With the increasing crossover between quantum information and machine learning, quantum simulation of neural networks has drawn unprecedentedly strong attention, especially for the simulation of associative memory in Hopfield neural networks due to their wide applications and relatively simple structures that allow easier mapping to the quantum regime. Quantum stochastic walk, a strikingly powerful tool to analyze quantum dynamics, has recently been proposed to simulate the firing pattern and associative memory with a dependence on the Hamming distance. We successfully map the theoretical scheme into a three-dimensional photonic quantum chip and realize quantum-stochastic-walk evolution through well-controlled detunings of the propagation constant. We demonstrate a good match rate of the associative memory between the experimental quantum scheme and the expected result for Hopfield neural networks. The ability of quantum simulation for an important feature of a neural network combined with the scalability of our approach through low-loss-integrated-chip and straightforward Hamiltonian engineering provides a primary but steady step toward photonic artificial-intelligence devices for optimization and computation tasks with greatly increased efficiencies.

The silicon monovacancy in $4H$-$\mathrm{Si}\mathrm{C}$ is a promising candidate for solid-state quantum information processing. We perform high-resolution optical spectroscopy on single $V2$ defects at cryogenic temperatures. We find favorable low-temperature optical properties that are essential for optical readout and coherent control of its spin and for the development of a spin-photon interface. The common features among individual defects include two narrow, nearly lifetime-limited optical transitions that correspond to $m_s=\pm 3/2$ and $m_s=\pm 1/2$ spin states with no discernable zero-field-splitting fluctuations. Initialization and readout of the spin states are characterized by time-resolved optical spectroscopy under resonant excitation of these transitions, showing significant differences between the $\pm 3/2$ and $\pm 1/2$ spin states. These results are well described by a theoretical model that strengthens our understanding of the quantum properties of this defect.

Optical fabrication of waveguides in a volume is limited by diffraction in the writing beams. We demonstrate the use of nondiffracting waves in the form of Bessel beams to fabricate scalable optical wiring through direct writing in a photosensitive volume. Experiments are performed in paraelectric potassium-lithium-tantalate-niobate (KLTN), where writing occurs through photogenerated space charge while guiding and electro-optic functionality are supported by the quadratic electro-optic effect. The method allows components to be integrated sequentially without interfering with each other during fabrication, an intrinsic superposition property that is used to realize single, double, and multiple waveguides, and $1\times 2$, $1\times 3$, and $1\times 4$ splitters, and electrically controlled optical switching.

We argue that a high-purity $\mathrm{Yb}$-doped silica glass can potentially be cooled via anti-Stokes-fluorescence optical refrigeration. In order to achieve net solid-state optical refrigeration, it is necessary for the cooling efficiency to be positive. This requires the pump wavelength to be greater than the mean fluorescence wavelength and the internal quantum efficiency as well as the absorption efficiency to be near unity. Our conclusion is reached by showing, using reasonable assumptions for the host-material properties, that the nonradiative decay rate of $\mathrm{Yb}$ ions can be made substantially smaller than the radiative decay rate. Therefore, an internal quantum efficiency of near unity can be obtained. Moreover, the background absorption coefficient in high-quality silica glass lies in an acceptable range to guarantee a near-unity absorption efficiency at room temperature. Using spectral measurements of the fluorescence from a $\mathrm{Yb}$-doped silica optical fiber at a range of temperatures, we estimate the minimum achievable temperature in $\mathrm{Yb}$-doped silica glass for different values of the internal quantum efficiency.

Spiking neural networks (SNNs) offer an event-driven and more-biologically-realistic alternative to standard artificial neural networks based on analog information processing. This can potentially enable energy-efficient hardware implementations of neuromorphic systems that emulate the functional units of the brain; namely, neurons and synapses. Recent demonstrations of ultrafast photonic computing devices based on phase-change materials (PCMs) show promise for addressing limitations of electrically driven neuromorphic systems. However, scaling these stand-alone computing devices to a parallel in-memory computing primitive is a challenge. In this work, we use the optical properties of the PCM ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ to propose a photonic SNN computing primitive, comprising a nonvolatile synaptic array integrated seamlessly with previously explored “integrate-and-fire” neurons. The proposed design realizes an “in-memory” computing platform that leverages the inherent parallelism of wavelength-division multiplexing. We show that the proposed computing platform can be used to emulate a SNN inferencing engine for image-classification tasks. The proposed design not only bridges the gap between isolated computing devices and parallel large-scale implementation but also paves the way for ultrafast computing and localized on-chip learning.

Testing the limits in performance of device setups with respect to sizes and oscillation frequencies by trying every single combination of the available media can give a set of superior designs with novel properties. This approach is followed for the absorption and scattering of visible light by core-shell nanowires, whose optical thickness is simultaneously large enough to individually perturb the background field and small enough to support coherent collective operation. Numerous maximally performing particles are reported, one for each pair of the bulk materials used, providing a set of limits for the absorbed and scattered power in the presence of realistic losses. This procedure detects the supported resonances at core-shell nanorods, finds the best designs for such a simple geometry, and thus offers additional degrees of freedom in particle or metasurface modeling.

Strong nonlinear optical effects generally require giant optical fields interacting with the nonlinear media. Doped graphene hosts electrically tunable plasmons with long lifetimes that interact strongly with light. We investigate a graphene plasmonic coupler and explore two mechanisms to pursue highly efficient third-harmonic generation (THG): (1) phase matching of graphene plasmons at fundamental- and third-harmonic frequencies and (2) peak third-order nonlinear susceptibility of doped graphene. The third-harmonic wave is mainly converted from the evanescent mode of the incident light and the THG efficiency is found to be enhanced by over 10 orders of magnitude compared with a bare monolayer graphene. The significantly enhanced nonlinear optical responses in the graphene plasmonic coupler make this configuration an ideal platform for the development of alternative frequency generators and for signal processing at midinfrared and terahertz frequencies.

The interfacing of quantum platforms via photonic links is a precursor for establishing scalable quantum networks. The connection of different types of quantum memories for hybrid networking requires the overcoming of the disparate photon wavelengths emitted by each quantum memory. Given achievements in quantum information processing with trapped-ion and neutral-atom architectures, a hybrid system with modular interconnectivity is advantageous. Here, we use a trapped ${}^{138}{\mathrm{Ba}}^{+}$ ion and a periodically poled lithium-niobate (PPLN) waveguide, with a fiber-coupled output, to demonstrate 19% end-to-end efficient quantum frequency conversion (QFC) of single photons from 493 to 780 nm and use fluorescence of the ion to produce light at the ${}^{87}\mathrm{Rb}{D}_2$ transition wavelength. To demonstrate the quantum nature of both the unconverted 493 nm photons and the converted photons near 780 nm, we observe strong quantum statics in their respective second-order intensity correlations. This work extends the range of intralaboratory networking between ions and networking between disparate quantum memories.

The rapid developments in orbital-angular-momentum-carrying Laguerre-Gaussian ${\mathrm{(LG}}_l^0)$ modes in recent years have facilitated progress in optical communication, micromanipulation, and quantum information. However, it is still challenging to efficiently generate bright, pure, and selectable ${\mathrm{LG}}_l^0$ laser modes in compact devices. We demonstrate a low-threshold solid-state laser that can directly output selected high-purity ${\mathrm{LG}}_l^0$ modes with high efficiency and controllability. Spin-orbital angular momentum conversion of light is used to reversibly convert the transverse modes inside a cavity and determine the output mode index. The generated ${\mathrm{LG}}_1^0$ and ${\mathrm{LG}}_2^0$ laser modes have purities of approximately 97% and approximately 93% and slope efficiencies of approximately 11% and approximately 5.1%, respectively. Our cavity design can also be easily extended to produce higher-order Laguerre-Gaussian modes and cylindrical vector beams. Such a compact laser configuration features flexible control, low threshold, and robustness, making it a practical tool for applications in superresolution imaging, high-precision interferometers, and quantum correlations.

The operation and readout of a nonvolatile current-modulated four-state magnetoplasmonic metal-insulator-metal (M-I-M) waveguide device is modeled as a function of device and material parameters. Current pulses applied to electrically isolated stacks containing a ferromagnet and an active layer (either a metal with large spin-orbit coupling or a topological insulator) embedded within a plasmonic metal ($\mathrm{Au}$) situated on either side of the insulating core can switch the orientation of the magnetic layers via the spin-transfer torque mechanism. The four possible magnetic-orientation combinations interact with the surface-plasmon-polariton mode to produce four distinguishable values (on the order of 0.1–1%) of the effective index of refraction. By overcoming the optical losses traditionally associated with magnetic materials within a plasmonic environment, this four-state optical memory element is able to exploit both the robust magnetic stability of metallic magnets and the spin-torque switching mechanism that can reorient these magnets via on-chip currents without requiring external magnetic fields. Using this element, we propose a 2-stage plasmonic memory register with a range of states that can scale as $4^n$ for $n$ stages.

Dephasing induced by residual thermal photons in the readout resonator is a leading factor limiting the coherence times of qubits in the circuit quantum electrodynamics architecture. This residual thermal photon, on the order of ${10}^{-1}$–${10}^{-3}$, is suspected to arise from noise impinging on the resonator from its input and output ports. To address this problem, we designed and tested a new type of band-pass microwave attenuator that consists of a dissipative cavity well thermalized to the mixing chamber stage of a dilution refrigerator. By adding such a cavity attenuator in-line with a three-dimensional superconducting cavity housing a transmon qubit, we have reproducibly measured increased qubit coherence times. At base temperature, through a Hahn echo experiment, we measured $T_{2e}/2T_1=1.0\begin{array}{c}+0.0\\ -0.1\end{array}$ for two qubits over multiple cooldowns. Through noise-induced dephasing measurement, we obtained an upper bound of $2\times {10}^{-4}$ on the residual photon population in the fundamental mode of the readout cavity, which to our knowledge is the lowest value reported so far. These results validate an effective method for protecting qubits against photon noise, which can be developed into a standard technology for quantum circuit experiments.

Accurate characterization of thermal emitters can be challenging due to the presence of background thermal emission from components of the experimental setup and the surrounding environment. This is especially true for an emitter operating close to room temperature. Here, we explore the characterization of near-room-temperature thermal emitters using Fourier-transform infrared (FTIR) spectroscopy. We find that the thermal background arising from optical components placed between the beam splitter and the detector in an FTIR spectrometer appears as a “negative” contribution to the Fourier-transformed signal, leading to errors in thermal-emission measurements near room temperature. Awareness of this contribution will help properly calibrate low-temperature thermal-emission measurements.

Theoretical predictions indicate that the n-type $\mathrm{Ge}/\mathrm{Si}\text{−}\mathrm{Ge}$ multi-quantum-well system is the most promising material for the realization of a $\mathrm{Si}$-compatible THz quantum cascade laser operating at room temperature. To advance in this direction, we study, both experimentally and theoretically, asymmetric coupled multi-quantum-well samples based on this material system, that can be considered as the basic building block of a cascade architecture. Extensive structural characterization shows the high material quality of strain-symmetrized structures grown by chemical vapor deposition, down to the ultrathin barrier limit. Moreover, THz absorption spectroscopy measurements supported by theoretical modeling unambiguously demonstrate inter-well coupling and wavefunction tunneling. The agreement between experimental data and simulations allows us to characterize the tunneling barrier parameters and, in turn, achieve highly controlled engineering of the electronic structure in forthcoming unipolar cascade systems based on n-type $\mathrm{Ge}/\mathrm{Si}\text{−}\mathrm{Ge}$ multi-quantum-wells.

We report on the challenges and limitations of direct coupling of the magnetic field from a circuit resonator to an electron spin bound to a donor potential. We propose a device consisting of a trilayer lumped-element superconducting resonator and a single donor implanted in enriched ${}^{28}\mathrm{Si}$. The resonator impedance is significantly smaller than the practically achievable limit obtained with prevalent coplanar resonators. Furthermore, the resonator includes a nanoscale spiral inductor to spatially focus the magnetic field from the photons at the location of the implanted donor. The design promises an increase of approximately 2 orders of magnitude in the local magnetic field, and thus the spin-to-photon coupling rate $g$, compared with the estimated rate of coupling to the magnetic field of coplanar transmission line resonators. We show that by use of niobium (aluminum) as the resonator’s superconductor and a single phosphorous (bismuth) atom as the donor, a coupling rate of $g/2\pi =0.24$ MHz (0.39 MHz) can be achieved in the single-photon regime. For this hybrid cavity-quantum-electrodynamic system, such enhancement in $g$ is sufficient to enter the strong-coupling regime.

We investigate thermal properties of a $\mathrm{NbN}$ single-photon detector capable of unit internal detection efficiency. Using an independent calibration of the coupling losses, we determine the absolute optical power absorbed by the $\mathrm{NbN}$ film and, via resistive superconductor thermometry, the temperature dependence of the thermal resistance $Z(T)$ of the $\mathrm{NbN}$ film. In principle, this approach permits simultaneous measurement of the electron-phonon and phonon-escape contributions to the energy relaxation, which in our case is ambiguous because of the similar temperature dependencies. We analyze $Z(T)$ with a two-temperature model and impose an upper bound on the ratio of electron and phonon heat capacities in $\mathrm{NbN}$, which is surprisingly close to a recent theoretical lower bound for the same quantity in similar devices.

A method for reconstructing joint photon-number distributions of twin beams from the experimental photocount histograms is suggested and experimentally implemented. Contrary to the standard reconstruction methods, it incorporates spatial noise reduction based on spatial pairing of photons. Superior performance of the method above the usual one for the maximum-likelihood approach is demonstrated.

Manipulating the topological property of iso-frequency contour (IFC) will provide a unique control for the interaction between light and matter, such as increased rates of spontaneous emission and negative refraction. Different from the aforementioned topological transition that is realized by controlling the in-plane anisotropic electromagnetic parameters in a two-dimensional case, we theoretically propose and experimentally demonstrate another topological transition by tuning the vertical electromagnetic parameters of the anisotropic medium. Specially, the critical IFC in this transition corresponds to two intersecting lines and can be called linear-crossing metamaterial (LCMM). Based on the directional propagation and negative refraction in LCMM, we demonstrate the focusing and super-resolution with partial cloaking in transmission-line-based LCMMs. The realization of LCMM will open up another avenue for controlling light propagation and verifying unusual phenomena involving zero-index media and hyperbolic metamaterials.

Reflective metasurfaces (or meta-mirrors) composed of artificially engineered subwavelength structures provide an alternate paradigm for arbitrarily controlling wave reflection with high efficiency. We introduce the concept of frequency-selective coding of a meta-mirror to perform desired scattering properties in different frequency bands by tailoring the phase dispersion of the metasurface. As a particular example, broadband backward-scattering reduction and high-efficient specular reflection are combined by a single meta-mirror in a frequency-selective manner. Two meta-atoms with optimized metallic patterns are used as the binary coding elements to realize an out-of-phase function in two side-bands, while the in-phase function is in the center frequency window. Then, we experimentally demonstrate that polarization-insensitive diffusion-like scatterings can be achieved by the proposed meta-mirror with randomly distributed meta-atoms in two side-bands from 7.5 to 9.5 GHz and 11.6 to 15 GHz, while in the selected center frequency window around 10.7 GHz, a high-efficient specular reflection property is realized. The experimental results are in good agreement with the full-wave simulations. Compared with the conventional metasurface designs for scattering reduction, the proposed concept exhibits a frequency-selective effect and hence can be used in many practical applications, for example, working as the ground plane of antennas to enable a high-efficient in-band radiation with out-of-band low-backward scattering.

Active polarization rotators are desirable for many practical applications such as liquid crystal displays, fiber communication, etc. Ultrathin metasurfaces incorporating phase-change or magneto-optical materials provide active tuning of polarization, but suffer from a long response time or the requirement for a strong magnetic field. In this study, a tunable plasmonic polarization rotating system is proposed, which consists of two coupled metal screens perforated with subwavelength apertures. The polarization rotating mechanism can be attributed to the near-field coupling of metal screens and its dependence on the screen’s lateral displacement. The theory and experiment in the microwave band demonstrate that, by using the x polarized incident wave and displacing one metal screen at the millimeter scale, a continuous and efficient polarization rotation of the outgoing wave (with a nearly linear polarization state) from −90° to 90° can be achieved. This arbitrarily directional and tunable polarization rotating effect may be extended to the THz regime as well and find potential applications in constructing versatile polarization rotators and signal modulators.

In conventional materials, Cherenkov radiation (CR) due to a moving charged particle is associated with a broad frequency range and velocity threshold. Here, we show that using a periodic grating structure, which possesses photonic bound states in the continuum (BICs) and quasi-BICs, unusual CR can be generated in a very narrow frequency band at a particle velocity below the common threshold. This effect arises from an interesting light-amplification process in the BIC structure, which can dramatically improve the efficiencies of evanescent-to-propagating wave conversion and CR generation. Our results present a mechanism for realizing Cherenkov lasing at low electron velocity and may find applications in compact free-electron lasers.