Long-distance matter-matter entanglement is pivotal for scalable quantum communication, distributed quantum computing, and sensing. In this Letter, we report entanglement between two atomic ensemble quantum memories over 420 km. We employ the Duan-Lukin-Cirac-Zoller (DLCZ) scheme for remote entanglement generation and convert photons emitted from the memories to telecom S band that enable us to harness ultralow transmission loss in fiber. We stabilize a photon's relative phase between the two memories using full-time far-off-resonant locking to reduce high-frequency noise and intermittent dual-band locking to compensate low-frequency drift jointly. Furthermore, we demonstrate that the memory-memory entangling probability beats the repeaterless channel capacity for direct entanglement distribution. Our experiment provides a test bed of studying quantum network applications beyond metropolitan scale.

Quantum frequency conversion (QFC) is essential for bridging the spectral gap between stationary qubits and low-loss optical communication channels. In this work, we demonstrate a short-wavelength-pumping QFC with the first-order quasi-phase matching period of 3.07 um on thin-film lithium niobate, converting ultraviolet photons to the telecom C-band. By constructing a theoretical model that correlates the normalized conversion efficiency with domain defects in the short-period phase-matched waveguide, we found the critical tolerance of domain defects along the waveguide should be ≤ 2 (excluding the ends). Based on this, we achieved a theoretical limit normalized conversion efficiency of 839

The mid-infrared spectral window is essential for molecular fingerprinting and atmospheric sensing, yet unlocking its full potential is currently constrained by a fundamental instrumental trade-off: existing systems cannot simultaneously deliver broad bandwidth, high photon flux, and metrological frequency fidelity. Here, we resolve this bottleneck by demonstrating a metrology-grade spectroscopic system based on difference frequency generation, driven by widely tunable, near-infrared diode lasers traceable to atomic standards. Our system achieves continuous tunability across the 3-3.7 μm atmospheric window and delivers output power exceeding 45 mW with an absolute frequency accuracy of 7.2 MHz. We harness this convergence to overcome a critical barrier in integrated photonics, unambiguously identifying and eliminating hydrogen-induced absorption in silicon nitride microresonators to achieve an 88-fold reduction in optical loss. We further reveal multi-phonon absorption in the silica cladding as the fundamental limit to mid-infrared integrated photonics. Finally, we demonstrate the system's versatility through scattering-resilient LiDAR capable of penetrating optically dense fog, and dual-modality sensing that simultaneously retrieves target distance and chemical composition. By unifying the rigor of frequency metrology with the versatility of broadband sensing, this architecture establishes a new paradigm for multi-dimensional perception in complex environments.

Device-independent quantum key distribution (DI-QKD) is a key application of the quantum internet. We report the realization of DI-QKD between two single-atom nodes linked by 100-kilometer (km) fibers. To improve the entangling rate, single-photon interference is leveraged for entanglement heralding, and quantum frequency conversion is used to reduce fiber loss. A tailored Rydberg-based emission scheme suppresses the photon recoil effect on the atom without introducing noise. We achieved high-fidelity atom-atom entanglement and positive asymptotic key rates for fiber lengths up to 100 km. At 11 km, 1.2 million heralded Bell pairs were prepared over 624 hours, yielding an estimated extractable finite-size secure key rate of 0.112 bits per event against general attacks. Our results close the gap between proof-of-principle quantum network experiments and real-world applications.

Quantum frequency conversion (QFC) plays a crucial role in constructing seamless interconnection between quantum systems operating at different wavelengths. To advance future quantum technology, chip-scale integrated QFC components, featuring high efficiency, small footprint, low power consumption, and high scalability, are indispensable. In this work, we demonstrate the first hybrid integrated QFC chip on a thin-film lithium niobate platform that connects the telecom and visible bands. Benefiting from the periodically poled microring resonator with ultra-high normalized conversion efficiency of 386,000%/W, an ultra-low pump power of 360 mu W is achieved, which is more than two orders of magnitude lower than the traditional straight waveguide scheme. By injecting current into the chip, an on-chip quantum efficiency of 57% and a noise count of similar to 7000 counts per second are achieved. Such an electrically pumped, integrated, and scalable QFC chip would significantly advance the integration of quantum networks and the development of chip-scale quantum optical systems.

Silicon single-photon detectors (Si SPDs) play a crucial role in detecting single photons in the visible spectrum. For various applications, photon detection efficiency (PDE) is the most critical characteristic for effectively collecting photons. Here, we present a Si SPD with a remarkable PDE of up to 84.4

Objective Lithium niobate (LN) has emerged as a cornerstone material in the fields of nonlinear optics and integrated photonics. While traditional bulk lithium niobate crystals have been extensively utilized for laser frequency conversion, they face significant limitations due to the walk-off effect and weak light field confinement during the process. These challenges necessitate extremely high pump light power levels to achieve substantial nonlinear conversion efficiency, making them less suitable for on-chip integration applications. The development of lithium niobate-on-insulator (LNOI) thin films has addressed these limitations through advanced bonding and thinning processes. LNOI not only retains the advantageous properties of bulk lithium niobate crystals but also offers a high refractive index contrast and the micron-scale light field confinement capability. Although Ti-diffused or proton-exchanged waveguides can achieve high frequency conversion efficiency, their relatively low damage thresholds (below 100 MW/cm2) significantly limit their applicability in high-power scenarios. In contrast, ridge waveguides directly fabricated in LNOI thin films through etching or other manufacturing techniques not only avoid introducing lattice defects but also maintain the excellent physical properties of the bulk crystals. While nanoscale thin film waveguides demonstrate ultra-high normalized conversion efficiency, their submicron mode field dimensions result in significant mismatch with standard fiber modes, leading to excessive coupling losses. To realize commercially viable high-reliability and high-power frequency conversion devices, the use of micron-scale thin film waveguides is imperative. The micron-scale LNOI ridge waveguides offer a promising balance between efficient nonlinear interaction and practical coupling performance, making them particularly suitable for demanding high-power applications. Further research should focus on optimizing their structural design and fabrication processes to fully unlock their potential in integrated photonics systems. Methods As illustrated in Fig. 1(a), the ridge waveguide utilized in this work features a ridge height of 7 mu m, a ridge width of 7 mu m, an etching depth of 3.5 mu m, and sidewall inclination of 70 degrees.The waveguide length is 15 mm, with a poling period of 18.18 mu m for the 1560 nm second-harmonic generation (SHG) process. Our platform is based on z-cut LNOI thin film, comprising a 7 mu m-thick LN layer, a 2 mu m-thick SiO2 layer, and a silicon substrate. The fabrication process involves two key steps: the periodically poling and the waveguide etching. As detailed in Fig. 2(a), the procedure begins with the deposition of aluminum layers on both sides of the LNOI wafer using electron beam evaporation. A periodic poling electrode pattern is then defined through spin-coating lithography, followed by the wet etching to reveal the electrode areas. Following the completion of the periodic poling process, a chromium layer is plated onto the sample surface to serve as a mask. The waveguide path is subsequently defined using spin-coating lithography, and the ridge waveguides are etched using the inductively coupled plasma. After completing the inductively couple plasma etching process, the chromium mask is removed using a chromium etchant solution. Finally, the waveguide sidewalls are polished to reduce the transmission loss, and both end facets of the waveguide are polished and coated with an anti-reflection layer to minimize the Fresnel reflection loss. Results and Discussions Under low-power conditions, the normalized conversion efficiency of the SHG waveguide is measured to be 61 % /(Wcm(2)). The relationship between the output power of the frequency-doubled light and the input power of the fundamental light follows a quadratic dependence. As the input power of the fundamental light increases, the output power of the frequency-doubled light scales linearly with it. Ultimately, an output of frequency-doubled light at 780 nm exceeding 5 W is achieved (Fig. 5). The internal conversion efficiency of the waveguide is determined to be 84.8 %, while the overall device conversion efficiency reaches 52.6 %. Analysis reveals that the residual fundamental light in higher-order modes and the temperature gradient generated by thermal effects at high power are the primary factors limiting the internal conversion efficiency from reaching 100 %. Finally, a 24 h high-power stability test is conducted, which demonstrates excellent stability with the output power fluctuation within +/- 3 %. Conclusions This paper presents a high-power periodically poled lithium niobate (PPLN) waveguide frequency conversion device with an all-fiber structure. Based on the 7 mu m-thick magnesium-doped LNOI thin films, the PPLN ridge waveguides are fabricated using dry etching technology. Under an input power of 9.5 W at the fundamental wavelength of 1560 nm, the device achieves a SHG output of 5 W at 780 nm, corresponding to an overall conversion efficiency of 52.6 degrees o. Notably, after continuous operation for 24 h under high-power conditions, the SHG output power fluctuation remains within +/- 3 %. Additionally, this study experimentally identifies the higher-order modes of the fundamental light as the principal factor constraining the device conversion efficiency from reaching its theoretical limit. The proposed single-mode condition provides a clear guideline for optimizing waveguide dimensions to enhance the frequency conversion efficiency. This high-power PPLN waveguide frequency conversion device successfully combines the high conversion efficiency with the low fiber coupling loss, delivering a significant output power. These advancements support the development of commercial integrated photonic devices and hold promising application prospects in quantum information processing and quantum light sources.

Long-distance entanglement is pivotal for quantum communication, distributed quantum computing and sensing. Significant progresses have been made in extending the distribution distance of entangled photons, either in free space or fiber. For future quantum network applications, matter-based entanglement is more favorable since the capability of storage is essential for advanced applications. Extending entanglement distance for memory qubits was partially hindered by the mismatch of its photonic emission wavelength with the low-loss transmission window of optical fiber. By incorporating quantum frequency conversion, memory-memory entanglement has been successfully extended to several tens of kilometers. Here, we make a significant step further by reporting the entanglement between two atomic ensemble quantum memories over 420 km. We convert photons emitted from the memories to telecom S-band, which enable us to exploit the significantly low transmission loss in fiber (0.17 dB/km). We employ the DLCZ scheme for remote entanglement generation, and delicately stabilize the relative phase between the two memories by using fulltime far-off-resonant locking to reduce high-frequency noise and intermittent dual-band locking to compensate low-frequency drift jointly. We demonstrate that the memory-memory entangling probability beats the repeaterless channel capacity for direct entanglement distribution. Our experiment provides a testbed of studying quantum network applications from metropolitan scale to intercity scale.

Silicon single-photon detectors (Si SPDs) play a crucial role in detecting single photons in the visible spectrum. For various applications, photon detection efficiency (PDE) is the most critical characteristic for effectively collecting photons. Here, we present a Si SPD with a remarkable PDE of up to 84.4% at 785 nm, supporting multiple operation modes. We design and fabricate a thick-junction Si single-photon avalanche diode (SPAD) that enhances the avalanche probability through a backside-illumination structure, while minimizing noise through the design of a doping-compensated avalanche region. To maximize PDE, we implement a readout circuit with a 50 V quenching voltage, enabling operation in free-running, gating, or hybrid modes. The SPAD, along with its readout circuits and affiliated circuits, is integrated into a compact SPD module. In free-running mode, the module achieves a maximum PDE of 84.4%, with a dark count rate of 260 cps, and an afterpulse probability of 2.9% at 268 K. This work provides a practical solution for applications requiring ultra-high-efficiency Si SPD with multiple operation modes.

To fully exploit the potential of quantum technologies, quantum networks are needed to link different systems, enhancing applications in computing, cryptography and metrology. Central to these networks are quantum relays that can facilitate long-distance entanglement distribution and quantum communication. In this work, we present a modular and scalable quantum relay architecture using a high-quality single-photon source. The proposed network incorporates three untrusted intermediate nodes and is capable of a repetition rate of 304.52 MHz. We use a measurement-device-independent protocol to demonstrate secure key establishment over fibres covering up to 300 km. This study highlights the potential of single-photon sources in quantum relays to enhance information transmission, expand network coverage and improve deployment flexibility, with promising applications in future quantum networks.

We demonstrate a vector spectrum analyzer for visible-light integrated photonics, which features a spectral bandwidth of 766 to 795 nm and 8.1 MHz frequency accuracy. This accuracy is achieved by referencing atomic hyperfine structures.

Entangled photon sources (EPSs) are essential for quantum science and technology. Despite advancements in integrated optical platforms like thin-film lithium niobate, a scalable, high-performance, chip-scale EPS has remained elusive. We address this by demonstrating an electrically pumped, post-selection-free polarization EPS, achieved through hybrid integration of a distributed feedback laser with a thin-film lithium niobate chip, which integrates periodically poled lithium niobate waveguides, a beam splitter, and a polarization rotator combiner. By injecting current into the chip, we realize a high-performance EPS with a bandwidth of 73 nm and an entanglement pair generation rate of 4.5×10^{10}  pairs/s/mW. The polarization entanglement shows Bell-state fidelities above 96% across frequency-correlated modes. This compact, integrated EPS enables key applications, including high-speed quantum key distribution via wavelength division multiplexing, satellite-based quantum communication, and entanglement-based quantum metrology.

Thin-film lithium niobate (TFLN) nanophotonics has revitalized traditional ferroelectric materials in recent years. The ultrahigh nonlinear efficiency and flexible dispersion engineering achievable on the TFLN platform have provided innovative solutions to challenges in both classical and quantum optics. Periodic poling of TFLN is essential for realizing efficient frequency conversion. However, fabricating periodically poled TFLN with high uniformity remains challenging due to phenomena such as imperfect converted domains and low reproducibility across different wafer batches. To achieve a uniform domain structure with high repeatability, we propose a surface poling technique for x-cut TFLN, utilizing an annealing process prior to applying an external electric field. Optimal parameters for a uniform domain structure were determined by comparing different annealing temperatures and durations, with a treatment temperature of 430 degrees C and a treatment time of 13 h which improved the quality of lithium niobate film. These findings offer valuable insights into the dynamics of domain reversal in TFLN, which could enhance the performance of nonlinear devices.

Integrated photonics has been successfully established in the near-infrared (NIR) telecommunication bands. With the soaring demand in biosensing, quantum information and transportable atomic clocks, extensive endeavors have been stacked on translating integrated photonics into the visible spectrum. Demonstrations of visible-light lasers, frequency combs, and atom traps highlight the prospect of creating chip-based optical atomic clocks that can make timing and metrology ubiquitous. A pillar to the development of visible-light integrated photonics is characterization techniques featuring high frequency resolution and wide spectral coverage, which however remain elusive. Here, we demonstrate a vector spectrum analyzer (VSA) for visible-light integrated photonics, offering spectral bandwidth of 766-795 and 518-541 nm. The VSA is rooted in widely chirping, high-power, narrow-linewidth, mode-hop-free lasers that are frequency-doubled from the near-infrared via efficient, broadband CPLN waveguides. The VSA is further referenced to hyperfine structures of alkaline atoms and iodine molecules, enabling megahertz frequency accuracy. We apply our VSA to showcase the characterization of loss, dispersion and phase response of passive integrated devices, as well as densely spaced spectra of mode-locked lasers. Leveraging individual operations at 518-541, 766-795, 1020-1098, and 1260-1640 nm bands, our VSA achieves an aggregate characterization bandwidth exceeding one octave. This capability establishes the VSA as an invaluable diagnostic tool for spectroscopy, nonlinear optical processing, imaging, and quantum interfaces with atomic systems.

Entangled photons are crucial resources for quantum information processing. Here, we present an ultrabright polarization-entangled photon source based on a periodically poled lithium niobate waveguide designed for practical quantum communication networks. Using a 780 nm pump laser, the source achieves a pair generation rate of 2.4 × 10^10 pairs/s/mW. Remarkably, the entangled photons are bright enough to be detected by a power meter, reaching a power of 17.9 nW under a pump power of 3.2 mW. We demonstrate the practicality of the source by conducting quantum key distribution experiments over long-distance fiber links. Wavelength-division multiplexing was employed to enhance the key generation, and nonlocal dispersion compensation was implemented to ensure precise timing coincidence measurements across a broad spectral range. By utilizing nine pairs of wavelength channels, the system achieved the applicable secure key rates of up to 440.80 bits/s over 200 km with a 62 dB loss and extended the maximum secure key generation distance to 404 km. These results demonstrate the potential of wavelength-multiplexed polarization-entangled photon sources for high-speed, long-distance quantum communication, positioning them as key components for future large-scale quantum networks.

Integrated photonics has reformed our information society by offering on-chip optical signal synthesis, processing and detection with reduced size, weight and power consumption. As such, it has been successfully established in the near-infrared (NIR) telecommunication bands. With the soaring demand in miniaturized systems for biosensing, quantum information and transportable atomic clocks, extensive endeavors have been stacked on translating integrated photonics into the visible spectrum, i.e. visible-light integrated photonics. Various innovative visible-light integrated devices have been demonstrated, such as lasers, frequency combs, and atom traps, highlighting the capacity and prospect to create chip-based optical atomic clocks that can make timing and frequency metrology ubiquitous. A pillar to the development of visible-light integrated photonics is characterization techniques featuring high frequency resolution and wide spectral coverage, which however remain elusive. Here, we demonstrate a vector spectrum analyzer (VSA) for visible-light integrated photonics, offering spectral bandwidth from 766 to 795 nm and frequency resolution of 415 kHz. The VSA is rooted on a widely chirping, high-power, narrow-linewidth, mode-hop-free laser around 780 nm, which is frequency-doubled from the near-infrared via an efficient, broadband CPLN waveguide. The VSA is further referenced to hyperfine structures of rubidium and potassium atoms, enabling 8.1 MHz frequency accuracy. We apply our VSA to showcase the characterization of loss, dispersion and phase response of passive integrated devices, as well as densely spaced spectra of mode-locked lasers. Combining operation in the NIR and visible spectra, our VSA allows characterization bandwidth exceeding an octave and can be an invaluable diagnostic tool for spectroscopy, nonlinear optical processing, imaging and quantum interfaces to atomic devices.

State-of-the-art optical cavities are pivotal in pushing the envelope of laser frequency stability below 10−16. This is often achieved by extending the cavity length or cooling the system to cryogenic temperatures to reduce the thermal noise floor. In our study, we present a 30-cm-long cavity that operates at room temperature and is outfitted with crystalline coatings. The system has a predicted ultralow thermal noise floor of 4.4 × 10−17, comparable to what is observed in cryogenic silicon cavities. A 1397-nm laser is stabilized in this advanced cavity, and the stable frequency is then transferred to the clock transition in strontium optical lattice clocks via a frequency-doubling process. We have meticulously minimized and assessed the technical noise contributions through comparisons with an ultrastable reference laser that is locked to a commercially available 30-cm cavity. The frequency instability of the system is rigorously evaluated using a three-cornered-hat method. The results demonstrate that the laser frequency instability remains below 2 × 10−16 for averaging times ranging from 1 to 50 s. These findings underscore the significant potential of room-temperature cavities with crystalline coatings in high-precision metrology and pave the way for further improvements in optical lattice clocks.

Towards realizing the future quantum internet1,2, a pivotal milestone entails the transition from two-node proof-of-principle experiments conducted in laboratories to comprehensive multi-node set-ups on large scales. Here we report the creation of memory-memory entanglement in a multi-node quantum network over a metropolitan area. We use three independent memory nodes, each of which is equipped with an atomic ensemble quantum memory3 that has telecom conversion, together with a photonic server where detection of a single photon heralds the success of entanglement generation. The memory nodes are maximally separated apart for 12.5 kilometres. We actively stabilize the phase variance owing to fibre links and control lasers. We demonstrate concurrent entanglement generation between any two memory nodes. The memory lifetime is longer than the round-trip communication time. Our work provides a metropolitan-scale testbed for the evaluation and exploration of multi-node quantum network protocols and starts a stage of quantum internet research.

The lithium niobate on insulator (LNOI) platform has revolutionized lithium niobate materials, and a series of quantum photonic chips have exhibited unprecedented performances. Quantum frequency conversion (QFC) that enables quantum state preservation during frequency conversion is crucial in quantum technology. This work demonstrates a low-noise QFC process on the LNOI nanophotonic platform, connecting telecom and near-visible bands. An internal conversion efficiency of 73% and an on-chip noise count of 900 counts per second (cps) are achieved. Furthermore, the preservation of quantum statistical properties is verified, indicating the QFC chip’s promise for extensive applications of LNOI integrated circuits in quantum information. Based on this chip, we constructed an upconversion single-photon detector with a detection efficiency of 8.7% and a noise of 300 cps, paving the way to integrated on-chip single-photon detection. The realization of a low-noise QFC device also provide a pathway for practical chip-scale QFC-based quantum systems in heterogeneous configurations.

Towards realizing the future quantum internet, a pivotal milestone entails the transition from two-node proof-of-principle experiments conducted in laboratories to comprehensive, multi-node setups on large scales. Here, we report on the debut implementation of a multi-node entanglement-based quantum network over a metropolitan area. We equipped three quantum nodes with atomic quantum memories and their telecom interfaces, and combined them into a scalable phase-stabilized architecture through a server node. We demonstrated heralded entanglement generation between two quantum nodes situated 12.5 km apart, and the storage of entanglement exceeding the round-trip communication time. We also showed the concurrent entanglement generation on three links. Our work provides a metropolitan-scale testbed for the evaluation and exploration of multi-node quantum network protocols and starts a new stage of quantum internet research.