Strongly Interacting Bose-Einstein Condensates: Probes and Techniques

A dilute gas Bose-Einstein condensate (BEC) near a Feshbach resonance offers a system that
can be tuned from the well-understood regime of weak interactions to the complex regime of strong
interactions. Strong interactions play a central role in the phenomena of superfluidity in liquid He,
and theoretical treatments for this regime have existed since the 1950’s. However, these theories
have not been experimentally tested as superfluid He offers no similar mechanism with which to
tune the interactions. In dilute gas condensates near a Feshbach resonance, where interactions can
be tuned, strong interactions have proven difficult to study due to the condensate’s metastable
nature with respect to the formation of weakly bound molecules. In this thesis, I introduce an
experimental system and novel probes of the gas that have been specifically designed to study
strongly interacting BECs. I present Bragg spectroscopy measurements that have accessed this
regime, as well as proof-of-principle experiments using photon-counting for Bragg spectroscopy at
low-momentum. Finally, I show preliminary data using contact spectroscopy, which is a method
that could lead to the first measurements of the predicted interaction energies for a dilute Bose gas
of atoms in the strongly interacting regime.