In the paper "Double-Slit Quantum Eraser" by Walborn, Cunha, Padua, and Monken (see http://grad.physics.sunysb.edu/~amarch/Walborn.pdf [Broken] and discussion of the paper at http://grad.physics.sunysb.edu/~amarch/ [Broken]), a double-slit quantum erasure experiment is performed with a delayed eraser. When the polarizer POL1 and Detector Dp are moved away from the beta-barium borate (BBO) crystal such that s-photon hits the detector Ds before the entangled p-photon hits the POL1 and Dp, an interference pattern is observed due to delayed quantum erasure. As such, one would surmise that the POL1 has some faster-than-light influence on the p-photon, which ultimately affects the entangled s-photon. The authors discuss that there was no choice available to an observer to influence the result in the time period after the detection of the s-photon and before the detection of p-photon. What would happen if there was a choice to affect the outcome after the s-photon hits Ds, but before the p-photon hits the POL1?(adsbygoogle = window.adsbygoogle || []).push({});

That is, assume POL1 and Dp are moved even further away from the BBO. Assume also that (1) after the s-photon hits the Ds and the s-photon strikes the Ds consistent with interference, the POL1 is moved away from the path of the p-photon before the p-photon hits the POL1, and (2) after the s-photon hits the Ds and the s-photon strikes the Ds consistent with no interference, the POL1 is maintained in the path of the p-photon such that the p-photon hits the POL1. What will happen?

In (1), we have a situation in which the s-photon hits the Ds consistent with interference, but the POL1 is moved before the p-photon hits the POL1 so that the polarity of the p-photon can be determined by the detector Dp. As such, the polarity of the s-photon can be determined and therefore the slit the s-photon went through can be determined. Such a result would imply that the s-photon would strike Ds consistent with no interference. In (2), we have a situation in which the s-photon hits the Ds consistent with no interference even through there is a delayed quantum erasure. Such a result would imply that delayed erasure does not work in this case.

Is it possible that (1) cannot happen and that there are limits to the delayed erasure in (2)? That is, if we have the ability to move the POL1, will the s-photon always strike Ds consistent with no interference (because if the polarity of the p-photon were to be detected, the s-photon cannot possibly strike the Ds consistent with interference)? If there is faster-than-light influence on the p-photon, then one would expect that the s-photon would always strike the Ds consistent with interference, as one could not move the POL1 out of the way fast enough. But then, the Dp would tell us the polarity of the p-photon, and hence the polarity of the s-photon could be observed, which would allow one to know the slit the s-photon passed through, which would prevent the s-photon from striking the Ds consistent with interference.

Or, is it possible that (2) cannot happen and that entanglement is destroyed by the movement of the POL1? That is, will the s-photon always strike the Ds consistent with interference? If that is the result, then (2) would not occur and therefore no limits on delayed quantum erasure would be implied. However, such a result would imply that moving the POL1 would destroy the entanglement of the photons, as determining the slit that the s-photon passed through should not then be possible given that the s-photon struck Ds consistent with interference. What is strange about this result is that the entanglement is destroyed after the s-photon strikes the detector, but before the polarity of the p-photon can be detected.

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# Delayed Quantum Erasure Double Slit with Observer Choice Available

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