  We report an experimental investigation of momentum diffusion in the
delta-function kicked rotor where time symmetry is broken by a two-period
kicking cycle and spatial symmetry by an alternating linear potential. We
exploit this, and a technique involving a moving optical potential, to create
an asymmetry in the momentum diffusion that is due to the classical chaotic
diffusion. This represents a realization of a type of Hamiltonian quantum
ratchet.
