The Research group of Prof. GUO Guangcan, from the Key Laboratory of Quantum Information, CAS, University of Science and Technology of China, has achieved a significant progress in quantum key distribution. Based on the self-developed active switching technology, professor HAN Zhengfu and associated professor CHEN Wei et al. fulfilled the world’s longest (more than 90 km) round-robin differential phase shift (RRDPS) quantum key distribution experiment. The experiment results verify the feasibility of this novel protocol, and exhibit its potential advantage in practical scenarios. This work has been published on Nature Photonics with the title of “Experimental demonstration of a quantum key distribution without signal disturbance monitoring”. [Nature Photonics (2015) doi:10.1038/nphoton.2015.209]
Quantum key distribution (QKD) provides an optimal way for distant parties to share their secrets based on the laws of quantum physics. However, the imperfect characteristic of real-life systems and devices may compromise its practical security. In almost all of existing QKD protocols, some security parameters must be estimated or monitored to obtain the final secure key rate. Thus, the signal disturbance may increase the system complexity, reduce the key generation efficiency, or cause vulnerabilities in practical QKD systems.
RRDPS protocol, proposed by T. Sasaki et al., doesn’t require these parameters to bound the secure key rate. In this novel protocol, the sender divides the light pulses into a series of L-bit packets, and encodes the random signals as a superposition of time bins. The receiver randomly chooses the time delay to measure the interference results and its position. The possibility of eavesdropper to correctly guess the bit value correctly is very small when L is large, and the estimation of eavesdropping information is independent of the signal disturbance. Due to its independence with security parameters, RRDPS system is potentially applicable in complicated environments, while the implementation of a large L is a critical technical challenge.
By promoting the Faraday-Michelson interferometers with self-owned intellectual property rights and developing the high-speed optical active switching technology as well as the active phase compensating technology, Prof. HAN Zhengfu’s group implemented a high accuracy, high stability and low insertion loss interferometer, which can actively choose the time delay from 1 to 64 bits to measurement the quantum signals. With the help of the high-speed and low-noise InGaAs/InP detection unit, the whole system worked at 1 GHz repetition rate with the packet length of 65, and can obtain secure key bits under 90 km distance with the security parameter of 3×2-80 and taking the finite-key analysis into account.
This is the first active switching and the longest RRDPS experiment around the world. The stable interferometer is extendible and the experimental realization is excellent. The results demonstrate the feasibility of implementing long distance quantum key distribution without monitoring the signal disturbance, and exploit a new branch of practical QKD technologies.
Associate researcher WANG Shuang and associate professor YIN Zhenqiang contributed equally to this work. This work was supported by the Strategic Priority Research Program (B) of the Chinese Academy of Sciences, the National Basic Research Program of China and the National Natural Science Foundation of China.
Figure 1 Experiment setup for the RRDPS QKD system. The self-developed “Faraday-Michelson” interferometers are used in the 64-bit actively selectable measurement components, which is very stable and has a small insertion loss.
Figure 2 The performance of the measurement component in the condition of different parameters. (a)delay, (b)insertion loss, (c)bit error rate