M5L21b_cq.txt

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Before I talk about manifestations of coherence,
I have two bigger questions, because the first form
of coherence I want to discuss is
the coherence involved in exciting atoms and the atom
emitting light.
It's related to the spontaneous emission and scattering
problem.
So just to sort of figure out what you know already,
let me ask you something about the nature
of spontaneous emission.

So the first choice is, spontaneous emission
is nothing else than a unitary transformation,
(unitary transformation or unitary time evolution)
of the wave function of the total system.
And option B is, spontaneous emission introduces-- well,
maybe it's through a master equation,
through optical Bloch equation-- introduces a random phase
into the time evolution of the quantum mechanical system.
So what is the picture you have of spontaneous emission?