Analysis of background light with the Optical Ground Station Oberpfaffenhofen for satellite QKD scenarios

Kurzfassung

Quantum key distribution (QKD) allows secure data transfer in a post-quantum era by exploiting the laws of quantum mechanics to generate a secure-key for data encryption. Satellite based QKD is the gateway to a global communication network due to its quadratic transmission loss scaling opposed to the exponential losses in fiber-based QKD. However, the free-space nature of satellite QKD imposes a new challenge due to the additional noise source of background light (BGL). BGL heavily influences the feasibility of a secure QKD protocol, and its impact has to be mitigated through temporal, spatial and spectral filtering techniques. To design optical systems for satellite QKD, the contributions of BGL must be characterized, but current observations of BGL in the C-band are limited to daytime measurements with collimator setups. These measurements are only translated to the receiving systems of QKD downlinks, Optical Ground Stations (OGSs), via theoretical models. This work presents an optical setup built and calibrated for BGL measurements in the C-band with single mode fiber (SMF) coupling during the day and night utilizing the Optical Ground Station Oberpfaffenhofen (OGSOP) designed for satellite QKD downlinks. The transmission loss of the optical system and the efficiency of the deployed Superconducting Nanowire Single Photon Detector (SNSPD) are determined and BGL measurement data in the C-band acquired during the day and night. In addition the measured BGL is compared with BGL simulations and prior measurements with collimator setups for verification. During the night, the measured BGL only raises slightly above the noise floor of the detector with larger radiances observed only close to the surface with up to 0.00023 (mW nm-1 m-2 sr-1) and close to the full Moon with up to 0.0043 (mW nm-1 m-2 sr-1). In daytime BGL measurements radiances between 1.09 (mW nm-1 m-2 sr-1) and 4.17 (mW nm-1 m-2 sr-1) were observed, exhibiting an offset from simulation tools. The results highlight the importance of system calibration for Optical Ground Station (OGS) based BGL measurements, while also providing insights into the magnitude of BGL usable for the system design in future satellite QKD missions.