  The interaction between an atom and the electromagnetic field inside a cavity
has played a crucial role in the historical development of our understanding of
light-matter interaction and is a central part of various quantum technologies,
such as lasers and many quantum computing architectures. The emergence of
superconducting qubits has allowed the realization of strong and ultrastrong
coupling between artificial atoms and cavities. If the coupling strength $g$
becomes as large as the atomic and cavity frequencies ($\Delta$ and
$\omega_{\rm o}$ respectively), the energy eigenstates including the ground
state are predicted to be highly entangled. This qualitatively new regime can
be called the deep strong-coupling regime, and there has been an ongoing debate
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 $g/\omega_{\rm o}$ ranging
from 0.72 to 1.34 and $g/\Delta\gg 1$. Using spectroscopy measurements, we have
observed unconventional transition spectra, with patterns resembling masquerade
masks, 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.
