This paper reports the first experimental realization of a superconducting circuit QED system in the ultrastrong coupling regime, where the normalized coupling rate \( g/\omega_r \) reaches up to 12%. The authors achieve this by enhancing the inductive coupling of a flux qubit to a transmission line resonator using the nonlinear inductance of a Josephson junction. They present direct evidence for the breakdown of the Jaynes-Cummings model, which describes the coherent exchange of a single excitation between the atom and the cavity mode. In the ultrastrong coupling regime, counterrotating terms in the Hamiltonian become significant, leading to phenomena such as anticrossings in the transmission spectra. These anticrossings are attributed to the simultaneous creation or annihilation of two excitations in the qubit and the resonator, respectively, and are not predicted by the Jaynes-Cummings model. The results demonstrate the potential of superconducting circuits for exploring ultrastrong light-matter interactions and open up new avenues for future research, including squeezing, switchable ultrastrong coupling, and the generation of bound states of qubits and photons.This paper reports the first experimental realization of a superconducting circuit QED system in the ultrastrong coupling regime, where the normalized coupling rate \( g/\omega_r \) reaches up to 12%. The authors achieve this by enhancing the inductive coupling of a flux qubit to a transmission line resonator using the nonlinear inductance of a Josephson junction. They present direct evidence for the breakdown of the Jaynes-Cummings model, which describes the coherent exchange of a single excitation between the atom and the cavity mode. In the ultrastrong coupling regime, counterrotating terms in the Hamiltonian become significant, leading to phenomena such as anticrossings in the transmission spectra. These anticrossings are attributed to the simultaneous creation or annihilation of two excitations in the qubit and the resonator, respectively, and are not predicted by the Jaynes-Cummings model. The results demonstrate the potential of superconducting circuits for exploring ultrastrong light-matter interactions and open up new avenues for future research, including squeezing, switchable ultrastrong coupling, and the generation of bound states of qubits and photons.