Chinese scientists have realized the distribution of quantum keys without relay optical fibers for thousands of kilometers

Pan Jianwei and Zhang Qiang of University of Science and Technology of China, Wang Xiangbin of Tsinghua University, Liu Yang of Jinan Institute of Quantum Technology, You Lixing and Zhang Weijun of Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, realized 1002 km point-to-point long-distance quantum key distribution in optical fiber by developing technologies such as low crosstalk phase reference signal control and extremely low noise single-photon detector, which not only set a world record for optical fiber relayless quantum key distribution distance, but also provided a scheme for high-rate backbone links for intercity quantum communication. On May 25, the relevant research results were published in Physical Review Letters.

Schematic diagram of long-distance quantum key distribution experimental system Courtesy of China University of Science and Technology

Quantum Key Distribution (QKD) is based on the basic principles of quantum mechanics, which can be used for secure key distribution between users, combined with “one secret at a time” encryption, so as to achieve the highest security of confidential communication. However, the distance over which quantum keys are distributed has always been limited by factors such as the inherent loss of communication fibers and detector noise. The two-field quantum key distribution (TF-QKD) protocol uses the characteristics of single-photon interference to improve the relationship between code rate and distance from the linear relationship of general quantum key distribution to the level of square root, so that the coding distance far exceeds that of general quantum key distribution schemes.

In this work, the research team adopted the “send-not-send” two-field quantum key distribution protocol proposed by Wang Xiangbin et al., which effectively improves the working distance of the quantum key distribution system under realistic conditions. In order to carry out extremely long-distance quantum key distribution, the research team cooperated with Changfei Fiber Optic Cable Co., Ltd. to use ultra-low loss optical fiber based on “pure silica core” technology to achieve a quantum channel fiber link below 0.16 dB per kilometer. The Shanghai Institute of Microsystems and Information Technology, Chinese Academy of Sciences, has developed a very low-noise superconducting single-photon detector, which suppresses the dark count caused by thermal radiation by multi-stage filtering in the 40 Kelvin and 2.2 Kelvin temperature regions, reducing the noise of the single-photon detector to 0.02 per second.

The research team also developed a dual-wavelength phase estimation scheme for time-division multiplexing, which avoided noise effects such as secondary Rayleigh scattering of reference light of the same wavelength and spontaneous Raman scattering of reference light of different wavelengths, and reduced the link noise to less than 0.01 Hz.

Based on the above technological developments, the work achieves two-field quantum key distribution at a distance of up to 1002 kilometers, obtaining a code rate of 0.0034 bits per second. After optimizing the system parameters, a code rate of 47.06 kbits per second was obtained at a fiber distance of 202 km, and the code rate obtained at 300 km and 400 km fiber distance was improved by 6 orders of magnitude compared with the original “measurement device independent” quantum key distribution.

Long-distance quantum key distribution experiment code rate result diagram Photo courtesy of China University of Science and Technology

According to the researchers, this work not only verifies the feasibility of the two-field quantum key distribution scheme at extremely long distances, but also verifies that the protocol can achieve high-bit-rate quantum key distribution at intercity fiber distance, which is suitable for intercity quantum communication backbone links.

The work was highly praised by reviewers as “an extremely important advance in the field, a new milestone in quantum key distribution technology.” (Source: Wang Min, China Science News)

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