A numerical security proof technique is used to analyze the security of continuous-variable quantum key distribution (CV-QKD) protocols with phase-shift keying (PSK) modulation against collective attacks in the asymptotic limit. We argue that it is sufficient to consider protocols with a maximum of eight signal states and analyze different postselection strategies for protocols with four (QPSK) and eight (8PSK) signal states for untrusted ideal and trusted nonideal detectors. We introduce a cross-shaped postselection strategy, and show that both cross-shaped and radial and angular postselection clearly outperform a radial postselection scheme (and no postselection) for QPSK protocols. For all strategies studied, we provide analytical results for the operators defining the respective regions in phase space. We outline several use cases of postselection, which can easily be introduced in the data processing of both new and existing CV-QKD systems. Motivated by the high computational effort for error correction, we study the case when a large fraction of the raw key is eliminated by postselection and observe that this can be achieved while increasing the secure key rate. Postselection can also be used to partially compensate the disadvantage of QPSK protocols over 8PSK protocols for high transmission distances, while being experimentally less demanding. Finally, we highlight that postselection can be used to reduce the key rate gap between scenarios with trusted and untrusted detectors while relying on less assumptions on Eve’s power.

Due to its potential for implementing a scalable quantum computer, the multiqubit Toffoli gate lies in the heart of quantum information processing. In this paper, we demonstrate a multiqubit blockade gate with atoms arranged in a three-dimensional spheroidal array. The gate performance is greatly improved by the method of optimizing control-qubit distributions on the spherical surface via evolutionary algorithm, which leads to an enhanced asymmetric Rydberg blockade. This spheroidal configuration, not only preserves the dipole blockade energy well between arbitrary control-target pairs, which keeps the asymmetric blockade error at a very low level, but also manifests an unprecedented robustness to the spatial position variations, leading to a negligible position error. Taking account of intrinsic errors and using typical experimental parameters, we numerically show that a c${}_6$not Rydberg gate can be created with a fidelity of 0.992, which is only limited by the Rydberg state decays. Our protocol opens up a platform of higher-dimensional atomic arrays for achieving multiqubit neutral-atom quantum computation.

To determine the effect of depolarization and the critical thickness in improper-ferroelectric hexagonal-ferrite thin films, we investigate the polarization switching of a ferroelectric/dielectric bilayer in capacitor structures at 20 K. Experimentally, we show that the spontaneous polarization persists throughout the studied thickness range (3 to 80 unit cell), even with a thick (10-nm) dielectric layer, suggesting no practical thickness limit for applications. By fitting the effect of depolarization using the phenomenological theory, we show that the spontaneous polarization remains finite when the thickness of the ferroelectric layer approaches zero, providing a hint for the absence of critical thickness. We also find that the interfacial effects limit the multidomain formation and govern the polarization switching mechanisms.

A plasmonic based nanocircuit transplants the lumped property of classical electronic circuits to optical frequencies, providing a circuit paradigm for optics and photonics. To broaden the material and frequency selections, the concept of a waveguide metamaterial is applied to design lumped elements by utilizing the waveguide’s structural dispersion to emulate the natural material’s dispersion. Here, as an important application of the physics of waveguide metamaterials, a general type of impedance-matching network directly integrated with a waveguide is investigated for various matching occasions of guided waves, outperforming the classical ones using waveguide membranes. Three representative matching examples are analyzed to validate the feasibility, namely, transmission discontinuities, active power amplifiers, and antennas. The numerical and experimental results confirm the impedance-matching applications of the waveguide metamaterial and suggest the potential merits of low loss and low crosstalk for the proposed waveguide-integrated matching networks in terahertz integrated-circuit designs.

The spatial resolution of an optical fiber bundle is limited by the size of a mode propagating in an individual fiber, which can be as small as a single wavelength approximately $\lambda$ for conventional low-to-medium refractive-index optical fibers. High-refractive-index optical fiber bundles have a potential for the subwavelength-resolution imaging due to strong mode confinement in a fiber core, but they suffer from inconvenient image readout from the bundle output end, which is done by energy-inefficient near-field probes. To address this issue, this work introduces a tapered high-refractive-index terahertz (THz) optical fiber bundle. It comprises the subwavelength-diameter sapphire optical fibers, that are stacked tightly in the object plane to sample the near field with a subwavelength resolution. The fibers diverge from the object plane and, thus, stretch the captured near field for its further read out from the output bundle end using conventional diffraction-limited lens. Such a tapered fiber bundle was investigated numerically and then implemented experimentally at $0.33$ THz. The subwavelength resolution of this bundle was confirmed, and varies over its aperture and can be as high as $0.35\lambda$. The developed imaging principle allows the $\simeq 0.5\lambda$ Abbe resolution limit of a free-space focusing to be overcome and almost any common diffraction-limited optics for the near-field applications to be adapted.

Dual-comb interferometry is based on self-heterodyning two optical frequency combs, with corresponding mapping of the optical spectrum into the radio-frequency domain. The dual comb enables diverse applications, including metrology, fast high-precision spectroscopy with high signal-to-noise ratio, distance ranging, and coherent optical communications. However, current dual-frequency-comb systems are designed for research applications and typically rely on scientific equipment and bulky mode-locked lasers. Here we demonstrate a fully integrated power-efficient dual-microcomb source that is electrically driven and allows turnkey operation. Our implementation uses commercially available components, including distributed-feedback and Fabry-Perot laser diodes, and silicon-nitride photonic circuits with microresonators fabricated in commercial multiproject wafer runs. Our devices are therefore unique in terms of size, weight, power consumption, and cost. Laser-diode self-injection locking relaxes the requirements on microresonator spectral purity and $Q$ factor, so that we can generate soliton microcombs resilient to thermal frequency drift and with pump-to-comb sideband efficiency of up to 40% at mW power levels. We demonstrate down-conversion of the optical spectrum from 1400 to 1700 nm into the radio-frequency domain, which is valuable for fast wide-band Fourier spectroscopy, which was previously not available with chip-scale devices. Our findings pave the way for further integration of miniature microcomb-based sensors and devices for high-volume applications, thus opening up the prospect of innovative products that redefine the market of industrial and consumer mobile and wearable devices and sensors.

Surface acoustic waves (SAWs) coupled to quantum dots (QDs), trapped atoms and ions, and point defects have been proposed as quantum transduction platforms, yet the requisite coupling rates and cavity lifetimes have not been experimentally established. Although the interaction mechanism varies, small acoustic cavities with large zero-point motion are required for high efficiencies. We experimentally establish the feasibility of this platform through electro- and optomechanical characterization of tightly focusing, single-mode Gaussian SAW cavities at approximately 3.6 GHz on $\mathrm{Ga}\mathrm{As}$. We explore the performance limits of the platform by fabricating SAW cavities with mode volumes approaching $6{\lambda }^3$ and linewidths less than 1 MHz. Employing strain-coupled single $\mathrm{In}\mathrm{As}$ QDs as optomechanical intermediaries, we measure single-phonon optomechanical coupling rates $g_0\approx 2\pi \times 1.2$ MHz. Sideband scattering rates thus exceed intrinsic phonon loss, indicating the potential for quantum optical readout and transduction of cavity phonon states. To demonstrate the feasibility of this platform for low-noise ground-state quantum transduction, we develop a fiber-based confocal microscope in a dilution refrigerator and perform single-QD resonance fluorescence sideband spectroscopy at millikelvin temperatures. These measurements show conversion between microwave phonons and optical photons with sub-natural linewidths.

Topological insulators have attracted great interest in recent years because of their robust transports of topological boundary states. Usually, the topological boundary states are sharply localized at the boundaries, which limits the capacity of transmissions and the interfacing with existing bulk devices in applications. Here, based on the acoustic topological insulators with pseudospin, we realize topological heterostructures that host a pair of helical waveguide modes. With the freedom of width, the waveguide modes are of adjustable capacity of transmissions, and convenient for applications. The waveguide modes are revealed to be robust against disorders and structural defects. We also demonstrate the partition and beam collimation of the topological waveguide modes. These results pave a way to explore the topological transports in versatile designs of heterostructures for acoustic waves.

The manipulation of magnetism with a gate voltage is expected to lead to the realization of energy-efficient spintronics devices and high-performance magnetic memories. Exploiting magnetoionic effects under micropatterned electrodes in solid-state devices adds the possibility of modifying magnetic properties locally, in a nonvolatile and reversible way. Tuning magnetic anisotropy, magnetization and Dzyaloshinskii-Moriya interaction allows the modification “at will” of the dynamics of nontrivial magnetic textures such as skyrmions and chiral domain walls in magnetic race tracks. In this work, we illustrate efficient magnetoionic effects in a ferrimagnetic $\mathrm{Pt}$/$\mathrm{Co}$/$\mathrm{Tb}$/${\mathrm{Al}\mathrm{O}}_x$ stack using a ${\mathrm{Zr}\mathrm{O}}_2$ thin layer as a solid-state ionic conductor. When a thin layer of terbium is deposited on top of cobalt, it acquires a magnetic moment that aligns antiparallel to that of cobalt, reducing the effective magnetization. Below the micropatterned electrodes, the voltage-driven migration of oxygen ions in ${\mathrm{Zr}\mathrm{O}}_2$ toward the ferrimagnetic stack partially oxidizes the $\mathrm{Tb}$ layer, leading to the local variation not only of the magnetization, but also of the magnetic anisotropy and of the Dzyaloshinskii-Moriya interaction. This leads to a huge increase in the domain wall velocity, which varies from 10 m/s in the pristine state to 250 m/s after gating. This nonvolatile and reversible tuning of the domain wall dynamics may lead to applications to reprogrammable magnetic memories or other spintronic devices.

A strong superconducting diode effect (SDE) is revealed in a thin superconducting film periodically nanostructured with magnetic dots. The SDE is caused by the current-activated dissipation mitigated by vortex-antivortex pairs (VAPs), which periodically nucleate under the dots, move and annihilate in the superconductor—eventually driving the system to the high-resistive state. Inversing the polarity of the applied current destimulates the nucleation of VAPs, the system remains superconducting up to far larger currents, leading to the pronounced diodic response. Our dissipative Ginzburg-Landau simulations detail the involved processes, and provide reliable geometric and parametric ranges for the experimental realization of such a nonvolatile superconducting diode, which operates in the absence of any applied magnetic field while being fluxonic by design.

We analyze the cross-resonance effect for fluxonium circuits and investigate a two-qubit gate scheme based on selective darkening of a transition. In this approach, two microwave pulses at the frequency of the target qubit are applied simultaneously with a proper ratio between their amplitudes to achieve a controlled-not operation. We study in detail coherent gate dynamics and calculate gate error. With nonunitary effects accounted for, we demonstrate that gate error below ${10}^{-4}$ is possible for realistic hardware parameters. This number is facilitated by long coherence times of computational transitions and strong anharmonicity of fluxoniums, which easily prevents excitation to higher excited states during the gate microwave drive.

We perform comprehensive experimental tests of various composite pulse sequences using one of the open-access IBM quantum processors, based on superconducting transmon qubits. We implement explicit pulse control of the qubit by making use of the opportunity of low-level access to the backend, provided by IBM Quantum. We obtain the excitation profiles for a huge variety of broadband, narrowband, and passband composite pulses, producing any prechosen target probabilities, ranging from zero to one. We also test universal composite pulses that compensate errors in any experimental parameter. In all experiments, we find excellent agreement between theoretical and experimental excitation profiles. This proves both the composite pulses as a very efficient and flexible quantum control tool and the high quality of the IBM quantum processor. As an extreme example, we test and observe a pronounced narrowband excitation profile for a composite sequence of as many as 1001 pulses.

Due to the similarities in electrochemical and physicochemical properties of sodium and lithium, sodium-ion batteries are an attractive substitute for lithium-ion batteries. The selection of appropriate electrode materials for $\mathrm{Na}$-ion batteries is an important step to achieve high stability and theoretical capacity. Here, the effects of heteroatom doping and charge variation on the ability of the ${\mathrm{Mo}\mathrm{S}}_2$ substrate to capture $\mathrm{Na}$ ions are studied using first-principles calculations. The results show that the interaction between $\mathrm{Na}$ ions and the ${\mathrm{Mo}\mathrm{S}}_2$ substrate is enhanced by electron deficiency or substituting a $\mathrm{S}$ atom with a dopant atom (X) with fewer valence electrons (X = $\mathrm{B}$, $\mathrm{Al}$, $\mathrm{Ga}$, $\mathrm{In}$, $\mathrm{C}$, $\mathrm{Si}$, $\mathrm{Ge}$, $\mathrm{Sn}$, $\mathrm{N}$, $\mathrm{P}$, $\mathrm{As}$, $\mathrm{Sb}$). In contrast, adding electrons to the ${\mathrm{Mo}\mathrm{S}}_2$ substrate weakens its interaction with $\mathrm{Na}$. We further analyze the diffusion barrier, open-circuit voltage, and theoretical capacity of hypothetical $\mathrm{Na}$-ion batteries using doped $X\text{−}{\mathrm{Mo}\mathrm{S}}_2$ (X = heteroatom) as electrode materials. We find that $\mathrm{Al}\text{−}$ or $\mathrm{Si}$-doped ${\mathrm{Mo}\mathrm{S}}_2$ can serve as good electrode materials for $\mathrm{Na}$-ion batteries, with a high theoretical capacity that exceeds 700 mAh/g. This work provides practical guidance for the development of electrode materials of $\mathrm{Na}$-ion batteries using two-dimensional materials.

Quantum-optical coherence tomography (QOCT) is an optical sectioning modality based on the quantum interference of entangled photon pairs in the well-known Hong-Ou-Mandel interferometer. Despite its promise, the technique is far from being competitive with current classical OCT devices due to the long required acquisition times, derived from the low photon-pair emission rates. In this work, we, on the one hand, demonstrate a quantum optical coherence microscopy technique, based on full-field QOCT, which employs entangled collinear photon pairs in a Linnik interferometer designed to overcome some of the limitations of previous QOCT implementations, and, on the other hand, test it on representative samples, including glass layers with manufactured transverse patterns and metal-coated biological specimens. In our setup, while the idler photon is collected with a multimode fiber, the signal photon is detected by an intensified charge-coupled device camera, leading to full-field transverse reconstruction through a single-axial acquisition sequence. Interestingly, our setup permits concurrent OCT and QOCT trace acquisition, the former with greater counts and the latter with the benefit of quantum-conferred advantages. We hope that our current results will represent a significant step forward towards the actual applicability of QOCT, e.g., in clinical settings.

Antineutrinos are an inextricable element of the fission process. The kiloton-scale KamLAND experiment has demonstrated a capability to detect reactor antineutrinos at a range of a few hundred kilometers. But to detect or rule out the existence of a single small reactor over many kilometers requires a large detector, so large in fact that the optical opacity of the detection medium itself becomes an important factor. If the detector is so large that photons cannot traverse the detector medium to an optical detector, then it becomes impractical. For this reason, gadolinium-doped-water Cherenkov detectors have been proposed for large volumes, due to their appealing light-attenuation properties. Even though Cherenkov emission does not produce many photons and the energy resolution is poor, there may be a place for Gd-doped-water detectors in far-field nuclear-reactor monitoring. In this paper, we focus on the reactor-discovery potential of large-volume Gd-doped-water Cherenkov detectors for nuclear-nonproliferation applications. Realistic background models for the worldwide reactor flux, geoneutrinos, cosmogenic fast neutrons, and detector-associated backgrounds are included. We calculate the detector run time required to detect a small 50-MWt reactor at a variety of stand-off distances as a function of detector size. We highlight that, at present, the photomultiplier-tube dark rate and event reconstruction algorithms are the limiting factors to extending such detectors beyond a fiducial mass of approximately 50 kt.

Integration of active elements into silicon wafers is the first step towards their usage in modern electronic devices based on nanometric structures. Spintronic terahertz emitters, typically composed of nanometer-thin magnetic multilayer, have the outstanding capability of producing high-quality, broadband terahertz pulses using extremely simple heterostructures. A question remains on whether an efficient and cheap integration with other silicon-based technologies can be achieved. We show here that simply having a ferromagnetic layer on silicon produces remarkably efficient spintronic terahertz emission despite the low spin-orbit coupling of the individual components. We achieve this by leveraging on the natural formation of silicides at the interface of a transition metal and silicon. The cobalt silicide layer has good spin-to-charge conversion efficiency that reaches around 1/6 as that of the prototypical spintronics THz-emitter heterostructure cobalt/platinum.

Hyperspectral imaging (HSI) records a series of two-dimensional (2D) images for different wavelengths to provide the chemical fingerprint at each pixel. Combining HSI with a tomographic data-acquisition method, we can obtain the chemical fingerprint of a sample at each point in three-dimensional (3D) space. The so-called 3D HSI typically suffers from low imaging throughput due to the requirement of scanning the wavelength and rotating the beam or sample. In this paper, we present an optical system that captures the entire four-dimensional (4D; i.e., 3D structure and one-dimensional spectrum) data set of a sample with the same throughput of conventional HSI systems. Our system works by combining snapshot projection optical tomography (SPOT), which collects multiple projection images with a single snapshot, and Fourier-transform spectroscopy (FTS), which results in superior spectral resolution by collecting and processing a series of interferogram images. Using this hyperspectral SPOT system, we image the volumetric absorbance of dyed polystyrene microbeads, oxygenated red blood cells (RBCs), and deoxygenated RBCs. The 4D optical system demonstrated in this paper provides a tool for high-throughput chemical imaging of complex microscopic specimens.

An ultrathin underwater metasurface with low frequency, broadband, high-efficiency absorption, and high-hydrostatic-pressure-resistance performance is proposed based on the integrated principle of local resonance and cavity resonance. The design-parameter complexity is overcome by the global-optimization capability of the genetic algorithm. As a demonstration, an average sound-absorption coefficient of 92.3% at 500–10 000 Hz with an ultrathin thickness of 32 mm is achieved, including more than 99% sound-absorption coefficient at 4670–8630 Hz. In the proposed metasurface, the complex surface impedances of the three subsurfaces provide unique views for the high-efficiency sound-absorption coupling mechanism by the equivalent-circuit model. Further investigation demonstrates that the excellent property of high-efficiency sound absorption is maintained under 4.5-MPa hydrostatic pressure. The proposed metasurface provides more possibilities for underwater noise suppression.