Why macroscopic quantum tunnelling in Josephson junctions differs from tunnelling of a quantum particle

We show that the macroscopic quantum tunnelling of a fluxon in a Josephson junction cannot be described, even qualitatively, as the tunnelling of a quantum particle in a potential U(rho), where the phase difference plays the role of particle position, if the length of the junction d exceeds a fluxon length. We calculate the probability per unit time of tunnelling (or escape rate), G, which has a form Gamma= Aexp(- B). In contrast to particles, where the B is proportional to d, our field-theory predicts a di. fferent behavior of B for either usual, 0- pi, or stacks of Josephson junctions, giving rise to a renormalization of G by many orders of magnitude. Copyright (C) EPLA, 2007.