  We investigate experimentally a two-dimensional rocking ratchet for cold
atoms, realized by using a driven three-beam dissipative optical lattice. AC
forces are applied in perpendicular directions by phase-modulating two of the
lattice beams. As predicted by the general theory [S. Denisov et al., Phys.
Rev. Lett. 100, 224102 (2008)], we observe a rectification phenomenon unique to
high-dimensional rocking ratchets, as determined by two single-harmonic
drivings applied in orthogonal directions. Also, by applying two bi-harmonic
forces in perpendicular directions, we demonstrate the possibility of
generating a current in an arbitrary direction within the optical lattice
plane.
