Master thesis and internship[BR]- Master's thesis : Study and control of polarization in a ground-based quantum communication telescope[BR]- Internship

Abstract

The vulnerability of classical encryption protocols to the increasing power of quantum computers is driving research into new and secure methods of communication. One promising candidate to address this problematic is quantum key distribution, which takes advantage of the properties of quantum mechanics to ensure a secure exchange of the encryption key, thus guaranteeing secure communication between the entities. Quantum key exchange algorithms exploit the properties of quantum objects, typically the polar- ization of photons, to set up the secure exchange. To this end, the polarization state of the photons must be maintained throughout the exchange. Reflections of the telescope mirrors at the receiver, however, modify the polarization state. The present work recalls the concept of light polarization, and the different types with the as- sociated mathematical description. A general introduction to quantum key distribution, and to the MOCA project, then follows to introduce the reader to the main parts of this work, which focuses on characterizing the changes in polarization caused by the telescope, and in particular by the third mirror where the high angle of incidence causes polarization aberrations. Based on the Jones formalism, it is shown that the phase shift between the orthogonal components of the polarization at the wavelengths relevant to this project is too large to satisfy the requirements in terms of polarization extinction ratio. As a result of the unsatisfactory performance of the telescope, an in-depth study of the use of birefringent crystals to correct the excessive phase shift is carried out, showing the need to use three crystals of different birefringence to achieve the desired polarization extinction ratios at the three relevant wavelengths for MOCA. Results also show that if there is an order of priority for optimizing the wavelengths of the application, the use of one or two crystals proves to be worthwhile. Eventually, based on ellipsometric measurements, the phase shift and change in polarization orientation generated by a mirror of configuration comparable to the third mirror of the telescope are determined. A comparison of the results provided by models to real measurements confirms the implementation of the Jones formalism and highlights the need to characterize a mirror on the basis of experimental measurements.