Practical underwater quantum key distribution based on decoy-state BB84 protocol

TL;DR

This paper demonstrates a practical underwater quantum key distribution system using decoy-state BB84 protocol, achieving secure key generation over a water channel with potential for longer-distance ocean communication.

Contribution

It introduces a compact, water-channel-compatible QKD system with innovative modulation, wireless synchronization, and multiplexing techniques, advancing underwater quantum communication.

Findings

Final key rate of 245.6 bps over 2.4m water channel

Average QBER of 1.91% during tests

System can tolerate up to 21.7dB channel loss

Abstract

Polarization encoding quantum key distribution has been proven to be a reliable method to build a secure communication system. It has already been used in inter-city fiber channel and near-earth atmosphere channel, leaving underwater channel the last barrier to conquer. Here we demonstrate a decoy-state BB84 quantum key distribution system over a water channel with a compact system design for future experiments in the ocean. In the system, a multiple-intensity modulated laser module is designed to produce the light pulses of quantum states, including signal state, decoy state and vacuum state. The classical communication and synchronization are realized by wireless optical transmission. Multiple filtering techniques and wavelength division multiplexing are further used to avoid crosstalk of different light. We test the performance of the system and obtain a final key rate of 245.6 bps with an average QBER of 1.91% over a 2.4m water channel, in which the channel attenuation is 16.35dB. Numerical simulation shows that the system can tolerate up to 21.7dB total channel loss and can still generate secure keys in 277.9m Jelov type 1 ocean channel.