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UBC Theses and Dissertations

Determination of the atom’s excited-state fraction in a magneto-optical trap Shen, Yue


This thesis introduces an empirical method for determining and controlling the excited-state fraction of atoms in a magneto-optical trap (MOT), which is essential for the use of cold atoms as a sensor when they are held in a MOT since their interactions with other particles and fields are quantum state dependent. A four-level theoretical atomic model was used to describe the transitions of the atoms in a MOT, and the fluorescence emitted from a fixed number of atoms under different laser conditions were measured to determine the saturation parameters empirically. Two saturation parameters P_sat= 1.15 (0.06) mW and P_r,sat= 2.05 (0.59) mW were successfully extracted from the model, and the excited-state fraction in the four-level model was accurately calculated as a function of the MOT trap parameters, which ranges from 0.045 to 0.415 for the experimental settings currently available. We also observed minor deviations from the four-level model for the photon scattering rate, and a hypothesis of atom pinning under high powers was proposed to explain the problem. We plan to use this simple excited-state fraction determination method to distinguish the ground and excited state collision cross section of Rubidium atoms with species in residual gas of the vacuum. This is the first step to establishing atom loss rates from a MOT as an atomic primary pressure standard.

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Attribution 4.0 International