Relaying Schemes and Rate-distance trade-off for Multihop QKD

In recent years, quantum communications have garnered considerable attention for their potential to enhance network security. Quantum Key Distribution (QKD) protocols, which promise theoretically secure key generation for remote nodes, have been a primary focus. However, QKD faces significant challenges due to its limited range, especially for fiber-based channels, until quantum repeaters are technologically feasible. Consequently, research efforts have been redirected towards se-cure communication via QKD nodes within multi-hop networks, as a first step towards realizing the quantum internet. In this paper, we propose and analyze a key relay protocol (KRP) for secure quantum key generation within a three-node network, which can be extended to more than two hops in a straightforward manner. We analyzed security under two different sets of assumptions at each node plus accurately evaluated the achievable key rate via numerical methods. Finally, we discuss the advantages and limitations of the proposed protocol.