Trusted Reconfigurable Intelligent Surface for Multi-User Quantum Key Distribution

We consider a multi-user quantum key distribution (QKD) system based on free-space optics (FSO) in terrestrial environment. Due to obstacles in the signal path, FSO-QKD is heavily affected by decoherence and other harmful effects. In order to avoid the performance degradation, trusted nodes (TNs) are introduced in the QKD network. However, in absence of alternative signal routes, the classical data and the quantum key are relayed by the same TN, which makes it a promising target for an eavesdropping attack. In order to address this issue, we explore the use of a trusted reconfigurable intelligent surface (RIS) that passively reflects the classical signals and manipulates them to make the encryption consistent with the distributed keys. The concept is supported by numerical simulations with multiple FSO-QKD users and an eavesdropper. We demonstrate that the level of data protection is very high with the proposed concept even in case of a successful attack on the trusted RIS and a complete key acquisition by the eavesdropper.