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