Multiphoton Antiresonance and Quantum Activation in Driven Systems

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We show that nonlinear response of a quantum oscillator displays antiresonant dips and resonant peaks with varying frequency of the driving field. The effect is a consequence of special symmetry and is related to resonant multiphoton mixing of several pairs of oscillator states at a time. We also discuss escape from a metastable state of forced vibrations. Two important examples show that the probability of escape via diffusion over quasienergy is larger than via dynamical tunneling provided the relaxation rate exceeds both of them. Diffusion dominates even for zero temperature, so that escape occurs via quantum rather than thermal activation. The effects can be studied using Josephson junctions and Josephson-junction based systems.