%0 Journal Article
%T Broken selection rule in the quantum Rabi model
%A P. Forn-D¨Şaz
%A G. Romero
%A C. J. P. M. Harmans
%A E. Solano
%A J. E. Mooij
%J Physics
%D 2015
%I arXiv
%X 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 a superconducting qubit. The eigenstates of the system consist of a superposition of bare qubit-oscillator states with a relative sign. In the limit of low qubit-oscillator coupling strength, the matrix element between excited eigenstates of different sign is very small in presence of an oscillator 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.
%U http://arxiv.org/abs/1510.03379v1