Steering Bose-Einstein condensates despite time symmetry
  A Bose-Einstein condensate in an oscillating spatially asymmetric potential is shown to exhibit a directed current for unbiased initial conditions despite time symmetry. This phenomenon occurs only if the interaction between atoms, treated in mean-field approximation, exceeds a critical value. Our findings can be described with a three-mode model (TMM). These TMM results corroborate well with a many-body study over a time scale which increases with increasing atom number. The duration of this time scale probes the validity of the used mean-field approximation. 
Dario Poletti, Giuliano Benenti, Giulio Casati, Peter Hanggi, Baowen   Li