This thesis presents the theoretical modeling of the experiment FORCA-G (FORce de CAsimir et Gravitation à courte distance) currently in progress at Paris Observatory. The purpose of this experiment is to measure short-range interactions between an atom and a massive surface. This interaction are of two kind : quantum electrodynamical (Casimir-Polder effect) and gravitationnal. The work presented here was to calculate the atomic states in the context of the experiment such that we can predict results and performances of the experiment. This has allowed to optimize the experimental scheme both for the high-precision measurement of the Casimir-Polder effect and for the search of deviation from the Newton's law of gravity predicted by unification theories.
