Macroscopic quantum mechanics: experiments on Josephson junctions

Abstract We present evidence of macroscopic quantum effects in Josephson junction at temperature well above the classical-quantum crossover temperature. The clear observation of quantum effects up to high temperatures has been obtained by measuring the tunneling rate out of a metastable state while the external energy was changing with time. Thermal mixing of eingenstates does not destroy quantum effects if the rate of change of the external energy is fast with respect to the rate of thermally induced energy transitions between levels. In our experiments this condition has been fulfilled by a suitable choice of the sweeping frequency of the external bias.