  In a quantum ratchet accelerator system, a linearly increasing directed
current can be dynamically generated without using a biased field. Generic
quantum ratchet acceleration with full classical chaos [Gong and Brumer, Phys.
Rev. Lett. 97, 240602 (2006)] constitutes a new element of quantum chaos and an
interesting violation of a sum rule of classical ratchet transport. Here we
propose a simple quantum ratchet accelerator model that can also generate
linearly increasing quantum current with full classical chaos. This new model
does not require a bichromatic lattice potential. It is based on a variant of
an on-resonance kicked-rotor system, periodically kicked by two optical lattice
potentials of the same lattice constant, but with unequal amplitudes and a
fixed phase shift between them. The dependence of the ratchet current
acceleration rate on the system parameters is studied in detail. The cold-atom
version of our new quantum ratchet accelerator model should be realizable by
introducing slight modifications to current cold-atom experiments.
