On-chip quantum key distribution over field-deployed fiber using lithium niobate photonic circuit
Hyungjun Heo, Min Ki Woo, Chang-Hoon Park, Hyeong‐Soon Jang, Hyeon Hwang, Hansuek Lee, Min‐Kyo Seo, Sangin Kim, Hyounghan Kwon, Hojoong Jung, Sang-Wook Han
Quantum key distribution (QKD) systems have proven their theoretically unconditional security by quantum mechanics, but the scalability and cost barriers limit the rapid growth of the QKD system industry. The integration of QKD systems on chips has enabled their widespread adoption in secure quantum communication technologies, but the optimized platforms and designs are still being studied. Herein, we fabricated monolithic quantum photonic circuits for the BB84 QKD protocol using thin-film lithium niobate (TFLN), which enables flexible design in organizing both active and passive elements on one chip based on its superior material properties. The proposed circuit design for both transmitter and receiver parts are identical, which facilitates stable operation and mass production. Using our device, we demonstrated QKD over a field-deployed quantum channel, and its performance is comparable to state-of-the-art. This result proved the potential of TFLN for quantum communication technology.
Quantum key distribution (QKD) systems have proven their theoretically unconditional security by quantum mechanics, but the scalability and cost barriers limit the rapid growth of the QKD system industry. The integration of QKD systems on chips has enabled their widespread adoption in secure quantum communication technologies, but the optimized platforms and designs are still being studied. Herein, we fabricated monolithic quantum photonic circuits for the BB84 QKD protocol using thin-film lithium niobate (TFLN), which enables flexible design in organizing both active and passive elements on one chip based on its superior material properties. The proposed circuit design for both transmitter and receiver parts are identical, which facilitates stable operation and mass production. Using our device, we demonstrated QKD over a field-deployed quantum channel, and its performance is comparable to state-of-the-art. This result proved the potential of TFLN for quantum communication technology.
Abstract Integrated photonics-based coexistence systems enable classical data channels and quantum channels to share the same optical waveguide by integrating quantum key distribution (QKD) into existing telecommunications infrastructure, reducing the costs and complexities of deploying quantum-secure links. Achieving this requires photonic components that efficiently reject intense classical light while preserving the integrity of ultra-weak single-photon quantum signals. Here, we demonstrate the first on-chip quantum-classical coexistence platform based on thin-film lithium niobate (TFLN). The chip integrates a polarization splitter-rotator (PSR) with two four-channel wavelength-division multiplexers, creating eight distinct channels encoded by wavelength and polarization in a highly compact millimeter-scale footprint. The device features ultra-low on-chip insertion loss ( IL ), high polarization isolation, and robust suppression of nonlinear noise, minimizing crosstalk from classical to quantum channels. Single-photon experiments verify its compatibility with QKD operations under realistic scenarios of simultaneous classical and quantum transmission. Its fabrication-robust design eases lithography constraints, enabling scalable and cost-effective manufacturing. Overall, this chip offers a viable path to integrate QKD into dense optical networks and advances the development of scalable quantum-secure communications.
Photonic integration in quantum communication holds significant potential for miniaturization and enabling commercial applications. Among various platforms, thin-film lithium niobate (TFLN) stands out due to its exceptional combination of high electro-optical efficiency, low propagation loss, and compact footprint. Here, we demonstrate a 2.5 GHz chip-to-chip fully integrated quantum key distribution (QKD) system based on a TFLN platform, which incorporates high-speed dual-polarization time-bin phase encoding and decoding functionalities. We achieve an extremely low quantum bit error rate of 0.53% and a secret key rate exceeding 10 Mbps over 25 km fiber spools. The design of cascaded Mach–Zehnder modulators effectively suppresses the patterning effect in high speed QKD. Notably, the TFLN chips used in both the transmitter and receiver share a similar architecture, highlighting the potential for creating a homogeneous transceiver. This work paves the way for high-speed, miniaturized QKD systems based on the lithium niobate integrated platform.
Quantum key distribution (QKD) is moving from laboratory demonstrations toward network deployment, and integrated photonic chips provide a hardware route for compact, stable and scalable systems. This review summarizes progress in integrated photonic chips for QKD from protocol requirements to deployable systems. It first examines how decoy-state BB84, measurement-device-independent QKD (MDI-QKD), twin-field QKD (TF-QKD), continuous-variable QKD (CV-QKD) and device-independent QKD (DI-QKD) constrain chip metrics such as loss, modulation accuracy, state indistinguishability, phase stability, synchronization and security monitoring. It then compares planar lightwave circuit (PLC), silicon photonics, silicon nitride, thin-film lithium niobate, indium phosphide (InP) and heterogeneous platforms in terms of routing loss, modulation speed, stable interference, source integration and detector interfaces. Transmitter and receiver architectures, coherent detection, single-photon detection interfaces and on-chip monitoring are discussed as system-level building blocks. The review further analyses phase stability, packaging coupling, thermal management, polarization control, network coexistence and reliability, emphasizing that deployable QKD chips must remain calibratable and compatible with security models under realistic operation. Finally, system demonstrations, network deployment and practical security are discussed within a unified evaluation framework. Throughout, we argue that integration benefits QKD only when compact routing, stable interference, calibration access and manufacturable packaging jointly keep the device inside the assumptions of its security proof; accordingly, chip performance should be judged by mapped, calibratable security parameters rather than by integration density alone.
We demonstrate a thin-film lithium niobate (TFLN) integrated photonic chip for time-binencoded quantum key distribution (QKD). The chip enables on-chip preparation of both time- and phase-basis quantum states required by the BB84 protocol. By exploiting the linear Pockels effect in TFLN, high-fidelity phase modulation is achieved without phase-dependent loss, resulting in stable and low-noise quantum state generation. The measured interference visibility reaches up to 98.74% for phase-basis states, while extinction ratios exceeding 22 dB are obtained for time-basis states. Furthermore, a chip-to-chip QKD system is implemented using TFLN integrated circuits at both the transmitter and the receiver. Finite-key analysis based on a one-decoy protocol demonstrates positive secret key generation over 20 km of standard single-mode fiber.
Time-bin encoded quantum states of light are crucial for quantum technology applications. The integration of manipulation functionalities into chip-scale devices is essential for deploying scalable, high-performance, and cost-effective quantum networks. Here we develop a fully integrated, high-throughput quantum receiver based on the thin-film lithium niobate (TFLN) platform, capable of high-speed electro-optic manipulation of time-bin encoded quantum states. The device's novel architecture enables active switching of time-bin quantum states with an electro-optic bandwidth exceeding 30 Ghz, while supporting real-time arbitrary projective measurements with a bandwidth of over 1 GHz. We showcase its versatility and performance through several applications, including the certification of entanglement with Bell's inequality violation by 38 standard deviations and with >95% visibility. We then apply it to a fiber-based quantum communication scenario, where we experimentally demonstrate an entanglement-based quantum key distribution (QKD) protocol, achieving stable finite-size secure key rates exceeding 25 kbit/s over 12 hours of continuous operation. By leveraging a high-speed active switching scheme, the system overcomes the need for temporal post-selection, eliminating a fundamental loophole that compromises the security of time-bin entanglement-based QKD protocols and relaxes the temporal resolution requirements of single-photon detectors. Moreover, it enables active selection of the projection basis, increasing the flexibility for communication parties. This approach establishes a versatile and scalable architecture for time-bin encoded quantum communication, enabling practical protocols on industry-grade photonic technology.
Improvement in secure transmission of information is an urgent need for governments, corporations and individuals. Quantum key distribution (QKD) promises security based on the laws of physics and has rapidly grown from proof-of-concept to robust demonstrations and deployment of commercial systems. Despite these advances, QKD has not been widely adopted, and large-scale deployment will likely require chip-based devices for improved performance, miniaturization and enhanced functionality. Here we report low error rate, GHz clocked QKD operation of an indium phosphide transmitter chip and a silicon oxynitride receiver chip-monolithically integrated devices using components and manufacturing processes from the telecommunications industry. We use the reconfigurability of these devices to demonstrate three prominent QKD protocols-BB84, Coherent One Way and Differential Phase Shift-with performance comparable to state-of-the-art. These devices, when combined with integrated single photon detectors, pave the way for successfully integrating QKD into future telecommunications networks.