Quantum key distribution enables the secure exchange of cryptographic keys based on physical principles. Superconducting nanowire single-photon detectors (SNSPDs) are key components in these systems. However, their detection properties can depend on the polarization of the incoming light, potentially affecting the security assessment of practical QKD systems.

The PolSecure project investigates these effects in entanglement-based quantum key distribution following the E91 protocol. The aim is to systematically characterize polarization-dependent detection differences in NbN-based SNSPD crossbar architectures, evaluate their impact on Bell test statistics, and develop suitable compensation methods.

For this purpose, SNSPD crossbars fabricated at Leibniz IPHT are optimized and integrated into an E91 measurement system using polarization-entangled photon pairs at 1550 nanometers. By calibrating individual detector pixels, distortions in CHSH Bell tests are to be quantified and reduced. The results will contribute to a better understanding and minimization of detector-induced security limitations in practical quantum communication systems.