Dual-Species Bose-Einstein Condensate of ${}^{87}$Rb and ${}^{133}$Cs

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