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A. Neumaier said:Its inaccurate to talk about a pair of photons traveling. What travels is a single quantun system in an entangled 2-photon state. In QFT it is impossible to separate this system into two photons. This is called inseparability.
I couldn't agree more. Although it's common practice to reference each as subsystems (or components) simply for shorthand purposes (since they eventually result in 2 particles).
However, you eventually have the combined system breaking out into what becomes 2 independent single particle systems (for example an entangled pair of electrons becomes 2 unentangled electrons). We don't know exactly when or how that happens (from experimental considerations), just that at some later time, that's what we find.
[entangled system with particle number=2] -> [independent particle A] + [independent particle B]
But while it is an inseparable system: it has both spatial and/or temporal extent... and therefore it should not be thought of as a localized quantum object. And clearly, the nature of an observation on one of the components somehow affects the other - or vice versa. Or it is mutual. I don't know. But clearly trying to describe it initially as 2 independent components can't be right (as you rightfully object); and trying to describe the correlated results as independently arrived at also can't be right (as Bell would object).
So I again ask: if we start with an entangled system which has spatial extent, and we measure one of the components A: how does the other B come into being with some observable strongly correlated to the A component?
And of course, I am asking how QFT would handle that, as vanhees71 asserts it offers an explicit local mechanism. (And I don't think it does, because of Bell.)