Multi-Photon, Multi-Level Dynamics in a Superconducting   Persistent-Current Qubit
  Single-, two-, and three-photon transitions were driven amongst five quantum states of a niobium persistent-current qubit. A multi-level energy-band diagram was extracted using microwave spectroscopy, and avoided crossings were directly measured between the third and fourth excited states. The energy relaxation times between states connected by single-photon and multi-photon transitions were approximately 30 - 100 microseconds. Three-photon coherent oscillations were observed between the qubit ground and fourth excited states with a decoherence time of approximately 50 nanoseconds. 
Y. Yu, W. D. Oliver, J. C. Lee, K. K. Berggren, L. S. Levitov, and T.   P. Orlando