  We have observed multiphoton transitions between two macroscopic
quantum-mechanical superposition states formed by two opposite circulating
currents in a superconducting loop with three Josephson junctions. Resonant
peaks and dips of up to three-photon transitions were observed in spectroscopic
measurements when the system was irradiated with a strong RF-photon field. The
widths of the multiphoton absorption dips are shown to scale with the Bessel
functions in agreement with theoretical predictions derived from the Bloch
equation or from a spin-boson model.
