Photon number splitting is observed in a transmon coupled to a superconducting quasi-lumped-element resonator in the strong dispersive limit. A thermal population of 5.474 GHz photons at an effective resonator temperature of T = 120mK results in a weak n = 1 photon peak along with the n = 0 photon peak in the qubit spectrum in the absence of a coherent drive on the resonator. Two-tone spectroscopy using independent coupler and probe tones reveals an Autler-Townes splitting in the thermal n = 1 photon peak.
The observed effect is explained accurately using the four lowest levels of the dispersively dressed qubit-resonator system and compared to results from numerical simulations of the steady-state master equation for the coupled system.

Observation of Autler-Townes effect in a dispersively dressed Jaynes-Cummings system

(a) A lumped element resonator (blue) and transmon qubit (red) are coupled to a coplanar waveguide transmission line (violet) and surrounded by a perforated ground plane (white). The resonator consists of a meandering inductor and an interdigitated capacitor. The transmon has two Josephson junctions in parallel the allow the transition frequency to be tuned with an external magnetic field and an on-chip flux bias (green). (b) Detailed view of transmon’s Josephson junctions and flux bias line.