Vortices and Frictionless Flow in Bose–Einstein Condensates

Barbara Goss Levi

The earliest experiments on gaseous Bose–Einstein condensates established that they are coherent collections of atoms, all described by a single wavefunction—just like the known superfluids, liquid helium-4 and superconductors. It was natural to wonder if they would also exhibit some of the behavior that is so characteristic of those larger-scale Bose condensates, such as quantized circulation (vortices) or frictionless flow. Many theorists have proffered ideas on how one would produce vortices in a Bose–Einsten condensate and what their properties might be. One of those ideas has now been experimentally realized. The approach was suggested by James Williams and Murray Holland of JILA and implemented by their experimental colleagues there. (JILA is a jointly operated lab of the University of Colorado at Boulder and the National Institute of Standards and Technology. Williams is now at the University of Toronto.)

Nicholas G. Parker, B. Jackson, A. Martin +1 more

Frédéric Chevy

Chao Feng

This thesis examines the manifestation of the critical velocity for superfluidity in dilute gas Bose-Einstein condensates (BECs). Superfluidity is a macroscopic quantum phenomenon characterised by unique properties such as frictionless flow, high thermal conductivity and the presence of quantised vortices. Our study focuses on what is arguably the most well-known attribute: the ability for an object to move through a superfluid without experiencing a drag force when travelling below a threshold critical velocity.The Landau criterion for superfluidity predicts the critical velocity in a BEC is equal to the speed of sound, and this conclusion is supported by a large body of theoretical work. However, a number of experiments have observed lower critical velocities than theory predicts. In the first part of this thesis, we numerically model experiments on the superfluid critical velocity of an elongated, harmonically trapped Bose-Einstein condensate as reported by [P. Engels and C. Atherton, Phys. Rev. Lett. 99, 160405 (2007)]. These experiments swept a repulsive obstacle, formed by a optical dipole potential, through the long axis of a condensate at constant velocity. Their results found an increase in the resulting density fluctuations of the condensate above an obstacle velocity of v _ 0:3 mm s

Shaun Johnstone

Lars Onsager noted that turbulent fluids constrained to flow in two dimensions tend to develop large-scale features. Quantum fluids, such as atomic Bose–Einstein condensates, also feature turbulent flows, however their vortices behave as discrete point-like particles. This thesis presents the results of experiments on the distribution of quantised vortices in highly oblate, uniform Bose-Einstein condensates. Initially randomly distributed vortices were observed to rearrange themselves to become more ordered, resembling Onsager’s description of large-scale flow features. These results add to our current understanding of the physical processes involved in turbulent flows of superfluids.

Haljan, ���������������������������������������������������������������������������������������� Anderson, Brian Matthews +3 more