We study numerically the onset of higher-level excitations and resonance frequency shifts in the generalized multilevel Rabi model with dispersive coupling under strong driving. The response to a weak probe is calculated using the Floquet method, which allows us to calculate the probe spectrum and extract the resonance frequency. We test our predictions using a superconducting circuit consisting of transmon coupled capacitively to a coplanar waveguide resonator. This system is monitored by a weak probe field, and at the same time driven at various powers by a stronger microwave tone. We show that the transition from the quantum to the classical regime is accompanied by a rapid increase of the transmon occupation and, consequently that the qubit approximation is valid only in the extreme quantum limit.
@article{arxiv.1710.00588,
title = {Multilevel effects in a driven generalized Rabi model},
author = {I. Pietikäinen and S. Danilin and K. S. Kumar and J. Tuorila and G. S. Paraoanu},
journal= {arXiv preprint arXiv:1710.00588},
year = {2018}
}