Professor Christopher Foot

Abstract:

We report the first experimental observation of Beliaev coupling between collective excitations of a Bose-Einstein condensed gas. Beliaev coupling is not predicted by the Gross-Pitaevskii equation and so this experiment tests condensate theory beyond the mean field approximation. Measurements of the amplitude of a high frequency scissors mode show that the Beliaev process transfers energy to a lower-lying mode and then back and forth between these modes, unlike Landau processes which lead to a monotonic decrease in amplitude. To enhance the Beliaev process we adjusted the geometry of the magnetic trapping potential to give a frequency ratio of 2 to 1 between the two scissors modes.

Abstract:

We report on the realization of a stiff magnetic trap with independently adjustable trap frequencies, ω-z$/ and ω-r$/ in the axial and radial directions, respectively. This has been achieved by applying an axial modulation to a time-averaged orbiting potential (TOP) trap. The frequency ratio of the trap, ω-z$//ω-r$/, can be decreased continuously from the original TOP trap value of 2.83 down to 1.6. We have transferred a Bose-Einstein condensate (BEC) into this trap and obtained very good agreement between its observed anisotropic expansion and the hydrodynamic predictions. Our method can be extended to obtain a spherical trapping potential, which has a geometry of particular theoretical interest.

Abstract:

A novel dipole force trap was developed for trapping caesium atoms in the lower hyperfine level of the ground state and in preliminary experiments a cloud of ≈1000 atoms was trapped. The viability of the trap is discussed, including suggestions for amendments to the trap design and problems that might occur which are intrinsic to the caesium atom.