Scientists implement 658 km of quantum key distribution and fiber optic sensing

Pan Jianwei, Zhang Qiang, etc. of the University of Science and Technology of China cooperated with Wang Xiangbin and Liu Yang of the Jinan Institute of Quantum Technology to develop a set of experimental systems that integrate quantum key distribution and optical fiber vibration sensing, and realized 658 km long-distance optical fiber sensing while completing the dual field quantum key distribution of optical fiber, with a positioning accuracy of 1 km, which greatly broke through the limitation of traditional optical fiber vibration sensing distance of more than 100 km. The results were published in the Physical Review Letters on May 2.

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Schematic diagram of an experimental device integrating quantum key distribution and optical fiber vibration sensing Courtesy of the University of Science and Technology of China

Optical fiber vibration sensing uses optical fiber in optical signal time, phase, amplitude and other information to monitor the vibration information in the link, with the advantages of high sensitivity, fast response, simple structure, uniform distribution, etc., and has a wide range of application prospects in the engineering fields such as building structure health monitoring, oil and gas pipeline leakage monitoring, and seismic monitoring. At present, the mainstream optical fiber vibration sensing scheme adopts distributed acoustic wave sensing technology, which can accurately measure the vibration information generated by each position of the optical fiber, but the sensing distance is difficult to exceed 100 kilometers.

Quantum key distribution is based on the basic principles of quantum mechanics, combined with “one secret at a time” encryption method can achieve unconditionally secure and confidential communication. The two-field quantum key distribution protocol can not only achieve very high real-world security, but also provide the possibility of exceeding the working distance of the general quantum key distribution protocol, which is considered to be the best solution to achieve ultra-long-distance optical fiber quantum key distribution.

However, the two-field quantum key distribution technology is demanding, requiring single-photon interference of two remote independent lasers, a slight deviation in the frequency of the light source, and any fluctuation in the fiber link, which will accumulate phase noise and reduce the quality of the single-photon interference. Therefore, in a two-field quantum key distribution experiment, it is necessary to detect rapid changes in phase caused by fiber vibrations, etc., and compensate for them.

In general, information about these phase changes is “discarded” after the experiment is completed. But in fact, this “redundant” information comes from phase changes caused by fiber vibration. By analyzing this information, the fiber optic link vibration signal can be obtained. According to the time correlation of the measurement results of the two sides of the communication, the vibration signal can be located to achieve ultra-long-distance optical fiber vibration sensing.

Based on the “send or not send” two-field quantum key distribution protocol proposed by Wang Xiangbin, Pan Jianwei and Zhang Qiang used key technologies such as time-frequency transmission to accurately control the frequency of two independent lasers. In collaboration with Chen Yang and Dongfeng Zhao of the University of Science and Technology of China, they used additional phase reference light to estimate the rapid relative phase drift of the optical fiber, and restored the external disturbance caused by the artificially controllable vibration source loaded on the fiber channel. Combined with the high-count rate, low-noise single-photon detector developed by the Youlixing team of the Shanghai Institute of Microsystems of the Chinese Academy of Sciences, the double-field quantum key distribution of the extremely long-distance optical fiber was finally realized, and the optical fiber sensing test was successfully carried out in the system.

Hoi-Kwong Lo, an internationally renowned expert in quantum cryptography and a professor at the University of Toronto, commented on the work: “It is impressive to detect vibrations at such a long distance. Giuseppe Marra, an expert in quantum precision measurements and the UK’s National Physical Laboratory, believes that “future quantum cryptographic networks based on this new technology can provide a lot of information about earthquakes.” (Source: China Science Daily Wang Min)

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