Lanterna Rally

Author: Yudan Guo (Physics researcher)

Publisher:

Published: 2021

Total Pages:

ISBN-13:

Engineering complex and exotic many body states is a long standing goal for ultracold atom experiments, for which strong and tunable interaction is a crucial ingredient. In this thesis, I will describe our experimental efforts using photons in a multimode confocal cavity to mediate effective interaction among atoms confined within the cavity. In contrast with traditional optical cavities where only a single fundamental Gaussian mode is coupled to the atoms, multimode cavities can support near-degenerate resonances that are well approximated by a continuum of photon modes. Superposition of cavity modes with incommensurate spatial profiles then gives rise to supermodes that adapt to atom configurations and allow for further shaping of the interatomic potential landscape. In a single mode cavity, Raman coupling through atom internal states with the cavity mode is demonstrated, leading to a novel spinor organization and Ising type cavity mediated interaction. As a first experiment in a multimode cavity, I will show that the interference of near-degenerate cavity modes leads to a strong and tunable-range local interaction between Bose-Einstein condensates (BECs) trapped within the cavity. Exploiting the symmetry of a confocal cavity, we measure the interaction between the BEC and its mirror image thus avoiding the complications of overlapping atom densities. I will then discuss how a confocal cavity also hosts a non-local sign-changing interaction due to the differing longitudinal Gouy phases of cavity modes. Using holographic imaging techniques, we reconstruct the complex intra-cavity electric field and confirm the spatial character of this non-local interaction. Finally, I will discuss additional control of the relative strength of the local and the non-local interaction by pumping on two degenerate resonances of the cavity. By canceling the non-local interaction, we restore the translational symmetry of the system. The strong local interaction can then induce a crystallization transition of the BEC where the resulting optical lattice responds dynamically to local changes in atomic density and accommodates a phonon excitation. Bragg spectroscopy measurements then reveal the dispersion of the phonon mode dependent on the BEC-photon coupling strength. I will conclude by briefly describing future experiments enabled by both the local and the non-local interactions.