Preparation and Coherent Control of a Rydberg Qutrit in an Ultra-Cold Atomic Ensemble

This thesis presents the preparation, coherent control and interferometric readout of a single qutrit encoded in a cold Rydberg ensemble. A single photon is stored as a collective Rydberg excitation and subsequently manipulated by applying microwave fields coupling different Rydberg levels, before being retrieved after a programmable delay. A time-resolved sequential readout protocol is developed to determine the qutrit state populations. Coherence of the qutrit is investigated using a Ramsey interferometer implemented between Rydberg states, with the resulting fringe pattern demonstrating phase control and multilevel coherence. Following a vacuum failure, the system was fully dismantled, cleaned and reassembled with a new 2D MOT cell. A programme of optimisation and characterisation was carried out using time-of-flight thermometry, spatially resolved absorption imaging and temperature based magnetic field compensation. Sub-Doppler cooling yields a minimum ensemble temperature of , and the highest measured optical depth is . This work establishes a robust and flexible platform for investigating high-dimensional quantum information protocols using collective Rydberg states and demonstrates the feasibility of fast, coherent control of single qutrits encoded in an atomic ensemble.