Quantum Relay-Assisted Free-Space Optical Communication

Relay-based quantum communication utilizes intermediate nodes to extend the transmission distance of quantum signals and mitigate the masking effect caused by obstacles or turbulence in free-space optical (FSO) channels. In this study, we investigate a three-node quantum communication system, where Alice, the transmitter, sends quantum-encoded binary information to Bob, the ultimate receiver, via an intermediate relay node, Charlie. By introducing Charlie, the end-to-end transmission is divided into two shorter FSO links, improving signal stability and reliability in the presence of severe atmospheric turbulence and noise. This work proposes a quantum relaying method specifically designed for FSO links and examines the impact of two-hop relaying in terms of the probability of error. The Helstrom error bound is derived to assess the system's detection limits, while density operator calculations are performed to characterize the quantum relaying setup. In addition, a closed-form expression is obtained for key system performance metrics. The findings offer valuable insights into optimizing quantum relaying strategies for long-distance quantum communication over FSO channels.