Advancing quantum key distribution on intermodal channels with fiber and free space links

Achieving secrecy in communications has been a challenging task that people tried to accomplish since civilization has born. Starting from simple methods like the Caesar cipher, the science of cryptography kept evolving over time, reaching the security that modern cryptographic schemes can guarantee thanks to the emergence of information theory. Moreover, the second quantum revolution, with the study of quantum information and the invention of quantum computing, is threatening the security of standard encryption schemes based on computationally hard to solve problem. Countermeasures have been proposed, and two main approaches emerged, post-quantum cryptography and quantum key distribution. The first one aims to guarantee security against an attacker with quantum computing resources available by using problems that are supposed to be hard to solve even for a quantum computer. The second approach instead uses the features of quantum mechanics to make the security of the protocol rely on the most fundamental features of physics. In this thesis, the practicality of quantum key distribution is investigated, in a number of experimental tests in laboratory environment and in more practical scenarios. First, quantum information and the quantum mechanical description of light is introduced, then the quantum key distribution protocol used in the experiments described in the following chapters is described. After that, a detailed description of the experimental activity and of the obtained results is given, dividing between two main different approaches in the chosen encoding of the used quantum state. The last two chapters are describing activities other activities, related to the development of quantum key distribution experiments, such as study of free space links and hardware design. The objective is to investigate the feasibility and the readiness of this technology as a future key element to guarantee the security of the communication infrastructure.