  Physical properties of single-electron devices in the week Coulomb blockade
regime are significantly dependent on non-perturbative effects. They arise as
instanton solutions of equations of motion for the corresponding
Ambegaokar-Eckern-Schoen action. In equilibrium those solutions are known as
Korshunov instantons. In this paper we study non-equilibrium Ambegaokar-Eckern-
Schoen action using Keldysh technique. We found instantons for the most general
stationary out-of equilibrium state. We also found that action saddle-point
value assumes a universal value irrespective of the stationary non-equilibrium
state.
