Polarization-Encoded BB84 over a 56 dB Loss Channel using Superconducting Nanostrip Single-Photon Detectors

Quantum key distribution represents one of the most promising advancements in secure communications, enabling the exchange of cryptographic keys between distant users with security guaranteed by quantum mechanics. Long-distance quantum key distribution, based on commercial devices, relies on the use of trusted nodes placed between communicating parties. However, the deployment of high-performance superconducting nanowire single-photon detectors increases the communication distances without the need for trusted nodes, which are potential points of vulnerability. In this work, we report the description of a polarization-encoded quantum key distribution system based on the BB84 three-state protocol, implemented via the ThinkQuantum QuKy platform, and interfaced with superconducting nanowire single-photon detectors. The experiments were conducted with the objective of testing the system’s performance under increasing channel attenuation. Special attention was paid to detector tuning strategies, polarization alignment, and the system’s response in high-count and high-loss regimes. Secure key exchange was demonstrated up to 56 dB attenuation.