Rabi oscillations with quantized light: which is the "quantum" effect, collapse, revival or both? In wikipedia http://en.wikipedia.org/wiki/Jaynes%E2%80%93Cummings_model#History it is stated that 

It was later discovered that the revival of the atomic population inversion after its collapse is a direct consequence of discreteness of field states (photons).[2][3] This is a pure quantum effect that can be described by the JCM but not with the semi-classical theory.

A similar claim has been made by my teacher during a lecture: he said that the revival, and not the collapse alone, in Rabi oscillations is a striking feature of the EM field quantization, not explainable with the quantization of the atom alone.
I do not understand why, since the corresponding (Rabi) model predicts periodical oscillation in the population, so that even observing just the collapse should be a proof of the field quantization.
 A: Both Rabi oscillations and the revivals are quantum mechanical effects. However, they consequences of the quantization of two different systems. Rabi oscillations can be explained and derived with a semi-classical theory in which the atomic system has quantized energy levels but the incident light fields are classical. Rabi oscillations do not require quantized fields. Quantum revivals however, in which the Rabi oscillations decay to zero before rising again, can only be explained with quantized fields via the Jaynes-Cummings model. 
A: "is a direct consequence of discreteness"
"he said that the revival, and not the collapse alone, in Rabi oscillations is a striking feature of the EM field quantization"
Discreteness and quantization are not exactly the same thing : if you don't put your atom inside a box but just let it free, there will be a continuum in the EM field (you can make the size of the box go to infinity) : the atom will go from excited to ground state and give its energy to the EM field, but there will be no revival.
Inside a finite box the EM energy level are discrete and you can have revival.
In both cases EM field is quantified, but if the size of the box goes to infinity (free atom) the EM energy levels will tend to a continuum.
So I think what your teacher said was : "the revival, and not the collapse alone, in Rabi oscillations is a striking feature of the EM field DISCRETENESS", because as you said, if the EM field is not quantized there is no collapse either.
