Quantum Optics Systems for Long-Distance Cryptography and Quantum Networks

The Field of Quantum Information has drawn a lot of scientific attention in the last decades, due to the variety of both theoretical open questions and experimental implementations of quantum technologies in different architectures. This thesis was part of two scientific projects, and it is hence divided into two parts, which both aims at implementing new quantum information technologies.

In the first part, we investigated the feasibility of performing Quantum Key Distribution with Continuous Variables, (CVQKD) between an orbiting Satellite and a ground station at the Earth’s surface. The implementation in continuous variables avoids the use of single-photon detectors, as current off-the-shelf telecommunication components could be used instead, which increases the applicability of the protocol. Furthermore, the link with an orbiting satellite opens the door to the potential construction of a global network since it allows to overcome the long distance communication problem, due to both the inherent losses in the optical fibers and the fact that signal amplification without introducing noise is not possible (no-cloning theorem). We therefore performed a theoretical study where we considered realistic physical parameters in this scenario to show the conditions for the feasibility of satellite CVQKD with state-of-the-art technology.

In the second part of the manuscript, we describe the design and the experimental results showing the performance of a continuous variable optical source for the generation of graph states at telecom wavelengths, that we built from scratch. The preparation of graph states, also called cluster states, can be used for implementing quantum cryptographic protocols, measurement-based quantum computation (MBQC) or quantum simulation. We map the graph state nodes and links to the multimode state from a non-linear interaction of light in a waveguide. After the waveguide, we obtain a set of uncorrelated squeezed vacuum states, than can be manipulated into a desired graph by an appropriate passive unitary transformation, hence multiport interferometry. We study the amount of the squeezing levels of the multimode squeezed vacuum state and the number of squeezers, that are related to the number of nodes and the amount of EPR correlations in the potential graph states after the appropriate basis change. We prepared an experiment where we directly measured, via homodyne detection, multimode squeezing after the light interaction in the non-linear waveguide, hence showing the functionality of the source. We finally give the short-term prospects for the optimization of the source and some of its long-term potentialities.