We couple a ladder-type three-level superconducting artificial atom to a cavity. Adjusting the artificial atom to make the cavity be resonant with the two upper levels, we then probe the lower two levels of the artificial atom. When driving the cavity to a coherent state, the probe spectrum shows energy level splitting induced by the quantized electromagnetic field in the cavity. This splitting size is related to the coupling strength between the cavity and the artificial atom and, thus, is fixed after the sample is fabricated. This is in contrast to the classical Autler–Townes splitting of a three-level system in which the splitting is proportional to the driving amplitude, which can be continuously changed. Our experiment results show the difference between the classical microwave driving field and the quantum field of the cavity.