Honours Project

Since the first experimental realization of a Bose-Einstein condensate (BEC), many of its properties have been thoroughly investigated. Superfluidity is one of the properties of a BEC which is yet not fully understood. Previous experimental investigations have gathered evidence for the existence of a critical velocity by moving tightly focussed optical dipole potentials or impurities through harmonically trapped Bose-Einstein condensates. However, their interpretation in terms of a critical velocity is somewhat problematic as either the potential was accelerating, or the speed of sound varied as a function of position due to the inhomogeneity of the condensate density.

Specifically tailored potentials are helpful tools when investigating properties of ultra-cold gases. We will use our versatile, time-averaged trapping scheme to create a ring-shaped potential for a BEC in order to quantitatively determine the critical velocity of such a system. Here, a flat-bottomed trap is generated by rapidly scanning the focussed laser in a circle, and the barrier is formed by reducing the laser power at a dynamically controllable position. By varying the speed of the barrier and observing the behavior of the BEC, we will quantitatively determine the critical velocity of this system.