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Optics Express

High efficiency polarization encoding using a single thermal and electro-optic TFLN-based chip for quantum key distribution

Chunxue Zhang, Hanming Yang, Junchi Ma, Liyong Guo, Jianguang Li, Jiashun Zhang, Yue Wang, Xiaojie Yin, Wei Chen, Yuanda Wu, Xiangyang Dai, Weihua Guo, Jian Li, Junming An

Published Jul 22, 2025• 2 citations
Publisher

Abstract

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.

DOI
10.1364/oe.567189

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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.
Quantum key distribution (QKD) ensures unconditional security for public key encryption by utilizing the principles of quantum mechanics. This study designs and fabricates a QKD chip based on the silicon-on-insulator (SOI) platform, employing a time-bin encoding scheme with decoy states. In the design, we integrate a slow thermo-optic phase modulator with a carrier-depletion modulator to ensure high fidelity of quantum states and high-speed encoding capabilities, enabling precise and flexible time-bin encoding. We achieve the encoding and decoding of four BB84 quantum states at a repetition rate of 100 MHz. In the experiment, the visibility of the interference fringes for the phase state |+⟩ is 93.66%, and for the phase state |−⟩, it is 92.36%. The extinction ratios for the time states |0⟩ and |1⟩ are 19.33 and 18.72 dB, respectively. The experimental results demonstrate that the chip has efficient quantum state preparation capability, providing significant support for the practical implementation of quantum key distribution technology. Additionally, we propose a new chip structure, to our knowledge, using SOI and Si 3 N 4 heterogeneous integration. The SOI waveguide is used for high-speed modulation encoding, while the delay line is composed of Si 3 N 4 waveguides. This structure is designed to address the stability issues of the chip caused by temperature variations.
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.
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.
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.
Silicon-based polarization-encoding quantum key distribution (QKD) has been widely studied, owing to its low cost and robustness. However, prior studies have utilized off-chip devices to demodulate the quantum states or perform polarization compensation, given the difficulty of fabricating polarized independent components on the chip. In this paper, we propose a fully chip-based decoder for polarization-encoding QKD. The chip realizes a polarization state analyzer and compensates for the BB84 protocol without requiring additional hardware. It is based on a polarization-to-path conversion method that uses a polarization splitter-rotator. The chip was fabricated using a standard silicon photonics foundry; it has a compact design and is suitable for mass production. In the experimental stability test, an average quantum bit error rate of 0.56%0.56\%0.56% was achieved through continuous operation for 10 h without any polarization feedback. Furthermore, using the developed feedback algorithm, the chip enabled the automatic compensation of the fiber polarization drift, which was emulated by a random fiber polarization scrambler. In the case of the QKD demonstration, we obtained a finite-key secret rate of 240 bps over a fiber spool of 100 km. This study represents an important step toward the integrated, practical, and large-scale deployment of QKD systems.