# Coherent interaction

Hi! I would really appreciate you to help me answer this question:

What is the physical meaning of coherent interaction (in the Dicke model for example)?

I mean, coherence is well defined in classical optics, but what does it means when you ask for a system of N atomic dipoles to interact coherently with an electromagnetic field?

Thanks!

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I'm not sure but I suppose it means that the N dipoles are all interacting with the field while in phase with each other.

I don't know about the Dicke model, as I'm struggling to understand Dickes papers, but for a system interacting with field :
E=A cos(kx-ωt)
Assume that the wavelength of E is large enough that all N dipoles see ~ the same E,
so
E=A cos(ωt)
Then coherent if the response of the dipoles is the same and has the same time evolution,
Dipole 1 has moment
P1 = e cos(ωt)
Same as dipole 2
P2= e cos(ωt)
If they interacted incoherently then
P2= e cos(ωt + p(t))
Where p(t) is some time varying phase difference in the dipoles responce.