  Understanding the interaction between light and matter is very relevant for
fundamental studies of quantum electrodynamics and for the development of
quantum technologies. The quantum Rabi model captures the physics of a single
atom interacting with a single photon at all regimes of coupling strength. We
report the spectroscopic observation of a resonant transition that breaks a
selection rule in the quantum Rabi model, implemented using an $LC$ resonator
and an artificial atom, a superconducting qubit. The eigenstates of the system
consist of a superposition of bare qubit-resonator states with a relative sign.
When the qubit-resonator coupling strength is negligible compared to their own
frequencies, the matrix element between excited eigenstates of different sign
is very small in presence of a resonator drive, establishing a sign-preserving
selection rule. Here, our qubit-resonator system operates in the ultrastrong
coupling regime, where the coupling strength is 10\% of the resonator
frequency, allowing sign-changing transitions to be activated and, therefore,
detected. This work shows that sign-changing transitions are an unambiguous,
distinctive signature of systems operating in the ultrastrong coupling regime
of the quantum Rabi model. These results pave the way to further studies of
sign-preserving selection rules in multiqubit and multiphoton models.
