(color online) (a) S₂₁ measurement results with increasing driving field amplitude for the resonator. The Xmon is biased to satisfy the resonance condition that . Continuous microwave is applied to resonantly drive the λ/4 coplanar waveguide resonator, preparing it in the coherent state. Weak sweeping probe microwave is injected from port 1. As the driving power increases and the average number of photons in the resonator increases, the central peak at frequency weakens, and two side peaks appear beside the central peak. The central peak of the S₂₁ spectrum corresponds to zero photon in the resonator, while the two side peaks correspond to one photon in the resonator. Because of the resonance coupling of the resonator and the two upper levels of Xmon, as shown in Fig. 5, there is an energy level splitting induced by this resonance single photon. (b) Spectrum section at different resonator driving power. There are three peaks while applying a strong enough microwave drive to the λ/4 coplanar waveguide resonator, and the middle peak corresponds to the transition between |g0〉 ↔ |e0〉, labeled with red double arrows in Fig. 5, and the two side peaks besides the middle peak correspond to the blue transitions shown in Fig. 5.