| This thesis presents experiments with a quantum-degenerate gas of bosonic atoms where interparticle interactions can be precisely controlled using a magnetic-field tunable Feshbach resonance. Using our new apparatus, we have the capability to study either single species Bose-Einstein condensates of 85Rb or 87Rb, or a mixture of both species. With such a mixture we have created and studied the first ultracold heteronuclear Feshbach molecules. Further work with two-component gases includes the creation of a dual-species BEC in which the miscibility of the two quantum fluids can be controlled. Finally, with a single species 85Rb condensate we have performed the first detailed measurements of the excitation spectrum of a BEC as a function of interaction strength. With strong interparticle interactions, we have observed that the excitation energy is smaller than that predicted by mean-field theory. |
