Implementation of quantum key distribution surpassing the linear rate-transmittance bound
Nature Photonics 14(7):422-+Jul2020 

Fang, Xiao-Tian; Zeng, Pei; Liu, Hui; Zou, Mi; Wu, Weijie; Tang, Yan-Lin; Sheng, Ying-Jie; Xiang, Yao; Zhang, Weijun; Li, Hao; Wang, Zhen; You, Lixing; Li, Ming-Jun; Chen, Hao; Chen, Yu-Ao; Zhang, Qiang; Peng, Cheng-Zhi; Ma, Xiongfeng; Chen, Teng-Yun; Pan, Jian-Wei


Quantum key distribution (QKD)1,2 offers a long-term solution to secure key exchange. Due to photon loss in transmission, it was believed that the repeaterless key rate is bounded by a linear function of the transmittance, O(η) (refs. 3,4), limiting the maximal secure transmission distance5,6. Recently, a novel type of QKD scheme has been shown to beat the linear bound and achieve a key rate performance of \(O(\sqrt{\eta })\) (refs. 7,8,9). Here, by employing the laser injection technique and the phase post-compensation method, we match the modes of two independent lasers and overcome the phase fluctuation. As a result, the key rate surpasses the linear bound via 302 km and 402 km commercial-fibre channels, over four orders of magnitude higher than existing results5. Furthermore, our system yields a secret key rate of 0.118 bps with a 502 km ultralow-loss fibre. This new type of QKD pushes forward long-distance quantum communication for the future quantum internet. 

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