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So, in Young's double slit experiment, we see where the particle lands on the back wall.

What if we were standing at the back wall, looking toward the slits? Could we gain more information not by simply observing the light bouncing at the back wall, but looking at it our selves?

Of course, for electrons or lasers, that ain't a great idea. And the slits are too small for our eyes to really make out, I'd gather.

Frogs can possibly detect a single photon.

What if we trained a frog to lick its left eye if it detects a photon from the left slit, and its right eye if it detects a photon from the right slit.

Or better yet, build a tiny robot with a camera.

Would that robot see both slits illuminate as it detects the photon?

Seems testable in theory.

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  • $\begingroup$ Eyes measure the energy of photons (approximately) but they don't measure the momentum of the photon. So the frog wouldn't be able to tell which slit the photon had come from. $\endgroup$ – John Rennie Sep 1 '14 at 5:52
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If the frog/robot could tell which slit the photon came through, this is really identical to measuring the photon at the slits, which is discussed in every treatment of the double slit experiment. In short, the diffraction pattern is lost.

If it cannot tell, then there is no new information you can get anyway.

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In our macroscopic world it's possible to "measure" something without changing it by a significant amount. Usually when you take a measurement, you're throwing something (eg, incident light) into the system and then you catch the stuff that comes out (eg, reflected light). This works because the momentum of the incident light changes the momentum of the macroscopic object you're measuring by a negligible amount.

If you try to measure a single photon this way, you can put an atom at one of the slits and make it absorb the photon. But then that photon is gone, you can't have it pass undisturbed through the slit and hit the other side. I don't think that's what you want.

So, yes, it's possible to detect a single photon, but not to detect it and leave the resulting system undisturbed.

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Sure no problem, You could put a fiber bundle behind each slit and send the light to a PMT or other single photon counter. You'll get a pulse at one PMT or the other. And no interference pattern. In principle you can make the slits far enough apart that you can get rid of the fiber bundle and just have the single photon detector behind each slit.

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