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Under the terms of the Creative Commons Attribution license.-- et al. ; 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. ©2016 American Physical Society ; Thiswork is supported by the German Research Foundation through SFB 631 and FE 1564/1-1; Spanish MINECO FIS2012-36673-03-02, MAT2014-53432-C5-1-R, FIS2014- 55867-P and FIS2012-33022; CAM Research Network QUITEMAD+; UPV/EHU UFI 11/55, UPV/EHU PhD Grant, and Basque Government IT472-10; the Fondo Nacional de Desarrollo Cientifico y Tecnológico (FONDECYT, Chile) under Grant 1150653; the EU projects CCQED, PROMISCE, and SCALEQIT. We further acknowledge GEFENOL. ; Peer Reviewed

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).-- et al. ; We realize a device allowing for tunable and switchable coupling between two frequency-degenerate superconducting resonators mediated by an artificial atom. For the latter, we utilize a persistent current flux qubit. We characterize the tunable and switchable coupling in the frequency and time domains and find that the coupling between the relevant modes can be varied in a controlled way. Specifically, the coupling can be tuned by adjusting the flux through the qubit loop or by controlling the qubit population via a microwave drive. Our measurements allow us to find parameter regimes for optimal coupler performance and quantify the tunability range. ; This work is supported by the German Research Foundation through SFB 631, Spanish MINECO FIS2012-36673-C03-02; UPV/EHU UFI 11/55; Basque Government IT472-10; CCQED, PROMISCE, and SCALEQIT EU projects. B.P. acknowledges support from the STC Center for Integrated Quantum Materials, NSF Grant No. DMR-1231319. ; Peer reviewed