Characterization of Polarization Dispersion in a Stabilized Quantum Link

This thesis investigates the stability of polarization states in a 20 km deployed underground fiber network between KTH Royal Institute of Technology and Ericsson in Stockholm, intended for the study of quantum communication. Polarization-encoded quantum states are sensitive to environmental disturbances, making active stabilization essential for maintaining fidelity. The study evaluates the performance of Qunnect’s Automated Polarization Controller (Qu-APC) through three experiments: measuring passive long-term polarization drift, analyzing wavelength-dependent polarization effects, and implementing triggered compensation using an Arduino-based control scheme. Results show that polarization drift occurs consistently over time, with a baseline drift of approximately 1.7% in fidelity per hour, emphasizing the need for active compensation. Triggered compensation every 30 minutes maintained fidelity above 95% and achieved over 99% network up-time. Although the wavelength sweep experiment produced inconclusive results, the findings confirm the feasibility of maintaining stable polarization states in real-world fiber networks, providing a foundation for future quantum communication applications.