Recently, the team of Academician Pan Jianwei of the University of Science and Technology of China cooperated with the team of Jinan Institute of Quantum Technology to realize an experimental system that integrates quantum key distribution and optical fiber vibration sensing.
While completing the optical fiber double field quantum key distribution (TF-QKD), the above team realized long-distance optical fiber sensing of 658 kilometers, and the positioning accuracy reached 1 kilometer, which greatly broke through the distance limit of 100 kilometers that is difficult for traditional optical fiber vibration sensing technology to exceed 100 kilometers. The results were published in the Form of "Editor's Recommendation" in the Physical Review Letters and reported on physics, a website of the American Physical Society (APS).
Image courtesy of Physical Review Letters
Optical fiber vibration sensing with optical fiber as a sensor for vibration sensing, through the use of a single optical fiber simultaneously to achieve vibration monitoring and signal transmission, due to the advantages of high sensitivity, fast response, simple structure, uniform distribution, etc., in the structural health monitoring, oil and gas pipeline leakage monitoring, perimeter protection and seismic monitoring and other engineering fields have a wide range of application prospects, so it has attracted widespread attention and research. At present, optical fiber vibration sensing mostly uses distributed sound wave sensing technology, and its sensing distance is limited to less than 100 kilometers, and an important technical challenge is how to overcome the distance limit and achieve long-distance optical fiber vibration sensing.
Quantum key distribution (QKD) is based on the basic principles of quantum mechanics, combined with the encryption method of "one secret at a time", which can achieve unconditionally secure and confidential communication. Because of its important practical significance, QKD has been a research hotspot in the international academic community for decades. The TF-QKD protocol proposed in 2018 can break through the linear limit of QKD bit rate and is considered to be the best solution to achieve ultra-long-distance optical fiber QKD. However, the TF-QKD technology is quite demanding, requiring single-photon interference from 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.
Fiber art image from Sono Creative/stock.adobe .com
In practical applications, noise such as sound and vibration along the fiber link are inevitable, so it is necessary to detect the phase change of the fiber caused by environmental noise in real time during the TF-QKD experiment and compensate for it in real time or after data processing. In general, information about these phase changes is discarded after the QKD experiment is completed. But in fact, this "redundant" information reflects real-time phase changes in the transmitted light in the fiber, which may come from vibration disturbances or temperature drift 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 achieve ultra-long-distance fiber vibration sensing.
This time, the research group of Pan Jianwei and Zhang Qiang of the University of Science and Technology of China is based on the "send" or "do not send" TF-QKD (SNS-TF-QKD) protocol proposed by Wang Xiangbin of jinan Quantum Technology Research Institute, and uses key technologies such as time-frequency transmission to accurately control the frequency of two independent lasers (Alice and Bob). The team cooperated with Chen Yang and Zhao Dongfeng of the University of Science and Technology of China to use additional phase reference light to estimate the rapid drift of the relative phase of the optical fiber, recovered the external disturbance generated by the artificial controllable vibration source loaded on the optical fiber channel, and combined with the high-count rate and low-noise single-photon detector developed by the Youlixing team of the Shanghai Institute of Microsystems of the Chinese Academy of Sciences, finally realized the 658 km fiber double field quantum key distribution and fiber vibration sensing, and located the disturbance position of the artificial vibration source on the link, with an accuracy of better than 1 km.
Security key rates for SNS-TF-QKD experiments, picture from the paper
"We showed the possibility of collecting vibration sensing data without adding new fibers or hardware to the TF-QKD network." Zhang Qiang said.
The TF-QKD experiment consists of two optical devices located at each end of the fiber, named Alice and Bob. Each device generates a random string of bits and sends it as an optical signal through an optical fiber to an intermediate node named Charlie, where the two signals interfere with each other, and the resulting optical signal is then transmitted back to Alice and Bob, and a shared key is generated using the interference result.
Schematic diagram of the experimental apparatus, picture from the paper
The research team said that the experimental system can be used to detect seismic vibrations by installing a piezoelectric device that causes Alice's fibers to vibrate at specific locations, with frequencies set between 1 and 1000 Hz (hertz), within the seismic sensing range. The phase changes produced by its vibrations are between 0.9 and 50 radians (1 radian is 57.296 angles), and the system can capture these changes. Zhang Qiang said that seismic waves should produce greater phase changes, between a few hundred and a few thousand radians. The team conducted a similar test on the frequency calibration link, which locks the frequencies of Alice and Bob lasers with a separate fiber on the TF-QKD system, and uses this link to pinpoint the location of the vibration source with an accuracy of 1 km.
"In the future, QKD networks based on this new technology can provide useful seismic-related information from existing fibers." Giuseppe Marra, a quantum expert at the UK's National Physical Laboratory (NPL), said.
Vibration test results through the QKD network, picture from the paper
The above results show that the TF-QKD network architecture can not only realize the distribution of security keys over ultra-long distances, but also be applied to ultra-long-distance vibration sensing, realize the fusion of wide-area quantum communication networks and optical fiber sensing networks, and be used to detect and locate earthquakes.
The first author of the paper is Chen Jiupeng and Zhang Chi, doctoral students of the University of Science and Technology of China, and the above research has been funded by the Ministry of Science and Technology, the Natural Science Foundation, the Chinese Academy of Sciences, Shandong Province and Anhui Province.