Coherent two photon excitation within an extended cloud of Rubidium 85 for the purposes of atomic interferometry and cooling

Abstract

Cold atom samples, at temperatures of the order 100 µK, are useful for a wide reaching array of new and exciting technological and scientific endeavours. Atoms are conventionally cooled by Doppler cooling, which relies on the continuous absorption and re-emission of photons in a closed optical cycle. This requirement is difficult to achieve when there are many allowed decay paths for the excited atom, making Doppler cooling only feasible for a handful atoms with simple energy level structures. More exotic energy level structures, such as those found in molecules, are notoriously difficult to cool. Coherent cooling schemes in comparison, offer advantages such as insensitivity to frequency detuning or a higher number of photon momenta which can be imparted for each spontaneous emission event making them promising candidates for the optical cooling of particles with more general energy level structures. In order to demonstrate these schemes, we have explored the coherent manipulation of an atomic cloud of Rubidium cooled using a two photon Raman resonance. Despite the long spontaneous decay times of such systems, we find a significant decay in the fidelity of the coherent manipulations, which we have characterised using the techniques of Raman spectroscopy, Rabi oscillations, Ramsey interferometry and spin echo. We have found the minimum time constant for the decay in the decoherence to be 2.1fi0:2 ms, which is a result of non-radiative and partially non-stochastic dephasing mechanisms. Due to a high level of decoherence during the spin-echo experiments further investigation is required to determine the exact ratio of stochastic to non-stochastic dephasing.

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