Recently, Pan Jianwei and Zhang Qiang from the University of Science and Technology of China cooperated with Wang Xiangbin and Liu Yang from the Jinan Institute of Quantum Technology to realize a set of experimental system that combines quantum key distribution and fiber optic vibration sensing. While completing fiber optic two-field quantum key distribution (TF-QKD), the 658 km long distance fiber optic sensing was realized, and the positioning accuracy reached 1 km. It greatly breaks through the limitation that the distance of traditional fiber optic vibration sensing technology is difficult to exceed 100 kilometers. The results were published as an "editor's recommendation" in the journal Physical Review Letters and reported by Physics, an affiliate of the American Physical Society (APS).

Fiber optic vibration sensing uses fiber as a sensor for vibration sensing, and realizes vibration monitoring and signal transmission by using a single fiber at the same time. Due to its advantages of high sensitivity, fast response, simple structure and uniform distribution, it has wide application prospects in structural health monitoring, oil and gas pipeline leakage monitoring, perimeter protection and earthquake monitoring and other engineering fields. So it has aroused people's extensive attention and research. At present, fiber optic vibration sensing mostly uses distributed acoustic wave sensing technology, and its sensing distance is limited to 100 km. An important technical challenge is how to overcome the distance limitation and realize long-distance fiber optic vibration sensing.

Quantum key distribution (QKD) is based on the basic principles of quantum mechanics, combined with the "one time secret" encryption method, can achieve unconditional security of confidential communication. Because of its important practical significance, QKD has been the focus of international academic research in the past decades. The TF-QKD protocol proposed in 2018 can break through the linear limit of the QKD bit rate, and is considered to be the best scheme to achieve ultra-long distance fiber QKD. However, TF-QKD technology is quite demanding, requiring the single-photon interference of two remote independent lasers, small deviations in the frequency of the light source and any fluctuations in the fiber link will accumulate phase noise and reduce the quality of the single-photon interference.

In practical applications, noise such as sound and vibration along optical fiber links is unavoidable, so it is necessary to detect the phase change of optical fiber caused by environmental noise in real time and compensate it in real time or data post-processing during TF-QKD experiment. In general, information about these phase changes is discarded after the QKD experiment. But in fact, this "redundant" information reflects real-time phase changes in the transmitted light in the fiber, which may result from vibration perturbations or temperature drifts along the fiber link. By analyzing these phase change information, combined with some characteristics of vibration, vibration information can be obtained and positioned, so as to realize ultra-long distance fiber optic vibration sensing.

Based on the "send" or "do not send" TF-QKD (SNS-TF-QKD) protocol proposed by Wang Xiangbin of Jinan Institute of Quantum Technology, the research team of Pan Jianwei and Zhang Qiang used time-frequency transmission and other key technologies to quasi-control the frequency of two independent lasers. In cooperation with Chen Yang and Zhao Dongfeng of the University of Science and Technology of China, they used additional phase reference light to estimate the relative phase fast drift of optical fibers. The external disturbance caused by the artificially controlled vibration source loaded on the fiber channel was recovered, and combined with the high-count rate and low-noise single-photon detector developed by the Youlixing team of Shanghai Microsystems Institute of the Chinese Academy of Sciences, the optical fiber two-field quantum key distribution and optical fiber vibration sensing were finally realized, and the disturbance location of the artificial vibration source on the link was located with an accuracy of better than 1 km.

The above research results show that TF-QKD network architecture can not only realize ultra-long distance distribution of security keys, but also be applied to ultra-long distance vibration sensing, and realize the fusion of wide-area quantum communication network and optical fiber sensing network.

The first authors of the research paper are Chen Jiupeng and Zhang Chi, PhD students at the University of Science and Technology of China.

Paper link:https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.128.180502

Physics Report Links：https://physics.aps.org/articles/v15/63

The work was supported by the Ministry of Science and Technology, the Natural Science Foundation, the Chinese Academy of Sciences, and the provinces of Shandong and Anhui.

(Hefei National Research Center for Microscale Matter Science, Institute of Quantum Information and Quantum Technology Innovation, School of Physics, Department of Scientific Research, Chinese Academy of Sciences)