The round-robin-differential-phase-shift (RRDPS) is a new quantum key distribution protocol proposed by Japanese and American scientists in 2014. This protocol can estimate the information leakage without monitoring signal disturbance parameters, which breaks through the design of quantum key distribution protocol.
In practical applications, free of monitoring channel disturbance also brings the advantages of simplified system and high error rate tolerance. However, there are still some key problems not solved in the protocol.
Quantum Cryptographic Research Group of University of Science and Technology of China (USTC) of Chinese Academy of Sciences perfected the security proof of RRDPS quantum key distribution theoretically. The RRDPS protocol with the lowest number of packet and the longest achievable distance was realized in the world for the first time. The problems of large pulse number and low efficiency were solved. The results were published in Nature Communications.
The researchers first improved the security proof of the RRDPS protocol theoretically. By constructing the eavesdropper's general collective attack model and fully considering the decoherence effect caused by the random phase on the eavesdropper's auxiliary state in each pulse of the encoding state, a tight bound of the eavesdropper's information was given.
This theory further optimized the estimation of eavesdropping information by combining the parameters of channel disturbance. The novel security proof clearly demonstrated the security mechanism of the RRDPS protocol, and the performance of the RRDPS was significantly improved. This new idea provided by this security proof is also useful for other high-dimensional QKD protocols.
The simulation results showed that based on the new security proof, the secret key rate and the security distance of RRDPS protocol were significantly improved. The pulse number of each packet L was greatly reduced compared with the original one.
In order to verify this theory, researchers also realized the simplest RRDPS demonstration experiment with L = 3. Key distribution without monitoring signal disturbance is achieved on 30 km fiber channel. If combined with channel disturbance parameters, this distance can reach 140 km.
These results had important reference value for enriching the theories and methods of security analysis of high dimensional quantum key distribution and improving the practicability of the system.
The study was funded by Ministry of Science and Technology, National Natural Science Foundation of China and Chinese Academy of Sciences.
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