Thesis title: Experimental Implementation and Performance Modeling of QKD in Fiber, Free-Space and Satellite Networks

Abstract: Quantum Key Distribution (QKD) has matured from a theoretical curiosity into a practical technology capable of securing future communication infrastructures against the threats posed by quantum computing. Positioned alongside post-quantum cryptography, QKD provides a complementary approach for enabling information-theoretic secure communications guaranteed by the laws of physics. However, the transition from isolated laboratory demonstrations to widespread, scalable deployment remains challenging, primarily due to the high cost of dedicated equipment, the difficulty of integration with existing infrastructure, and the complexity of global-scale coverage. This thesis addresses these challenges through a comprehensive study spanning fundamental principles, experimental implementations, and system-level modelling, covering the entire development chain from hardware-level optimization to network integration and satellite-based QKD system modelling.

Building upon the theoretical foundations of quantum mechanics and key QKD protocols, the experimental foundation of this work focuses on the development and characterization of robust QKD hardware. A key milestone is the design and implementation of the Single Photon Polarization-Encoded System (SPPES), which serves as a versatile testbed for evaluating QKD performance through single photon-level exchange over fiber or free-space links. Building on this platform, a polarization-encoded QKD source was developed and optimized for practical deployment. Moving beyond the laboratory, this work demonstrates the practical integration of the QKD source into operational environments, targeting the critical challenge of quantum-classical coexistence. In the optical fiber domain, the thesis investigates the integration of QKD signals within deployed backhaul and fronthaul network segments, verifying the system's ability to operate alongside high-power classical data traffic without necessitating expensive dark fibers. Complementing this, a terrestrial Free-Space Optical (FSO) link was established in a metropolitan environment, demonstrating the successful transmission of quantum signals over a turbulent, noisy channel under nighttime and daylight conditions. This field trial highlights the potential of FSO technology to serve as a flexible "last-mile" solution for urban quantum connectivity.

Finally, to extend QKD beyond terrestrial limitations and address the challenge of global-scale distribution, the thesis provides extensive modelling of Satellite-to-Ground QKD links. A comprehensive software simulation tool was developed to model the complex interplay between orbital dynamics, atmospheric channel effects, and protocol performance. Through this tool, a rigorous feasibility analysis was conducted for hybrid satellite-fiber network architectures to define the strict system requirements necessary for future satellite missions. The results offer critical insights for mission planning, effectively de-risking future deployments and establishing a roadmap for the realization of global satellite-based quantum networks.

In summary, this work bridges the gap between fundamental component design and large-scale network deployment. By advancing the field from the optimization of 'inside-the-box' critical hardware details to the modeling and planning of global QKD networks via satellite links, this thesis lays the essential groundwork for the next generation of quantum-secured networks. Collectively, these experimental and theoretical contributions provide a pathway for integrating quantum cryptography into real-world systems, effectively building the ground towards future global quantum communication networks.

Supervisor: Hercules Avramopoulos

PhD Student: Argiris Ntanos