Soliton Project: Research.

We are currently developing a new experiment in Durham to form Bose-Einstein condensates using Rb-85. This isotope has a broad Feshbach resonance, permitting the controlled modification of the interactions in the condensate. However, evaporative cooling of this species to quantum degeneracy is complicated by the presence of high inelastic collision rates. We plan to alleviate this problem by implementing some of the latest advances in laser and evaporative cooling. Atomic interactions are responsible for much of the richness of BEC physics. The effects of interactions may be modelled in a mean-field treatment by a term proportional to the condensate density and the s-wave scattering length, as.

Feshbach resonances enable the experimentalist to modify the scattering length and therefore offers unprecedented control of the condensate wavefunction. Such resonances arise when the energy of a pair of colliding atoms matches that of a quasi-bound molecular state. Variation of an external magnetic field shifts the molecular state into and out of resonance with the atomic state leading to dramatic variations in the scattering length.

To gain familiarity with the Feshbach resonance and to develop techniques to load condensates into a range of optical potentials, we plan initially to study the formation of bright matter wave solitons. Such solitons may be produced from a BEC by using attractive interactions to compensate the usual dispersion of finite wavepackets by switching from a positive to a negative scattering length.

Soliton solutions to the GPE exist in the quasi-1D regime, where the radial harmonic oscillator state spacing hwr is larger than the mean-field energy. The equation for a propagating soliton is then

where lz = 2h(m|g1D|N).

The immediate plan for the experiment in Durham is to investigate the relationship between soliton formation and condensate collapse. The next step will be to characterise the lifetime, coherence, and phase evolution of solitons. It may be possible to then engineer soliton collisions to probe the interactions between solitons. Ultimately we aim to use the solitons to measure atom-surface interactions and to demonstrate quantum reflection.

Content © Simon L. Cornish, Durham University 2007