Entanglement through

IWonder

Dear group,

I keep asking me how coherence can be kept for entangled pairs (say) which
undergo multiple interactions with their environment. In an experiment
where two photons are fired back to back and detected a few meters away, I
could imagine that in some probability, they go through a vacuum and never
experience anything in the middle, thus holding their entanglement. But
there are these reports of entangled photons which have been through
several kilometers of waveguides. When a photon propagates in a
dielectrics, it keeps being absorbed and reemited by the medium so that it
looks like it is a slower photon, when in fact it is a huge and continuous
chain of absorptions and reemissions. How can entanglement be sustained in
such conditions, as as been reported experimentally by several groups? Why
is not a photon in one branch building an exponentially growing amount of
entanglement with all the particles it meets as it flies by, effectively
washing away completely its initial entanglement with the other photon?

I am grateful if you can enlighten me on this point,

Best,

G.M.