short-range test of the gravitational inverse-square law (ISL) using a torsion pendulum, the residual electrostatic force is usually the major disturbance. We have built a torsion pendulum apparatus to verify the coupling effect of the residual electrostatic force with the separation variation between the test mass and the shielding membrane due to the environmental vibrations. We present an alternative solution for reducing the disturbance by suspending the shielding membrane, which has a common mode response as the torsion pendulum to the seismic translational motion and ground tilt. The experimental results with a fixed membrane are in good agreement with the theoretical calculations. By replacing the fixed membrane with a suspended membrane, the sensitivity of the torsion pendulum has been improved by at least a factor of 3 from 0.5 to 20 mHz, which is very useful for the further improvement of the ISL test in short range. This work may be helpful for other small-force measurement experiments.

Recently, the spectral manipulation of single photons has been achieved through spatial-temporal modulation of the optical refractive index. Here, we utilized the spatial-temporal modulation of the potential to manipulate the motion of massive particles, i.e., realizing the MeV-magnitude electron acceleration and atom decelerator on a millimeter scale. With the language of spatial-temporal modulation, manipulation of the photons and massive particles are connected and clearly investigated. The spatial-temporal modulation approach is also powerful for the optimization of multistage modulation with a large number of single stages on a photonic integrated chip and can be generalized to a wide range of systems. Thus, this approach may play a significant role in hybrid photonic chips and microscale particle manipulation.

We present a heuristic mathematical model of the relation between the geometry of a photonic crystal waveguide and the Purcell-enhancement factor at a particular wavelength of interest. We use this model to propose approaches to the design of a photonic crystal waveguide maximizing the Purcell enhancement at a target wavelength. Numerical simulations indicate that the proposed model allows controlled variation of Purcell-factor peak width and height, which can be used to achieve increased robustness with regard to fabrication defects.

We investigate the effects of photon loss and spectrally impure sources on Gaussian boson sampling (GBS) when used in dense-subgraph-finding algorithms. We find that the effectiveness of these algorithms is remarkably robust to such errors, to such an extent that there exist classical algorithms that can efficiently simulate the underlying GBS. These results suggest that, unlike the GBS problem itself, the speed-up of GBS-based algorithms over classical approaches when applied to the dense-subgraph problem is not exponential. They do suggest, however, that any advantage offered could be achieved on a quantum device with far less stringent requirements on photon loss and purity than general GBS.

Integrating measurement-device-independent quantum key distribution (MDI QKD) with classical networks guarantees security and economy. Unfortunately, the performance of MDI QKD, suppressed though it is by network asymmetry, is further degraded by the addition of integration noise. As a consequence, symmetric systems are always believed to be better for a higher key rate. However, we reveal a counterintuitive fact that asymmetric MDI QKD has better performance in integrated systems. Modeling the mechanism of the asymmetric effect in integrated MDI systems and analyzing the noise-suppression effect of asymmetric structures, we observe an unanticipated increase of the key rate up to 260%, giving a conclusion diametrically opposite that for the original system. The phenomenon found and the proposed analytical model provide a method for noise suppression and performance improvement, permitting high adaptability for realistic asymmetric networks.

Measurement-device-independent quantum key distribution (MDI QKD) is a crucial step for the quantum Internet, as it removes all detector side channels and enables secure QKD with untrusted nodes. However, MDI QKD faces a significant challenge from polarization alignment, since drift of the state of polarization affects the efficiency of key generation, especially when the quantum states are sent from remote locations. Conventional methods for polarization alignment either require additional resources, such as classical lasers or reference pulses, or interrupt the QKD process, which reduces the efficiency of the system. In this paper, an efficient polarization-alignment method in MDI QKD is proposed that leverages intrinsic events from MDI to directly calculate how to compensate for polarization drift. The proposed method does not require additional resources such as reference pulses, thereby eliminating bandwidth limitations, and it is fast, as it can directly calculate how to correct polarization with a small criterion fluctuation. We also introduce the polarization dimension in addition to the encoding dimension and establish a response-rate model for MDI QKD with additional polarization dimensions. Overall, the proposed method is a resource-efficient solution for polarization drift correction in MDI QKD, which can enhance the practicality and scalability of QKD networks.