Design and Implementation of Free-Space QKD Systems: Challenges and Solutions for Terrestrial and Satellite Links

n the Information Age, ensuring secure communication has become a cornerstone of modern society. Quantum Key Distribution (QKD) emerges as a revolutionary technology, leveraging the principles of quantum mechanics to enable theoretically unbreakable encryption. This doctoral thesis contributes to the advancement of QKD technology by addressing key challenges in the implementation of free-space quantum communication systems across varying distances and architectures. A major focus of this research is the development of a remote transmitter designed for long-distance free-space quantum links. The thesis presents experimental results on turbulence characterization and single-mode fiber coupling efficiency by means of an adaptive optics system. The impact of adaptive optics on overall system efficiency and its potential to improve key distribution rates over extended distances is thoroughly analyzed. In addition to academic research, the thesis also details the development of a freespace QKD system for a start-up of the University of Padova, demonstrating the current maturity of the technology for real-world applications. The system designed for the start-up highlights the growing capability of industries to implement QKD solutions outside the lab