COLLECTIVE EXCITATIONS OF A BOSE-EINSTEIN CONDENSATE IN A MAGNETIC TRAP

Collective excitations of a dilute Bose condensate have been observed. These excitations are analogous to phonons in superfluid helium. Bose condensates were created by evaporatively cooling magnetically trappe d sodium atoms. Excitations were induced by a modulation of the trappi ng potential, and detected as shape oscillations in the freely expandi ng condensates. The frequencies of the lowest modes agreed well with t heoretical predictions based on mean-field theory. Before the onset of Bose-Einstein condensation, we observed sound waves in a dense ultrac old gas.