Towards a Digital Twin for the Ground to QEYSSat Quantum Link

Simulations of physical systems require high-fidelity models to accurately represent reality. Simple models may be analytically tractable, but may not be sufficiently representative of reality for the given application. The cost of this simplicity is accuracy, or in the case of quantum key distribution, provable security. Sources of this accuracy gap include the difficulty of modelling physical effects which do not lend themselves well to analytical descriptions, such as afterpulsing. Here, we introduce a novel Monte Carlo based photon emission, transmission, and detection simulator, designed in the context of the Quantum Encryption and Science Satellite (QEYSSat) mission. Within this simulator, every major physical effect a photon may experience during an experiment, from emission to detection, can be accounted for in a probabilistic manner. This methodology allows for the inclusion of experimental parameters which are relevant for a satellite mission, and their impacts on secure key lengths. This simulator serves as a comprehensive baseline to predict and validate experimental data for the upcoming QEYSSat mission.