Phase fluctuations in elongated Bose-Einstein condensates

Summary form only given. We have systematically studied BECs of /sup 87/Rb atoms in the regime of phase-fluctuating condensates. We observe the phase fluctuations by measuring the density distribution of the released cloud after ballistic expansion. By varying the temperature, the number of atoms in the condensate and the aspect ratio of the trapping potential, we studied the continuous transition from the usual 3D regime, where phase fluctuations are low, into the regime of strong phase fluctuations in the condensate. To determine the amount of phase fluctuations experimentally, we measured the formation and structure of stripes in the atomic density distribution after ballistic expansion as a function of the above mentioned parameters. The appearance of stripes after switching off the trapping fields can be understood as conversion of phase fluctuations respect. phase gradients into axial velocity fields and consecutive density distributions. These results are in good agreement with theoretical and numerical studies and set clear limitations on applications of BECs e.g. in interferometric measurements.