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.
@article{arxiv.1412.7372,
title = {Ultrastrong coupling in two-resonator circuit QED},
author = {A. Baust and E. Hoffmann and M. Haeberlein and M. J. Schwarz and P. Eder and J. Goetz and F. Wulschner and E. Xie and L. Zhong and F. Quijandria and D. Zueco and J. -J. Garcia Ripoll and L. Garcia-Alvarez and G. Romero and E. Solano and K. G. Fedorov and E. P. Menzel and F. Deppe and A. Marx and R. Gross},
journal= {arXiv preprint arXiv:1412.7372},
year = {2016}
}