Design and performance of satellite-based quantum communication network

Quantum Key Distribution (QKD) allows the establishment of ultra-secured symmetric encryption keys. Unlike asymmetric cryptography relying on mathematical complexity and vulnerable to Quantum Computing (c.f., Shor algorithm), QKD is a theoretically proven unbreakable method, ensuring security in depth based on the use of quantum phenomena. However, optical fibre based QKD implementation suffers from severe physical restriction due to absorption losses in optical fibre that restricts the distance between the end-users to typically less than 150 km. In 2017, the Chinese successfully demonstrated the possibility to perform entanglement based QKD between end-users over a slant range varying from 1600 to 2400 km using a Low Earth Orbit (LEO) satellite, called Micius. This overtaking of the distance between end-users on the ground thus requires free space optical communications between a satellite and a pair of Optical Ground Station (OGS), inducing perturbative effects during the propagation through the atmosphere. Moreover, in the perspective of an operational system with many OGS, the accesses schedule to define the mission planning and to guarantee the service level agreements has to be studied too. This work focuses on the analysis of the space based QKD design and performance using the entanglement based BBM92 protocol. In the first part the calculation of the secret key rate and the evaluation of the link budget with different models are performed: these two concepts are then both utilized to provide a performance result for different BBM92 QKD protocol cases of study. In the second part multiple algorithms are proposed for the definition of the mission accesses schedule. The unconstrained algorithm to prioritize the accesses democratization among all pairs is presented and the distinction between the daytime and the night-time and also the presence of cloud coverage are taken into account. Moreover, a constrained algorithm and a Monte Carlo simulation are studied to provide an absolute control converging towards the user desired operational performances. The results of all the previous algorithms are reported for different orbital situations and system parameters.