Quantum transitions of a small Josephson junction array

We have experimentally studied a small Josephson junction array in the pres ence of microwave irradiation. The array has comparable energy scales for s ingle-charge effects and the Josephson effect, resulting in a discrete set of macroscopic eigenenergy levels. Excitation of the array by low-power mic rowaves is possible at frequencies where the photon energy matches the leve l spacing. The microwave frequency and amplitude dependence show that the e xcitation mechanism involves resonant quantum coherent dynamics of the arra y. (C) 2000 Elsevier Science B.V. All rights reserved.