Tristan has successfully defended his thesis with the title “HIGH-DIMENSIONAL QUANTUM COMMUNICATION” on 20th July 2018 and has been awarded the degree of doctor (Ph.D.). The topic of the thesis is secure quantum communication in free-space line-of-sight applications and multimode fibers by exploiting the high dimensional spatial hilbert space for better security. The work was carried out in the Complex Photonic Systems (COPS) group in the MESA+ Institute for Nanotechnology under the supervision of Pepijn Pinkse. We are grateful to Prof. Dr. D. Bouwmeester (Universiteit Leiden), Dr. C. Marquardt (Max Planck Institute for the Science of Light, Erlangen), Prof. Dr. Ir. J.W.M. Hilgenkamp (University of Twente) and Prof. Dr. A. Lagendijk (University of Twente) for joining the committee.