A novel free-space optical communication link with structured light for quantum key distribution and single photon transmissions.

Since the rise of modern optical telecommunications, the need to use the same channel to connect multiple users or to enhance the capacity of transmitted data has driven the investigation of the various degrees of freedom of light. This effort has led to the development of several division multiplexing strategies that exploit wavelength, time, polarization, phase/amplitude, and more recently spatial modes. Space division multiplexing (SDM) takes advantage of the structuring of the intensity or phase profile of electromagnetic waves into a set of orthogonal spatial channels, which can serve as independent information carriers at the same frequency alongside conventional modulation formats. This approach provides a possible solution to the pressing issue of “optical crunch”, that is, the network saturation. Moreover, it can expand the available alphabet of states for single-photon transmissions. In this thesis, an innovative scenario will be considered and investigated for SDM, based on the exploitation of a new type of beams characterized by multipole phase structure. This thesis focuses on the conception and numerical validation of a free-space optical communication scheme based on multipole-phase division multiplexing. The study addresses the generation, propagation, and separation of multipole-phase beams, combining theoretical modeling with computational simulations to outline compact and efficient all-optical architectures for their manipulation and control. Beyond the ideal transmission scenario, the investigation will extend to conditions affected by atmospheric turbulence and to the single-photon regime, broadening the analysis carried out during my Master’s thesis. By exploiting this novel form of spatial multiplexing, the work introduces a promising alternative to existing approaches—such as those relying on orbital angular momentum—capable of supporting higher transmission capacities and offering a practical route towards robust free-space links.