Quantum transitions of a small Josephson junction array

We have experimentally studied a small Josephson junction array in the presence of microwave irradiation. The array has comparable energy scales for single-charge effects and the Josephson effect, resulting in a discrete set of macroscopic eigenenergy levels. Excitation of the array by low-power microwaves is possible at frequencies where the photon energy matches the level spacing. The microwave frequency and amplitude dependence show that the excitation mechanism involves resonant quantum coherent dynamics of the array.