Circuit quantum electrodynamics in the ultrastrong-coupling regime

In circuit quantum electrodynamics (QED), where superconducting artificial atoms are coupled to on-chip cavities, the exploration of fundamental quantum physics in 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. Here, we report on the first experimental realization of a circuit QED system operating in the ultrastrong-coupling limit, where the atom-cavity coupling rate g reaches a considerable fraction of the cavity transition frequency ‰ _r . Furthermore, we present direct evidence for the breakdown of the Jaynes-Cummings model. We reach remarkable normalized coupling rates g/ ‰ _r of up to 12% by enhancing the inductive coupling of a flux qubit to a transmission line resonator. Our circuit extends the toolbox of quantum optics on a chip towards exciting explorations of ultrastrong light-matter interaction.