M5L21d_cq.txt

#
# File:   content-mit-8-421-5x-subtitles/M5L21d_cq.txt
#
# Captions for 8.421x module
#
# This file has 58 caption lines.
#
# Do not add or delete any lines.
#
#----------------------------------------

OK-- and this is now the next question--
there is a certain randomness in spontaneous emission
when we go to the laboratory and look
at the spontaneously emitted photons.
And this is actually what I want to work out
with you maybe even today.
I think ten minutes may be enough.
What is really the information, the phase information,
which we have in a photon which has been spontaneously emitted?

First a very big disclaimer.
This question does not contradict the first one.
The fact that we have a unitary evolution with this operator
is 100% or 110% true.
But this operator will actually lead
to final states of the photon field, which
may not have a specific phase.
So if you say there is some intuition that there
is something going on with the phase, you may be correct.
But everything which is going on with the phase
is the result of a unitary time evolution.
The system itself is described by an operator,
by a Schrodinger equation, for the total system.
But the question I have for you now is, if you detect,
let's say, the photon emitted in spontaneous emission,
the randomness of spontaneous emission, the, let me call it,
loss of phase or at least the diminishment of the read out--
there will be situations where we have a laser beam which has
a well-defined phase, photons are scattered,
and we just cannot retrieve the phase of the laser beam
by looking at the photons.
So this is what I mean here.
Also the photons can come out of a unitary time evolution.
My question now i about the randomness
or this loss of phase of spontaneously emitted photons.

And now I want to know your best guess.

What is it due to?

Is it only due to the measurement process
of the photon?

Or is it due to performing sort of a partial trace,
averaging over certain states?

So if you're interested in the photon,
maybe tracing out the states of the atom
or averaging over modes of the electromagnetic field.
And question C is:  both is actually possible?

So if you look at spontaneously emitted photons
and they're not perfectly phase coherent, they're not
reproducing the phase of the laser who has created them,
what is the reason for that?
Is it always the fundamental reason or is there
no fundamental reason, it's only a kind of reason of ignoring information, taking a partial trace?