Novel ground states of Bose-condensed gases

Abstract

Bose-Einstein condensates (BEC) provide a novel tool for the study of macroscopic quantum phenomena and condensed matter systems. Two of the recent frontiers, quantized vortices and ultracold molecules, are the subject of this thesis. The formation of highly-ordered vortex lattices in a Bose-condensed gas has been observed. These triangular lattices contain more than 150 vortices with lifetimes of several seconds. The vortices were generated by rotating the condensate with a scanning blue-detuned laser beam. Depending on the stirrer size, vortices were either nucleated at discrete surface-mode resonances (large beams) or over a broad range of stirring frequencies (small beams). Additionally, the dynamics of the lattices have been studied at finite temperature by varying the condensed fraction of atoms in the system. The decay of angular momentum is observed to be strongly temperature-dependant, while the crystallization of the lattice appears to be insensitive to temperature change. Recently, the field of BEC has been extended to include cold molecules. Here ultra-cold sodium molecules were produced from an atomic BEC by ramping an applied magnetic field across a Feshbach resonance. These molecules were used to demonstrate coherent molecular optics. In particular, we have extended Kapitza-Dirac and Bragg diffraction to cold molecules. By measuring the Bragg spectrum of the molecules immediately after their creation, the conversion from atoms to molecules was shown to be coherent - the matter wave analog to frequency doubling in optics. In addition, the more general process of sum-frequency generation was demonstrated.

(cont.) Atoms prepared in two momentum states, prior to creating molecules, were observed to cross-pair, generating a third momentum state. Finally, molecular matter-wave interference was realized using an autocorrelation technique.