Photon-assisted Cooper-pair tunneling in a superconducting single-electron transistor has been used to investigate the energy gap between two charge states. We observed a symmetric-antisymmetric energy-level splitting between two charge statescoherently superposed by Josephson coupling. A mixing effect between two microwaves with different frequencies was observed by experiment. This experiment in principle can be used for observing photo-induced Rabi splitting between two charge states.
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