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This dissertation studies spin squeezing, entanglement and decoherence inlarge ensembles of cold, trapped alkali atoms with hyperfine spin f interactingwith optical fields. Restricting the state of each atom to a qutrit embedded inthe 2f+1 dimensional hyperfine spin enables us to efficiently model thecoherent and dissipative dynamics of the ensemble. This formalism also allowsus to explore the effects of local control on the internal hyperfine spins ofthe atoms. State preparation using such control increases the entangling powerof the atom-light interface for f>1/2. Subsequent control of the internal spinsconverts entanglement into metrologically relevant spin squeezing. In the caseof squeezing by quantum nondemolition measurement, we employ a numerical searchto find state preparations that maximize spin squeezing in the presence ofdecoherence. Dissipative dynamics on our system include optical pumping due tospontaneous emission. While most works ignore optical pumping or treat itphenomenologically, we employ a master equation derived from first principles.This work is extended to the case of an atomic ensemble interacting with anon-homogeneous paraxial probe. The geometries of the ensemble and the probeare optimized to maximize both spatial mode matching and spin squeezing.

Internal Spin Control, Squeezing and Decoherence in Ensembles of Alkali Atomic Spins