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

TL;DR

This dissertation explores how internal control, squeezing, and decoherence affect large ensembles of alkali atomic spins interacting with light, proposing methods to enhance entanglement and spin squeezing despite decoherence effects.

Contribution

It introduces a formalism for modeling hyperfine spin dynamics, optimizing state preparation and control for improved entanglement and squeezing in atomic ensembles.

Findings

State preparation with control enhances entanglement for f>1/2.

Optimized geometries improve spatial mode matching and spin squeezing.

Numerical methods identify maximal squeezing under decoherence.

Abstract

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