Ratchets are periodic structures lacking spatial inversion symmetry. Despite the asymmetry, no preferential motion occurs for particles moving in a ratchet potential and being only subject to equilibrium thermal noise, in agreement with the second law of thermodynamics. However, directed motion can occur under the combined effect of thermal noise and unbiased driving forces.
We investigate this so-called ratchet effect for Brownian particles moving in one-dimensional ratchet structures. We focus on temperatures such that tunnelling is the dominant transport mechanism. We discuss the occurrence of current reversals as the parameters of the bath or of the driving force are varied.
We finally report on the recent experimental observation of the quantum ratchet effect for vortices in quasi one-dimensional arrays of Josephson junctions.