Modifying spin diffusion in a nondegenerate ultracold gas

Diffusion plays a major role in describing how particles and heat move in any nonequilibrium system. The diffusion of quantum properties is not as well understood as the diffusion of classical properties, especially at low temperatures or high densities. Exploring the diffusion of the quantum property known as spin is beneficial for explaining quantum effects that arise at low temperatures or high densities, and this knowledge acquired could assist in the development of new ultra-low-power devices. This thesis examines spin diffusion at various temperatures and densities using the highly-tunable experimental platform of ultracold atoms. Around one million rubidium-87 atoms are cooled to nanokelvin temperatures to create an ultracold gas, where quantum interactions between atoms can significantly modify spin diffusion compared to classical diffusion. One-dimensional spin diffusion is observed for an initial two-domain spin profile. Remarkably, diffusion of this spin profile is slowed at temperatures above quantum degeneracy, where the ultracold gas is largely classical but with quantum collisions. We demonstrate that this slowing of spin diffusion is due to the presence of spin coherence between spin domains, and that the removal of coherence speeds up spin diffusion to classical timescales. Spin diffusion is further modified by applying a linear differential potential that can speed, slow, or stop spin diffusion of a two-domain spin profile. Differential potentials spatially alter the precession of spins, which then alters the spin-rotating quantum collisions that modify spin diffusion. For a linear differential potential with a specific sign and magnitude, stabilized spin domains in an ultracold gas are observed for 40 times longer than classical diffusion timescales. In addition to modifying spin diffusion with coherence and differential potentials, we demonstrate arbitrary control of one-dimensional spin diffusion using temporally varying differential potentials and three-domain spin profiles. These spin diffusion modifying techniques could be useful for manipulating spin in other nonequilibrium systems, and set the stage for simulating spin-based devices in ultracold atoms.