Re: how do a photon reflecting in a mirror stay coherent and not entangle ?

First let us consider what a mirror does:

Our standard picture of a perfect mirror is that photons incident on it cause
electrons (inside it) to oscillate in phase with the incoming photons
in such a way as to screen out any propagating EM field within the
mirror itself. A wave is therefore reflected. This phenomenon is
discussed at length in standard texts on Electromagnetism and optics. See,
for example,
 
Optics
by Eugene Hecht 

Modern
Optics by B. D. Guenther

Electromagnetic
Fields and Waves by Lorrain and Corson

However, maybe we can think about the direct interaction with the
electrons. The first thing to note is that a photon doesn't just
interact with one electron. The electrons in a metal are all
interacting and so the photon sees the combined effect of more than
millions of billions of electrons.  

The electrons initially (assuming that the mirror is cold) have very
little energy (compared to the energy of a photon). The photon has
a well-defined energy and phase. We know that energy is never
transferred to the electron for a long period of time because the
mirror is 100% reflective so what happens to the phase? Well, if all
of the energy is passed back to the reflected photon, this photon must
remain coherent since the electrons are no longer oscillating and
there have no phase.

Even if the mirror is not perfect and maybe absorbs a little or
scatters, the fraction of incoming photons that is reflected remain
coherent since the very short-lived oscillation they caused has
re-emited its light. It might be expected that absorbed photons would
transfer some of their phase to the electrons. However,
absorption in a metal causes heating which is not a phase-conserving
process.

Since none of the phase can passed onto the electrons (at least not
for long periods of time) it is maybe obvious that there can be no
kind of entanglement and certainly none of the sort of entanglement
which occurs in quantum mechanics which is actually quite difficult to
generate. A tutorial on this kind of entanglement can be found at 
this site.