  We consider the interaction between a single-mode quantized perturbing
external field and a Bose-Einstein condensate (BEC) out of equilibrium. The
usual Rabi type oscillations between the ground and the excited state of the
coherent topological modes are observed with a Rabi frequency modified by the
two-body atomic interactions. Taking into account the granular structure of the
external perturbing field reveals the well know phenomena of collapse and
revival of the Rabi oscillations. In particular we find that atomic
interactions reduce the Rabi frequency and also affect the collapse and revival
sequence.
