Dispersive Probing of Quantum State Preparation in Ultracold 87Rb

Abstract

This thesis describes the use of microwave frequency sweeps across the resonance of a magnetic dipole transition to induce adiabatic rapid passage of an ultracold (<1 μK) atomic ensemble between Zeeman sub-levels of the 87Rb ground state. Atoms begin in the |F = 2, mF = 2> quantum state, and efficient preparation of the sample in the |F = 1, mF = 1> state is demonstrated. A dispersive probing system is developed using the technique of frequency modulation spectroscopy, which involves a heterodyne detection scheme. Dispersive probing is implemented in parallel to the state preparation, such that the real-time evolution of coherent transfer processes can be monitored. The dispersive probe is used as a tool to quickly and accurately determine the microwave resonance frequency to within a 30 kHz range. Resonant coherent manipulations (Rabi oscillations) between the |F = 1, mF = 1> and |F = 2, mF = 2> states are then demonstrated and probed dispersively. A proof of concept application of the dispersive probing system is also presented. It is used to monitor an RF evaporative cooling sequence, providing a precursor measurement on a trapped atomic cloud. This leads to the ability to terminate the experimental sequence in the case of an unfavourable starting point, and thus a reduction of classical number fluctuations in the final atom number.