Circuit quantum electrodynamics in the ultrastrong-coupling regime

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In circuit quantum electrodynamics (QED), where superconducting artificial atoms are coupled to on-chip cavities, the strong-coupling regime has greatly evolved. In this regime, an atom and a cavity can exchange a photon frequently before coherence is lost. Nevertheless, all experiments so far are well described by the renowned Jaynes–Cummings model. In this work, the group at the Walter Meissner Institut in Garching, Germany collaborated closely with the SOLID theorist E. Solano to report on the first experimental realization of a circuit QED system operating in the ultrastrong-coupling limit, where the atom–cavity coupling rate reaches a considerable fraction of the cavity transition frequency. The results contain direct evidence for the breakdown of the Jaynes–Cummings model and remarkable normalized coupling rates g/ωr of up to 12% are reached by enhancing the inductive coupling of a flux qubit to a transmission line resonator. The circuit developed extends the toolbox of quantum optics on a chip towards exciting explorations of ultrastrong light–matter interaction.

T. Niemczyk, F. Deppe, H. Huebl, E. P. Menzel, F. Hocke, M. J. Schwarz, J. J. García-Ripoll, D. Zueco, T. Hümmer, E. Solano, A. Marx, and R. Gross
Nature Physics 6, 772–776 (2010)