Many-body quantum ratchet in a Bose-Einstein condensate
  We study the dynamics of a dilute Bose-Einstein condensate confined in a toroidal trap and exposed to a pair of periodically flashed optical lattices. We first prove that in the noninteracting case this system can present a quantum symmetry which forbids the ratchet effect classically expected. We then show how many-body atom-atom interactions, treated within the mean-field approximation, can break this quantum symmetry, thus generating directed transport. 
Dario Poletti, Giuliano Benenti, Giulio Casati and Baowen Li