Abstract
Particles in a ratchet, that is, a potential without spatial inversion symmetry, can move in one direction even in the absence of macroscopic forces, provided that there is a source of energy. In this paper, a quantum ratchet, based on an asymmetric (triangular) quantum dot, is investigated experimentally and theoretically. We find that coherent electron transport through such a device depends on the sign of the applied voltage. In this way a net current can be obtained even when the applied ac voltage is zero on average. Strikingly, the direction of the current depends on the amplitude at which the quantum dot ratchet is rocked.