Ultrastrong coupling in two-resonator circuit QED
A. Baust, Eric A. Hoffmann, M. Haeberlein, M. J. Schwarz, Peter Eder, Jan Goetz, F. Wulschner, E. Xie, Ling Zhong, Fernando Quijandría, David Zueco, J.-J. García Ripoll, Laura García-Álvarez, Guillermo Romero, E. Solano, Kirill G. Fedorov, E. P. Menzel, Frank Deppe, Achim Marx, R. Gross · Physical review. B./Physical review. B · 2016
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 $17.5%$ 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 approximation. In contrast to earlier experiments, the high coupling strength is achieved without making use of an additional inductance provided by a Josephson junction.