Device Characterization and Network Integration of Quantum Key Distribution Components

Quantum technologies are expected to become essential in the near future for secure communication, sensing, and computation. In the Chapter 1: State of the Art of my thesis a description of the fundamental principles of quantum communication, where photons act as qubits, the basic units of quantum information, exploiting superposition and entanglement to enable tasks beyond classical capabilities, and of single-photon sources and detectors as critical components for quantum communication is provided. Then, Quantum Key Distribution (QKD) protocols are illustrated to emphasize their reliance on physical laws for security and their vulnerability to practical implementation challenges. In addition, the architecture of telecommunication networks, from historical developments to modern optical infrastructures, as well as multiplexing techniques, optical fiber characteristics, and nonlinear effects are described. During my three-years PhD I had the opportunity to visit different institutions and companies, exploring and participating in a variety of activities. Deterministic Single-Photon Sources (SPSs), specifically color centers in diamond, which combine Room-Temperature (RT) operation with excellent photostability, have been characterized (Chapter 2: Color centers in diamond) at the Solid State Physics Group laboratories of the University of Turin. The experimental activities focus on the fabrication and characterization of germanium-vacancy (GeV-) color centers in diamond. Deterministic implantation was achieved using Focused Ion Beam (FIB) technology, followed by high-temperature annealing in vacuum conditions to activate defects. Nanopillar structures were fabricated and coupled to GeV- emitters to enhance photon collection efficiency. These emitters were characterized using confocal Photoluminescence (PL) microscopy and Hanbury-Brown and Twiss (HBT) interferometry to verify single-photon emission and extract optical parameters such as lifetime, saturation power, and polarization behavior. In addition, the characterization of QKD components (Chapter 3: Characterization of QKD detectors and sources) carried out at INRiM (Istituto Nazionale di Ricerca Metrologica), included the calibration of silicon - Single-Photon Avalanche Diodes (Si-SPAD) detectors using SI-traceable substitution methods, the analysis of backflash emission in InGaAs/InP- SPADs to assess security vulnerabilities, and the implementation of quantum state tomography to validate the purity of polarization-encoded qubits in a commercial QKD source. The performance of QKD devices under different conditions as well as a specific study on Spontaneous Raman Scattering (SpRS) for the integration of QKD systems into existing networks (Chapter 4: QKD integration in existing networks) were performed at FiberCop Fixed Access & Device Engineering & Innovation laboratories, providing quantitative baselines for quantum network design. Finally, sustainability considerations are integrated throughout the work, highlighting the importance of energy-efficient designs, room-temperature operation, and eco-design principles for scalable quantum networks. This thesis aims to bridge the gap between fundamental quantum research and practical deployment, providing a roadmap for secure, interoperable, and sustainable quantum communication infrastructures