Dual-species Bose-Einstein condensate of 87Rb and 133Cs

We report the formation of a dual-species Bose-Einstein condensate of Rb-87 and Cs-133 in the same trapping potential. Our method exploits the efficient sympathetic cooling of Cs-133 via elastic collisions with Rb-87, initially in a magnetic quadrupole trap and subsequently in a levitated optical trap. The two condensates each contain up to 2 x 10(4) atoms and exhibit a striking phase separation, revealing the mixture to be immiscible due to strong repulsive interspecies interactions. Sacrificing all the Rb-87 during the cooling, we create single-species Cs-133 condensates of up to 6 x 10(4) atoms.