Towards High Dimentional Practical Quantum Communication

Quantum communication promises information-theoretic security grounded in the laws of physics, addressing the computational vulnerability of classical public-key cryptosystems such as RSA to quantum algorithms. However, deployable quantum communication systems are still not optimised for practical conditions due to high optical complexity and cost, very low key-rate exchange over long distances, and limited information capacity in two-dimensional encodings.

This thesis develops and experimentally validates a unified prepare-and-measure platform that addresses the challenges above. At its core is a polarization-encoded optical setup based on a self-compensating SAGNAC loop modulator and a time-multiplexed measurement stage requiring only two single-photon detectors, achieving 99.78\% fidelity across two mutually unbiased bases. Using this optical system, three demonstrations of increasing practical relevance are presented. (i)

A simplified BB84 quantum key distribution (QKD) protocol benchmarked over a deployed 7 km dark fibre link on the Instituto Superior Técnico campus with both SPAD and SNSPD detectors, marking it the first demonstration using top-of-the-line SNSPD detectors in Portugal; (ii) A numerical feasibility study of a CubeSat-based downlink for satellite-to-ground QKD and quantum keyless private communication (QKPC), optimising protocol parameters across a realistic LEO overpass and yielding a peak transmission rate of 80.8 kHz and 700 MHz at zenith, respectively; (iii) A quantum-secure time transfer protocol that uses the quantum signals for both key generation and time transfer, and further uses the QKD-generated key to authenticate timing data over a 4.2 m bidirectional fibre link, achieving a clock offset precision of 0.105 ± 0.018 ps over 8 s averaging windows.

The remaining work extends the prepare-and-measure scenario to high-dimensional quantum states by combining time-bin and polarization degrees of freedom, exploiting the increased information capacity, enhanced noise robustness, and higher security thresholds of qudit encodings to push practical quantum communication beyond the qubit regime.