What, explicitly, happens as EM radiation travels from A to B? I'm in my fourth year of a Masters Physics course, and am quite concerned that I don't fully understand this.
Suppose a photon is emitted at point A and absorbed at point B one light-minute away. We 'look at' the system 20s after the photon is emitted. Define point C as the point one-third of the way between A and B.


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*How should the light be described? I want to say "the photon is at point C" but I know Fock states are unphysical and coherent states are how best to describe light. But, is this superposition of photon numbers still at point C, or is there a spatial distribution too? If there is a spatial distribution, does that not give a chance that it will be absorbed at B at eg 59s or 61s?

*Where are the magnetic and electric fields modified? Is it just at point C ; everywhere along the path from A to B (which would seem to have information travel faster than light) ; everywhere between A and C ; some Gaussian-like volume around C ; something else?

*How are the E and B fields modified? I "know" that it's by sin(kx-wt), but depending on the answer to Q2 that may mean different things. (Eg, if E and B change only between A and C, do they just "switch off" once it's absorbed at B (again raising locality questions), or does the magnitude dampen with time after it's initially altered? In general, how do they change with time, and at various events (emission, absorption, 'the phtoton passing point C'?)

*What does it mean to say that E and B are perpendicular? Do they literally vary spatially, ie if I move upwards from C with my detector I read a B but no E field, while if I move sideways then I get an E but no B field? If not, to what does the perpendicularity refer?


The bold is my questions (obviously); after that is my guesses at answers and/or hints at where I'm confused, but feel free to ignore these and talk about anything that's relevant to answering.
To be clear, I'm not asking about if I actually made a measurement at point C, I'm asking what we can deduce from the equations etc we know describing light and the EM field. (Think calculating where a ball I throw in the air will be after 0.2s, as opposed to catching and measuring it after 0.2s and hence changing the rest of the motion.)
Also, I'm looking for a model of what happens, and (as far as possible) the simplest full explanation why. (I tried asking one lecturer but he started talking about Fourier transforms and stuff I didn't really understand/see the relevance of, and didn't give a direct "they're modified like this" answer, hence this note.)
Finally, if you refer to modes of EM waves/fields in your answer (or are feeling particularly generous), please define exactly what you mean by this as well - many times I've Googled and still I don't entirely understand these.
Much much thanks!
 A: 
Suppose a photon is emitted at point A and absorbed at point B one
  light-minute away. We 'look at' the system 20s after the photon is
  emitted. Define point C as the point one-third of the way between A
  and B.

It looks as if you believed that the photon goes from A to B following
the straight line between those points. I also have some doubt about
your wording "a photon is emitted at point A". You don't say it but it
would seem that photon's emission does happen at a precise instant of
time.
I would prefer a more elaborate statement, something like this:
"An atom is placed in position A of an inertial frame. Assume the atom
has only two (nondegenerate) energy levels, $E_0$, $E_1$ (or the others
may be neglected). Since a long time the atom has stayed in its lowest
level $E_0$. By a properly chosen laser pulse it is brought to level
$E_1$, in a very short time. So we may say that at time $t=0$ it is in
that state. Afterwards it will decay emitting a photon, e.g. via an
electric dipole transition whose mean life is short, say $10\,\mathrm{ns}$.
At a distance of 1 light-minute is placed, in A's frame, an array of detectors. At time $t=1\,\mathrm{min}$ one of them, say B, detects the photon."
Then your questions could follow. We'll see presently.

We 'look at' the system 20s after the photon is emitted. Define point
  C as the point one-third of the way between A and B.

This makes sense.

I want to say "the photon is at point C"

Surely you understand this isn't a reasonable option. At that time
the photon doesn't occupy a precise position in space. Photon emission
isn't a "Nadelstrahlung" (needle radiation) as Einstein believed.
You could find the photon everywhere at a distance of about 20
light-secs from A. Detector B too has only a (small) probability of
detecting the photon. It could be detected instead by any other
detector.

but I know Fock states are unphysical and coherent states are how
  best to describe light.

Wait a moment. Where did you learn that Fock states are unphysical?
This comes novel to me. Maybe the exact argument was different. Could
you give some reference?
As to coherent states, they are OK to represent a state of e.m. field
resembling a macroscopic e.m. wave. But in our case, where we know
exactly one photon is present, this is far from a coherent state.
In a coherent state the number of photons is undetermined. it is a superposition of states with different photon numbers. Maybe you know there is an uncertainty relation between photon number and field phase?
Going on: I can't understand your question 2. Sorry.


  
*How are the E and B fields modified? 
  

Modified wrt what? I'm afraid you have some misunderstandings of the
whole matter of quantum fields. Did you take courses on that matter? E
and B are operators defined on Fock space (sorry :-) ). It makes no
sense to speak of "modifying" them. Your quantum system is expressed
mathematically as a Hilbert space (Fock space) and operators defined
in it (e.g. E, B, but also the Hamiltonian and so on). 
We are reasoning about free fields (with some abuse). In Schrödinger
picture the state of e.m. field evolves in time. In my description I
vaguely described the state at time near $t=0$. The only certain thing is that after - say - $t=50\,\mathrm{ns}$ it is an eigenstate of the observable "photon number", at eigenvalue 1. The state changes with $t$, exhibiting photon propagation from source outwards.


  
*What does it mean to say that E and B are perpendicular?
  

Well, E and B are vectors. This means they are operators on Fock
space, more precisely two triplets of operators to represent
components of e.m. field. These operators are so defined that an
identity $\vec E\cdot\vec B=0$ holds. I can't dwell on it here, but you
will find it explained in Ch. from 1 to 3 (according book's
organization) of every book on QFT.
