Thermally activated escape from the zero-voltage state in long Josephson junctions

We have measured the rate of thermally induced escape from the zero-voltage state in long Josephson junctions of both overlap and in-line geometry as a function of applied magnetic field. The statistical distribution of switching currents is used to evaluate the escape rate and derive an activation energy $\Delta U$ for the process. Because long junctions correspond to the continuum limit of multidimensional systems, $\Delta U$ is in principle the difference in energy between stationary states in an infinite-dimensional potential. We obtain good agreement between calculated and measured activation energies for junctions with lengths a few times the Josephson penetration depth ${\lambda }_J$.