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Optica Quantum

Integrated lithium niobate photonics for high-speed quantum key distribution

Zhihao Lin, Yuanfei Gao, Lai Zhou, Huihong Yuan, Yuntao Zhu, Zhongjin Lin, Wei Zhang, Yidong Huang, Xinlun Cai, Zhiliang L. Yuan

Published Mar 25, 2025• 31 citations
Publisher

Abstract

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.

DOI
10.1364/opticaq.551726

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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.
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.
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.
Quantum key distribution (QKD) relies on quantum state manipulation to achieve unconditionally secure key transmission. Polarization encoding, a mainstream QKD scheme, faces practical limitations due to the bulkiness and instability of discrete optical components. This work presents a polarization-encoding chip based on hybrid thin-film lithium niobate (TFLN) and silicon-on-insulator (SOI) integration, enabling dynamic control of four polarization states | R 〉, | A 〉, | L 〉, and | D 〉 with extinction ratios over 16.78 dB, using only a single thermal and electro-optic phase shifter in a compact architecture. Experimental results demonstrate that the chip achieves a secure key rate (SKR) of 328 kbps and a quantum bit error rate (QBER) of 1.96% at 100 MHz repetition rate in a 10 km fiber-based QKD system. Compared to conventional silicon-based carrier-depletion modulators (CDMs), the TFLN-SOI hybrid integration design avoids phase saturation and dynamic loss induced by carrier depletion, significantly enhancing the stability and scalability of polarization encoding.
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.
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 Quantum photonic technologies, such as quantum key distribution, are already benefiting greatly from the rise of integrated photonics. However, the flexibility in design of these systems is often restricted by the properties of the integration material platforms. Here, we overcome this choice by using hybrid integration of ultra-low-loss silicon nitride waveguides with indium phosphide electro-optic modulators to produce high-performance quantum key distribution transceiver chips. Access to the best properties of both materials allows us to achieve active encoding and decoding of photonic qubits on-chip at GHz speeds and with sub-1% quantum bit error rates over long fibre distances. We demonstrate bidirectional secure bit rates of 1.82 Mbps over 10 dB channel attenuation and positive secure key rates out to 250 km of fibre. The results support the imminent utility of hybrid integration for quantum photonic circuits and the wider field of photonics.