Across 4,600 kilometers! Space-ground integrated quantum communication

The space-ground integrated quantum communication network is a large-scale space-ground demonstration connecting the "Mozi" and the "Beijing-Shanghai Trunk Line". Unlike the first brief encounter in 2017, this communication network is the world's first wide-area quantum communication network integrating more than 700 ground-based optical fiber quantum key distribution links and two satellite-ground free-space high-speed links. It has achieved a large-scale, multi-user quantum confidentiality key distribution across the ground with a span of 4,600 km. It has also conducted more than two years of stability and security testing, standardization research, and application demonstrations in different fields such as government affairs, finance, and electricity, successfully verifying the feasibility of building a space-ground integrated quantum communication network.

On January 7, 2021, Pan Jianwei and his colleagues Chen Yuao, Peng Chengzhi and others from the University of Science and Technology of China, in collaboration with Wang Jianyu's research group from the Shanghai Institute of Technical Physics, Chinese Academy of Sciences, the Jinan Institute of Quantum Technology and China Cable Television Network Co., Ltd., published a paper titled "A space-ground integrated quantum communication network spanning 4,600 km" in the international academic journal Nature, proving that the conditions for the practical application of wide-area quantum secure communication technology have initially matured.

Reviewers of Nature magazine commented that this is the largest and most advanced quantum key distribution network on Earth and a "huge engineering achievement" in quantum communications.

The integrated quantum communication network covers 32 nodes in four provinces and three cities in my country through the "Beijing-Shanghai Trunk Line", including four quantum metropolitan area networks in Beijing, Jinan, Hefei and Shanghai. At the same time, it connects four ground stations in Tibet Ali Station, Yunnan Lijiang Station, Urumqi Nanshan Station and Qinghai Delingha Station through the "Mozi". Beijing Xinglong Station has been connected to the "Beijing-Shanghai Trunk Line" before, and is the connection point between "Mozi" and the "Beijing-Shanghai Trunk Line", as shown in Figure 4-22. "4600km" is because this network covers from Beijing to Shanghai through the Beijing-Shanghai Trunk Line, with a total optical fiber length of more than 2000km, and is connected from Beijing Xinglong Station to Urumqi Nanshan Station through satellites, spanning about 2600km. By building the prototype of an integrated quantum communication network, Chinese researchers have laid a scientific and technological foundation for the realization of a global quantum secure communication network in the future.

After the completion of the "Beijing-Shanghai Trunk Line", more than two years of relevant technology verification, application demonstration and a large number of stability tests, security tests and related standardization research were carried out. It passed security tests such as photon number separation attacks, blinding attacks, time-shift attacks, wavelength-dependent attacks and some potential Trojan horse attacks. The results show that the "Beijing-Shanghai Trunk Line" can resist all currently known quantum hacker attack schemes. At the same time, the key distribution capacity of the "Beijing-Shanghai Trunk Line" network can support more than 12,000 users at the same time.

The space-ground integrated quantum communication network has been connected to more than 150 industry users such as finance, electricity, and government affairs. At the beginning of 2019, State Grid Corporation of China established a quantum key distribution channel spanning 2,600 km from the Beijing headquarters to State Grid Xinjiang Electric Power Co., Ltd. based on this network, realizing encrypted transmission of power communication data, and for the first time engineering-verified the feasibility of satellite-ground quantum communication to carry out actual business. Based on the results of a large number of tests and standardization research on the space-ground integrated quantum communication network, the International Organization for Standardization/International Electrotechnical Commission (ISO/IEC), one of the world's three major standardization organizations, is compiling the international standard "Security Requirements, Tests and Evaluation Methods for Quantum Key Distribution" based on the practice of the "Beijing-Shanghai Trunk Line". Another international organization, the International Telecommunication Union (ITU), is also drafting international standards such as trusted relay security requirements based on the construction model of the "Beijing-Shanghai Trunk Line", and has released the quantum key distribution (QKD) network functional architecture standard.

The related technologies developed in this work have also laid the foundation for the miniaturization, low cost and localization of quantum communication systems. Researchers are constantly exploring ways to make optical ground stations miniaturized and mobile, so as to achieve more flexible and efficient docking between satellites and the ground. The team successfully developed a small ground station with a mass of about 100 kilograms, realized the satellite-to-ground quantum key distribution experiment between the small ground station and the "Mozi", and conducted satellite-to-ground quantum key distribution experiments with multiple international ground stations. In the future, small ground stations are expected to be further able to be carried by one person. At the same time, under the premise of ensuring the key distribution rate, a small space quantum key distribution payload of dozens of kilograms has been successfully developed. These achievements have also laid the foundation for the formation of international technical standards for satellite quantum communications.

——Excerpt from "Starry China, Our Quantum Science Satellite"