Although quantum key distribution (QKD) can offer secure private communication, there are still some technical limitations onpractical long distance quantum communication. Among them, channel lossand the detector noise are two most severe limitations, given that quantumsignals cannot be amplified. The new-found twin-field QKD (TFQKD)increases the relationship between code rate and distance from the linear relationship of general quantum key distribution to the square root level. Therefore, the code distance far beyond the general quantum key distribution scheme can be obtained, and theoretically, the code rate far higher than the general quantum key distribution scheme can be obtained, which provides a new direction for long-distance, high-performance quantum key distribution.However, the conditions under which the experimental implementation of the two-field quantum key distribution scheme are extremely demanding and hard to realize.

Recently, the research team led by Prof. PAN Jianwei, Prof. ZHANG Qiang and Prof. LIU Yang from the University of Science and Technology of China (USTC), cooperates with Prof. WANG Xiangbin from the Tsinghua University and Prof. YOU Jinxing from Shanghai Institute of Microsystem and Information Technology, has demonstrated twin-field quantum key distribution through the sending-or-not-sending protocol with a realistic phase drift over 300 km optical fiber spools, on the basis of a single photon detector with a high detection rate. The research was online published in Physical Review Letters on September 5th.

In the research, two-field quantum key distribution is realized on a 300-kilometer fiber channel with a sharp phase change in the real environment. After considering important theoretical requirements such as statistical fluctuation and finite length analysis, the key generation rate is reached at 300 km. The 2016 experiment was 50 times and broke the theoretical limit of the highest rate of code for the general non-relay quantum key distribution scheme. Compared with the existing published two-field QKD experiment, this study is the only one that considers the finite code length effect. In addition, the joint team also analyzed that the program can perform long-distance quantum key distribution over 700 kilometers under conditions such as improved detector performance.

Fig1: Schematic of sending or not sending twin-field QKD. (Image by PAN Jianwei’s team)

The research demonstrated the generation of secure keys at fiber distances of up to 300 km, yielding a higher key rate than the repeater less secret key capacity. The key rate calculation has fully considered finite size effects, thus, guaranteeing the security in a practical situation.With currently available technology and the results of theoretical simulations with practical parameters, the research team expects that distribution distances of more than 500 km will be achieved in the near future.

This work verifies the feasibility of the long-distance two-field quantum key distribution scheme, and proves that the protocol has long-distance, high-code-rate performance, and is very suitable for using in inter-city quantum key distribution backbone links. The work was reviewed by the reviewer of the Physical Review Letter as an important milestone in the practical dual-field quantum key distribution, and selected as the "Editor's Choice" by the Physical Review Letter, and featured highlights by Physics of American Physical Society.

Paper link:

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.123.100505

(Written by LI Xiaoxi, edited by LI Xiaoxi, USTC news center)