Ultrastrong coupling in two-resonator circuit QED
  We report on ultrastrong coupling between a superconducting flux qubit and a resonant mode of a system comprised of two superconducting coplanar stripline resonators coupled galvanically to the qubit. With a coupling strength as high as 17% of the mode frequency, exceeding that of previous circuit quantum electrodynamics experiments, we observe a pronounced Bloch-Siegert shift. The spectroscopic response of our multimode system reveals a clear breakdown of the Jaynes-Cummings model. In contrast to earlier experiments, the high coupling strength is achieved without making use of an additional inductance provided by a Josephson junction. 
A. Baust, E. Hoffmann, M. Haeberlein, M. J. Schwarz, P. Eder, J.   Goetz, F. Wulschner, E. Xie, L. Zhong, F. Quijandria, D. ...
cond-mat.mes-hall quant-ph