Collective Excitations of a Bose-Einstein Condensate in a Dilute Gas

We observe phononlike excitations of a Bose-Einstein condensate (BEC) in a dilute atomic gas. ${}^{87}$Rb atoms are optically trapped and precooled, loaded into a magnetic trap, and then evaporatively cooled through the BEC phase transition to form a condensate. We excite the condensate by applying an inhomogeneous oscillatory force with adjustable frequency and symmetry. We have observed modes with different angular momenta and different energies and have studied how their characteristics depend on interaction energy. We find that the condensate excitations persist longer than their counterparts in uncondensed clouds.