Ultrastrong coupling in two-resonator circuit QED
- Baust, A.
- Hoffmann, E.
- Haeberlein, M.
- Schwarz, M. J.
- Eder, P.
- Goetz, J.
- Wulschner, F.
- Xie, E.
- Zhong, L.
- Quijandria Diaz, Isaac Fernando
- Zueco, D.
- Ripoll, J. J. G.
- Garcia-Alvarez, L.
- Romero, G.
- Solano, E.
- Fedorov, K. G.
- Menzel, E. P.
- Deppe, F.
- Marx, A.
- Gross, R.
DOIAbstract
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
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