Advanced Quantum Communication in Free-Space

Quantum mechanics have been the catalyst of exciting recent advances in various scientific fields, particularly in quantum computing, sensing, metrology, and communication. One of the most promising applications of the latter is information-theoretically secure communication using light. As in the classical case, the range of quantum communication over optical fibers is limited because of their inherent losses. At the same time, it is impossible, based on the principles of quantum mechanics, to copy and amplify quantum states. For that reason, free-space channels, that have lower losses compared to fibers, are an alternative for extending the applicability of quantum communication towards the so-called Quantum Internet. Atmospheric random media, however, come with their own challenges that need to be addressed. This thesis focuses on mitigating the challenges, while enhancing the range and efficiency of free-space quantum secure communication, focusing on polarization-based quantum key distribution (QKD), polarization being a degree-of-freedom of photons that is preserved in atmospheric channels. We explore different scenarios of free-space communication under daylight conditions, critical in the development of global quantum networks which have to operate without interruptions and will most likely be composed of heterogeneous nodes. Beginning from the idea of an intercity high-altitude balloon quantum network, we develop a simulator for modeling horizontal, uplink and downlink free-space channels, which allows us to assess the feasibility of various quantum protocols. After laying out the theoretical foundations, we present channel sensing techniques and measurements of real-time estimates of the atmospheric turbulence strength. This knowledge aids both in the design of free-space systems, but also in the emulation of atmospheric channels in a laboratory setting. This enables the optimization of receiving systems designed to mitigate the adverse effects of atmospheric turbulence, before their actual deployment. We then showcase quantum key distribution in an urban intermodal network that combines both fiber and a short free-space channel of less than one kilometer. To extend the range QKD over tens of kilometers, we detail the design and development of a free-space receiver using adaptive optics. The performance of the receiver is investigated with a series of preliminary tests. Finally, we attempt to characterize the polarization transformation of a Coudé telescope to be used for satellite-based quantum key distribution. Through these efforts, this thesis contributes to the development of robust free-space quantum communication systems and paves the way for their practical application in global quantum networks.