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Physics
Quantum Physics
Quantum Interpretations and Foundations
What does delayed choice entanglement swapping require?
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[QUOTE="vanhees71, post: 6829095, member: 260864"] I think, in principle, there is no contradiction between our views. Indeed the preparation alone doesn't entangle photons 1 and 4. To the contrary, it's made precisely such that photons 1 and 4 are not entangled. The important point to understand is that even [B]when[/B] doing the measurements on photons 2 and 3 nothing happens to the full ensemble of photons 1 and 4 but that when choosing each of the four subensembles you prepare a specific entangled state of photons 1 and 4 without a causal interaction with either of these photons. This possibility of entanglement swapping is due to the preparation of the 4 photons in the original state, i.e., photons 1 and 2 as well as photons 3 and 4 in a Bell state, but these two pair states in a product state. The point is that, if you accept the validity of the microcausality constraint of relativistic QFT, then there can be no causal effect of the measurement on photons 2 and 3 on the photons 1 and 4. It just enables the (post-)selection of specific subensembles for which photons 1 and 4 are entangled. I don't see, whether we still disagree or not and if so, about which detail. [/QUOTE]
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Quantum Interpretations and Foundations
What does delayed choice entanglement swapping require?
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