Two-dimensional rocking ratchet for cold atoms

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, Y. Zolotaryuk, S. Flach, and O. Yevtushenko, 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 biharmonic forces in perpendicular directions, we demonstrate the possibility of generating a current in an arbitrary direction within the optical lattice plane.