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If the experiment is done with single photons and a "detector" at one of the slits the interference pattern breaks down. What happens if three slits are used with single photons and a "detector" at, say, the right hand slit. Does the interference pattern occur in the two remaining slits or does the interference pattern breaks down for all the slits ? Thanks for any response

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In much the same way that putting a photon-absorbing detector at one slit in a double-slit experiment gives you the single-slit interference pattern back, putting a detector at one slit out of three will give you the two-slit pattern back.

It might help to think of the detector as blocking photons, in the same way that the material in which the slits are cut blocks photons. A detector doesn't have magical powers different from other physical objects, and blocking one slit doesn't do anything to the remaining slits. You could get the same visual result by putting chewing gum in one slit, you just wouldn't get any data from your gum. :)

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You are ignoring the fact that, in principle, the detector need not block the photons at the observed slit. – Arnold Neumaier Aug 11 '12 at 10:55
True, but so does the original question. – Colin Fredericks Aug 11 '12 at 16:03
The original question says nothing about how the photon is detected, hence nothing about blocking. – Arnold Neumaier Aug 11 '12 at 16:07
But it does state that blocking one of the slits in the double-slit destroys the pattern. To my understanding, detecting the passing photon with minimal disruption to it does not destroy the double-slit pattern. – Colin Fredericks Aug 11 '12 at 20:16
He doesn't talk about blocking. Indeed, to the extent that a detection mechanism is efficient, the interference pattern becomes less pronounced. With 100% efficiency, it disappears completely. en.wikipedia.org/wiki/… – Arnold Neumaier Aug 12 '12 at 10:51
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As detection necessarily decoheres the input signal, one gets a combination of the pattern with a single slit (for the observed slit) and of the pattern with two slits (for the unobserved slits).

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I agree with arnold, the detector need not interfere with the photon as it passes through the detector; e.g. delayed response quantum erasure experiments have gotten around this. And really, you can't talk about blocking a photon, because if it exhibits an interference pattern, it probably didn't pass through both slits, just a man made probability wave to estimate its final position. A detector doesn't "block" a particle, it just means that the particle had to exist to serve the purpose of the detector at that point in space and time, to service consciousness and measurement. For if you were to erase the which path information detected by the detector, after it passes through the slits, the interference pattern comes back. I am of the opinion, that a photon doesn't exist in any form until it absolutely had to, it's just in superposition of infinite possibility, until required by some conscious purpose. Quantum mechanics and the "laws of nature" just sound a bit like a marvelous type of technology if you ask me. But I ramble.

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