Resumen: 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. Idioma: Inglés DOI: 10.1103/PhysRevB.93.214501 Año: 2016 Publicado en: Physical Review B. Condensed Matter and Materials Physics 93, 21 (2016), 214501 [8 pp] ISSN: 1098-0121 Factor impacto JCR: 3.836 (2016) Categ. JCR: PHYSICS, CONDENSED MATTER rank: 18 / 67 = 0.269 (2016) - Q2 - T1 Factor impacto SCIMAGO: 2.339 - Electronic, Optical and Magnetic Materials (Q1) - Condensed Matter Physics (Q1)