We show that the information about a quantum system contained in the coherent evolution is still available for times much longer than the spontaneous lifetime. We consider the deflection of an atomic wave packet by a resonant quantized light field in the presence of spontaneous emission. We present an analytical expression for the momentum distribution of the deflected atoms which allows us to monitor the dynamics of the relaxation quantum system ‘‘two-level atom plus field’’ on a macroscopic scale.

• Received 17 September 1993

DOI:https://doi.org/10.1103/PhysRevA.49.3127