A circuit that beats Jaynes–Cummings model

In circuit quantum electrodynamics1–10 (QED), where superconducting
artificial atoms are coupled to on-chip cavities, the exploration of fundamental quantum physics in the strongcoupling 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 model11. Here, we report on the first experimental realization of a circuit QED system operating in the ultrastrong-coupling limit12,13, 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 coupling14 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. [DOI: 10.1038/NPHYS1730]