Phase-Diffusion in Switching Process of Underdamped Josephson Junctions

We measured the switching from a superconducting state to a finite voltage state in a dc SQUID, which is equivalent to a single Josephson junction with tunable critical current I_c. The width and the mean of the switching current distribution depend on the effective temperature T_eff and I_c. The phase-diffusion was observed by investigating the switching current distribution as a function of T_eff. It is found that the crossover from the thermal activation to the phase-diffusion scaling with the ratio of I_c and T_eff, clarifying the physical picture of the phase-diffusion. We developed an analytical multi-retrapping model which can address the experimental results very well.