Two-dimensional tunneling in a SQUID

Traditionally quantum tunneling in a superconducting quantum interference device (SQUID) is studied on the basis of a classical trajectory in imaginary time under a two-dimensional potential barrier. The trajectory connects a potential well and an outer region crossing their borders in perpendicular directions. In contrast to that main-path mechanism, a wide set of trajectories with components tangent to the border of the well can constitute an alternative mechanism of multipath tunneling. The phenomenon is essentially nonone-dimensional. Continuously distributed paths under the barrier result in enhancement of tunneling probability. A type of tunneling mechanism (main path or multipath) depends on character of a state in the potential well prior to tunneling. A temperature dependence of the tunneling probability in a very asymmetric (different capacitances) SQUID has a finite slope at zero temperature. A transition between thermally assisted tunneling and pure activation can be not smooth depending on current through a very asymmetric SQUID.