Superconducting qubit-oscillator circuit beyond the ultrastrong-coupling regime

The interaction between an atom and the electromagnetic field inside a cavity(1-6) has played a crucial role in developing our understanding of light-matter interaction, and is central to various quantum technologies, including lasers and many quantum computing architectures. Superconducting qubites(7,8) have allowed the realization of strong(9,10) and ultrastrong(11-13) coupling between artificial atoms and cavities. If the coupling strength g becomes as large as the atomic and cavity frequencies (A and omega(of) respectively), the energy eigenstates including the ground state are predicted to be highly entangled(14). There has been an ongoing debate(15-17) over whether it is fundamentally possible to realize this regime in realistic physical systems. By inductively coupling a flux qubit and an LC oscillator via Josephson junctions, we have realized circuits with g/omega(o)), ranging from 0.72 to 1.34 and g/Delta >>1. Using spectroscopy measurements, we have observed unconventional transition spectra that are characteristic of this new regime. Our results provide a basis for ground-state-based entangled pair generation and open a new direction of research on strongly correlated light-matter states in circuit quantum electrodynamics.