From this experiment it is apparent that interference is destroyed by a "which-way" marker and that it can be restored through erasure of the marker, accomplished by making the appropriate measurement on the entangled partner photon p.
In this set up, the "which-way" measurement does not alter the momentum or position of the photons to cause destruction of the interference pattern. We can think of the loss of interference as being due only to the fact that the photons are entangled and that the presence of the quarter wave plates changes this entanglement. The interference pattern can be brought back through the erasure measurement because of the entanglement of the photons, and the way that the presence of the quarter wave plates and polarizer changes the entanglement.
It is very obvious I need some time to digest and listen carefully here. Of course I draw from Wiki quite regularly and I hate to think such efforts to destroy a concerted effort by those whose hearts are pure, for leading others into the correct methods, would not resort ot the efforts and likes of those Lubos has brought to our attention.
ON the note below taken from Lubos update, it is without thinking that I might have lured others into the state of complacency without fully understanding, and hence my part in this effort less than kind? So I'll draw back for a bit here and try and digest what I learnt and see if I can get it together.
Lubos Motl:
Note added later: let me mention that Kastner has submitted another paper criticizing Afshar's conclusions. In my opinion both Unruh as well as Kastner replace Afshar's experiment by a completely different experiment that does not capture the main flaw of Afshar's reasoning. The main flaw is that Afshar does not realize that for a tiny grid, only a very tiny percentage of photons is used to observe the wave-like properties of light; these are essentially the photons for which the which-way information is completely lost. Because most photons go through the lens without any interactions and interference, Afshar is not allowed to say that he observes the wave-like phenomena with visibility close to one. In fact, it is close to zero if a consistent set of photons is used to define both V and K.
Here is the paper >Kastner talks too in regards to the content of Afshar experiment.
Why the Afshar Experiment Does Not Refute ComplementarityR. E. Kastner
ABSTRACT. A modified version of Young’s experiment by Shahriar Afshar demonstrates that, prior to what appears to be a “which-way” measurement, an interference pattern exists. Afshar has claimed that this result constitutes a violation of the Principle of Complementarity. This paper discusses the implications of this experiment and considers how Cramer’s Transactional Interpretation easily accomodates the result. It is also shown that the Afshar experiment is analogous in key respects to a spin one-half particle prepared as “spin up along x”, subjected to a nondestructive confirmation of that preparation, and post-selected in a specific state of spin along z. The terminology “which-way” or “which-slit” is critiqued; it is argued that this usage by both Afshar and his critics is misleading and has contributed to confusion surrounding the interpretation of the experiment. Nevertheless, it is concluded that Bohr would have had no more problem accounting for the Afshar result than he would in accounting for the aforementioned pre- and
post-selection spin experiment, in which the particle’s preparation state is
confirmed by a nondestructive measurement prior to post-selection. In addition,
some new inferences about the interpretation of delayed choice experiments are
drawn from the analysis.
1. Introduction.The Young two-slit experiment is a famous illustration of wave-particle duality: a quantum particle emitted toward a screen with two small slits will produce an interference pattern on a detecting screen downstream from the slits. On the other hand, as has been repeatedly demonstrated, if one tries to obtain “which-way” or “which slit” information, the downstream interference
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