Measurement of the effect of moderate dissipation on macroscopic quantum tunneling

The escape rate {Γ}(T) from the zero voltage state has been measured for a current-biased Josephson tunnel junction shunted with a normal metal resistor. Values of {Γ}(T) measured at the lowest temperature of the experiment, 18 mK, and extrapolated to T=0 are in excellent agreement with the predictions for macroscopic quantum tunneling that include a reduction by a factor of about 300 for the effect of dissipation. Above the crossover temperature, the enhancement of {Γ}(T) over the rate for thermal activation is in good agreement with predicted quantum corrections.