A point-to-point long-distance quantum key distribution (QKD) over a distance of 1,002 km has been achieved by the scientists from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS), and their collaborators from Tsinghua University, Jinan Institute of Quantum Technology, and Shanghai Institute of Microsystem and Information Technology (SIMIT) of CAS.  

This milestone not only sets a new world record for non-relay QKD but also provides a solution for high-speed intercity quantum communication. The study was published in Physical Review Letters. 

QKD is based on the principles of quantum mechanics and enables secure key distribution between two remote parties. When combined with the "one-time pad" encryption method, it can achieve the highest level of security for confidential communication. However, the distance of QKD has been limited by factors such as the channel loss and system noise. 

The twin-field QKD (TF-QKD) using sending-or-not-sending (SNS) protocol has been demonstrated in the experiment to improve the relation between the key rate and channel transmittance from a linear η to its square root η. Therefore, it can achieve a much longer secure distance than traditional QKD protocols. 

To achieve long-distance QKD, the scientists, collaborating with Yangtze Optical Fibre and Cable Joint Stock Limited Company (YOFC), achieved a maximum attenuation of 0.16 dB/km using ultra-low-loss fiber based on pure sillica core technology.  

SIMIT developed ultra-low-noise superconducting single-photon detectors. By implementing multiple filters at temperatures of 40 K and 2.2 K to suppress dark counts caused by thermal radiation, the noise of the single-photon detectors was reduced to around 0.02 cps. Furthermore, the scientists developed a dual-band phase estimation scheme to avoid the spontaneous Raman scattering noise, reducing the system noise to below 0.01 Hz. 

Based on their previous findings, the scientists achieved TF-QKD over a record distance of 1002 km, with a key rate of 0.0034 bps.  

This study not only verifies the feasibility of the SNS-TF-QKD scheme at extremely long distances but also demonstrates that this protocol can achieve high key rates in many practical scenarios. It paves the way for the realization of high-speed intercity quantum communication networks.