Advances in free-space quantum key distribution

In the last decades, quantum mechanics has revolutionized research for innovative applications in plenty of different areas, promising and providing results that cannot be achieved by its classical counterpart. Today, the possibility of generating and manipulating quantum states at will allows researchers to put their hands in the development of these new technologies, leading to what is already being called \emph{the second quantum revolution}. As the most ready quantum technology, quantum key distribution (QKD) is rapidly growing toward global networks to secure classical communication against any type of collective attacks. Because of the unavailability of quantum repeaters, free-space satellite communications must be considered to achieve longer connections, as fiber channels are limited in distance due to higher losses. Additionally, a network of quantum nodes requires a different approach than point-to-point communication, resembling the classical internet structure and presenting new challenges such as hybrid connections. The work described in this doctoral thesis aims to contribute to the development of a global quantum infrastructure by demonstrating different configurations of small quantum networks, both in fiber and in free-space. A model for free-space quantum channels is presented, necessary to understand the behavior of quantum states through random media and verify the reliability of our experiments. Furthermore, an enhanced version of the decoy-states method for QKD protocols is introduced, which enables the exploitation of different quantum sources as needed by recent developments.