Supercurrent decay in extremely underdamped Josephson junctions

We present an experimental study of the effective dissipation relevant in the thermally activated supercurrent decay of extremely underdamped Josephson junctions. Data referring to the supercurrent decay of Nb/AlOx /Nb Josephson junctions are compared with the Kramers theory. Our measurements allow us to obtain the ''effective'' resistance to be used in the resistively shunted junction model that results to be the subgap resistance due to the presence of thermally activated quasiparticles. The extremely low dissipation level obtained at low temperatures renders our result quite interesting in view of experiments in the quantum limit.