What was the key experiment conducted by Michel H. Devoret, John Clarke, and John M. Martinis at Berkeley?

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In the 1980s Michel H. Devoret, along with mentor John Clarke and postdoctoral researcher John M. Martinis, carried out an experiment that provided direct evidence of macroscale quantum behavior, indicated by a measurable voltage. The voltage resulted from a transition in energy that occurred as electrons underwent macroscopic quantum tunneling from a zero-voltage state.