Performance of Underwater Quantum Key Distribution with polarization encoding

Shi-Cheng Zhao, Xin-Hong Han, Ya Xiao, Yuan Shen, Yong-Jian Gu, Wen-Dong Li

Published 2019-03-04Version 1

Underwater quantum key distribution (QKD) has potential applications for absolutely secure underwater communication. However, the performance of underwater QKD is limited by the optical elements, the background light and dark counts of the detector. In this paper, we propose a modified formula of the quantum bit error rate (QBER), in which the effect of detector efficiency on QBER caused by the background light is considered. Then we calculate the QBER of the polarization coding BB84 protocol in Jerlov type seawater by analyzing the effect of background light and optical components in closer to reality situation. Lastly we further analyze the final key rate and the maximum secure communication distance in three propagation modes i.e., upward, downward and horizontal modes. We find that secure QKD can be carried out in the clearest Jerlov type seawater at a distance of hundreds of meters, even in the worst downward propagation mode. Specifically, by optimizing the system parameters, it is possible to securely transmit information with a rate of 67kbits/s at the distance of 100 meters in the seawater channel with a attenuation coefficient 0.03/m at night, and the maximum secure distance can attain more than 300m. For practical underwater QKD, the performance can be improved if decoy state is used according to the calculated secure bit rate for underwater QKD. Our results are helpful to long distance underwater quantum communication.