Quantum key distribution (QKD) can offer secure private communication. However, there are still some technical limitations on practical long distance quantum communication, among which channel loss and the detector noise are two most severe limitations, given that quantum signals cannot be amplified. 

The new-found twin-field QKD (TFQKD) increases the relationship between code rate and distance from the linear relationship of general QKD to the square root level. Therefore, the code distance far beyond the general QKD scheme can be obtained, and theoretically the code rate far higher than the general QKD scheme can be obtained, which provides a new direction for long-distance, high-performance QKD. However, the demanding conditions for the experimental implementation of the TFQKD scheme are hard to realize. 

Recently, a research team led by Prof. PAN Jianwei, Prof. ZHANG Qiang and Prof. LIU Yang from the University of Science and Technology of China of the Chinese Academy of Sciences (CAS), and the collaborators from Tsinghua University and Shanghai Institute of Microsystem and Information Technology of CAS, demonstrated TFQKD 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 researchers realized the TFQKD on a 300-kilometer fiber channel with a sharp phase change in the real environment. Taking theoretical requirements such as statistical fluctuation and finite length analysis into consideration, they made the key generation rate reach at 300 km. The key generation rate is 50 times than that of the 2016 experiment and broke the theoretical limit of the highest rate of code for the general non-relay QKD scheme.

This study 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 guaranteed the security in a practical situation. With existing technology and the results of theoretical simulations with practical parameters, the researchers expect that distribution distances of more than 500 km can be achieved in the near future. 

Besides, this study verified the feasibility of the long-distance TFQKD scheme, and proved that the scheme has long-distance, high-code-rate performance and is very suitable for using in inter-city quantum key distribution backbone links.

The study, published in Physical Review Letters, was selected as the "Editor's Choice" by the Physical Review Letter, and featured highlights by Physics of American Physical Society.