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Tenth International Conference on Information Optics and Photonics

High-speed and robust modulation of arbitrary polarization states based on Sagnac interferometer for quantum key distribution

Zexu Wang, Huaxing Xu, Bo Liu, Feifei Yin, Kun Xu, Yitang Dai

Published Dec 8, 2025• 0 citations
Publisher

Abstract

In polarization-encoding quantum key distribution (QKD) systems, achieving high-speed, robust, and low-error polarization modulation helps to increase the secure key rate (SKR). Moreover, the advanced QKD protocols place higher demands on the system’s photon preparation capability, which implies providing additional polarization states for practical implementation. Here, we redesign the Sagnac interferometer and utilize the non-reciprocity of the lithium niobate modulator to eliminate the reverse modulation that limits the modulation speed. The proposed scheme enables high-speed, robust, and low-error modulation with arbitrary polarization states. The average bit error rate (BER) for the 10 min measurement is 0.2332% with a system repetition frequency of 16.5 GHz.

DOI
10.1117/12.3085145

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In polarization-encoding quantum key distribution (QKD) systems, achieving high-speed, robust, and low-error polarization modulation helps to increase the secure key rate (SKR). Moreover, the advanced QKD protocols place higher demands on the system’s photon preparation capability, which implies providing additional polarization states for practical implementation. Here, we redesign the Sagnac interferometer and utilize the non-reciprocity of the lithium niobate modulator to eliminate the reverse modulation that limits the modulation speed. The proposed scheme enables high-speed, robust, and low-error modulation with arbitrary polarization states. The average bit error rate (BER) for the 10 min measurement is 0.2332% with a system repetition frequency of 16.5 GHz.
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Polarization modulation plays a key role in polarization-encoding quantum key distribution (QKD). Here, we report a new, to the best of our knowledge, polarization modulation scheme based on an inherently stable Sagnac interferometer. The presented scheme is free of polarization mode dispersion and calibration as well as insensitive to environmental influences. Successful experiments at a repetition frequency of 1.25 GHz have been conducted to demonstrate the feasibility and stability of the scheme. The measured average quantum bit-error rate of the four polarization states is as low as 0.27% for 80 consecutive minutes without any adjustment. This high-speed intrinsically stable polarization modulation can be widely applied to many polarization-encoding QKD systems, such as BB84, MDI, etc.
Satellite-based quantum key distribution (QKD) has made significant strides in the past few years, and an international space race toward practical quantum networks has begun. Being one major encoding method, highly robust, high-performance and miniaturized polarization modulation is critical to meet the needs of future satellite applications. Here, we employ the Sagnac-based method to realize inherent-stable polarization modulation and propose a novel optical design to achieve a state-of-the-art repetition rate of 2 GHz. This value approaches the theoretical repetition frequency limit with commercially-available phase modulators. Furthermore, the polarization encoder is miniaturized into a robust 6.0 cm × 6.5 cm module and two commercially mature phase modulators. A 90-min consecutive test was successfully performed, verifying the robustness of this scheme with an average quantum bit error rate as low as 0.42%. The presented intrinsically stable high-speed polarization modulation provides an effective solution for future satellite applications.
Quantum key distribution (QKD) allows distant parties to exchange cryptographic keys with unconditional security by encoding information on the degrees of freedom of photons. Polarization encoding has been extensively used for QKD along free-space, optical fiber, and satellite links. However, the polarization encoders used in such implementations are unstable, expensive, and complex and can even exhibit side channels that undermine the security of the protocol. Here we propose a self-compensating polarization encoder based on a lithium niobate phase modulator inside a Sagnac interferometer and implement it using only commercial off-the-shelf (COTS) components. Our polarization encoder combines a simple design and high stability reaching an intrinsic quantum bit error rate as low as 0.2%. Since realization is possible from the 800 to the 1550 nm band using COTS devices, our polarization modulator is a promising solution for free-space, fiber, and satellite-based QKD.
Non-reciprocal modulation in bidirectional links eliminates the modulation speed limitation caused by preventing electrical signals from simultaneously modulating light in both directions, thereby enabling the system to operate at higher speeds. Conventional lithium niobate modulators only support this functionality under specific conditions. The periodically poled lithium niobate (PPLN) based modulator we propose supports non-reciprocal modulation across expanded application scenarios. Simulation results demonstrate that the PPLN modulator achieves high-speed modulation for both continuous wave and pulsed light. A polarization encoder based on a Sagnac interferometer using the PPLN modulator enables random modulation of four polarization states at a system repetition frequency of 10GHz, with an average polarization extinction ratio exceeding 20dB.