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The presence of inter-particle interactions in a system elevates the physics involved from fundamentally single-body, with each particle following its own trajectory according to external forces, to many-body, with collective behaviour emerging from the interplay between the multitude of particles. Furthermore, the stronger the interactions, the more richly many-body the behaviour becomes; weak interactions can be modelled as a background ‘mean field’ whereas the presence of strong interactions introduces fluctuations and correlations that cannot be simplified in this fashion. This Thesis is concerned with two experiments utilising tunable interactions in three-dimensional thermal Bose gases, with particular emphasis on the unitary regime of maximal interactions. We use 39K, a bosonic isotope, in anisotropic harmonic optical trapping potentials. In the unitary Bose gas we find many-body complexity introduced by the possibility of three particles coming into close proximity. However, the preparation of Bose gases with such strong interactions is hampered by this, due to destructive three-body recombination events which eject particles and heat the cloud. To mitigate this, we perform a spin-flip that changes the internal atomic state from one with weak intra-state interactions to one with strong intra-state interactions. This technique constitutes an interaction quench since it takes place on a short timescale compared to other evolution timescales of the ultracold cloud. One experiment concerns the hydrodynamic expansion of a gas after release from an anisotropic trapping potential. When interactions are sufficiently strong, a pronounced inversion of the anisotropy during expansion can be observed which is a manifestation of interaction-driven collective flow. We show that this elliptic flow is intimately linked to thermalisation, and show that it is highly dependent on the microscopic details of the collisions involved. The other experiment addresses the two- and three-body contacts in the Bose gas. These contact parameters arise from the corresponding two- and three-body correlations present in Bose gases, and underpin a large number of macroscopic thermodynamic variables. We map out the two-body contact across the full range of interactions from weak to strong, including the unitary regime, and reveal the three-body contact at unitarity. |
