Rabi nutations: immediate test of the field quantization in a cavity

Circular Rydberg atom and electromagnetic field stored in superconducting cavity constitute a simple system, isolated from the environment and allow us to study fundamental effects of the mater-field interaction. This work presents an experiment in a resonant regime. The Rabi nutation signal corresponding to the evolution of the atomic populations presents Fourier components at frequencies proportional to square root of successive integers; it's a direct evidence of field quantization in a cavity. We demonstrate also that the analysis of the signal gives the distribution of the photons number in the held stored in the cavity.