Asymmetric Long Josephson Junction Acting as a Ratchet for a Quantum Field

We study the escape rate of flux quanta in a long Josephson junction having an asymmetric spatial inhomogeneous critical current density. Such a junction can behave as a ratchet when driven by an ac current in the presence of a magnetic field. This rectification gives rise to a dc voltage V_dc across the junction. The usual approach of particlelike tunneling cannot describe this rectification, and a quantum field theory description is required. We also show that, under specific conditions, the rectification direction, and consequently V_dc, can change its sign when varying the temperature T near the crossover temperature T^* between the quantum and classical regimes.