Active manipulation on absorption takes an essential place in nanophotonic light modulators, which always suffer from static tuning ability, limited depth, and insufficient modulating efficiency. In this study, we propose an alternative strategy utilizing the symmetry-protected-type bound state in the continuum (SPBIC) to satisfy a tunable critical-coupling absorber with high modulating mobility. To demonstrate our concept, we illustrate an ultrathin critical-coupling absorbing system composed of germanium metasurface and monolayer graphene through elaborately maneuvered quasi-SPBIC, which provides substantial local field enhancement to boost the light absorption of graphene. The intensity of such critical-coupling absorption can be efficiently tuned by modulating the polarization of the incident waves with an efficiency of 97.1%. Via this polarization control, we demonstrate digital switch “0-1” and switchable and selectable image-displaying functions in simulation. Such functions are obtained in one single-designed array by incident polarization while requiring no additional structural change. Our results may pave the way for next-gen nanophotonic metadevices.

We demonstrate an experimental protocol for the preparation and control of heralded single photons in inhomogeneously polarized states, such as vector vortex and full Poincaré beam states. A laser beam is shaped by a voltage-controlled spin-to-orbital angular momentum converter $q$-plate device, which eliminates the need for an interferometer for the robust preparation of high-quality inhomogeneously polarized beams. Such a beam is then used as a pump in a spontaneous parametric down-conversion (SPDC) photon-pair source. We demonstrate the full pump to heralded single-photon transfer of the intensity and phase distributions, as well as of the vector polarization structure. Additionally, we show that by controlling the polarization to which the heralding idler photon is projected before detection, we can toggle between the direct and basis-switched pump-single-photon transfer. We show that this nonlocal control of the heralded single photon pertains also to the topological class of the resulting heralded single photon. We believe that our work will lead to opportunities in photon-based quantum information processing science.

Entanglement enables many promising applications in quantum technology. Devising alternative generation methods and harnessing entanglement are prerequisites for practical applications. Here, we realize a distinct polarization-entangled source by simultaneously achieving type-0 and type-I backward quasiphase matching (BQPM) through spontaneous parametric down-conversion in a single bulk crystal, which is different from all previous entangled-source configurations. Pumping the crystal with a single polarized beam generates a nonmaximally polarization-entangled state, which can be further projected to a maximal Bell state with a pair of Brewster windows. Hong-Ou-Mandel interference experiments are done on polarization-degenerate photon pairs for both type-0 and type-I BQPM processes for the first time. The emitted photons in both processes have a bandwidth as narrow as 15.7 GHz. The high quality of this source is characterized by various methods. The rather simple configuration, narrow bandwidth, and high entanglement quality make the source very promising for many quantum information tasks.

Recent advances in planar optics with geometric phase superstructures have brought an alternative paradigm in the control of structured light and, in particular, have substantially enhanced the capabilities of generating and detecting orbital angular momentum (OAM) states of light and associated spatial modes. However, the structured modal interface that can reciprocally link OAM states via adiabatic control and access an associated higher-order geometric phase remains absent in planar optics. In this work, we propose and experimentally demonstrate a planar optical astigmatic retarder fabricated with a liquid-crystal (LC) geometric phase. The LC superstructure is designed based on the principle of fractional Fourier transformation and is capable of reciprocal conversion between all possible OAM states on the same modal sphere. Such a planar device paves the way towards an easily deployed modal interface of paraxial OAM states, unlocks the resource of a higher-order geometric phase, and has promising applications in high-dimensional classical and quantum information.

The radiation transfer equation is widely used for simulations, for example, heat transfer in engineering, diffuse optical tomography in healthcare, and radiation hydrodynamics in astrophysics. By combining the lattice Boltzmann method, we propose a quantum algorithm for radiative transfer. This algorithm encompasses all the essential physical processes of radiative transfer: absorption, scattering, and emission. Although a sufficient number of measurements are required to precisely estimate the quantum state, and the initial encoding of the quantum state remains a challenging problem, our quantum algorithm exponentially accelerates radiative-transfer calculations compared to classical algorithms. In order to verify the quantum algorithm, we perform quantum circuit simulation using IBM Qiskit Aer and find good agreement between our numerical result and the exact solution. The algorithm paves the way for applications, such as fault-tolerant quantum computers for plasma engineering, telecommunications, nuclear-fusion technology, healthcare, and astrophysics.

A setup that simulates ground states of spin graphs would allow one to solve computationally hard optimization problems efficiently. Current optical setups to this goal have difficulties decoupling the amplitude and phase degrees of freedom of each effective spin, which risks that the resultant mapping will be invalid, a problem known as amplitude heterogeneity. Here, we propose a setup with coupled active optical cavity modes, where this problem is eliminated through their particular geometric arrangement. Acting as an effective Monte Carlo solver, the ground state can be found exactly. By tuning a parameter, the setup solves $XY$ or Ising problems.

We show that an oscillator array prefers to operate at an exceptional point of degeneracy (EPD) occurring in a waveguide periodically loaded with discrete nonlinear gain and radiating elements. The concept of the EPD is employed to conceptualize an exceptional synchronization regime, which leads to enhanced radiating power efficiency. The system maintains a steady-state degenerate mode of oscillation at a frequency of 3 GHz, even when the small-signal nonlinear gain values are nonuniform along the array. We designed the system using small-signal gain to work at the EPD of zero phase shift in consecutive unit cells. Contrarily to the original expectation of zero phase shift, after reaching saturation, the time-domain signal in consecutive unit cells displays a $\pi$ phase shift. Hence, we demonstrate that the saturated system tends to oscillate at a distinct EPD, associated to a $\pi$ phase shift between consecutive cells, than the one at which the system was originally designed using small-signal gain. This alternative EPD at which the nonlinear system is landing is associated to higher radiating power efficiency with respect to power provided by nonlinear gains. Finally, we demonstrate that the oscillation frequency is independent of the length of the array, contrarily to what happens ordinary oscillating systems based on one-dimensional cavity resonances. These findings may have a high impact on high-power radiating arrays with distributed active elements.

On-chip efficient nonlinear functions are instrumental in escalating the utilities and performance of photonic integrated circuits, especially for a wide range of classical and quantum applications, such as tunable coherent radiation, optical frequency conversion, spectroscopy, quantum science, etc. Here, we fabricated a $Z$-cut lithium tantalate on insulator (LTOI) microdisk with high quality ($Q\sim {10}^6$) factors. Cascaded three- and four-wave mixing generation simultaneously were achieved in the LTOI microdisk cavity, due to the high $Q$ factor with a strongly confined light field, relatively large nonlinear coefficient, and optical damage threshold of the LTOI microdisk. The strict mode phase-matching condition and the second-harmonic efficiency of the whole three-wave mixing processes were analyzed. Our work shows that the LTOI microresonators with the high $Q$ factor and its high optical damage threshold and wide transparency range can support a variety of on-chip optical nonlinear processes, which heralds its application potential in integrated nonlinear photonics.

Skyrmions are topologically protected quasiparticles, originally studied in condensed-matter systems and recently in photonics, with great potential in high-density information storage. Despite the recent attention, most optical solutions require complex systems yet produce limited topologies. Here we demonstrate an extended family of quasiparticles beyond normal skyrmions that are controlled in compact photonic gradient-index media, extending to higher-order members such as multiskyrmions and multimerons, with increasingly complex topologies. We introduce multiple topological numbers (centrality, radiality, vorticity, and polarity) in addition to the skyrmion number to describe these photonic quasiparticles. Our compact creation system lends itself to integrated and programmable solutions of complex particle textures, with potential impacts on both photonic and condensed-matter systems for revolutionizing topological informatics and logic devices.

The increasing complexity of recent photonic experiments challenges the development of efficient multichannel coincidence counting systems with high-level functionality. Here, we report a coincidence unit able to count detection events ranging from single to 16-fold coincidences with full channel-number resolution. The device operates within sub-100-ps coincidence time windows, with a maximum input frequency of 1.5 GHz and an overall jitter of less than 10 ps. The unit high-level timing performance renders it suitable for quantum photonic experiments employing low-timing-jitter single-photon detectors. Additionally, the unit can be used in complex photonic systems to drive feed-forward loops. We demonstrate the developed coincidence counting unit in photon-number-resolving detection to directly quantify the statistical properties of light, specifically coherent and thermal states, with a fidelity exceeding 0.999 up to 60 photons.

Quantum microwave photonics (QMWP) technology, which harnesses energy-time-entangled biphotons as optical carriers and employs time-correlated single-photon detection for high-speed rf-signal recovery, has demonstrated remarkable potential in achieving nonlocal rf-signal coding and efficient rf-signal recovery from significant dispersion interference. In this paper, we expound on the principle of maximizing the recovered rf signals in QMWP systems. Through theoretical analysis and experimental validation, we establish a precise relationship between the recovered rf signal’s magnitude, the input rf signal’s frequency, involved dispersions, and the width of the coincidence selection window. This enables us to determine the optimal width for the coincidence selection window and the corresponding magnitude of the maximized rf signal given the dispersion. These findings offer valuable guidance for improving QMWP systems, thus expanding their potential applications.

Coherent high-quality laser sources are important for a variety of photonic devices and applications, yet they are especially challenging for high-power semiconductor lasers, because high-power and single-mode lasing present conflicting requirements; high power drives nonlinear processes causing multimode lasing and instabilities. Here, we present a distributed feedback (DFB) microring laser coupled to a uniform ring, which displays single-mode lasing up to 10 times the threshold value. This is the first microring DFB laser in gallium nitride ($\mathrm{Ga}\mathrm{N}$) and the first monolithic coupled microring $\mathrm{Ga}\mathrm{N}$ lasers. Our work paves the way for future designs of high-quality high-power coupled lasers for applications, such as topological lasers, high-power monolithic frequency combs, optical clocks, gyros, sensing, and quantum systems.

Programmable linear optical processors (LOPs) can have widespread applications in computing and information processing due to their capabilities of implementing reconfigurable on-chip linear transformations. A conventional LOP that uses a mesh of Mach-Zehnder interferometers (MZIs) requires $2N+3$ stages of phase shifters for $N\times N$ matrices. However, it is beneficial to reduce the number of phase-shifter stages to realize a more compact and lower-loss LOP, especially when long and lossy electro-optic phase shifters are used. In this work, we propose a structure for LOPs that can implement arbitrary matrices as long as they can be realized by previous MZI-based schemes. Through numerical analysis, we further show that the number of phase-shifter stages in the proposed structure can be reduced to $N+2$ and $N+3$ for a large number of random dense matrices and sparse matrices, respectively. This work contributes to the realization of compact, low-loss, and energy-efficient programmable LOPs.

Most modern high-data-rate optical communication systems require at each end a light source to transmit the data and a detector to receive the data. In order to realize a communication scheme where the transmitter unites with the receiver, we take advantage of a coupled injection-locked semiconduct. From the theoretical description of two mutually coupled diode lasers using a set of six nonlinear differential-delay equations we outline the stable compound cavity solutions of this system and investigate their dependencies in terms of locking bandwidth, coupling strength and coupling phase. From these modeling results we then design a remarkably simple communication system without any additional detectors that exploits compound laser states of different frequency detunings of the coupled laser system as information carriers with simultaneous upstream information of the first laser and downstream information of the second laser. The data recognition is accomplished by evaluating the compound state utilizing the change in the terminal voltage of both lasers (laser-as-detector principle) with respect to their solitary case (i.e., the terminal voltage of the uncoupled lasers), yielding the message via the agreed decoding scheme. The functionality of the principle had been demonstrated by a table-top experiment with good error-free transmission and potential to improve the transmission speed of this proof-of-principle demonstration. Our concept reduces the complexity compared to conventional transmission systems, thus providing the potential to significantly reduce the costs of the overall transmission setup an,d in particular, transferring the concept to other challenging wavelengths and other semiconductor lasers. Since the optical signal then no longer needs to be detected by an optical detection unit, the amount of optical components required for ordinary data transmission is reduced.

We present an experimental realization of a noise-free and high-bandwidth quantum memory scheme using a rubidium vapor that is confined within the hollow core of a photonic crystal fiber. We achieve the same internal efficiencies as similar free-space experiments (30%) for 4.5-ns-long optical pulses but with a 100-fold reduction in the control-field power required. Modeling indicates that this efficiency could be improved to 88% with higher control powers and the implementation of techniques such as light-induced atomic desorption to increase the optical depth. The compactness, robustness, and low drive power of this approach lends itself to direct integration into large-scale fiber-based quantum processors.

Valley kink states and valley-polarized chiral edge states, whose topologically protected properties have great application potential in high-speed information processing and communication, have recently become important frontiers in topological photonics. However, valley photonic topological insulators mainly support single-type topological states through breaking lattice spatial symmetry. It remains difficult for different types of topological states to coexist in a specific photonic system. In this paper, we propose judiciously designed substrate-integrated topological photonic crystals (but without metal vias) with $C_3$ symmetry that can support valley kink states and valley-polarized chiral edge states in different waveguide channels. Via numerical simulations and measurements, we demonstrate the robust transmission of both valley kink states and valley-polarized chiral edge states through sharp corners and we show high-efficiency coupling between classical guided waves and topological waves. More importantly, we achieve smooth transitions between chiral and trivial edge states, trivial edge states and valley kink states, as well as valley kink states and valley-polarized chiral edge states by bridging different waveguide channels. Our proposed design is beneficial for interfacing with existing functional photonic waveguides or devices and opens up an alternative pathway for advanced planar integrated topological photonic systems with subwavelength thicknesses.

In light of recent achievements in optical computing and machine learning, we consider the conditions under which all-optical computing may surpass electronic and optoelectronic computing in terms of energy efficiency and scalability. When considering the performance of a system as a whole, the cost of memory access and data acquisition is likely to be one of the main efficiency bottlenecks not only for electronic, but also for optoelectronic and all-optical devices. However, we predict that all-optical devices will be at an advantage in the case of inference in large neural network models, and the advantage will be particularly large in the case of generative models. We also consider the limitations of all-optical neural networks, including footprint, strength of nonlinearity, optical signal degradation, limited precision of computations, and quantum noise.