DrChinese said:
That is what vanhees71 is also asserting. And I say these experiments flat out exclude that possibility. Here is what happens:
Photon pairs of 1 & 4 start out uncorrelated (in all sub-ensembles). They become correlated (entangled) IF AND ONLY IF systems 1 & 2 and 3 & 4 are allowed to interact. If they interact, in all cases they will be entangled. Therefore the remote state of the 1 & 4 pairs physically changed (from uncorrelated to correlated).
This physical change is demonstrated by an abrupt change in the statistics for 4 fold coincidences. That change occurs exactly as the 2 & 3 arrival times in the remote beamsplitter become coincident so the 2 & 3 arrival times do not distinguish each other. That is the only change necessary to change the 1 & 4 stats from random to perfectly correlated (the 2 & 3 outcomes remaining constant as part of the 4 fold coincidences). You are selecting the same 4 fold groups in all scenarios, using the exact same criteria. The only variable is the difference in arrival times of 2 and 3, which certainly shouldn't matter to distant 1 & 4 according to those who think this is only about selection.
There is no swapping paper that will say otherwise to the above, other that to acknowledge there are different viable interpretations (and I do not dispute those). Every paper refers to the swap as an actual action that depends on the observer bringing remote systems into contact at a single point in spacetime. The decision to perform the swap physically changes the outcomes for 1 & 4, and is variously referred to by "project", "cast", "swap" and not by terms such as "reveal".
And in no swapping paper is what is occurring said to be restricted by locality, relativity, etc. So again, the reference to the construction of QFT to swapping papers is inappropriate. The experiment is objective; interpretations of QFT are subjective and must bow to what the experiment says.
The misunderstanding is on your side!
What interacts in Alice's selective filter measurement are the photons 2&3 (note that
@DrChinese now again flips the notation to the other older paper by Zeilinger et al he didn't want to discuss anymore, because it's not available as a preprint or a legal open source, but that other paper is just the very same experiment only that the four photons are now labeled with 1234 instead of 01234; just to avoid further confusion) with the beam splitter and the detectors. Since photons 1&2 as well as 3&4 are entangled, these photons are parts of the partially inseparable four-photon state, and that leads to the entanglement swapping and teleportation through selection. It's not that there are spooky actions at a distance via Alice's measurements on her photons 2&3. In this sense everything is local, as it's described by relativistic QFT. What's "nonlocal" in the very specific sense of QFT are the correlations described by entanglement. Taken both properties of QFT/photons together you have a causal description without spooky actions at a distance. That delicate balance between the nonlocal aspects of entanglement (describing strong correlations between inseparable parts of a quantum system) and the local description of interactions (microcausality) makes relativistic QFT (or more precisely stated this class of relativistic QFTs, namely local QFTs underlying the Standard Model) consistent with both the causality structure of relativity (no faster-than-light signals leading to causal effects) and the inseparability of correlations between far-distant parts of a quantum system, as described by entangled photon states.
Nobody denies that there are interactions leading to the (post-)selection of specific sub-ensembles as done in the two papers, discussed here. Just to cite one interpretational sentence from the more recent PRL by Jennewein et al (PRL
88, 017903 (2002)), discussing the variant of the experiment, where Alice's filtering manipulations are done after the photons 0&3 are registered, i.e., the post-selection or delayed-choice variant of the experient, which in my opinion clearly shows the correctness of the above features of standard QFT rather than non-local actions at a distance of some alternative models, which
@DrChinese seems to prefer (I still don't know what precisely his model for the findings is, because he doesn't give a clear formulation, which indeed can only be given in a mathematical way). For the following note that this paper labels the four photons as 0123:
===================Quote Jennewein et al ====================================
A seemingly paradoxical situation arises — as suggested
by Peres [4]— when Alice’s Bell-state analysis is delayed
long after Bob’s measurements. This seems paradoxical,
because Alice’s measurement projects photons 0 and 3 into
an entangled state after they have been measured. Nev-
ertheless, quantum mechanics predicts the same correla-
tions. Remarkably, Alice is even free to choose the kind
of measurement she wants to perform on photons 1 and 2.
Instead of a Bell-state measurement she could also mea-
sure the polarizations of these photons individually. Thus
depending on Alice’s later measurement, Bob’s earlier re-
sults indicate either that photons 0 and 3 were entangled
or photons 0 and 1 and photons 2 and 3. This means that
the physical interpretation of his results depends on Alice’s
later decision.
Such a delayed-choice experiment was performed by
including two 10 m optical fiber delays for both outputs
of the BSA. In this case photons 1 and 2 hit the de-
tectors delayed by about 50 ns. As shown in Fig. 3, the
observed fidelity of the entanglement of photon 0 and pho-
ton 3 matches the fidelity in the nondelayed case within
experimental errors. Therefore, this result indicates that
the time ordering of the detection events has no influence
on the results and strengthens the argument of Peres [4]:
This paradox does not arise if the correctness of quantum
mechanics is firmly believed.
========================= end of quote ================
The cited reference by Peres is:
A. Peres, J. Mod. Opt. 47, 139 (2000).
