The nitrogen-vacancy ($\mathrm{N}$-$V$) center in diamond has a wide range of potential applications in quantum metrology, communications, and computation. The key to its use lies in how large the optical spin contrast is and the associated fidelity of the spin-state readout. In this paper, we propose a mechanism for improving contrast with a spin-to-charge protocol that relies on the use of an external electrode and cryogenic temperatures to discretize the diamond conduction band for spin-selective resonant photoionization. We use effective-mass theory to calculate the discrete eigenenergies in this system and use them to formulate a spin-to-charge protocol that involves resonant photoionization out of the $\mathrm{N}$-$V$ ground state into the diamond conduction band. The major sources of broadening, which guide the design of the experiment, are also addressed. With this mechanism, we calculate an optical spin contrast that and an associated spin-readout fidelity of 85%. This significant improvement can be applied to a number of cryogenic quantum technologies.

Single-photon detectors operating on satellites for use in a quantum communications network can incur large dark-count-rate increases from the natural radiation environment of space. Displacement damage to the material lattice of a detector from the ionizing radiation can result in a permanent dark-count increase in the detector. In this work, we analyze the radiation-induced dark-count rate of a silicon single-photon avalanche diode onboard a satellite at different orbiting altitudes, as well as, the additional radiation from a nuclear-disturbed environment caused by a high-altitude nuclear explosion. For detectors on low Earth orbit satellites, protons are the biggest source of radiation damage and are best mitigated by choosing an orbit that minimizes exposure when passing through the South Atlantic Anomaly and Polar Cusps. Detectors on medium Earth orbit and geostationary orbit satellites, if shielded by more than 10 mm of aluminum, provide the best platform in terms of the least amount of radiation damage to the detectors. In the event of a high-altitude nuclear explosion, the artificial radiation belts produced by the explosion will cause too much damage to silicon single-photon detectors on low Earth orbit satellites and render them unfit for quantum communications in less than a day. Higher orbit satellites will only suffer minor dark-count-rate increases from the artificial radiation belts.

We present a simple approach to produce multiple correlated photon pairs through spontaneous parametric down-conversion. We develop Gaussian masks to subdivide the pump beam before passing it through a nonlinear medium. In this way, we are able to observe simultaneous separate down-converted emission cones with spatial overlap. The technique we employ could be used to greatly increase the dimensionality of entangled photonic systems generated from spontaneous parametric down-conversion, affording greater scalability to optical quantum computing than previously explored.

We show scalable and complete suppression of the recently reported terahertz-pulse-induced switching between different resistance states of antiferromagnetic $\mathrm{Cu}\mathrm{Mn}\mathrm{As}$ thin films by ultrafast gating. The gating functionality is achieved by an optically generated transiently conductive parallel channel in the semiconducting substrate underneath the metallic layer. The photocarrier lifetime determines the timescale of the suppression. As we do not observe a direct impact of the optical pulse on the state of $\mathrm{Cu}\mathrm{Mn}\mathrm{As}$, all observed effects are primarily mediated by the substrate. The sample region of suppressed resistance switching is given by the optical spot size, thereby making our scheme potentially applicable for transient low-power masking of structured areas with feature sizes of about 200 nm and even smaller.

Conventional polarized light interference is realized by using the natural crystals sandwiched between two linear polarizers, which has applications in optical signal modulation and photomechanics. However, due to a shortage of suitable birefringent materials, a similar effect has not been reported in the low-frequency range. In this paper, a compact and tunable polarization interference system working in the microwave band has been proposed, employing artificially structured planar materials, i.e., two metal screens perforated with subwavelength apertures. One screen splits the incident linear polarization into two orthogonal electric components and the other combines the parallel electric components (in a certain direction) to induce an interference. We find that the near-field coupling between the two metal screens plays a crucial role and the polarization interference effect is strongly dependent on the lateral dislocation of the system. The theory and experiment demonstrated that, by laterally displacing one metal screen (at the millimeter scale), a nearly linear modulation of transmission intensity (93.1%–1.4%) can be achieved. The proposed effect can be extended to the THz range and may be valuable for constructing the microwave or THz modulators.

The coherent transduction between microwave and optical frequencies is critical to interconnect superconducting quantum processors over long distances. However, it is challenging to establish such a quantum interface with high efficiency and small added noise based on the standard direct conversion scheme. Here, we propose an electro-optic transduction system based on continuous-variable quantum teleportation. Reliable quantum-information transmission can be realized with an arbitrarily small cooperativity, in contrast to the direct conversion scheme, which requires a large minimum cooperativity. We show that the teleportation-based scheme maintains a significant rate advantage robustly for practical thermal noise and all values of cooperativity. We further investigate the performance in the transduction of complex quantum states such as cat states and Gottesman-Kitaev-Preskill (GKP) states and show that a higher fidelity can be achieved with the teleportation-based scheme. Our scheme significantly reduces the device requirement, and makes quantum transduction between microwave and optical frequencies feasible in the near future.

Self-injection locking is a dynamic phenomenon, which provides passive stabilization of a laser emission frequency via resonant optical feedback. The stabilization coefficient depends on the level of the feedback and quality factor of the external resonant structure creating the feedback. A conventional self-injection locked laser (SIL) based on a dielectric cavity involves barely tunable resonant Rayleigh scattering (RRS). In this work we study theoretically a scheme of a SIL via a high-$Q$ microresonator with a drop-port coupled mirror, in which the optical feedback level is optimally adjusted by tuning the drop-port mirror coupling. We show that the additional reflector can improve the laser stabilization and power handling efficiency if compared with the classic RRS-based scheme.

Establishing and approaching the fundamental limit of orbital-angular-momentum (OAM) multiplexing are necessary and increasingly urgent for current multiple-input multiple-output research. In this work, we elaborate the fundamental limit in terms of independent scattering channels (or the degrees of freedom of scattered fields) through angular-spectral analysis, in conjunction with a rigorous Green’s function method. The scattering-channel limit is universal for arbitrary spatial-mode multiplexing, which is launched by a planar electromagnetic device, such as antenna, metasurface, etc., with a predefined physical size. As a proof of concept, we demonstrate both theoretically and experimentally the limit by a phase-only metasurface hologram that transforms orthogonal OAM modes to plane-wave modes scattered at critically separated angular-spectral regions. Particularly, a minimax optimization algorithm is applied to suppress angular-spectrum aliasing, achieving good performances in both full-wave simulation and experimental measurement at microwave frequencies. This work offers a theoretical upper bound and corresponding approach route for engineering designs of OAM multiplexing.

We investigate the ultrastrong tunable coupler for coupling of superconducting resonators. The obtained coupling constant exceeds 1 GHz, and the wide-range tunability is achieved for both antiferromagnetics and ferromagnetics from $-1086$ to 604 MHz. The ultrastrong coupler is composed of a rf superconducting quantum interference device (SQUID) and dc SQUID as tunable junctions, which connect to resonators via shared aluminum thin-film meander lines enabling such a huge coupling constant. The spectrum of the coupler obviously shows the breaking of the rotating-wave approximation, and our circuit model treating the Josephson junction as a tunable inductance reproduces the experimental results well. The ultrastrong coupler is expected to be utilized in quantum annealing circuits and/or noisy intermediate-scale quantum devices with dense connections between qubits.

We propose a one-step scheme to implement the time-optimal nonadiabatic holonomic three-qubit controlled gates in Rydberg atoms by modulating Rabi frequencies of driving fields on the target atom. We perform the time-optimal operations following the brachistochrone curve, after which the scheme manifests completely different characteristics in contrast to the conventional nonadiabatic holonomic quantum computation (NHQC) that all operations require exactly the same evolution time, even for small angular rotations. Since the present three-qubit controlled-gate scheme significantly reduces the evolution time, the influence of decoherence can be greatly reduced. On the other hand, the robustness on the systematic error can also be guaranteed by the feature of geometric phase. The simulation through master equation verifies the high fidelity and robustness of our scheme. Although we use the Rydberg-atom system to verify the feasibility of our scheme, any quantum platforms capable of achieving the same form of effective Hamiltonian is feasible, which means the proposed three-qubit time-optimal-control nonadiabatic holonomic gates is independent of the particular physical platform and may have the potential for wider application.

We approach the inverse problem of imaging as a task of automated feature learning of the underlying wave equation. While feature extraction with unsupervised machine learning is widely used in analyzing complex data on clustering, classification, and visualization, we show its direct usage on discovering interpretable physical concepts and then obtaining images from wave-propagation data. By using a spring-mass lattice as a simplified model in acoustic imaging, a variational autoencoder is trained to extract features that govern the dynamics of wave propagation from configurations with uncorrelated random material parameters. The extracted features are transformed to images of spring constants and masses by an additional linear regression. The current scheme of extraction-based inverse imaging of features is robust against noise in wave-propagation data and incomplete accessibility in the case of inverse scattering in practice. Our approach requires minimal prior knowledge of the wave-scattering mechanism with applications from the extraction of physical constants and defect detection to discovering physical phenomena.

We study the impact of experimental imperfections in integrated optical circuits. We discuss the emergence of a moderate biased error in path encoding, and investigate its correlation with properties of the optical paths. Our analysis connects and deepens previous studies in this direction, revealing potential issues in high-precision tests and optical implementations of machine learning.

Nonlocal point-to-point correlations between two photons have been used to produce “ghost” images without placing the camera towards the object. Here we theoretically demonstrate and analyze the advantage of non-Gaussian quantum light in improving the image quality of a ghost imaging system over a traditional Gaussian light source. For any squeezing degree, the signal-to-noise ratio (SNR) of the ghost image can be enhanced by the non-Gaussian operations of photon addition and subtraction on a two-mode squeezed light source. We find striking evidence that using non-Gaussian coherent operations, the SNR can be promoted to a high level even within the extremely weak squeezing regime. The resulting insight provides experimental recipes of quantum imaging using non-Gaussian light for illumination.

The canonical way to sustain photonic topological edge states relies on the interface formed by topologically trivial and nontrivial photonic crystals that share a common band gap to prevent the coupling of edge states from the radiation continuum of free space. Herein, we report a scheme to achieve bound topological edge states in the continuum at the air edge of a single topologically nontrivial photonic crystal utilizing the physics of bound states in the continuum. These localized states, though existing in the continuum, exhibit topological features inherited from the nontrivial topology of bulk photonic bands, such as pseudospin-momentum locking unidirectional propagation. Microwave experiments are provided to further validate the theoretical predictions. Our work not only has practical importance but also opens an avenue where the nontrivial topology of bulk photonic bands could be exploited to enrich the physics of bound states in the continuum.

Conventional acoustic metasurfaces are constructed with “locally” gradient phase-shift profiles provided by subunits. The local strategy implies the ignorance of the mutual coupling between subunits, which limits the efficiency of targeted sound manipulation, especially in complex environments. By taking into account the “nonlocal” interaction among subunits, nonlocal metasurface offers an opportunity for accurate control of sound propagation, but the requirement of the consideration of gathering coupling among all subunits, not just the nearest-neighbor coupling, greatly increases the complexity of the system and therefore hinders the explorations of functionalities of nonlocal metasurfaces. In this work, empowered by deep-learning algorithms, the complex inverse gathering coupling can be learned efficiently from the preset dataset so that the inverse mechanism of nonlocal metasurfaces can be described effectively. As an example, we demonstrate that nonlocal metasurfaces, which can redirect an incident wave into multichannel reflections with arbitrary energy ratios, can be accurately predicted by deep-learning algorithms. Compared to the theory, the relative error of the energy ratios is less than 1%. Furthermore, experiments witness three-channel reflection with three types of energy ratios of (1, 0, 0), (1/2, 0, 1/2), and (1/3, 1/3, 1/3), proving the validity of the deep-learning-enabled nonlocal metasurfaces. Our work might blaze an alternative trail in the design of acoustic functional devices, especially for the cases containing complex wave-matter interactions.

Planar structure and small atomic mass usually induce high lattice thermal conductivity in a crystalline solid. Here, we present a first-principles study on the lattice dynamics and phonon-transport properties of a series of ABX honeycomb compounds and show that replacing cations with lighter atoms may yield lower lattice thermal conductivity $({\kappa }_l)$. The anomalous mass-trend ${\kappa }_l$, which is experimentally observed, is found to be inherent to the lattice vibration of the specific layered honeycomb structure, using A$\mathrm{Cu}\mathrm{Sb}$ (A = $\mathrm{Ca}$, $\mathrm{Sr}$, and $\mathrm{Ba}$) as prototypical compounds. We reveal that a lighter A atom bonds more weakly with the $\mathrm{Cu}\text{−}\mathrm{Sb}$ honeycomb ring, giving rise to stronger vibrational anharmonicity, which induces ultralow ${\kappa }_l$ in $\mathrm{Ca}\mathrm{Cu}\mathrm{Sb}$ that is unexpected of light elements and a high-symmetry planar structure. Combined with the high degeneracy and strong anisotropy of the bottom conduction band, $\mathrm{Ca}\mathrm{Cu}\mathrm{Sb}$ exposes an ultrahigh n-type zT of about 2.2 at 700 K. These findings elucidate the mechanism governing anomalous phonon-transport behavior, which also offers guidance in discovering low-cost and low-mass-density materials for advanced thermoelectric applications.

We demonstrate a method to reduce number fluctuations in an ultracold atomic sample using real-time feedback. By measuring the Faraday rotation of an off-resonant probe laser beam with a pair of avalanche photodetectors in a polarimetric setup, we produce a proxy for the number of atoms in the sample. We iteratively remove a fraction of the excess atoms from the sample to converge on a target proxy value in a way that is insensitive to environmental perturbations and robust to errors in light polarization. Using absorption imaging for out-of-loop verification, we demonstrate a reduction in the number fluctuations from $3\mathrm{\%}$ to $0.45\mathrm{\%}$ for samples at a temperature of $16.4\mu \mathrm{K}$ over the time scale of several hours, which is limited by temperature fluctuations, beam-pointing noise, and photon shot noise.

We develop a concept of metasurface-assisted ghost imaging for nonlocal discrimination between a set of polarization objects. The specially designed metasurfaces are incorporated in the imaging system to perform parallel state transformations in general elliptical bases of quantum-entangled or classically correlated photons. Then, only four or fewer correlation measurements between multiple metasurface outputs and a simple polarization-insensitive bucket detector after the object can allow for the identification of fully or partially transparent polarization elements and their arbitrary orientation angles. The approach can find applications for real-time and low-light imaging across diverse spectral regions in dynamic environments.

Quantum holonomic gates hold built-in resilience to local noises and provide a promising approach for implementing fault-tolerant quantum computation. We propose to realize high-fidelity holonomic $(N+1)$-qubit controlled gates using Rydberg atoms confined in optical arrays or superconducting circuits. We identify the scheme, deduce the effective multibody Hamiltonian, and determine the working condition of the multiqubit gate. Uniquely, the multiqubit gate is immune to systematic errors, i.e., laser parameter fluctuations and motional dephasing, as the $N$ control atoms largely remain in the very stable qubit space during the operation. We show that $C_N$-not gates can reach the same level of fidelity at a given gate time for $N\le 5$ under a suitable choice of parameters, and the gate tolerance against errors in systematic parameters can be further enhanced through optimal pulse engineering. In the case of Rydberg atoms, the proposed protocol is intrinsically different from typical schemes based on Rydberg blockade or antiblockade. Our study paves an alternative way to build robust multiqubit gates with Rydberg atoms trapped in optical arrays or with superconducting circuits. It contributes to current efforts to develop scalable quantum computation with trapped atoms and fabricable superconducting devices.