Studying a dual-species BEC with tunable interactions

We report on the observation of controllable spatial separation in a dual-species Bose-Einstein condensate (BEC) with ${}^{85}$Rb and ${}^{87}$Rb. Interparticle interactions between the different components can change the miscibility of the two quantum fluids. In our experiments, we clearly observe the immiscible nature of the two simultaneously Bose-condensed species via their spatial separation. Furthermore the ${}^{85}$Rb Feshbach resonance near 155 G is used to change them between miscible and immiscible by tuning the ${}^{85}$Rb scattering length. Our apparatus is also able to create ${}^{85}$Rb condensates with up to $8\times {10}^4$ atoms which represents a significant improvement over previous work.