eraser

Below links to a web page containing a nicely readable (tech-friendly) account of two-slit experiments with photons ("the quantum eraser") that show all the peculiar features that prompted Richard Feynman to advise, "Do not keep saying to yourself,' But how can it be that way?' "

Be sure you notice how the experiment involving a "which-way" marker mimics EPR entanglement to enable one to determine indirectly through which slit the photons go, and so the marker destroys the interference pattern. But (as in EPR) it does not disturb the photons that enter the two- slit apparatus. So here "disturbance" does not seem to explain the shift in the pattern of outcomes.

What makes this possible is that the photons that enter the two-slit device (s-photons) are paired with distant photons that do not (p-photons) so that a typical state of the pair looks like

where the Mark1 and Mark 2 states of the p-photons are orthogonal. As in EPR, this entanglement allows one to determine through which slit the s-photon has gone by measuring the marks on its distant p-photon twin.

As you read you will notice several comments about the metaphysics of the photon pairs; e.g., "It is not accurate to consider these photons as separate entities, but rather as one." These remarks and similar ones about the "deeper" meaning of quantum entanglement respond to what Feynman (above) suggests is pointless; namely, thinking that beyond QM lies some deep mystery we need to understand. If Feynman is right the remarks are just word music providing a "gentle pillow" (Einstein) to lull us into thinking we actually do understand: "how can it be that way?". (You might re-read my Do correlations need to be explained? [# 12]. Note that if a photon pair (s, p) did constitute one integral entity then a measurement of the p-part would indeed be a disturbance of the whole!)

Quantum Eraser Link