A spectrally uncorrelated biphoton state generated from the spontaneous nonlinear optical process is an important resource for quantum information. Currently, such a spectrally uncorrelated biphoton state can only be prepared from limited kinds of nonlinear media, thus limiting their wavelengths. In order to explore a wider wavelength range, here we theoretically study the generation of a spectrally uncorrelated biphoton state from 14 isomorphs of potassium dihydrogen phosphate (KDP) crystal. We find that 11 crystals from the “KDP family” still maintain similar nonlinear optical properties of KDP, such as KDP, DKDP, ADP, DADP, ADA, DADA, RDA, DRDA, RDP, DRDP, and KDA, which satisfy three kinds of the group-velocity matching conditions for spectrally uncorrelated biphoton-state generation from near-infrared to telecom wavelengths. Based on the uncorrelated biphoton state, we investigate the generation of a heralded pure-state single photon by detecting one member of the biphoton state to herald the output of the other. The purity of the heralded single photon is as high as 0.98 without using a narrow-band filter; the Hong-Ou-Mandel interference from independent sources can also achieve a visibility of 98%. This study may provide more and better single-photon sources for quantum-information processing at near-infrared and telecom wavelengths.

${\mathrm{VO}}_2$ undergoes metal-insulator transition (MIT) at the transition temperature (τ_c) of 68 °C, accompanied by a sharp optical response change. A unique $\mathrm{Au}/{\mathrm{VO}}_2$ half-sphere core-shell structure is fabricated to study the temperature-dependent plasmonic effects and the thermochromic response for smart window applications. By increasing the $\mathrm{Au}$ core size, the surface plasmonic resonance wavelength of the ${\mathrm{VO}}_2$ high-temperature rutile phase is tuned from 600 to 720 nm and confirmed by simulations. Meanwhile, ${\tau }_c$ of ${\mathrm{VO}}_2$ is reduced by up to 10 °C, which is thought to be associated with plasmon-induced or percolation-enhancement-related effects. Modification of ${\tau }_c$ occurs on the optical domain only and not in the electrical domain. The in-depth analysis suggests that the interaction of the plasmon between $\mathrm{Au}$ and ${\mathrm{VO}}_2$ in the near field triggers different mechanisms, which reduce the ${\tau }_c$ of ${\mathrm{VO}}_2$, in particular under high irradiation levels. However, under normal irradiation levels the main contributing effect is found to be thermal strain at the nanoparticles‘ surface.

We report on the experimental implementation of topological edge-state arrays with power confinement in the edge state as high as 29 dB relative to its farthermost channel. Average insertion losses of <2 dB in the edge channel vs total output power in the whole array are measured. The device follows a Su-Shrieffer-Heeger construction, consisting of seven waveguides fabricated on (100) liquid-phase-epitaxy-grown bismuth-substituted lutetium iron garnet films on a gadolinium gallium garnet substrate. The delocalization of the field can be realized by magnetizing only the single edge-state channel, thereby diverting the backscattered signal to serve as a high-performing optical isolator with a predicted isolation ratio as high as −50 dB. We discuss methods and challenges for full experimental implementation of a SSH topology-based optical isolator.

Wavelengths in the telecommunication window (approximately 1.25–1.65 $\mu \mathrm{m}$) are ideal for quantum communication due to low transmission loss in fiber networks. To realize quantum networks operating at these wavelengths, long-lived quantum memories that couple to telecom-band photons with high efficiency need to be developed. We propose coupling neutral ytterbium atoms, which have a strong telecom-wavelength transition, to a silicon photonic crystal cavity. Specifically, we consider the ${}^3{P}_0{\leftrightarrow }^3{D}_1$ transition in neutral ${}^{171}\mathrm{Yb}$ to interface its long-lived nuclear spin in the metastable ${}^3{P}_0$ “clock” state with a telecom-band photon at $1.4\mu \mathrm{m}$. We show that $\mathrm{Yb}$ atoms can be trapped using a short-wavelength (approximately $470$ nm) tweezer at a distance of 350 nm from the silicon photonic crystal cavity. At this distance, due to the slowly decaying evanescent cavity field at a longer wavelength, we obtain a single-photon Rabi frequency of $g/2\pi \approx 100$ MHz and a cooperativity of $C\approx 47$ while maintaining a high photon collection efficiency into a single mode fiber. The combination of high system efficiency, telecom-band operation, and long coherence times makes this platform well suited for quantum optics on a silicon chip and long-distance quantum communication.

Since their first synthesis in 2015, the all-inorganic lead halide perovskite nanocrystals ${\mathrm{Cs}\mathrm{Pb}\mathrm{X}}_3\left(\mathrm{X}=\mathrm{Cl},\mathrm{Br},\mathrm{I}\right)$ have attracted a great deal of attention due to their outstanding electronic and optical properties as well as their performances, which outclass the ones of their II–VI conterparts in many application fields. In addition to these properties, the understanding of the emission features in these systems at the single object scale is crucial, e.g., for nanophotonics and quantum optics devices. Here, the details of the band-edge excitonic emission are theoretically explored. The contribution of the long-range exchange interaction to the bright-exciton splittings is computed in strong and weak confinement regimes using group theory arguments and k$\cdot$p approximation. We show that the shape anisotropy with the real crystalline (cubic, tetragonal, or orthorhombic) structures of nanocrystals explain well their emission properties. In the weak confinement regime, splittings are inversely proportional to the cube of the exciton Bohr radius and we observe an increase of the splittings from iodide, to bromide, and then to chloride perovskite compounds. However, in the strong confinement regime, splittings increase inversely proportional to the nanocrystal volume and, for a given nanocrystal size, the splitting values are comparable for the three-halide-perovskite materials. The present theoretical developments lead to quantitative contributions in good agreement with available experimental data mainly in the weak confinement regime.

Waveguide-based spin-photon interfaces on the $\mathrm{Ga}\mathrm{As}$ platform have emerged as a promising system for a variety of quantum information applications directly integrated into planar photonic circuits. The coherent control of spin states in a quantum dot can be achieved by applying circularly polarized laser pulses that may be coupled into the planar waveguide vertically through radiation modes. However, proper control of the laser polarization is challenging since the polarization is modified through the transformation from the far field to the exact position of the quantum dot in the nanostructure. Here, we demonstrate polarization-controlled excitation of a quantum-dot electron spin and use that to perform coherent control in a Ramsey interferometry experiment. The Ramsey interference reveals an inhomogeneous dephasing time of $2.2\pm 0.1$ ns, which is comparable to the values so far only obtained in bulk media. We analyze the experimental limitations in spin initialization fidelity and Ramsey contrast and identify the underlying mechanisms.

The lithium niobate integrated photonic platform has recently shown great promise in nonlinear optics on a chip scale. Here, we report second-harmonic generation in a high-$Q$ lithium niobate microring resonator through modal phase matching, with a conversion efficiency of $1500\mathrm{\%}{\mathrm{W}}^{-1}$. Our device also allows us to observe difference-frequency generation in the telecom band. Our work demonstrates the great potential of the lithium niobate integrated platform for nonlinear wavelength conversion with high efficiencies.

We generate correlated photon pairs in an all-pass silicon microring resonator using continuous-wave pumped spontaneous four-wave mixing and we characterize in detail the optical bistability in the nonclassical single-photon regime. Special attention is given to the resonance-locked method by balancing the pump-induced heating and the active off-chip cooling, leading to stable operation of the photon-pair source. The maximal coincidence rate of $20.53\pm 0.34\mathrm{Hz}$, maximal coincidence-to-accidental ratio of $654\pm 125$, and minimal zero-delayed heralded second-order correlation of $0.14\pm 0.09$ are achieved. The wavelengths of photon pairs remain unchanged at different pump power, indicating that our source is more compatible with the common communication systems than those using conventional on-resonance strategies, that tunes the pump wavelength. This work not only experimentally validates the nonclassical bistability behaviors, but also puts forward a simple method of pump-resonance matching for high-quality performance.

Nitrogen-vacancy (N-$V$) centers in nanodiamond (ND) are a promising single-photon-source candidate for quantum technology. However, the poor N-$V$ emission rate and low outcoupling of light significantly hinder their effective use in practical implementations. To overcome this limit, we place NDs hosting N-$V$ centers on silver columnar thin films (CTFs) and measure an increase in emission by an order of magnitude. The CTFs consist of silver nanocolumns the length of which is chosen to be half the wavelength of the emitted light. The silver nanocolumns act as efficient optical antennas that couple to the N-$V$ centers via the optical near field and outcouple the excitation energy of the N-$V$ centers effectively into the optical far field. A large distribution of radiated powers from different NDs is observed. Computer simulations show this distribution to arise from the different orientations of the emitting dipoles with respect to the columnar axis. We also report that further structuring of the silver CTF into gratings yields higher photon emission.

Spin-orbit torque provides a powerful means of manipulating domain walls along magnetic wires. However, the current density required for domain-wall motion is still too high to realize low power devices. We experimentally demonstrate helicity-dependent domain-wall motion by combining synchronized femtosecond laser pulses and short current pulses in $\mathrm{Co}/\mathrm{Ni}/\mathrm{Co}$ ultrathin-film wires with perpendicular magnetization. The domain wall can remain pinned under one laser circular helicity while being depinned by the opposite circular helicity. Thanks to the all-optical helicity-dependent effect, the threshold current density due to spin-orbit torque can be reduced by more than 50%. Based on this joint effect combining spin-orbit torque and helicity-dependent laser pulses, an optoelectronic logic-in-memory device is experimentally demonstrated. This work enables an alternative class of low-power spintronic-photonic devices beyond the conventional approach of all-optical switching or all-current switching for data storage.

Entanglement between optical and microwave cavity modes is a critical issue in illumination systems. Optomechanical systems are utilized to introduce coupling between the optical and microwave cavity modes. However, due to some restrictions of the optomechanical system, especially sensitivity to the thermal photon noise at room temperature, an alternative optoelectronic system is designed to address the problem. We study a method by which it may be possible to remove the mechanical part of the previous systems to minimize the thermally generated photons. Unlike optomechanical systems, in our system, the optical mode is directly coupled to the microwave cavity mode through the optoelectronic elements without employing any mechanical parts. The utilized approach leads to generating the entangled modes at room temperature. For this purpose, the dynamics of the motion of the optoelectronic system is theoretically derived using the Heisenberg-Langevin equations from which one can calculate the coupling between optical and microwave cavity modes. The direct coupling between the optical and microwave cavity modes is the most important feature and is achieved through the combination of the photodetector and a Varactor diode. Hence, by controlling the photodetector current, that is, the photocurrent, depending on the optical cavity incident wave and the Varactor diode-biased voltage, the coupling between the optical and microwave cavity modes is established. The voltage across the Varactor diode also depends on the generated photocurrent. Consequently, our results show that the coupled modes are entangled at room temperature without the requirement for any mechanical parts.

We report that metamaterial-inspired one-dimensional gratings (or metagratings) can be used to control nonpropagating diffraction orders as well as propagating ones. By accurate engineering of the near field, it becomes possible to satisfy power-conservation conditions and achieve perfect control over all propagating diffraction orders with passive and lossless metagratings. We show that each propagating diffraction order requires 2 degrees of freedom represented by passive and lossless loaded thin “wires.” This provides a solution to the old problem of power management between diffraction orders created by a grating. The theory developed is verified by both three-dimensional full-wave numerical simulations and experimental measurements, and can be readily applied to the design of wavefront-manipulation devices over the entire electromagnetic spectrum as well as in different fields of physics.

We report on the isolation of single negatively-charged-silicon-vacancy ($\mathrm{Si}$-$V^{-}$) centers in nanodiamonds. We observe the fine structure of single $\mathrm{Si}$-$V^{-}$ centers with reduced inhomogeneous ensemble linewidth below the excited-state splitting, stable optical transitions, good polarization contrast, and excellent spectral stability under resonant excitation. On the basis of our experimental results, we develop an analytical strain model where we extract the ratio between strain coefficients of excited and ground states as well the intrinsic zero-strain spin-orbit splittings. The observed strain values are as low as the best values in low-strain bulk diamond. We achieve our results by means of $\mathrm{H}$-plasma treatment of the diamond surface and in combination with resonant and off-resonant excitation. Our work paves the way for indistinguishable, single-photon emission. Furthermore, we demonstrate controlled nanomanipulation by an atomic-force-microscope cantilever of one- and two-dimensional alignments with an accuracy of about 10 nm, as well as new tools including dipole rotation and cluster decomposition. Combined, our results show the potential to utilize $\mathrm{Si}$-$V^{-}$ centers in nanodiamonds for controlled interfacing via optical coupling of individually-well-isolated atoms for bottom-up assemblies of complex quantum systems.

A quantum photonic integrated circuit (QPIC) is a promising tool for constructing integrated devices for quantum-technology applications. In the optical regime, silicon photonics empowered by complementary-metal-oxide-semiconductor (CMOS) technology provides optical components useful for realizing large-scale QPICs. Optical nonlinearity at the single-photon level is required for QPICs to facilitate photon-photon interaction. However, to date, realization of optical elements with deterministic (i.e., not probabilistic) single-photon nonlinearity by use of silicon-based components is challenging despite the enhancement of the functionality of QPICs based on silicon photonics. In this study, we realize a strongly coupled $\mathrm{In}\mathrm{As}$/$\mathrm{Ga}\mathrm{As}$ quantum-dot–cavity-quantum-electrodynamics system on a CMOS-processed silicon photonic chip. The heterogeneous integration of the $\mathrm{Ga}\mathrm{As}$ cavity on the silicon chip is performed by transfer printing. The cavity-quantum-electrodynamics system on the CMOS photonic chip realized in this work is a promising candidate for an on-chip single-photon nonlinear element, which constitutes the fundamental component for future applications based on QPICs, such as coherent manipulation and nondestructive measurement of qubit states via a cavity, and an efficient single-photon filter and router.

Quantum key distribution (QKD) guarantees the secure communication between legitimate parties with quantum mechanics. High-dimensional QKD (HDQKD) not only increases the secret key rate but also tolerates higher quantum bit error rate (QBER). Many HDQKD experiments have been realized by utilizing orbital-angular-momentum (OAM) photons as the degree of freedom (DOF) of OAM of the photon is a prospective resource for HD quantum information. In this work, we propose that a high-quality HDQKD based on polarization-OAM hybrid states can be realized by utilizing state mapping between different DOFs and we characterize this HDQKD. Both the preparation and measurement procedures of the proof-of-principle verification experiment are simple and stable. Our experiment verifies that a $0.60\mathrm{\%}\pm 0.06\mathrm{\%}$ QBER and a $1.849\pm 0.008$-bit secret key rate per sifted signal can be achieved for a four-dimensional QKD with the weak coherent light source and decoy state method.

Midinfrared devices based on metalenses are of great utility for numerous applications, including free-space communication, imaging, and molecule sensing. However, because of the intrinsic dispersion of their building bricks, metalenses significantly suffer from large chromatic aberration. In this paper, we propose an achromatic metalens that can operate over a broad band of wavelengths in the infrared region from 3.7 to 4.5 $\mu \mathrm{m}$ for circularly polarized incidences. The Pancharatnam-Berry phase and the propagation phase are used to control the wavefront of light and eliminate the chromatic aberration, respectively. As a proof-of-principle demonstration, we numerically study two devices: an achromatic focusing lens and a beam deflector. The simulation results demonstrate the devices have a focusing efficiency of about $20.7\mathrm{\%}$ in the infrared region from 3.7 to 4.5 $\mu \mathrm{m}$. Our proposed approach may pave the way toward practical application of midinfrared devices.

We use localized surface-plasmon resonances in metallic nanoantennas to suppress the defect environment of colloidal quantum dots and to enhance and polarize their spontaneous emission. For this we study the interaction of such quantum dots with functional metal-oxide plasmonic metastructures consisting of an $\mathrm{Au}$/$\mathrm{Si}$ Schottky junction in close vicinity of a $\mathrm{Si}$/$\mathrm{Al}$ oxide charge barrier. We show that optically excited quantum dots can couple with such metastructures via their electric dipole fields, offering super-plasmonic-processes that include the impact of hot electron generation and fixed negative charges at the $\mathrm{Si}$/$\mathrm{Al}$ oxide junction. For metallic nanoantennas with small aspect ratios our results show that these metastructures can reduce the defect-induced nonradiative decay rates of quantum dots such that their lifetime can become significantly longer than those in the absence of such nanoantennas. For metallic nanoantennas with larger aspect ratios these structures lead to ultrahigh enhancement of exciton-plasmon coupling, making the spontaneous emission of quantum dots strongly polarized while increasing their emission intensities by about 50 times. These results show by keeping excitons in the cores of quantum dots, the metastructures suppress migration of photoexcited electrons to surface defects and, therefore, reduce Auger recombination. Coherent dynamics associated with the quantum-dot-induced exciton-plasmon coupling is theoretically investigated.

The investigation of the characteristic luminescent response of $\mathrm{Ce}$-doped silica fibers exposed to electrons in the 20–200-GeV energy range is reported in this work to explore the feasibility of using silica-based fibers for a simultaneous dual-readout approach. The sol-gel method allows the preparation of either doped or undoped fibers with high aspect ratio and high purity, providing good flexibility and spatial resolution for the realization of a dual-readout detector. The dual Cherenkov and scintillation light emitted by silica-based fibers potentially offers applications in high-energy-physics calorimetry as well as in other fields, such as radiation monitoring in medicine, security, and industrial control. The response of the fibers, embedded in a tungsten-copper absorber block to obtain a spaghetti-like geometry in a high-energy-physics environment, is investigated through a test-beam campaign at the CERN Super Proton Synchrotron facility. The discrimination of Cherenkov and scintillation light is demonstrated and discussed, along with a detailed investigation of the scintillation properties of the material: time-resolved spectroscopy, relative light output, and attenuation length are evaluated. The results presented in this study can pave the way for further material engineering and future applications.

Compression of electron pulses with terahertz radiation offers short pulse durations and intrinsic subcycle stability in time. We report the generation of 12-fs (rms), 28-fs (FWHM) electron pulses at a kinetic energy of 75 keV by using single-cycle terahertz radiation and a simple planar mirror. The mirror interface provides transverse velocity matching and spatially uniform compression in time with purely longitudinal field-electron interaction. The measured short-term and long-term temporal drifts are substantially below the pulse duration without any active synchronization. A simple phase-space model explains the measured temporal focusing dynamics for different compressor strengths and shows a path toward generating isolated attosecond electron pulses from beams of almost arbitrary transverse emittance.

We study spatial intensity distributions in plasmonic distributed feedback lasers (DFB) composed of metal nanoparticle arrays. Real-space distributions give direct access to “coupling-strength” parameters that quantify DFB performance in the framework of coupled-wave theory (CWT). We observe that CWT indeed parametrizes real-space intensity distributions and extract coupling-strength parameters that quantify the plasmonic feedback mechanism. These coupling-strength parameters differ from those required to parametrize the plasmonic band structures of the system, counter to the common result for dielectric DFB lasers, where CWT describes both real-space and $k$-space physics. Also, the measured coupling constants are significantly smaller than would be expected from estimates on the basis of the unit-cell geometry. We conclude that while CWT is successful as a generic description of any system with forward and backward waves with gain, matching this model to photonic band structures, or to common parameter-estimate approaches, fails because the underlying assumption that a perturbative plane-wave expansion applies is not valid for plasmonic antenna arrays.