USTC Realizes Small-Packet-and-Long-Distance Quantum Key Distribution


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USTC Quantum Cryptographic Research Group have perfected the security proof of Round-Robin-Differential-Phase-Shift (RRDPS) quantum key distribution theoretically. Based on this, 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 are solved.

RRDPS is a new quantum key distribution protocol proposed by Japanese and American scientists in 2014. The sender of the protocol encodes the random key on the phases of the optical pulses and forms a packet for every L pulse. 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, which aroused great interest in academia. However, there are still some key problems not solved in the protocol, such as rough or unknown information leakage proof or antecedents, large pulse number, complicated measurement device.

In order to solve these problems, the authors 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 is given.

This theory can further optimize the estimation of eavesdropping information by combining the parameters of channel disturbance. This novel security proof clearly demonstrates the security mechanism of the RRDPS protocol, and the performance of the RRDPS is significantly improved. This new idea provided by this security proof is also useful for other high-dimensional QKD protocols.

The simulation results show that based on the new security proof, the secret key rate and the security distance of RRDPS protocol have been significantly improved. The pulse number of each packet L is greatly reduced compared with the original one. It is meaningful for reducing the difficulty of implementing the RRDPS system and enhancing its practicability.

In order to verify this theory, HAN Zhengfu’s group has 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 have 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.

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