Observation of transition from escape dynamics to underdamped phase diffusion in a Josephson junction

We have investigated the dynamics of underdamped Josephson junctions with a relatively small Josephson coupling E-J. In addition to the usual crossover between macroscopic quantum tunneling and thermally activated (TA) behavior we observe, for the first time, the transition from TA behavior to underdamped phase diffusion. Above the crossover temperature, the threshold for switching into the finite-voltage state becomes extremely sharp. We propose a (T, E-J) phase diagram with various regimes and show that for a proper description of it, dissipation and level quantization in a metastable well are crucial.