Quantum Communication Beyond Qubits

Quantum Communication and particularly Quantum Key Distribution (QKD) has traditionally relied on qubits - two-level quantum systems - for secure information exchange, and has been explicitly designed to link two remote parties. This thesis explores the evolving landscape of quantum communication beyond qubits, addressing both high-dimensional protocols and multi-party scenarios. The first part of the thesis develops a flexible security proof framework for high-dimensional (HD) QKD protocols in both the asymptotic and the finite-size regimes. Unlike approaches based on theoretically convenient but, for many platforms, experimentally infeasible Mutually Unbiased Basis measurements, our framework only relies on measurements implementable in state-of-the-art laboratories. We analyse a general HD QKD protocol in the finite-size regime, proving security against both i.i.d. collective attacks and coherent attacks. Beyond the commonly used fixed-length approach, often impractical in fluctuating free-space channels, we also provide a variable-length security argument tailored to rapidly varying conditions. The second part of the thesis turns to Discrete Modulated (DM) Continuous-Variable (CV) QKD, a promising candidate for high-rate implementations in metropolitan area networks. We provide the first rigorous application of a composable finite-size security proof against i.i.d. collective attacks to a fully implemented experimental system, demonstrating the generation of secure finite-size key over a 20km fiber link. We further develop a security framework for DM CV-QKD in passive optical point-to-multipoint networks, a widely used telecommunication topology, involving multiple users, and discuss different trust scenarios. Finally, we implement a three-party passive optical network and analyse security with the developed framework.