Quantum key distribution (QKD) enables remote users to establish secret keys secured by quantum mechanics. Polarisation encoding offers simple state preparation and measurement; however, fibre birefringence, component imperfections, and high-speed polarisation modulation make stable, low-error operation challenging.
This thesis investigates the development of a high-speed polarisation-based QKD transmitter. The optical components, including intensity and phase modulators, interferometric systems, and electronic polarisation controllers (EPCs), are characterised experimentally. Gradient descent, coordinate gradient descent, and simulated annealing are compared for EPC-based state preparation. With a 50 mV step, coordinate gradient descent is approximately six times faster than conventional gradient descent, while simulated annealing achieves 100% success over the tested runs at the selected settings.
The polarisation-control approach is evaluated in a proof-of-concept BB84 feasibility experiment over a deployed 224 km undersea fibre link between the United Kingdom and Ireland, with a measured attenuation of 38.6 dB. Channel loss and polarisation stability are characterised, and the QBER is measured using SPAD and SNSPD detector configurations. At a mean photon number of one, the SNSPD configuration produces average QBERs of 2.98% and 1.55% for the horizontal and vertical states, respectively, substantially below the corresponding SPAD results. Separately, a field-trial-informed asymptotic decoy-state model projects a maximum secret-key yield of 2.93*10^-6 bit/clock.
Finally, several iterations of high-speed polarisation-encoders are developed and assessed, identifying extraction efficiency and fibre-axis alignment as key practical constraints. The resulting hybrid encoder prepares four polarisation states at 40 MHz, with measured overall system QBERs below 3%. Overall, the work establishes practical methods for polarisation control, long-distance feasibility assessment, and stable high-speed state preparation in polarisation-based QKD systems.
