Evidence of two-dimensional macroscopic quantum tunneling of a current-biased DC-SQUID

The escape probability out of the superconducting state of a hysteretic DC-SQUID has been measured at different values of the applied magnetic flux. At low temperature, the escape current and the width of the probability distribution are temperature independent but they depend on flux. Experimental results do not fit the usual one-dimensional (1D) Macroscopic Quantum Tunneling (MQT) law but are perfectly accounted for by the two-dimensional (2D) MQT behaviour as we propose here. Near zero flux, our data confirms the recent MQT observation in a DC-SQUID \cite{Li02}.