The evolution of the atomic state in the resonant Jaynes-Cummings model (a two-level atom interacting with a single mode of the quantized radiation field) with the field initially in a coherent state is considered. It is shown that the atom is to a good approximation in a pure state in the middle of what has been traditionally called the ‘‘collapse region.’’ This pure state exhibits no Rabi oscillations and is reached independently of the initial state of the atom. For most initial states a total or partial ‘‘collapse of the wave function’’ takes place early during the interaction, at the conventional collapse time, following which the state vector is recreated, over a longer time scale.

• Received 9 August 1990

DOI:https://doi.org/10.1103/PhysRevLett.65.3385