At 12:35 -0800 14/01/2003, Hal Finney wrote:
Tim May writes:
 This arises with quantum measurements of course. Once a measurement is
 made--path of a photon, for example--all honest observers will report
 exactly the same thing. There simply is no basis for disputing the
 past, for Alice to say "I saw the photon travel through the left slit"
 but for Bob to say "I saw it travel through the right slit."
That's an interesting example, because usually the point of two-slit
experiments is that there is no "fact of the matter" about which slit
the particle went through.  That's why you get interference from
the double-slit.  What would you say about the past in that case?
Are there two pasts, one where the particle went through each slit,
which have now recombined to form the present?  Or just one past, where
the particle managed to go through both slits at once?

An interesting paper related to this question, I think, is:
http://xxx.lanl.gov/abs/quant-ph/0210015


Quantum entanglement with acousto-optic modulators: 2-photon beatings
and Bell experiments with moving beamsplitters

Authors: Andre Stefanov, Hugo Zbinden, Antoine Suarez, Nicolas Gisin
Comments: 14 pages, 16 figures

       We present an experiment testing quantum correlations with frequency
shifted photons. We test Bell inequality with 2-photon interferometry where we
replace the beamsplitters by acousto-optic modulators, which are equivalent
to moving beamsplitters. We measure the 2-photon beatings induced by the
frequency shifts, and we propose a cryptographic scheme in relation. Finally,
setting the experiment in a relativistic configuration, we demonstrate that the
quantum correlations are not only independent of the distance but also of the
time ordering between the two single-photon measurements.

Bruno

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