Dissipationless flow and superfluidity in gaseous Bose-Einstein condensates

Superfluid flow is a manifestation of quantum mechanics at the macroscopic level. Phenomena like dissipationless flow and persistent currents can be traced back to the existence of a macroscopic wavefunction. The gradient of its phase gives the superfluid velocity. However, superfluid flow is only stable below a critical velocity. Above this velocity, excitations of the fluid can be generated, which can be either phonons, rotons (in liquid He) or vortices. The realization of Bose-Einstein condensation in dilute gases 3 has created a new test ground for the theory of quantum fluids. Early experiments confirmed the microscopic foundation of the phenomenon of superfluidity: the phonon nature of low-lying collective excitations was observed 6 and the coherence or macroscopic phase of the condensate. This picture became more complete during the last year with the observation of vortices, 9 transverse excitations, evidence for a critical velocity and suppression of collisions. Although the metastability and therefore limited lifetime of the quantum gas and its mesoscopic size may prevent spectacular observations of persistent flows, there is already a clear picture emerging of the many facets of superfluidity in a gas.