Realizing deterministic and bright single-photon sources with minimal two-photon probability at room temperature is essential for quantum communication, computation, and sensing. Solid-state quantum emitters, particularly nitrogen-vacancy (N-$V$) centers in diamond, are prime candidates for single-photon emission and spin addressability but suffer from limited photon extraction efficiency and higher two-photon probability. Here, we demonstrate efficient coupling of single N-$V$ centers in diamond nanowaveguides to guided optical modes, enabling enhanced single-photon outcoupling with low two-photon probability. By optimizing excitation and collection geometry, we achieve a single-photon count rate of approximately $2\text{Mcps}$ along with a low two-photon probability of $g^{(2)}(0)=0.014\pm 0.004$. The nanowaveguide architecture overcomes alignment and bandwidth constraints of resonant cavities, providing a scalable, fabrication-compatible route for bright single-photon sources.

Accurate characterization of nonlinear photonic integrated circuits is fundamentally limited by the inability of linear transmission measurements to distinguish input and output on/off-coupling efficiencies, which enter only through their product. We show that this degeneracy arises from an inherent input-output symmetry of linear optics and can be broken by exploiting the directional asymmetry of nonlinear processes. We introduce bidirectional nonlinear optical tomography (BNOT), which combines forward and backward pumping of complementary nonlinear interactions to uniquely infer individual on/off-coupling efficiencies from off-chip measurements. By jointly modeling the nonlinear response and detection statistics, BNOT enables unbiased reconstruction of on-chip nonlinear figures of merit with significantly reduced uncertainty, whereas conventional calibration yields systematically biased estimates. Monte Carlo simulations demonstrate reliable convergence of the inferred efficiencies across realistic noise and fluctuation regimes. Our results establish nonlinear directionality as a general symmetry-breaking resource for coupling-resolved characterization in integrated photonics, with immediate implications for scalable quantum light sources, frequency conversion, and precision optical metrology.

We present an approach to the molecular beam epitaxy of high-$Q$ planar GaAs-based microcavities in which the AlGaAs high-index layers of the distributed Bragg reflectors are replaced by short-period GaAs/AlAs superlattices (digital alloys) engineered to provide the same effective Al content. This design enables a significant reduction of interface roughness, precise control of both the $\lambda /4$ optical thickness and the effective Al content, suppression of the propagation of certain structural defects, and efficient tuning of the intrinsic absorption at the polariton emission wavelength through optimization of the superlattice parameters. Using this approach, we have grown a microcavity with a low polariton lasing threshold of $P_{\mathrm{th}}\approx 570\pm 90\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$ and a high experimental quality factor of $Q_{\mathrm{expt}}\approx (5.4\pm 0.1)\times {10}^4$. This value exceeds by a factor of 2 the theoretical estimate obtained within a model in which the digital alloy is replaced by a ternary AlGaAs alloy with the same effective Al content. We demonstrate that accurate modeling of the stop-band characteristics and the $Q$ factor requires incorporation of the modified electronic density of states in the superlattice, including quantum-confinement and excitonic effects.

Two-photon states generated from spontaneous parametric down-conversion (SPDC) can display entanglement in all degrees of freedom (DoFs) of light, including spatial, temporal, and polarization. The coupling between different DoFs of a two-photon state has been shown to display rich structures that enable novel and robust information processing schemes. While the existing literature has studied these couplings by postselecting the SPDC field, a comprehensive understanding of the inherent spatial-polarization coupling produced in the SPDC process is still lacking. This work produces a full spatial map of the polarization entanglement generated across the entire SPDC field, which contains an entire class of near-maximally polarization-entangled states with an average concurrence of $0.8303\pm 0.0004$. The spatial-polarization coupling manifests as radially or linearly varying polarization-entangled states, whose wavefunctions are dependent on the transverse momenta of the down-converted photons and the pump beam, respectively. Our study lays important groundwork for further exploiting the coupling between entanglement in different DoFs for future quantum technologies.

High-quality quantum communications that enable important capabilities, such as distributed quantum computing and sensing, will require quantum repeaters for providing high-quality entanglement. To realize high-rate heralded entanglement for quantum repeaters, Chen et al. [Phys. Rev. Appl. 19, 054029 (2023)] proposed a scheme for heralded-multiplexed generation of quasideterministic entangled photon pairs, called zero-added-loss multiplexing (ZALM). Here, we propose a design of the ZALM source using time-bin entanglement and spectral shearing. Additionally, we provide an analysis of experimentally relevant spectral-shearing parameters to optimize the spectral multiplexing. Moreover, we experimentally verify the compatibility of time-bin pulses and spectral shearing, as supported by observation of no appreciable phase shift when the same shearing is applied to both time bins. These results expand the benefits of applying a ZALM source to time-bin entanglement use cases. Moreover, more fully demonstrating time-bin and spectral shearing compatibility clears a path toward a broader use of spectral shearing that provides a deterministic frequency shift of high utility.

We demonstrate a double-layer all-optical inference platform for classification using cascaded nonlinear diffraction. The system utilizes a dual-pass configuration through a nonlinear optical crystal, shaping a superposition of Laguerre-Gaussian modes across two layers to steer optical intensity into class-specific detector regions. By optimizing via an in situ genetic algorithm with 10 trainable degrees of freedom, we achieve 88.65% accuracy on the ten-class Modified National Institute of Standards and Technology (MNIST) dataset, leveraging crystal temperature as a critical hyperparameter to optimize noncollinear mode mixing. This work establishes a robust pathway for high-accuracy, low-complexity all-optical computing through structured light and cascaded nonlinear optical interactions.

Near-field radiative heat transfer (NFRHT), primarily caused by evanescent wave tunneling in the near-field region, between a doped silicon metasurface (${\mathrm{M}}_{\mathrm{Si}}$) and a ${\mathrm{Si}\mathrm{O}}_2$ slab was investigated, employing rigorous coupled-wave analysis (RCWA) with the scattering matrix formalism. It was found that NFRHT is significantly enhanced in the frequency bands, owing to the hybrid modes coexcited by the surface plasmon polaritons of the ${\mathrm{M}}_{\mathrm{Si}}$ and the surface phonon polaritons of ${\mathrm{Si}\mathrm{O}}_2$. A transient all-optical method was proposed for NFRHT, based on focused ion beam (FIB) deposition, to measure the heat flux between patterned surfaces. It was experimentally demonstrated that, in a small near-field region, such as 100 nm, compared with RCWA, the effective medium theory (EMT) exhibits large deviations and cannot explain the physical mechanism of NFRHT well in artificial structures of a micrometer-scale period. In addition, the NFRHT mechanism was clarified by comparing the patterned ${\mathrm{M}}_{\mathrm{Si}}$ and unpatterned ${\mathrm{Si}\mathrm{O}}_2$, paving the way for the physical research and applications of NFRHT.

Escalating data volumes and security demands challenge traditional encryption, necessitating high-capacity, robust optical solutions. While optical knots offer topological protection, current constructions are restricted to simple torus knots. We overcome this limitation by introducing a framework for lemniscate knots, a generalized family that unlocks a rich variety of topologies through novel geometric control. Distinct from conventional realizations in the transverse component, we experimentally realize these knots within the longitudinal component—a largely unexplored polarization subspace—using inverse design. Crucially, we exploit the inherent faintness under paraxial conditions to establish an intrinsically covert topological carrier. Furthermore, we demonstrate a hybrid encryption scheme integrating these knots with the Advanced Encryption Standard. This scheme achieves hexadecimal encoding that provides multilayered protection, simultaneously expanding information capacity and bolstering cryptographic security. This work expands the topological toolkit and unlocks the longitudinal dimension for robust, high-capacity, and secure optical information processing.

Controlling the transport and nature of quantum excitations in low-dimensional systems is a key requirement for scalable quantum devices, including communication networks and quantum simulators. We propose a one-dimensional hybrid quantum lattice model, in which each lattice unit integrates a single-mode resonator that interacts with a two-level system, featuring direct coupling between adjacent two-level systems. This configuration enables the coherent propagation of excitations with tunable atomic, photonic, or polaritonic character. Beyond conventional single excitation transport, we demonstrate that appropriate impedance-matching and resonance conditions allow for the controlled swapping of excitation type as the excitation propagates along the lattice. We analyze the resulting dynamics using local observables and pairwise concurrence to track both transport and quantum correlations. Our results establish a minimal platform for controlled single excitation conversion, with direct relevance to hybrid quantum networks, on-chip quantum interconnects, and engineered quantum simulators.

Time-bin entanglement is a commonly used form of entanglement for quantum communication protocols over fiber networks, due to the natural resilience of this encoding scheme to thermal phase fluctuations and to birefringence in optical fibers. Projective measurements on some bases in the time-bin encoding need, however, postselection in the time domain of the measured events, introducing a loophole in Bell tests and requiring high temporal resolution. In this work, we demonstrate chip-integrated receivers for time-bin entanglement certification, including a high-speed optical switch to remove the temporal postselection loophole. The receivers are realized using thin-film lithium niobate and operate at a switching frequency of 5 GHz. We demonstrate a Bell inequality violation by more than 24 standard deviations without the need for time resolution at the time-bin separation level.

We experimentally realize an angular filter using a microwave network of coaxial cables, guided by the theoretical framework of quantum graphs with beyond-nearest-neighbor coupling. Our setup implements a resonant metamaterial interface that selectively transmits waves only at discrete output directions. The experimental results confirm the presence of angular filtering predicted in quantum graph models. A signal introduced at one side of a finite square lattice network is transmitted through the filter only along certain directional components at specific frequencies. The effect is caused by a resonance-induced switching of boundary conditions at the filter interface. Our findings validate theoretical predictions and demonstrate the practical applicability of graph-based metamaterial concepts in real-world wave systems.

Heterodyne interferometry for precision science often comes with an optical phase modulation, for example, for intersatellite clock noise transfer for gravitational-wave (GW) detectors in space, exemplified by the Laser Interferometer Space Antenna (LISA). The phase modulation potentially causes various noise couplings to the final phase extraction of heterodyne beatnotes by a phasemeter. In this paper, in the format of space-based GW detectors, we establish an analytical framework to systematically search for the coupling of various noises from the heterodyne and modulation frequency bands, which are relatively unexplored so far. In addition to the noise caused by the phase modulation, the high-frequency laser phase noise is also discussed in the same framework. The analytical result is also compared with a numerical experiment to confirm that our framework successfully captures the major noise couplings. We also demonstrate a use case of this study by taking the LISA-like parameters as an example, which enables us to derive requirements on the level of the laser and phase-modulation noises in the high-frequency regimes.

Hardware-algorithm co-design provides an effective route for implementing quantum optimization on near-term hardware. Counterdiabatic (CD) protocols can accelerate adiabatic optimization but are difficult to realize digitally due to increased circuit depth and noise sensitivity. Here, we report a photonic co-design strategy that directly implements CD dynamics on a programmable photonic integrated circuit based on a Mach-Zehnder interferometer mesh. By exploiting the ability of the photonic processor to realize arbitrary unitary transformations, our approach bypasses gate-level decomposition and significantly reduces circuit depth. We further enhance the protocol by mitigating higher-order many-body terms in conventional CD optimization. An experimental demonstration of integer factorization achieves success probabilities up to 86% with only three layers, together with accelerated energy convergence, highlighting programmable photonic platforms for quantum optimization.

Spin-decoupled metasurfaces hold significant potential for integrated photonics by enabling independent manipulation of the spin degree of freedom of light. However, achieving robust spin-decoupled performance over both a broad bandwidth and a wide range of incident angles remains a pivotal challenge. In this work, a spin-decoupled strategy incorporating the propagation phase with the geometric phase is implemented to design plasmonic metasurfaces capable of independently controlling wave fronts of orthogonally circularly polarized light in the visible spectrum. Two representative devices demonstrate distinct spin-decoupled functionalities: one generating a vortex beam under left circular polarization (LCP) and the other enabling LCP-selective bifocusing, while both maintain specular reflection under right circular polarization. Systematic experimental characterization using microscopic spectroscopy demonstrates robust spin-decoupled performance across an 80 nm bandwidth and a wide range of incident angles with a scanning range of up to $40\text{μ}\mathrm{m}$. This dual advantage in spectral and angular stability, surpassing typical dielectric counterparts, provides solutions for reliable metasurface devices in applications such as multiplexed optical communications and miniaturized high-resolution imaging.

The integration of photonic components into surface ion traps provides a scalable approach for trapped-ion quantum computing, sensing, and metrology, enabling compact systems with enhanced stability and precision. However, the introduction of optical apertures in the trap electrodes can distort the trapping electric field. This can lead to excess micromotion and ion displacement, which degrade the performance of quantum logic operations and optical clocks. In this work, we systematically investigate the electric field distortion in a surface ion trap with integrated waveguides and grating couplers that require apertures in the surface electrode using finite element method simulations. We analyze methods to reduce these distortions by exploiting symmetries and transparent conductive oxide materials.

A quantum network utilizes quantum entanglement and quantum error correction to effectively transmit quantum information. The development of such networks is a crucial step toward achieving secure and efficient quantum communication. This paper introduces a Bell inequality to verify nonlocality in continuous-variable (CV) multipartite chained entanglement-swapping networks, and proposes a CV-quantum key distribution (QKD) protocol for such networks with Bell-test gating, in which key generation proceeds only when the Bell test is passed. We analyze the achievable key rates under collective and coherent attacks. Numerical simulations show a clear scalability advantage, with the maximum secure distance increasing with the number of nodes. These results support the proposed CV-QKD protocol as a practical route to long-distance secure communication in CV quantum networks.

We report the fabrication of a barium magnesium fluoride (${\mathrm{Ba}\mathrm{Mg}\mathrm{F}}_4$) whispering-gallery-mode (WGM) microresonator demonstrating an intrinsic quality factor ($Q_0$) of about $2\times {10}^9$ at $1.55\text{μ}\mathrm{m}$, corresponding to a loaded quality factor ($Q_L$) of about $1\times {10}^9$ under critical coupling. At $398\mathrm{nm}$, the resonator exhibits a quality factor ($Q_L$) of $1.25\times {10}^8$ measured in the undercoupled regime, highlighting its potential for ultraviolet (UV) photonics. This performance was achieved in a 2.23-$\mathrm{mm}$-diameter resonator produced via a multistage process combining single-point diamond turning, chemomechanical polishing, and high-temperature annealing, yielding a surface roughness of approximately 1-$\mathrm{nm}$ RMS. Photoluminescence measurements under UV excitation revealed broadband visible emission, confirming the high optical quality of the crystal. When pumped in the near-infrared range, the resonator acted as a nonlinear element enabling the formation of Kerr optical frequency combs with a spectral width of about 30 nm and a line spacing of 42 GHz in the normal-dispersion regime. When incorporated into a fiber-laser cavity, it supported the generation of a frequency comb with a bandwidth of about 20 nm and a mode spacing of approximately 1 GHz at a pump power threshold of 86 mW. These results establish ${\mathrm{Ba}\mathrm{Mg}\mathrm{F}}_4$ WGM microresonators as a promising platform for nonlinear photonics across a broad spectral range, from the UV to the near-infrared.

Artificial atoms nonlocally coupled to waveguides—the so-called giant atoms—offer opportunities for the control of light and matter. In this work, we show how to use an array of nonlocally coupled transmon “molecules” to engineer a passive photonic controlled gate for waveguide photons. In particular, we show that a conditional elastic phase shift between counterpropagating photons arises from the interplay between direction-dependent couplings, engineered through an interplay of nonlocal interactions and molecular binding strength; and the nonlinearity of the transmon array. We analyze the conditions under which a maximal $\pi$-phase shift—and hence a CZ gate—is obtained, and characterize the gate fidelity as a function of key experimental parameters, including finite transmon nonlinearities, emitter spectral inhomogeneities, and limited cooperativity. Our work opens the use of giant atoms as key elements of microwave photonic quantum computing devices.