Effects of energy-level quantization on the supercurrent decay of Josephson junctions

We present measurements of the supercurrent decay in high quality Nb/AlOx/Nb Josephson junctions which show evidence of energy-level quantization within the washboard potential describing the junction. The effect of discrete energy levels leads to oscillations in the rate of escape from the zero voltage state as a function of the bias current, observed at different temperatures. The oscillation spacing fits that expected from the energy level quantization. Moreover, a fit with the quantum theory allows us to derive interesting conclusions about the effective dissipation of the system at low temperatures.