We investigate the robustness of bound states in the continuum (BICs) in a bilayer dielectric rod array against geometric and material perturbations. Our analysis focuses on both symmetry-protected and Fabry-Pérot BICs (FP BICs), examining their transformation into quasi-BICs under three structural modifications: (i) in-plane displacement of one layer, which breaks the ${\mathrm{C}}_2$ symmetry of the system; (ii) introduction of material losses that break time-reversal symmetry; and (iii) variation in the interlayer distance, which preserves structural symmetry. In particular, we demonstrate that material losses inevitably induce radiation in Fabry-Pérot BICs via second-order perturbation processes, converting them into quasi-BICs, while symmetry-protected BICs remain nonradiative. We further show that, despite the inherent instability of BICs under symmetry-breaking effects, their resilience can be significantly enhanced through proper design. Both Fabry-Pérot and symmetry-protected BICs exhibit exponentially weak sensitivity to ${\mathrm{C}}_2$-breaking perturbations as the interlayer distance increases. Finally, we show that additional FP BICs emerge under oblique incidence, originating from the interference of two high-$Q$ quasi-BICs near the symmetry-protected ones. Our findings pave the way for the development of BIC-based photonic devices with improved robustness against fabrication imperfections, environmental variations, and material losses.

We propose an in situ tunable photon blockade scheme based on a three-mode interaction model comprising two bosonic modes and a two-level system. In this scheme, the mediating bosonic mode, which is subjected to periodic modulation, serves as a Floquet coupler. This approach offers two significant advantages: (1) it enables universal photon blockade by increasing the number of coupling channels, thereby integrating the strengths of both conventional and unconventional photon blockades, while overcoming their coupling strength limitations; (2) it introduces additional controllable degrees of freedom and periodic characteristics, allowing the system to generate single photons with different frequencies on demand. The numerical results for the second-order correlation function of photons and the average single-photon number indicate that, in both the strong-coupling regime and the weak-coupling regime, the universal photon blockade surpasses conventional and unconventional photon blockades individually, reducing the photon second-order correlation function by 2 orders of magnitude. Our proposed scheme holds promise for designing integrated and versatile single-photon sources capable of performing multiple tasks.

Linear optical networks are fundamental to the advancement of quantum technologies, including quantum computing, communication, and sensing. The accurate characterization of these networks, described by unitary matrices, is crucial to their effective utilization and scalability. In this work, we present a method for reconstructing the transfer matrix of a linear optical interferometer based on the analysis of cross-correlation functions of photon counts between pairs of output modes. Our approach accounts for losses and photon indistinguishability, making it robust to experimental imperfections. By minimizing the requirements for the input states, the method simplifies the experimental implementation. We demonstrate the effectiveness of our technique through theoretical modeling and experimental validation in a four-mode programmable integrated optical interferometer. The results show high fidelity in matrix reconstruction and successful application in boson sampling experiments. In addition, we provide a comprehensive formalism for correlation functions and discuss the robustness of the method to measurement errors. This work offers a practical and efficient solution for characterizing linear optical networks, paving the way for scaling up photonic quantum technologies.

Controlling the directional emission of quantum dots (QDs) is essential for applications in lighting, optical communications, biomedicine, and quantum information technologies. However, most existing strategies fail to simultaneously achieve high directionality and strong emission efficiency. Here, we present a dual-layer concentric-ring metalens optimized through inverse design powered by a deep reinforcement learning algorithm. This approach not only boosts fluorescence and directional emission but also significantly reduces the computational overhead. The proposed structure enables nonreciprocal emission control of embedded QDs, driven by unique near-field coupling and constructive interference of Bloch surface waves. A fluorescence enhancement factor of up to 5371 is achieved, with an emission divergence angle below 5°. Moreover, both the emission wavelength and directionality are tunable by varying the thickness ratio of dual-layer concentric-ring or the QD position. These findings offer promising opportunities for integrated quantum light sources and advanced biomedical photonic systems.

Quantum transduction is essential for connecting quantum technologies across varied frequencies. Achieving a positive quantum capacity of a traditional quantum transduction channel is a long-pursued goal in the field. However, to overcome the high threshold remains a major challenge, owing to practical limitations. Recently, an entanglement-assisted transducer was proposed based on a cavity-optic system [Opt. Quantum 2, 475 (2024)], where a modified bosonic loss channel was obtained, and the transduction efficiency could be enhanced by properly tuning the squeezing parameters. In this paper, we further identify three types of quantum channel enabled by this design, offering additional options for implementing the proposed transduction schemes. Compared with transducers without entanglement assistance, the scheme also shows a great enhancement in the conversion bandwidth for achieving high quantum capacity, further increasing its value in practical applications.

The generation and dynamic control of optical skyrmions enable topological structured light. However, existing approaches require complex experimental setups or sophisticated fabrication technology, limiting their scalability. Here, we introduce a method to generate and manipulate optical skyrmions without nanofabrication or precise alignment, leveraging spin-orbit interaction in a uniaxial crystal. By focusing a fundamental Gaussian beam along the optical axis of the crystal, the skyrmion texture will be generated in the output field. Furthermore, we demonstrate controlled transformation between different topological states by modulating the incident polarization. This approach provides a direct route for manipulating compact topological light and paves the way for applications based on optical skyrmions.

Ultrafast excitation of nonequilibrium carriers under intense pulses offers unique opportunities for controlling the optical properties of semiconductor materials. In this work, we propose a scheme for ultrafast generation of surface plasmon-polaritons (SPPs) via a transient metagrating formed under two interfering optical pump pulses in a thin film of the semiconductor $\mathrm{Ga}\mathrm{As}$. The grating is formed due to modulation of the refractive index associated with the nonequilibrium carrier generation. The temporal grating structure formed enables the generation of SPP waves at the $\mathrm{Ga}\mathrm{As}$/$\mathrm{Ag}$ interface via weak probe pulse excitation. We propose a theoretical model describing nonequilibrium carriers formation and diffusion and their contribution to permittivity modulation via Drude and band-filling mechanisms. We predict that by tuning the parameters of the pump and probe, one can reach the critical coupling regime and achieve efficient generation of SPPs on timescales of 0.1–1 ps.

The recent emergence of bound states in the continuum (BICs) has established a powerful paradigm for achieving high-quality-factor resonances in metasurfaces. Here, we propose a silicon nitride metasurface structure capable of supporting polarization-sensitive quasi-BICs, to generate structural colors with a wide gamut, high saturation, and tunable brightness. By designing a periodic array of nanodisks with in-plane symmetry breaking, we successfully excite magnetic dipole quasi-BICs with polarization-sensitive properties. Owing to its high-quality factor, this resonance exhibits a narrow-linewidth reflection peak across the visible spectrum, thereby significantly enhancing color purity. We employ a synergistic approach that combines deep learning with reinforcement learning to efficiently predict the optimal structural parameters, resulting in coverage that approaches the simulated sRGB color gamut. Experimental results further demonstrate that the brightness of the structural colors can be dynamically tuned by controlling the polarization state of incident light. Based on this mechanism, we design and fabricate two categories of chromatic patterns with orthogonally arranged air holes, which exhibit clear polarization-dependent responses under both orthogonal linear polarizations and white-light illumination. Our work offers insights into the application of high-performance structural colors in color printing, dynamic information encryption, and optical anticounterfeiting.

Silicon nitride is one of the most promising and mature integrated photonics platforms, providing a gateway to the development of cutting-edge devices based on photonic integrated circuits (PICs) with enhanced performance capabilities. For a wide range of applications, integrated heaters remain the preferred method for controlling PICs through the thermo-optic effect. Here, we demonstrate in detail the fabrication process and the characterization of heaters based on a ${\mathrm{Sc}\mathrm{N}}_x$/${\mathrm{Nb}/\mathrm{Sc}\mathrm{N}}_x$ stack. The proposed approach addresses both of the main issues that cause heater failures: electromigration and oxidation (oxygen diffusion). The findings of the experimental investigation suggest that the proposed ${\mathrm{Sc}\mathrm{N}}_x$/${\mathrm{Nb}/\mathrm{Sc}\mathrm{N}}_x$ heaters exhibit a critical current density $j_c\sim 100{\mathrm{GA}/\mathrm{m}}^2$ and are capable of heating silicon nitride waveguides by more than $200{}^{\circ }\mathrm{C}$, providing a specific optical path change per heater length $\mathrm{\Delta }{L}_{\mathrm{optical}}/L_{\mathrm{heater}}\sim 5\text{μ}\mathrm{m}/\mathrm{mm}$ for silicon nitride waveguides in the C band. Thanks to the elevated reliability level and remarkably high efficiency, the proposed ${\mathrm{Sc}\mathrm{N}}_x$/${\mathrm{Nb}/\mathrm{Sc}\mathrm{N}}_x$ heaters potentially enable a significant reduction in the PIC footprint, without compromising performance and robustness. Additionally, we carried out a rough experimental assessment of the magnitude of thermal crosstalk.

The integration of elementary quantum objects (atoms, molecules, and quantum dots) with solid-state nanostructures lies at the forefront of nano-optics, nanophotonics, and quantum information science. However, the advancement of this field is hindered by the lack of a rigorous and feasible theoretical framework for describing atom-nanostructure interactions, which are inherently complex and multiparametric. In this work, we develop a theory of light transmission through a zero-mode waveguide (ZMW) containing a single atom. It is shown that the presence of a single atom inside the ZMW can lead to either a significant enhancement or suppression of light transmission, depending on the detuning of the excitation field frequency from the atomic resonance. This extraordinary transmission and blocking effect can be employed for studying the spatiotemporal dynamics of atoms in complex nanoscopic environments, probing quantum optical phenomena, and developing novel nano-optical devices.

Compact integrated optical parametric oscillators are of high demand in numerous cutting-edge applications within photonics. In particular, degenerate optical parametric oscillators attract flourishing research interest, owing to their unique features, which are essential for optical computing and quantum optics. In this study, we experimentally examine the optimal conditions for highly efficient degenerate optical parametric oscillations (DOPOs) in silicon nitride (${\mathrm{Si}}_3{\mathrm{N}}_4$) microring resonators (MRRs) excited by a dual-frequency pump. We demonstrate, both numerically and experimentally, that synchronous frequency scanning of pump lasers is an effective strategy to achieve optimal coupling of a pump power into a cavity, as well as to obtain efficient DOPOs. Additionally, we show, both experimentally and numerically, that a large thermal nonlinearity of a MRR can significantly increase a system’s overall tolerance with respect to pump frequency drift. Through our investigation, we obtain DOPO spectral line handling over 16% of the total output power. This study provides a technique for highly efficient and stable DOPO realization.

Realizing scalable quantum interconnects necessitates the integration of solid-state quantum memories with foundry photonics processes. While prior photonic integration efforts have relied upon specialized, laboratory-scale fabrication techniques, this work demonstrates the monolithic integration of a quantum memory platform with low-loss foundry photonic circuits via back-end-of-line deposition. We deposited thin films of titanium dioxide (${\mathrm{Ti}\mathrm{O}}_2$) doped with erbium ($\mathrm{Er}$) onto silicon nitride nanophotonic waveguides and studied $\mathrm{Er}$ optical coherence at subkelvin temperatures with photon echo techniques. We suppressed optical dephasing through ex-situ oxygen annealing and optimized measurement conditions, which yielded an optical coherence time of $64\text{μ}\mathrm{s}$ (a 5-kHz homogeneous linewidth) and slow spectral diffusion of 27 kHz over 4 ms, results that are comparable to state-of-the-art $\mathrm{Er}$ nanophotonic devices. Combined with second-long electron spin lifetimes and demonstrated electrical control of $\mathrm{Er}$ emission, our findings establish $\mathrm{Er}$:${\mathrm{Ti}\mathrm{O}}_2$ on foundry photonics as a manufacturable platform for ensemble and single-ion quantum memories.

Biphoton states have shown promising applications in quantum information processing, including quantum communications, quantum metrology, and quantum imaging. The generation and manipulation of biphoton entanglement in topologically distinct modes paves the way in this direction. Here we present a comprehensive method for regulating the topological properties of the system by combining the nonlinearity in waveguides, i.e., nonlinearity in the waveguide coupling materials, and the waveguide lattice structure. Our method enables the generation of topological biphoton states with injected pump activation on the topologically trivial modes. This is realized through self-induced manipulation on pump-dependent nonlinear couplings on the defects, which is unable to be realized while there are no such nonlinear couplings. Specifically, by including the nonlinear gain/loss mechanism in the coupling between the nearest-neighbor waveguides and the third-order Kerr nonlinearity effect along the waveguides, the injected pump power will be the controllable parameter for the manipulation of the topology in the defect states and the generation of biphoton entanglement states. We also present an experimental proposal to realize our scheme and its generalization in the contemporaneous “active” topological photonics time-bin platforms. Our method can be used in other Su-Schrieffer-Heeger (SSH) models with various defect configurations. Our method enables the reusability and versatility of SSH lattice chips and their application for fault-tolerant quantum information processing, promoting the industrialization process of quantum technology.

Chip-scale, electrically tunable, continuous-wave, coherent terahertz (THz) radiation sources are critical for emerging applications in sensing, imaging, spectroscopy, communications, space, and quantum technologies. Here, we demonstrate a robust source-on-a-chip THz emitter based on a layered high-temperature superconductor, engineered with an elliptical microcavity and capable of sustained coherent emission over an unprecedented operational lifetime exceeding 11 years. This compact THz source operates up to 60 K (with $T_{\mathrm{c}}$ ≈ 90 K), delivering stable radiation in the 0.7–0.8 THz range, with on-chip electrical tunability from 100 GHz to 1 THz. Coherence arises from the phase-locked oscillation of intrinsic Josephson junction arrays, resonantly coupled to transverse electromagnetic modes within the cavity, analogous to a laser cavity, yielding collective macroscopic oscillations. THz emission remains detectable across an approximately 0.5-m free-space open-air link at room temperature. We analyse the cavity-mode structure and extract THz photon generation rates up to approximately 503 photons ${\mathrm{fs}}^{-1}$ in cryogenic conditions and 50–260 photons ${\mathrm{ps}}^{-1}$ over the air. These results demonstrate, for the first time, sustained and electrically tunable coherent THz emission from superconductors over multiyear timescales, defining another class of robust, chip-integrated THz lasers with applications in scalable THz and quantum technologies.