Oscillations of level populations under the influence of an Incident Light field.

If Light interacts with a two-level System this can lead to a periodic exchange of Energy between the light field and the two-level system. These oscillations are associated with oscillations of the quantum mechanical expectations values of level populations and photon numbers. They can be interpreted as a periodic change between Absorption and stimulated emission of photons, and can be modeled using the Bloch vector formalism. A competing process, which can prevent the observation of these oscillations, is spontaneous emission.