Neutral-atom arrays serve as an ideal platform to study quantum simulation and quantum logic gates, where intense efforts have been devoted to improving the fidelity of two-qubit gates. We report our recent findings in constructing a different type of two-qubit controlled phase gate protocol with neutral atoms enabled by the Rydberg blockade, which aims at both robustness and high fidelity. It relies upon a modulated driving pulse with a specially tailored smooth waveform to gain appropriate phase accumulations for quantum gates. The major features include finishing a gate operation within a single pulse, not necessarily requiring individual site addressing, not being sensitive to the exact value of the blockade shift, and suppressing the population leakage error and rotation error. Building upon this progress, we further develop an upgrade in the form of dual-pulse off-resonant modulated driving with the major distinct feature of Doppler insensitivity, in order to address the challenge to the fidelity of two-qubit gates caused by residual thermal motion of the cold atoms. In principle, the gate fidelity remains reasonably high over a relatively significant velocity range of the qubit atoms. Moreover, we anticipate that this will inspire future improvements in gate protocols for other types of qubit platforms, and the strategies used here may find applications in the area of quantum optimal control.

The shot-noise unit in continuous-variable quantum key distribution plays an important and fundamental role in experimental implementation, as it is used as a normalization parameter that contributes to performing security analyses and distilling key information. However, the traditional calibration procedure and detector model cannot cover all system noise in practical applications, which results in loopholes and influences the practical security. What is more, the traditional procedure is also rather complicated and has difficulty in providing compatibility with automatic operating systems. In this paper, we propose a calibration model based on a proposed trusted detector model, which could naturally close the loopholes in practical applications. It can help to identify the shot-noise unit in only one step, which can not only effectively simplify the evaluation process but also reduce the statistical fluctuations, while two steps are needed in the traditional method. We prove its feasibility and derive a complete version of the corresponding entanglement-based model. A detailed security analysis against arbitrary collective attacks and numerous simulation results in both the asymptotic-limit regime and the finite-size regime are provided. A proof-of-principle experiment is implemented, and the results indicate that our one-time calibration model can be employed as a powerful alternative method for calibrating the shot-noise unit. Our method paves the way for the deployment of continuous-variable quantum key distribution with real-time calibration and automatic operation.

The relationship between real powder distributions and optical coupling is a critical building block for developing a deeper physical understanding of laser-additive manufacturing and for creating more reliable and accurate models for predictable manufacturing. Laser-light absorption by a metal powder is distinctly different from that of a solid material, as it is impacted by additional parameters, such as particle size, shape distribution, and packing. Here, we use x-ray computed tomography to experimentally determine these parameters in a thinly spread austenitic stainless-steel powder on a metal substrate, and we combine these results with optical absorptance measurements during a 1 ms stationary laser-light exposure to simulate the additive-manufacturing process. Within the thinly spread powder layer, the particle volume fraction changes continuously from near zero at the powder surface to a peak value of 0.72 at a depth of 235 µm, with the most rapid increase taking place in the first 100 µm. The relationship between this particle volume fraction gradient and optical absorptance is investigated using an analytical model, which shows that depth-averaged absorptance measurements can measure the predicted average value, but will fail to capture local effects that result from a changing powder density. The time-averaged absorptance remains at levels between 0.67 and 0.80 across a two orders of magnitude range in laser power, which is significantly higher than that observed in solid stainless-steel experiments. The dynamic behavior of the absorptance, however, reveals physical phenomena, including oxidation, melting, and vapor cavity (keyhole) formation, as well as quantifying the effect of these on the absorbed energy.

Millimeter-wave superconducting devices offer a platform for quantum experiments at temperatures above 1 K, and new avenues for studying light-matter interactions in the strong coupling regime. Using the intrinsic nonlinearity associated with kinetic inductance of thin-film materials, we realize four-wave mixing at millimeter-wave frequencies, demonstrating a key component for superconducting quantum systems. We report on the performance of niobium nitride resonators around 100 GHz, patterned on thin (20–50-nm) films grown by atomic layer deposition, with sheet inductances up to $212\mathrm{pH}/◻$ and critical temperatures up to 13.9 K. For films thicker than 20 nm, we measure quality factors from $1\times {10}^4$ to $6\times {10}^4$, and explore potential loss mechanisms. Finally, we measure degenerate parametric conversion for a 95-GHz device with a forward efficiency up to $+16\mathrm{dB}$, paving the way for the development of nonlinear quantum devices at millimeter-wave frequencies.

Spin wave, the precession of magnetic order in magnetic materials, is a collective excitation that carries spin angular momentum. Similar to acoustic or optical waves, the spin wave also possesses the polarization degrees of freedom. Although such polarization degrees of freedom are frozen in ferromagnets, they are fully unlocked in antiferromagnets or ferrimagnets. Here we introduce the concept of magnetic gating and demonstrate a spin-wave analog of the Datta-Das spin transistor in antiferromagnets. Utilizing the interplay between polarized spin wave and the antiferromagnetic domain walls, we propose a universal logic gate of pure magnetic nature, which realizes all Boolean operations in one single magnetic structure.

Active control of radiative heat transfer still remains open due to its exciting application potential. In view of this, we present a theoretical demonstration of dynamically tunable near-field radiative heat transfer (NFRHT) between two multilayer hyperbolic metamaterials (consisting of alternating layers of a magneto-optical (MO) material and a dielectric) using an external magnetic field. We show that magnetization-induced hyperbolic modes play a significant role in radiative heat transfer and allow a highly tunable heat flux. Moreover, the hybridization of intrinsic and magnetization-induced hyperbolic modes significantly enhances the radiative heat transfer. In particular, we find that polarization conversion due to nonzero off-diagonal elements enables the spectral heat transfer coefficient for the TE polarization to be as large as that for the TM polarization, which is vastly different from the case of two closely spaced bulk MO plates. In addition, to quantitatively characterize the thermal modulation, we show a negative thermal magnetoresistance effect in this system, which approaches $-59.6\mathrm{\%}$ when the magnetic field intensity reaches 10 T. Our findings may be beneficial for active noncontact thermal management at the nanoscale, and facilitate a deeper understanding of the mechanisms of MO hyperbolic metamaterials in terms of NFRHT.

Recently, signatures of nonlinear Hall effects induced by Berry-curvature dipoles have been found in atomically thin $1T^{\prime }/T_d$-${\mathrm{W}\mathrm{Te}}_2$. In this paper, we show that in strained polar transition-metal dichalcogenides (TMDs) with $2H$ structures, Berry-curvature dipoles created by spin degrees of freedom lead to strong nonlinear Hall effects. Under an easily accessible uniaxial strain of order $0.2\mathrm{\%}$, strong nonlinear Hall signals, characterized by a Berry-curvature dipole on the order of $1\AA$, arise in electron-doped polar TMDs such as $\mathrm{Mo}\mathrm{S}\mathrm{Se}$, and this is easily detectable experimentally. Moreover, the magnitude and sign of the nonlinear Hall current can be easily tuned by electric gating and strain. These properties can be used to distinguish nonlinear Hall effects from classical mechanisms such as ratchet effects. Importantly, our system provides a potential scheme for building electrically switchable energy-harvesting rectifiers.

Versatile spin-orbit-torque switching is potentially useful for spin logics and neuromorphic computing. Using two orthogonal spin-orbit torques, we nonvolatilely switch the magnetization of a $\mathrm{Pt}$/$\mathrm{Co}$/$\mathrm{Mg}\mathrm{O}$ crossbar with perpendicular magnetic anisotropy among multistates such as the spin-up, spin-down, and spin-random states. By electrical methods, we can even reverse the chirality of the current dependence of the magnetization from clockwise to counterclockwise. Realization of chirality-controllable switching among multistates could pave the way toward spin logics and spintronic neural computing.

Self-powered ${\mathrm{Ga}}_2{\mathrm{O}}_3$-based solar-blind photodetectors have received attention recently due to the increased demand for energy saving, miniaturization, and high efficiency in devices. An ideal device structure consisting of a ${\mathrm{Ga}}_2{\mathrm{O}}_3$-based p-n junction is still difficult to obtain, since p-type doping is a major challenge. Although self-powered devices based on heterojunction are promising, there are two fatal disadvantages: (1) photosensitivity of the non-solar-blind region, on account of the narrower band gap of the heterojunction materials; and (2) poor quality of the epitaxial film due to lattice mismatch. In view of the various polymorphs of ${\mathrm{Ga}}_2{\mathrm{O}}_3$, we propose constructing a structure consisting of a ${\mathrm{Ga}}_2{\mathrm{O}}_3$ phase junction with α and β phases (α/β phase junction) for self-powered solar-blind photodetectors. The small lattice mismatch and similar band gap between α- and β-${\mathrm{Ga}}_2{\mathrm{O}}_3$ will solve the two problems outlined above. The formation of α- and β-${\mathrm{Ga}}_2{\mathrm{O}}_3$ is expected to result in a type-II band alignment, promoting separation of photogenerated carriers, which transfer through the junction to the corresponding electrodes. Herein, the α/β phase junction of ${\mathrm{Ga}}_2{\mathrm{O}}_3$ vertically aligned nanorod arrays with a thickness-controllable β-${\mathrm{Ga}}_2{\mathrm{O}}_3$ shell layer are fabricated by a low-cost and simple process of hydrothermal and postannealing treatment. Two different types of self-powered α/β-${\mathrm{Ga}}_2{\mathrm{O}}_3$ phase junction-based photodetectors, in the form of solid-state type and photoelectrochemical type, are constructed and realized. Our analysis shows that the constructed photodetectors are capable of highly efficient detection of solar-blind signal without any bias voltage. This work demonstrates the usefulness of using the α/β-${\mathrm{Ga}}_2{\mathrm{O}}_3$ phase junction in a self-powered solar-blind photodetector, which is not only energy efficient, but also potentially workable in outer space, at the south and north pole, and other harsh environments without external power for a long time.

The development of transparent bipolar oxide devices is largely hampered by the lack of oxides with reliable p-type conductivity. Recent calculations show that rock salt (RS) structured $\mathrm{Zn}\mathrm{O}$ alloys are promising candidates for transparent p-type oxides. Here, we synthesize wide-gap ${\mathrm{Zn}}_{1-x}{\mathrm{Ni}}_x\mathrm{O}$ alloy thin films over the entire composition range on glass substrates by means of radiofrequency magnetron sputtering. We find that the ${\mathrm{Zn}}_{1-x}{\mathrm{Ni}}_x\mathrm{O}$ alloy thin films undergo a phase transition from wurtzite (WZ) to RS structure as the $\mathrm{Ni}$ content x increases to x ∼ 0.27. Interestingly, the band gap of RS-${\mathrm{Zn}}_{1-x}{\mathrm{Ni}}_x\mathrm{O}$ is about 4.6 eV at the WZ to RS transition composition (x ∼ 0.27) and decreases with x to the value of RS-$\mathrm{Ni}\mathrm{O}$ (∼3.8 eV). Nominally undoped alloy thin films sputtered in pure $\mathrm{Ar}$ are semi-insulating, while $\mathrm{O}$-rich RS-${\mathrm{Zn}}_{1-x}{\mathrm{Ni}}_x\mathrm{O}$ thin films with relatively high x (e.g., x ≥ 0.5) sputtered in ${\mathrm{Ar}}^{+}$ 1.4% ${\mathrm{O}}_2$ and/or with $\mathrm{Cu}$ doping exhibit good p-type conductivity at room temperature. The hole transport in these RS-${\mathrm{Zn}}_{1-x}{\mathrm{Ni}}_x\mathrm{O}$ alloy thin films follows a small polaron hopping process, with activation energy ranges from 0.19 to 0.32 eV. Drastic differences in the electronic band structure of the WZ and RS ${\mathrm{Zn}}_{1-x}{\mathrm{Ni}}_x\mathrm{O}$ alloys are also observed with a type II band offset for alloys at the WZ to RS transition composition. The valence band maximum (VBM) of the RS phase is >1 eV above that of the WZ phase, making them energetically more favorable for the formation of native acceptor defects. The much higher VBM position of RS alloys also favors their extrinsic p-type doping efficiency.

Quantum metrology derives its capabilities from the careful employ of quantum resources for carrying out measurements. This advantage, however, relies on refined data postprocessing, assessed based on the variance of the estimated parameter. When Bayesian techniques are adopted, more elements become available for assessing the quality of the estimation. Here we adopt generalized classical Cramér-Rao bounds for looking in detail into a phase-estimation experiment performed with quantum light. In particular, we show that the third-order absolute moment can give a superior capability in revealing biases in the estimation, compared to standard approaches. Our studies point to the identification of an alternative strategy that brings a possible advantage in monitoring the correct operation of high-precision sensors.

We present an ultrathin graphene metascreen that possesses dispersive optical activity in the early terahertz spectrum. The metascreen design consists of periodically etched L-shaped voids on a graphene substrate backed by a conductive plane. The specific unit-cell design is based on chirality and leads to highly asymmetric radiations from the plasmon-polariton surface currents, leading to linear and circular dichroism. Hence the incident linearly or circularly polarized electric fields are effectively absorbed by the metasurface in different proportions. Consequently, the metasurface assumes half- and quarter-wave-plate behaviors in different parts of the reflected optical spectrum. In particular, we show via full-wave simulations that the dichroic metascreen supports perfect linear-to-circular polarization conversion (circular dichroism) in two adjacent terahertz frequency bands. In two other terahertz bands, it rotates the incoming linearly polarized wave vector by ${90}^{\circ }$ (linear dichroism). Moreover, since graphene has a variable refractive-index dependence on its chemical potential, the dispersion characteristics can be shifted to neighboring frequencies within the early terahertz spectrum. We further demonstrate an angularly stable response for incident angles varying between $0^{\circ }$ and ${45}^{\circ }$. The tunable linear and circular dichroism characteristics are well suited for applications in sensing, imaging, and spectroscopy at terahertz frequencies.

The competition between plasmonic resonances and noble-metal interband transitions at near-ultraviolet (NUV) frequencies complicates nonlinear conversion processes at nanoscales. Here, we experimentally design a highly enhanced and reproducible plasmon-enhanced second-harmonic generation (PESHG) platform suitable for NUV frequencies by constructing three-dimensional silver mushroom arrays. The SHG emission from proposed platforms involved in geometrical asymmetry greatly outperforms that from unpatterned metallic films. Studies among mushroom arrays with various geometric parameters and weight ratios of component materials enable us to distinguish characteristics exhibited by plasmon-driven enhancement and interband-transition suppression, such that competitive effects on PESHG can be readily observed and quantitatively measured. Our work may provide insights for understanding the physical mechanism governing plasmon-enhanced nonlinear optical processes and may help attain efficient nonlinear wavelength conversions for quantum-optical and nonlinear metamaterial applications.

Here, the ${\mathrm{Bi}\mathrm{Fe}\mathrm{O}}_3/{\mathrm{Sr}\mathrm{Ru}\mathrm{O}}_3$ heterostructure is fabricated and the anomalous Hall effect (AHE) is investigated. The nonmonotonic anomalous Hall resistivity behavior in ${\mathrm{Bi}\mathrm{Fe}\mathrm{O}}_3/{\mathrm{Sr}\mathrm{Ru}\mathrm{O}}_3$ originates from the inhomogeneous ${\mathrm{Sr}\mathrm{Ru}\mathrm{O}}_3$ layer instead of the topological Hall effect. It is surprising that the AHE in the ${\mathrm{Bi}\mathrm{Fe}\mathrm{O}}_3/{\mathrm{Sr}\mathrm{Ru}\mathrm{O}}_3$ structure can be manipulated by switching the ferroelectric polarization of ${\mathrm{Bi}\mathrm{Fe}\mathrm{O}}_3$. Furthermore, the modulation of AHE in ${\mathrm{Sr}\mathrm{Ru}\mathrm{O}}_3$ under ferroelectric polarization is discussed with respect to first-principles calculations and originates from the change of band structure around the Fermi level. The ferroelectric-manipulated AHE suggests a pathway to realize nonvolatile, reversible, and low-energy-consuming voltage-controlled spintronic devices.

Sandstone mechanical stability is of key concern in projects involving injections of ${\mathrm{CO}}_2$ in sandstone geological reservoirs, for the purpose of long-term storage. We develop a method to measure nanometer-scale deformations of sandstones in real time. We demonstrate that Berea sandstone, when hydrated, changes dimensions with a relative deformation of the order of ${10}^{-4}$. If the moisture content increases, sandstone samples exhibit an extension and if the moisture content decreases then the samples shrink. We also discover that, immediately after exposure to water, the sandstone temporarily shrinks, just for a few seconds, after which time a slow extension begins and continues until about half of the fluid evaporates. Such shrinkage followed by an extension is also observed when the sample is exposed to acetone, mineral spirits, or vacuum oil. The results are obtained using a high-resolution nanopositioner technique and, in independent experiments, confirmed using the technique of coda wave interferometry.

We study the performance limit of single-mode operation in low-loss large-V-number planar waveguide lasers, with transverse spatial hole burning as the dominating mechanism for transverse-mode competition. By introducing normalized variables, we develop a simple semianalytical model to describe the universal output characteristics of single-mode emission before the onset of high-order modes, which can be easily scaled to a wide range of laser configuration parameters. Our model is validated using exact numerical solutions, which show applicability beyond that of the low-loss approximation. Our analysis establishes a minimum criterion of the loss ratio between the fundamental and first higher-order mode for a robust single-mode operation. This criterion is much weaker than that of conventional wisdom based on the pure wave-propagation argument, which we attribute to a resonator enhancement effect. Our universal outcome can be denormalized to establish the relationship between single-mode extraction efficiency and optimum output coupling in a multimode laser with arbitrary modal loss ratios over a wide range of single-pass unsaturated gain and loss.

Important discoveries have frequently been made through studies of matter under high pressure. Conditions of the pressure environment are important for the interpretation of experimental results. Due to various restrictions inside the pressure cell, detailed information relevant to the pressure environment, such as the pressure distribution, can be hard to obtain experimentally. Here we present the study of pressure distributions inside the pressure medium under different experimental conditions with $\mathrm{N}$-$V^{-}$ centers in diamond particles as the sensor. These studies not only show a good spatial resolution, wide temperature and pressure working ranges, compatibility of the existing pressure cell design with the proposed method, but also demonstrate the usefulness to measure with these sensors as the pressure distribution is sensitive to various factors. The method and the results will benefit many disciplines such as material research and phase transitions in fluid dynamics.