  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.
