Nonlinear effects for Bose-Einstein condensates in optical lattices

We present our experimental investigations on the subject of dynamical nonlinearity-induced instabilities and of nonlinear Landau-Zener tunneling between two energy bands in a rubidium Bose-Einstein condensate in an accelerated periodic potential. These two effects may be considered two different regimes (for small and large acceleration) of the same physical system and studied with the same experimental protocol. Nonlinearity introduces an asymmetry in Landau-Zener tunneling; as a result, tunneling from the ground state to the excited state is enhanced, whereas in the opposite direction it is suppressed. When the acceleration is lowered, the condensate exhibits an unstable behavior due to nonlinearity. We also carried out a full numerical simulation of both regimes by integrating the full Gross-Pitaevskii equation; for the Landau-Zener effect, we also used a simple two-level model. In both cases we found good agreement with the experimental results.