A research team led by Prof. PAN Jianwei and Prof. ZHANG Qiang from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences, collaborating with researchers from the Jinan Institute of Quantum Technology, developed a system that can not only promise twin-field quantum key distribution (TF-QKD) over a 658-km optical fiber, but also sense channel vibration with an accuracy better than one km.   

The study was published in Physical Review Letters.

Quantum key distribution (QKD) can realize unconditionally secure and confidential communication. The TF-QKD protocol, proposed in 2018, can break through the linear limit of the QKD bit rate, which is considered as the optimal solution for ultralong optical fiber QKD.  

However, TF-QKD requires single-photon interference of two remote independent lasers, and slight frequency deviations of light sources or any fluctuations in the fiber link will accumulate phase noise and degrade the interference invisibility.

The researchers analyzed the information and characteristics of vibration, and managed to locate the position of vibration and realize ultralong fiber-optic vibration sensing.  

Based on "sending-or-not-sending" TF-QKD (SNS-TF-QKD), they utilized critical techniques such as time-frequency transmission to precisely control the frequency of two independent lasers. They calculated relative phase drift by utilizing phase reference light, and recovered the external perturbations generated by artificial vibroseis.  

Moreover, assisted with a single photon detector, the researchers realized over-658-km TF-QKD with long-distance vibration sensing, and detected the position of perturbations with precision better than one km. 

The study revealed that a TF-QKD network can realize long-haul QKD, and it can also serve in remote vibration sensing, thus achieving the integration of wide-area quantum communication and optic-fiber sensor network.

Schematic illustration of TF-QKD (Image from unsplash.com)