500km: China creates new record for ground-based quantum key distribution

Recently, Pan Jianwei, Zhang Qiang, Chen Tengyun and other people of China University of Science and Technology, in cooperation with Wang Xiangbin and Ma Xiongfeng of Tsinghua University, broke through the long-distance independent laser phase interference technology, and realized the two-field quantum key distribution (TF-QKD), phase-matched quantum key distribution (PM-QKD) of 500 km of real-world optical fiber. The relevant research results have recently been published in the international authoritative academic journals “Physical Review Letters”, “Nature Photonics”, and the former selected as “editor recommendation” article.

It is reported that channel loss is the most serious limiting factor in the long-distance practical application of quantum key distribution (QKD). The existing measuring equipment unrelated quantum key distribution (MDI-QKD) uses dual photon composite event as an effective detection event, the security code rate decreases linearly with channel decay, and in the absence of quantum relay, the security code-code rate is bound by the linear boundary.

TF-QKD uses single photon interference as an effective detection event, so that the security code rate decreases linearly with the square root of channel decay, and can even easily break the linear limit of QKD code rate in the absence of relay.

However, TF-QKD is technically demanding because it requires single-photon-level interference from two remote independent lasers, and a precise estimate of the relative phase of long-range fiber link rapid drift required through single-photon detection results, as well as a single-photon detector that simultaneously meets high count rates, high efficiency, and ultra-low dark counts.

500km: China creates new record for ground-based quantum key distribution

In the latest study, Pan Jianwei’s experimental team based on Wang Xiangbin’s “send-do not send” TF-QKD protocol and Ma Xiongfeng’s PM-QKD protocol, respectively, the development of time-frequency transmission technology and laser injection locking technology, the two independent remote laser wavelength stab to the same, The relative phase of the fiber is estimated to drift rapidly by using additional phase reference light.

Combined with the high-count low-noise single-photon detector developed by the Shanghai microsystem of the Chinese Academy of Sciences, the safe code distance of QKD was pushed to 509 km in the laboratory.

This sets a new world record for the longest distance for quantum key distribution, breaking the code-generation limit set by traditional non-relay-free QKDs at a fiber code rate of more than 500 km, exceeding the non-relay QKD code-forming limit under the ideal detection device (the detector efficiency is 100%).

If the system repeat frequency is upgraded to 1GHz in long-distance quantum communication networks such as the Beijing-Shanghai mainline, the code-forming rate of 300 km can reach 5Kbps, which can greatly reduce the number of trusted trunks in the backbone fiber quantum communication network and greatly improve the security of the optical fiber quantum confidential communication network.