Critical exponents in metastable decay via quantum activation.

We consider decay of metastable states of forced vibrations of a quantum oscillator close to the bifurcation points where the states disappear. Decay occurs via quantum activation over a quasienergy barrier, a mechanism that differs from both tunneling and thermal activation. The decay probability W scales with the distance eta to the bifurcation point as /ln W/ proportional to eta(xi). The exponent xi is found for a resonantly driven oscillator and an oscillator modulated at nearly twice its eigenfrequency.