A cold-atom ratchet interpolating between classical and quantum dynamics
  We study the crossover between classical and quantum dynamics by observing the behavior of a quantum ratchet created by exposing a Bose-Einstein condensate to short pulses of a potential which is periodic in both space and time. Such a ratchet is manifested by a directed current of particles, even though there is an absence of a net bias force. We confirm that the ratchet behavior can under certain circumstances be the same in both regimes. We demonstrate that this behavior can be understood using a single variable containing many of the experimental parameters and thus the ratchet current is describable using a single universal scaling law. 
R. K. Shrestha, W. K. Lam, J. Ni, and G. S. Summy
quant-ph cond-mat.quant-gas