Performance of Underwater Quantum Key Distribution with Polarization Encoding

TL;DR

This paper analyzes the performance of underwater quantum key distribution using polarization encoding, proposing a modified QBER formula, and evaluates secure communication distances and rates in different seawater conditions and modes.

Contribution

It introduces a realistic QBER model considering detector efficiency and background light, and assesses secure QKD performance in various underwater propagation modes.

Findings

Secure QKD is feasible up to hundreds of meters in clear seawater.

A secure transmission rate of 67 kbps at 100 m in seawater with 0.03/m attenuation.

Performance can be enhanced using decoy states for practical applications.

Abstract

Underwater quantum key distribution (QKD) has potential applications in absolutely secure underwater communication. However, the performance of underwater QKD is limited by the optical elements, background light, and dark counts of the detector. In this paper, we propose a modified formula for the quantum bit error rate (QBER), which takes into account the effect of detector efficiency on the QBER caused by the background light. Then we calculate the QBER of the polarization encoding BB84 protocol in Jerlov type seawater by analyzing the effect of the background light and optical components in a more realistic situation. Finally, 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 performed 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 a distance of 100 m in the seawater channel with an attenuation coefficient of 0.03/m at night. For practical underwater QKD, the performance can also be improved by using decoy states. Our results are useful to long distance underwater quantum communication.