Quantum key distribution (QKD) offers a theoretically secure method to share secret keys, yet practical implementations face challenges due to noise and loss over long-distance channels. Traditional QKD protocols require extensive noise compensation, hindering their industrial scalability and lowering the achievable key rates. Alternative protocols encode logical qubits in noise-resilient states but at the cost of using many physical qubits, increasing susceptibility to loss and limiting transmission distance. In this work, we introduce a logical-qubit encoding that uses antisymmetric Bell states in the continuous photonic degrees of freedom, frequency and time. By leveraging the continuous space, we overcome this noise-loss robustness trade-off by minimizing the number of photons per logical qubit while optimizing the encoding resilience over noise fluctuations. We analyze the security of our encoding and demonstrate its robustness compared to existing state-of-the-art protocols. This approach provides a path toward scalable, efficient QKD implementations under realistic noise conditions.

Spatial filtering of optical fields has widespread applications, ranging from beam shaping to optical information processing; however, conventional spatial filters are bulky and alignment sensitive. Here, we present nonlocal non-Hermitian metasurfaces that can act as exceptionally effective optical spatial filters while being highly compact and insensitive to both lateral and longitudinal displacements. The design of these metasurfaces is based on a resonant waveguide grating in which radiative losses of the modes are tailored to realize a symmetry-protected bound state in the continuum in the middle of a non-Hermitian flat band. Using this design, we propose a compact spatial-filtering device operating over an angular range of approximately $1^{\circ }$ around normal incidence. In addition to being ultrathin and robust against translational misalignment, the proposed metasurfaces are easy to manufacture, which makes them an attractive alternative to conventional spatial filters, with the potential to become a widely used optical component.

Quantum networks and distributed quantum computing rely on entanglement generation between photons and long-lived stationary qubits. For large-scale architectures, one of the most crucial parameters is the efficiency at which entanglement can be created and detected. Here, we maximize the efficiency for the detection of hybrid entanglement between a nuclear spin qubit in diamond with a photonic time-bin qubit. Our approach relies on the optimal implementation of a photonic qubit analyzer, for which we use a high-speed electro-optical deflector to direct photons deterministically along the optimal interferometer paths. This way, we eliminate all cases in which photons are randomly lost due to propagation in the wrong interferometer path. We use nitrogen-vacancy centers in diamond, for which we demonstrate nuclear spin–photon entanglement with a fidelity of 0.6 reducing the experimental acquisition time by a factor of 2. An extension to other spin–photon entanglement systems and higher-dimensional entanglement schemes is outlined.

Because of its exceptional optical and nonlinear properties, ${\mathrm{Li}\mathrm{Nb}\mathrm{O}}_3$ has been widely adopted as a key semiconductor for optical communications, where effective thermal management is vital to ensure device performance and reliability. This study investigates the thermal conductivity of ${\mathrm{Li}\mathrm{Nb}\mathrm{O}}_3$ based on ab initio calculations. By considering the contribution of wavelike phonons and the effect of four-phonon scattering, the calculated thermal conductivity of ${\mathrm{Li}\mathrm{Nb}\mathrm{O}}_3$ at 300 K is $4.54\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$, which matches the experimental value of $4.63\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$. Wavelike mutually coherent phonons contribute significantly to thermal transport, with contributions as high as 25.6% to the total thermal conductivity at 900 K. Four-phonon scattering exhibits a limited but non-negligible impact on thermal conductivity, resulting in a reduction of 4.02%. Spectral analysis reveals that, for particlelike phonons, low-frequency modes from 0 to 6 THz contribute 81.5% to the thermal conductivity. This study clarifies phonon transport mechanisms in ${\mathrm{Li}\mathrm{Nb}\mathrm{O}}_3$, offering valuable guidance for device design.

High-dimensional entanglement, in the form of transverse spatial correlation between a pair of photons generated via spontaneous parametric down-conversion, is not only a valuable resource in many academic and real-life applications, but also provides access to several intriguing quantum phenomena. One such nonintuitive phenomenon is phase entanglement, in which the biphoton state is correlated in the complex phase of its wavefunction. This state, which emerges during the propagation of the biphoton wavefunction, exhibits no position correlation, yet retains full entanglement. In this work, we experimentally explore this state in two distinct ways. The first is by tracking the vanishing spatial photon number correlation over propagation distances lying in $[0,\mathrm{\infty })$, folded into a finite range using single-lens imaging. These observations show excellent agreement with our theoretical predictions based on the double Gaussian approximation of the biphoton state. The second approach involves performing a two-photon interference experiment using a double slit and this state, which reveals the correlated phase front. We show, both theoretically and experimentally, that the observed two-photon interference structure is markedly different from that produced by position-correlated photons, as confirmed by computing the joint probability distribution of photons and related metrics. Such interference using phase-entangled light has implications that open avenues for advanced experiments and applications in the field of spatial entanglement.

All-optical magnetization switching driven by femtosecond lasers offers a promising solution for data storage. To address power consumption, optical enhancement methods based on the two-temperature model can reduce energy requirements for magnetization reversal. To achieve this, we design a phase-matched optical resonant cavity formed between photonic crystals and multilayer ferrimagnetic films. This cavity with band filtering properties enables dual detection through transmission and reflection, similar to the all-optical switching films, facilitating high-density storage device integration. Finite element simulations are employed to adjust key photonic crystal parameters, ensuring theoretically optimal location of the pump laser. Experimental results demonstrate that over 87% of the 800-nm-pump laser is absorbed. A twofold pump laser fluence reductions for magnetization reversal are validated through static magneto-optical Kerr effect microscopy and time-resolved magneto-optical Kerr effect measurements. Our results provide substantial advances toward low-energy consumption and high-density compatibility characteristics for the next-generation ultrafast magneto-optical storage devices.

For quantum computation using photons, performing deterministic quantum gate operations is a challenge due to the probabilistic nature of the photon-photon interaction. Encoding qubits in multiple degrees of freedom of photons and controlling operations between them is one of the promising ways to navigate the probabilistic behavior. Using the single-photon discrete-time quantum walk in combination with polarization and path degrees of freedom, we experimentally demonstrate the realization of a universal set of quantum gates with high fidelity at room temperature. The deterministic realization of quantum gates through the photonic quantum walk are characterized via quantum state tomography. For a three-qubit system using a single photon, the first qubit is encoded using polarization information, and the other two qubits are encoded using path information, closely resembling a Galton-board setup. To generate a six-qubit Greenberger-Horne-Zeilinger state, entangled photon pairs are used to entangle the two three-qubit modules on which gate operations are performed. We also provide insights into the mapping of photonic quantum walk operations to quantum circuit representation and propose method to resourcefully scale. This demonstration marks significant progress towards using quantum walks for quantum computing and provides a framework for using fewer photons in combination with different degrees of freedom of photons to scale the number of qubits.

Integrated microring resonators have a broad range of applications in diverse fields with the potential to design compact, robust, energy-efficient devices crucial for quantum applications. Degenerate optical parametric oscillations (DOPOs) realized in dual-pumped microring resonator with third-order optical nonlinearity are of special interest. They demonstrate both bistability of the phase of the excited signal mode and generation of nonclassical light, which can be used for coherent photonic computing. Using coupled mode equations, we perform a comprehensive numerical analysis of DOPO conditions with normal group velocity dispersion and with bichromatic pumping. Through analytical and numerical approaches, we identify optimal setup parameter values that minimize the threshold power, highlighting the importance of considering the full spectrum of mode interactions. Additionally, we show that dispersion engineering, achievable in photonic molecules or photonic crystal microresonators, may provide a targeted frequency shift of specific microresonator modes resulting in pump power threshold reduction.

Recent progress has demonstrated that chiral-anomaly bulk states can achieve similar properties to those of topological valley-Hall edge states by using a reverse strategy of boundary-induced chiral transport phenomena in some bulk modes. However, the experimental attainment of chiral-anomaly bulk states in an electromagnetic system is rarely reported. In this paper, we experimentally confirm a magnetic resonance designer surface-plasmon crystal (SPC) composed of metallic patterns deposited on a dielectric substrate, which can demonstrate boundary-induced chiral-anomaly bulk states by exploiting appropriate perfect electrical conductor boundary conditions in a finite-size two-dimensional Dirac semimetal waveguide. The proposed magnetic resonance SPC is radically distinct from photonic crystals based on Bragg scattering. The dispersion and frequency responses of the chiral transport bulk states can be tuned by adjusting the magnetic resonance frequency. Moreover, the proposed SPC can utilize almost all available guiding space, which greatly improves the utilization of materials. It is expected that the present results can be scaled to other frequencies by changing the proportional size of the magnetic resonance unit cell, which could open the possibility of designing more compact, space-efficient, and robust electromagnetic wave and microfunctional devices.

Here we experimentally create a temporal waveguide doublet by introducing truncation boundaries in a synthetic temporal lattice, which is constructed using two fiber loops connected by a variable optical coupler. The optical pulses are confined to evolve and couple between the temporal lattice sites, akin to a directional waveguide coupler operating in the time dimension. Total energy transfer between the temporal waveguides is achieved only when the coupling ratio takes specific values. Nonlinearity is introduced into the temporal waveguides by incorporating an optoelectric conversion circuit into one of the fiber loops. We demonstrate a nonlinear Mach-Zehnder interferometer, where the output site of the optical pulse can be adjusted by varying the incident pulse intensity. The system has the potential to serve as a temporal switch and could find applications in optical pulse control and signal processing.

Photonic bound states in the continuum (BICs), possessing two unique properties of infinitely high quality ($Q$) factor and momentum-space polarization vortex, promise unprecedented opportunities for tailoring high-$Q$ resonances and enhancing light field manipulation. Recently, all-dielectric metasurfaces (MSs) underpinned by high refractive index and low intrinsic loss materials have unlocked the full potentials of BICs to realize ultrahigh-$Q$ resonances. However, experimental demonstrations of high-$Q$ terahertz BICs resonances in all-dielectric MSs with substrates remain a long-standing challenge. Here we propose and fabricate a paradigm of a crystalline silicon terahertz MS over a quartz substrate to demonstrate optical resonators with high-$Q$ quasi-BICs resonances. These resonators feature the simultaneous excitation of multiple topologically protected symmetry-protected BICs and accidental BICs. Under larger asymmetries, two additional BICs are observed in symmetry-broken terahertz MSs, thereby offering an extra degree of freedom for engineering BICs and easing fabrication. Our experimental results reveal two quasi-BICs modes characterized by sharp Fano resonances that boast an unparalleled $Q$ factor of up to 265, which is the highest reported experimental $Q$ factor in a terahertz MS with a substrate. We envision that these findings will not only hold enormous prospects for broadening the functional scope of terahertz metaphotonics but will also lay the groundwork for the realization of high-performance terahertz optoelectronic devices and metadevices.

Achieving compact on-chip pulsed lasers with attractive performance metrics and compatibility with the silicon photonics platform is an important, yet elusive, goal in contemporary nanophotonics. Here the fundamental question of whether two-dimensional (2D) materials can be used as both gain and saturable absorption media to enable compact integrated passively $Q$-switched nanophotonic lasers is posed and addressed by examining a broad range of 2D material families. The study is conducted by developing a temporal coupled-mode theory framework involving semiclassical rate equations that is capable of rigorously handling gain and saturable absorption by 2D materials, allowing us to perform stability and bifurcation analysis covering broad parameter spaces. The range of pulse-train metrics (repetition rate, pulse width, and peak power) that can be obtained via different 2D materials is thoroughly assessed. Our work illustrates that nanophotonic cavities enhanced with 2D materials can enable passive $Q$-switching with repetition rates ranging up to 50 GHz, short pulse duration down to a few picoseconds, and peak power exceeding several milliwatts. Such attractive metrics, along with the ultrathin nature of 2D materials and the ability to electrically tune their properties, demonstrate the potential of the proposed platform for compact and flexible integrated laser sources.

Fiber-based long-haul quantum communication would greatly benefit from a robust and deterministically integrated source of quantum states. Here, we report the design, fabrication, and optical characterization of $\mathrm{In}\mathrm{As}$/$\mathrm{In}\mathrm{P}$ quantum dots in the $\mathrm{In}\mathrm{P}$ H1 point-defect 2D photonic crystal cavity, integrated with a standard single-mode fiber employing microtransfer printing. The device is placed in a compact cryocooler maintaining a cryogenic temperature of 15 K, and it exhibits a low multiphoton contribution to emission, with $g^{(2)}(0)=0.14(14)$ in the all-fiber-based optical link between two laboratory nodes. In this way, we demonstrate a plug-and-play all-fiber single-photon source operating in the third telecom window, where standard telecommunication fiber networks can be used as a low-loss medium.

Optical modulators are essential building blocks for high-capacity optical communication and massively parallel computing. Among all types of optical modulators, traveling-wave Mach-Zehnder modulators (TW-MZMs) featuring high speed and efficiency are widely used, and have been developed on a variety of integrated material platforms. Existing methods to design and simulate TW-MZMs so far strongly rely on specific material properties, and thus inevitably involve complicated electrical-circuit models. As a result, these methods diverge significantly. In addition, they become increasingly inefficient and inaccurate for TW-MZMs with extended length and rising modulation speed, posing formidable challenges for millimeter-wave and terahertz operation. Here, we present an innovative perspective to understand and analyze high-speed TW-MZMs. Our perspective leverages nonlinear optics and complex band structures of periodic rf electrodes, and is thus entirely electromagnetic-wave-based. Under this perspective, we showcase the design, optoelectronic simulation, and experimental validation of high-speed TW-MZMs based on $\mathrm{Si}$ and ${\mathrm{Li}\mathrm{Nb}\mathrm{O}}_3$, and further demonstrate unambiguous advantages in simplicity, accuracy, and efficiency over conventional methods. Our approach can essentially be applied to almost any integrated material platform, including those based on semiconductors and electroabsorption materials. With high-frequency electrode designs and optoelectronic co-simulation, our approach facilitates the synergy and convergence of electronics and photonics, and offers a viable route to construct future high-speed millimeter-wave and terahertz photonics and quantum systems.

Nodal line semimetals exhibit band degeneracies that manifest themselves as one-dimensional ring structures in the momentum space. Here we experimentally demonstrate the existence of both gapless and gapped nodal rings in a two-dimensional photonic crystal composed of all-dielectric material. Breaking mirror symmetry that introduces a mass term causes the nodal ring degeneracies to transition into gapped states. By stacking two photonic crystals with opposite mass terms, we observe the emergence of Jackiw-Rebbi-like interface states within the gapped nodal rings. These interface states exhibit flat band dispersion, which is used to realize slow-light waveguides with low propagation loss. Additionally, a mode-tapering waveguide is designed by our modulating both the size of the gapless nodal ring structure and the bandwidth of the gapped nodal rings. Our results provide a simple two-dimensional platform for investigating nodal ring physics and exploring the potential applications of nodal rings in photonic devices.

Weak measurement is an effective technique for detecting the photonic spin Hall effect (SHE), but its performance is limited under strong coupling conditions or when the preselected and postselected states are nearly orthogonal. While modified weak measurements have been proposed to address these issues, the formation mechanism of the amplified displacements remains unclear, potentially impacting practical reliability. Therefore, exploring alternative methods for detecting the photonic SHE to compensate for the shortcomings of existing techniques is significant. In this paper, we develop a differential interference method to detect the photonic SHE. By leveraging differential interference, we reestablish the relationship between spin-dependent displacements and amplified displacements and elucidate the formation mechanism of the amplified displacements. We find that the measurement procedure using preselected and postselected ensembles is still effective. We compare the theoretical models of differential interference, conventional weak measurements, and modified weak measurements. We show that the experimental results coincide with the predictions of the differential interference theory. Due to the amplification effect of differential interference, the measurement resolution reaches ${10}^{-7}$ rad at small incidence angles, outperforming conventional polarimeters. Our method offers an alternative to weak measurement techniques for detecting the photonic SHE and holds promise for precision measurement.