Observation of phase diffusion in a two-dimensional SQUID potential

We report experiments detecting two different escape mechanisms of a dc-SQUID from the zero voltage to the gap voltage state. The two underdamped Josephson junctions of the SQUID have the same critical current but different shunting capacitors, corresponding to a model of a virtual particle with spatially anisotropic mass moving in a two-dimensional (2D) potential. The measurements show the coexistence of escape either directly from the superconducting to the gap voltage state or from the phase diffusion regime. The latter is dominated by repeated escape and retrapping events and an increased dissipation. This coexistence is explained qualitatively by analyzing the shape of the 2D potential as well as the effect of the anisotropic mass of the phase particle. Using numerical simulations, the motion of the particle in the two different regimes is examined and it is shown that the anisotropy in mass results in anisotropic dissipation. Measured and simulated switching current histograms and current-voltage characteristics are presented. A qualitative agreement between simulation and measurement is obtained, indicating simultaneously present escape by phase diffusion and the conventional switching from the superconducting state.