Multiphoton transitions in a macroscopic quantum two-state system
  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. 
S. Saito, M. Thorwart, H. Tanaka, M. Ueda, H. Nakano, K. Semba, and H.   Takayanagi