Decoy-State Quantum Key Distribution over a Long-Distance High-Loss Underwater Free-Space Channel

• Published in: arXiv.org
• Main Authors: Cheng-Qiu, Hu, Zeng-Quan, Yan, Gao, Jun, Zhan-Ming, Li, Zhou, Heng, Jian-Peng Dou, Xian-Min, Jin
• Format: Paper Journal Article
• Language: English
• Published: Ithaca Cornell University Library, arXiv.org 14.04.2020
• Subjects: Bit error rate; Coastal waters; Communication; Physics - Optics; Physics - Quantum Physics; Quantum cryptography; Qubits (quantum computing); Seawater; Underwater vehicles
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2004.06708

Atmospheric free space and fiber have been widely exploited as the channels for quantum communication, and have enabled inter-continent and inter-city applications. Air-sea free-space channel, being capable of linking the satellite-based quantum resource and underwater vehicle, has now become the last piece of the puzzle in building global quantum communication network. However, long-distance quantum communication penetrating water up to tens to hundreds of meters is extremely challenging due to the inevitable high loss. Here, we present an experimental demonstration of underwater decoy-state quantum key distribution against high loss, meanwhile keep a low quantum bit error rate less than 2.5% for different distances. By directly modulating blue-green lasers at a high speed of 50MHz and decoy-state protocol, we are able to for the first time reach a long-distance quantum key distribution that is unconditionally secure and can enable real-life air-sea quantum communication tasks. The demonstrated distance, even in coastal water of Jerlov types 2C, is up to 30 meters, about one-order improvement over the proof-in-principle demonstrations in previous experiments, and the channel loss is equivalent to 345-meter-long clean seawater of Jerlov type I, representing a key step forward to practical air-sea quantum communication.