In the Optical Bloch Equations (https://en.wikipedia.org/wiki/Maxwell%E2%80%93Bloch_equations) it is imposed that the populations decay at a rate $\gamma$, and that the coherences decay at $\frac{\gamma}{2}$.

I can see why the populations should decay at $\gamma$ (i.e. via Wigner-Weiskoppf theory) but how do we arrive at $\frac{\gamma}{2}$ for the coherences?

My motivation in asking this question is in trying to extend the OBE's to a three-level Vee system, where is is not obvious what decay rate to assign to the excited state coherence terms (i.e. $\rho_{e_1e_2}$).

This post imported from StackExchange Physics at 2016-11-24 08:30 (UTC), posted by SE-user user2640461