Thermal enhancement of macroscopic quantum tunneling: Derivation from noise theory

We recompute the thermal enhancement of quantum tunneling using a noise-theory derivation. This calculation is applied to the macroscopic quantum tunneling out of the zero-voltage state of a current-biased Josephson junction. The rate enhancement for T much less than the crossover temperature is shown to be due to the thermal current noise generated at low frequencies by the shunt admittance across the junction. The results of this theory are found to be in perfect agreement with earlier findings based on more formal functional integral techniques. The method is applied to circuits with dissipative elements of arbitrary frequency dependence. The method also gives the first analytical predictions for the effect of the prefactor on the rate enhancement