We study the role of time-reversal symmetry on the dynamical response of nonlinear optical systems that behave as unidirectional (one-way) devices. It is shown that lossless nonlinear materials, despite being nonreciprocal, are typically time-reversal invariant. This property raises an apparent paradox because time-reversal-invariant systems are forcibly bidirectional. Here, we present a solution for this conundrum, and theoretically explain why the one-way behavior can indeed be compatible with the time-reversal invariance. It is found that in the time-reversed problem the incident waves have a variation in time that is generally incompatible with the adiabatic approximation. For this reason, the adiabatic approximation fails to predict the bidirectional nature of nonlinear systems. We discuss the implications of this finding on the performance of practical nonlinear one-way devices.

Exciton-polariton arising from strong light-matter interaction between exciton and optical cavity has attracted considerable attention due to its potential applications in Bose-Einstein condensation, low-threshold lasing, and slow light. In recent years, two-dimensional lead halide perovskite has emerged as an ideal candidate for realizing exciton polariton at room temperature because it has large exciton binding energy and quantum yield. Here, we demonstrate that strong coupling could be enabled with a perovskite metasurface that supports multipolar Mie resonance, including magnetic quadrupole dominant, anapole, and toroidal resonances. For an array of perovskite nanodisks, the strong coupling behavior between these resonances and exciton is confirmed by the anticrossing features in absorption spectra mapping, while the Rabi splitting is increased from 230.7 meV in magnetic quadrupole-exciton strong coupling to 253 meV in both anapole-exciton and toroidal-exciton strong coupling. The enhanced Rabi splitting is attributed to the stronger field localization within the perovskite instead of within the air gap. In addition, we find that the Rabi splitting depends on the oscillatory strength of the exciton mode and can then be boosted to 362 meV in anapole-exciton strong coupling. Our results provide promising ways to improve the performance of optoelectronic devices such as low-threshold lasers and slow-light devices.

Integrated photonics capable of incorporating rare-earth ions with high optical coherence is desirable for realizing efficient quantum transducers, compact quantum memories, and hybrid quantum systems. Here we describe a photonic platform based on the SmartCut erbium-doped lithium niobate thin film, and explore its stable optical transitions at telecom wavelength in a dilution refrigerator. Optical coherence time of up to $180\mu \mathrm{s}$, rivaling the value of bulk crystals, is achieved in optical ridge waveguides and ring resonators. With this integrated platform, we demonstrate tunable light-ion interaction and flexible control of optical thickness by exploiting long waveguides, whose lengths are in principle variable. This unique ability to obtain high optical density using low-concentration ions further leads to the observation of multiecho pulse trains in centimeter-long waveguides. Our results establish a promising photonic platform for quantum information processing with rare-earth ions.

High-dimensional entanglement offers a variety of advantages for both fundamental and applied applications in quantum information science. A central building block for such applications is a programmable processor of entangled states, which is crucial for the certification, manipulation, and distribution of high-dimensional entanglement. The leading technology for processing photons is integrated multiport interferometers. However, such devices are incompatible with structured light and their scaling is challenging. Here, we unlock these limitations by demonstrating a reconfigurable processor of entangled photons in high dimensions that is based on multiplane light conversion (MPLC), a technology that has recently been developed for multiplexing hundreds of spatial modes for classical communication. We use our programmable MPLC platform to certify three-dimensional entanglement in two mutually unbiased bases, perform 400 arbitrary random transformations on entangled photons, and convert the mode basis of entangled photons for entanglement distribution.

The coupling between multiple modes in subwavelength scale has attracted a lot of attention due to the interesting physics favored by the enhancement of light-matter interaction. In this contribution, we propose a planar hybrid plasmonic-photonic crystal structure, which enables the tunable coupling between Tamm plasmon-polariton and Fabry-Perot cavity modes. Then, a strong coupling can be achieved through evoking the Tamm-mode dependence of the top-layer thickness of the hybrid structure, which is indicated by an anticrossing with a spectrum splitting of 9.298 meV. The effect enables the spatial confinement of polaritons and the formation of hybrid one-dimensional plasmon-polariton modes. In order to account for the splitting of the orthogonally polarized resonances, a detailed investigation on the TE-TM splitting is also performed through the angular-dependence analysis. We note that the value of the TE-TM splitting strongly depends on the thicknesses of dielectric layer and the distance between two distributed Bragg reflector (DBR) stacks. We show that the cavity modes are extremely sensitive to the distance between the DBR stacks and propose using this feature for strain sensing. Thus, our structure may not only offer a platform for study of interesting physics (e.g., spin optronics), but may also provide a route for high-accuracy strain sensors.

The fluctuational electrodynamic investigation of thermal radiation from nonequilibrium or nonisothermal bodies remains largely unexplored because it necessarily requires volume integration over the fluctuating currents inside the emitter, which quickly becomes computationally intractable. Here, we put forth a formalism combining fast calculations based on modal expansion and fluctuational electrodynamics to accelerate research at this frontier. We employ our formalism on a sample problem: a long silica wire held under temperature gradient within its cross section. We discover that the far-field thermal emission carries a nonzero spin, which is constant in direction and sign, and interestingly, is transverse to the direction of the power flow. We clearly establish the origin of this transverse spin as arising from the nonequilibrium intermixing of the cylindrical modes of the wire, and not from any previously studied or intuitively expected origins such as chiral or nonisotropic materials and geometries, magnetic materials or fields, and mechanical rotations. This finding of nonequilibrium spin texture of emitted heat radiation can prove useful for advancing the noninvasive thermal metrology or infrared-imaging techniques.

We introduce a nonlinear photonic system that enables asymmetric localization and unidirectional transfer of an electromagnetic wave through the second-harmonic generation process. Our proposed scattering setup consists of a noncentrosymmetric nonlinear slab with nonlinear susceptibility ${\chi }^{(2)}$ placed to the left side of a one-dimensional periodic linear photonic crystal with an embedded defect. We engineered the linear lattice to allow the localization of a selected frequency $2{\omega }_{\star }$ while frequency ${\omega }_{\star }$ is in the gap. Thus in our proposed scattering setup, a left-incident coherent transverse electric wave with frequency ${\omega }_{\star }$ partially converts to frequency $2{\omega }_{\star }$ and becomes localized at the defect layer while the unconverted remaining field with frequency ${\omega }_{\star }$ exponentially decays throughout the lattice and gets reflected. For a right-incident wave with frequency ${\omega }_{\star }$ there will not be any frequency conversion and the incident wave gets fully reflected. Our proposed structure will find application in designing optical components such as optical sensors, switches, transistors, and logic elements.

A single Bessel beam (BB) enables the scalable optical writing of waveguide devices with multiple inputs and outputs in a nonlinear medium. For this purpose, we experimentally propagate a diffracting zero-order BB in a strontium barium niobate photorefractive crystal. Such a configuration can be used for the self-writing of complex waveguiding structures. By tailoring different key parameters such as the diffraction phenomenon through the BB size, the nonlinearity strength through the applied electric field, and the photoinduced refractive-index modulation depth through input beam intensity, our optical platform enables not only the generation of optical splitters but also an all-optical control of the output intensity levels. We also analyze the stability in space and time of the different photoinduced channels. We show that adding a background illumination onto the medium permits a steady-state photoinduced refractive-index modulation with up to nine output channels. These results enrich the research on scalable optical writing techniques for the realization of complex interconnects and enlarge further possibilities for all-optical switching.

Known coherent metasurfaces control the interference of waves of a given frequency with other coherent waves at the same frequency, either illuminating from a different direction or created as intermodulation products. In this paper, we introduce a class of metasurfaces that are modulated in time coherently with the illuminating radiation. Such modulation opens a possibility to control reflection, absorption, and transmission at multiple frequencies, including illuminations by two or more incoherent waves. In particular, we study dynamic resistive layers and show how to use them to design thin multifrequency perfect absorbers that overcome the bandwidth limit for static linear absorbers. Furthermore, we demonstrate possibilities of remote tuning of the absorption level. We hope that this work opens up alternative avenues in wave engineering using coherent modulation of metasurface parameters.

We study the capabilities of time-modulated diffraction gratings as imaging devices. It is shown that a time-dependent but transversally homogeneous slab can be used to make a perfect image of an object in the far-field, since all the evanescent modes couple to propagative time-diffracted orders. It is found that, if the image to be obtained is axially symmetric, it can be recovered by measuring the time signal at a single point, without the need to perform a spatial scan, so that time gratings can act as well as single-pixel imaging devices. In the case of having an object without axial symmetry, the time grating can be combined with a spatial grating, and then the full image can be recovered again with a measurement at a single point. We apply the theory of compressive sensing to optimize the recovery method, and numerical examples are provided. We show, therefore, that time-modulated gratings can be used to perfectly recover the image of an object in the far-field and after measuring at a single point in space, making them a promising approach to superresolution and ultrafast imaging.

The theory and practical realization of a broadband quantum-noise generator based on an original integrated optical beam splitter in the form of a Mach-Zehnder interferometer is demonstrated. The beam splitter with a double output, made on a lithium niobate substrate, provides accurate electro-optical balancing of the homodyne quantum-noise detection circuit. The experimentally obtained excess of quantum noise over classical noise is 12 dB in the frequency band over 4 GHz, which is the best parameters of quantum-noise generators known from the literature.

The quantum Hall system possesses topologically protected edge states, which have enormous theoretical and practical implications in both fermionic and bosonic systems. Harnessing the quantum Hall effect in optical platforms with lower dimensionality is highly desirable with synthetic dimensions and has attracted broad interests in the photonics society. Here, we introduce an alternative way to realize the artificial magnetic field in a frequency dimension, which is achieved in a pump-probe configuration with cross-phase modulations in a one-dimensional four-waveguide array. The dynamics of the topological chiral edge state has been studied and the influence from the crosstalk of the pump fields has been explored. Our work shows an all-optical way to simulate the quantum Hall system in a photonic system and holds potential applications in manipulating light in waveguide systems.

Modeling and simulations of entanglement-based quantum-networking protocols commonly assume perfect entangled states. Some investigations have been performed, which show how imperfections cause the efficiency of the protocols to rapidly deteriorate. For polarization-entangled states created by spontaneous-parametric down-conversion (SPDC), the fundamentals of phase matching lead to a trade-off problem for the optimal properties of the state. We present a SPDC method, which circumvents the trade-off problem and allows one to obtain degeneracy, indistinguishability, and heralded-single-photon spectral purity greater than $99\mathrm{\%}$ for any target SPDC wavelength in the visible and near-infrared spectrum. Therefore, our method can, in principle, generate polarization-entangled states that are optimal for polarization-entanglement-based quantum-networking protocols.

Accurate control of light polarization represents a core building block in polarization metrology, imaging, and optical and quantum communications. Voltage-controlled liquid crystals offer an efficient way of polarization transformation. However, common twisted nematic liquid crystals are notorious for lacking an accurate theoretical model linking control voltages and output polarization. An inverse model, which would predict control voltages required to prepare a target polarization, is even more challenging. Here we report both the direct and inverse models based on deep neural networks, radial basis functions, and linear interpolation. We present an inverse–direct compound model solving the problem of control voltages ambiguity. We demonstrate an order of magnitude improvement in accuracy using deep learning compared to the radial basis function method and 2 orders of magnitude improvement compared to the linear interpolation. Errors of the deep neural network model also decrease faster than the other methods with an increasing number of training data. The best direct and inverse models reach average infidelities of $4\times {10}^{-4}$ and $2\times {10}^{-4}$, respectively, which are accuracy levels not reported yet. Furthermore, we demonstrate local and remote preparation of an arbitrary single-photon polarization state using deep learning models. The results will impact the application of twisted-nematic liquid crystals, increasing their control accuracy across the board. The presented bidirectional learning can be used for optimal classical control of complex photonic devices and quantum circuits beyond interpolation.

We propose a realistic setup, inspired by already existing experiments, within which we develop a general formalism for the implementation of distributed quantum gates. Mediated by a quantum link that establishes a bidirectional quantum channel between distant nodes, our proposal works both for inter- and intranode communication and handles scenarios ranging from the few to the many modes limit of the quantum link. We are able to design fast and reliable state transfer protocols in every regime of operation, which, together with a detailed description of the scattering process, allows us to engineer two sets of deterministic universal distributed quantum gates: gates whose implementation in quantum networks does not need entanglement distribution nor measurements. By employing a realistic description of the physical setup we identify the most relevant imperfections in the quantum links as well as optimal points of operation with resulting infidelities of $1-F\approx {10}^{-2}$–${10}^{-3}$.

Due to challenging efficiency limits facing conventional and unconventional electronic architectures, information processors based on photonics have attracted renewed interest. Research communities have yet to settle on definitive techniques to describe the performance of this class of information processors. Photonic systems are different from electronic ones, and the existing concepts of computer performance measurement cannot necessarily apply. In this paper, we quantify the power use of photonic neural networks with state-of-the-art and future hardware. We derive scaling laws, physical limits, and platform platform performance metrics. We find that overall performance takes on different dominant scaling laws depending on scale, bandwidth, and resolution, which means that energy efficiency characteristics of a photonic processor can be completely described by no less than seven performance metrics over the range of relevant operating domains. The introduction of these analytical strategies provides a much needed foundation and reference for quantitative roadmapping and commercial value assignment for silicon photonic neural networks.

The bull’s-eye structure in the terahertz (THz) frequency region has ample applications owing to its ability to focus free-propagating waves into subwavelength apertures, resulting in enhanced transmission, that is, extraordinary transmission. However, its coupling properties have been primarily discussed in terms of the normal plane-wave incidence to the structure. In this study, we investigate the multiple resonances in extraordinary transmission with normal and oblique incident waves. The experiment using a widely tunable and high-power THz wave source revealed two types of resonances. The main resonance split depends on the incident angle, and the other corresponds to the side lobe of the main resonances. The results are explained by a simple analytical model using a finite number of scattering media. The analysis is supported by the full-wave simulation using the finite-element method, which agrees with the experimental results. The coupling mechanisms will be applicable to design devices, such as THz biosensing devices or THz antennas for rapid communication systems.

Color centers in silicon carbide are emerging candidates for distributed spin-based quantum applications due to the scalability of host materials and the demonstration of integration into nanophotonic resonators. Recently, silicon vacancy centers in silicon carbide have been identified as a promising system with excellent spin and optical properties. Here, we fully study the spin-optical dynamics of the single silicon vacancy center at hexagonal lattice sites, namely V1, in 4H-polytype silicon carbide. By utilizing resonant and above-resonant sublifetime pulsed excitation, we determine spin-dependent excited-state lifetimes and intersystem-crossing rates. Our approach to inferring the intersystem-crossing rates is based on all-optical pulsed initialization and readout scheme, and is applicable to spin-active color centers with similar dynamics models. In addition, the optical transition dipole strength and the quantum efficiency of V1 defect are evaluated based on coherent optical Rabi measurement and local-field calibration employing electric field simulation. The measured rates well explain the results of spin-state polarization dynamics, and we further discuss the altered photoemission dynamics in resonant enhancement structures such as radiative lifetime shortening and Purcell enhancement. By providing a thorough description of the V1 center’s spin-optical dynamics, our work provides deep understanding of the system, which guides implementations of scalable quantum applications based on silicon vacancy centers in silicon carbide.