Implementation of Optimized Coherent-One-Way Quantum Key Distribution over a 70-km Outdoor Fiber Link

Sheng-Yao Huang^1*, Yen-Ju Chen², Chih-Wen Kuo¹, Chih-Sung Chuu¹

¹Darpartment of Physics, National Tsing Hua University, Hsinchu, Taiwan
²Material and Electro-Optics Research Division, National Chung-Shan Institute of Science and Technology, Taoyuan, Taiwan

* Presenter:Sheng-Yao Huang, email:legend401@gapp.nthu.edu.tw

Quantum key distribution (QKD), leveraging the principles of quantum superposition and non-cloning, offers a long-term solution for unconditionally secure communication. The optimized Coherent-One-Way (COW) protocol, utilizing solely intensity modulators and passive measurements, provides a highly practical approach to QKD. The secure key rate lower bounds for optimized COW under optimal collective and optimal coherent attacks can be numerically estimated through semidefinite programming. We implemented and field-tested the optimized COW protocol over a 70-km outdoor fiber link with a attenuation of -18.9 dB, achieving a quantum bit error rate (QBER) of 0.6%, an interference visibility of 98%, and a secure key rate of 3.66 x 10⁶ bits per pulse. Notably, our results indicate that optimizing the average photon number per pulse at the transmitter and applying time filtering at the receiver are both crucial for achieving a high secure key rate, particularly in light of practical imperfections such as modulator optical null depth and detector dark counts. This study lays a foundation for future long-distance QKD implementations and provides a platform for investigating potential eavesdropping strategies, advancing the robustness of quantum communication systems.

Keywords: Quantum Key Distribution, Coherent-One-Way Protocol, Semidefinite Programming, Time Filtering, Field Implementation