Extreme multiphoton phenomena in Josephson junctions: Euclidean resonance

Decay of a zero-voltage state of a Josephson junction at low temperature occurs via quantum tunneling through an effective potential barrier. An extremely small probability of quantum tunneling may become not very small under the action of an ac component of the bias current. The tunneling rate has a peak as a function of a dc component of the bias current (Euclidean resonance). An analysis of this extremely multiphoton process is done on the basis of classical trajectories in imaginary time. The studied phenomenon does not involve transitions between energy levels and, thus, is distinctly different from the well-known process of photon-assisted tunneling.