Dissipation enhanced asymmetric transport in quantum ratchets

Quantum mechanical motion of a particle in a periodic asymmetric potential is studied theoretically at zero temperature. It is shown based on a semi-classical approximation that the tunneling probability from one local minimum to the next becomes asymmetric in the presence of a weak oscillating field even though there is no average macroscopic field gradient. Dissipation enhances this asymmetry, and leads to a steady unidirectional current, resulting in a quantum ratchet system.