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.