Virtual electron diffusion during quantum tunneling of the electric charge

We calculate the rate of the elastic macroscopic quantum tunneling of the electric charge (q-MQT) in a system of two small-area, normal, tunnel junctions in the Coulomb blockade regime. Despite the fact that the intermediate electron state on the central electrode of the system during the tunneling is virtual, the rate of the q-MQT depends crucially on the character of real electron motion through this electrode. Typically this motion is diffusive, so that the tunneling rate is determined by the process of ‘‘virtual diffusion’’ of electrons on the time scale of the inverse Coulomb energy of the system, ħ/${\mathit{E}}_{\mathit{C}}$.