  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.
