I describe the atoms using a so called Lindblad master equation. The atoms are kept in this description, but the light field is eliminated using two assumptions:
- The coupling between the two is very weak.
- Correlations between the two decay so fast that this can be considered instantaneous.
The later produces white noise.
Typical transition frequencies between two levels of an atom are 10^15Hz. The coupling between atoms and light is on the order of the decay rate at which photons are transmitted, which sits at around 10^6Hz.