“Quantum key distribution (QKD) is presently attracting immense research interest on the global level due to its revolutionary potential to enable information-theoretically secure communication. However, the wide applications of QKD are curtailed by the channel loss since the encoded photons cannot be amplified or relayed via classical optical techniques to keep its security, which limits the increase of key rate and distance of QKD.
A research team led by Academician Guo Guang-Can from University of Science and Technology of China (USTC) extended the transmission distance of QKD to over 830 km optical fibers. They broke the record of 605 km achieved by researchers from Toshiba’s Cambridge Research Laboratory, and made an important step towards terrestrial quantum secure communication and networks over the scale of 1000 km. The work was published online in Nature Photonics on January 17.
Twin-field quantum key distribution (TF-QKD) protocol proposed in 2018 overcame the fundamental rate-distance limit of QKD, but its theoretical improvement and experimental technology development were extremely challenging. The group of Guo Guang-Can and Han Zheng-Fu first proposed no phase post-selection TF-QKD protocol in 2019 (Physical Review Applied, 11, 034053(2019)), and demonstrated the feasibility of TF-QKD over 300 km fiber channels for the first time (Physical Review X, 9, 021046(2019)).
Wang Shuang, Yin Zhen-Qiang, He De-Yong, and Chen Wei of this group have consistently explored TF-QKD for more than two years. They first proposed a four-phase TF-QKD protocol, and generalized it to the finite-size regime, to theoretically get long distance and high key rate simultaneously. Then they developed a corresponding high-speed and low-noise TF-QKD system and optimized it to set a fiber-based record of distance. This allowed for the first time QKD with a fiber distance over 830 km and an unprecedented channel loss budget beyond 140 dB (which is equivalent to almost 1000 km fiber with 0.1419 dB/km loss coefficient), even in the finite-size regime.
Besides the record of fiber distance and unprecedented channel loss budget, three significant outcomes of their experiment were: (a) its secure key rate substantially (50~1000 times) exceeded previous results of approximate distances; (b) high-quality twin fields were remotely (1000-km) generated without the assistance of optical amplifiers; (c) a stable interference visibility above 96% was achieved with a total fiber length of 1689 km. Their collaborators were from Jiangsu Hengtong Optical Fiber Technology Co., Shanghai University, and Scontel company of Russia.
This achievement not only indicated the great potential of TF-QKD, but also was of great importance for future deployment of quantum communication and network over 1000-km range. The reviewers highly praised on this work as “this represents a milestone in the field” and “the result in itself is certainly impressive showing that repeaterless QKD over long distances is indeed be a reality”.”