Detecting a photon passing through a slit without destroying it?

[Cato]

TL;DR

I occasionally read descriptions of the double slit experiment in which the author says something like, "If a detector is place at each slit so that we know which slit the photon passsed through, the interefence patten does not form." Is this even possible? Is it possible to detect a passing photon and have it remain a photon? An electron, yes, but a photon?

I cannot see how a photon can be detected and yet remain the same photon. I am thinking that the description "If a detector is place at each slit so that we know which slit the photon passsed through, the interefence patten does not form" is sloppy and in error.

 

[PeterDonis]

I cannot see how a photon can be detected and yet remain the same photon.

First, you don't have to detect the photon itself to know which slit it passed through. You could detect the photon's effect on the slit.

Second, there are nondestructive ways of detecting photons, as a simple Google search will show you.

 

[Cato]

Isn't detecting the photon's effect on the slit the same as detecting the photon? And wouldn't that change the energy of the photon? Same for the second -- wouldn't detecting a photon nondestructively change the photon? Is it possible to detect a photon without altering its energy? So that an interference pattern would not be created not because we "knew" which slit the photon passed through, but because the energy of the photon was changed in the process of being nondestructively detected?

 

[PeterDonis]

Isn't detecting the photon's effect on the slit the same as detecting the photon?

No. For example, you could measure the recoil of the slit as the photon passes through it and interacts with it. That does not involve any measurement of the photon.

wouldn't that change the energy of the photon?

The photon might not even have a definite energy to begin with. But it will certainly change the behavior of the photon, since detecting which slit the photon went through destroys the interference pattern.

wouldn't detecting a photon nondestructively change the photon?

See above.

So that an interference pattern would not be created not because we "knew" which slit the photon passed through, but because the energy of the photon was changed in the process of being nondestructively detected?

These aren't two different alternatives. They're two different descriptions of the same process.

 

[DrChinese]

I cannot see how a photon can be detected and yet remain the same photon. I am thinking that the description "If a detector is place at each slit so that we know which slit the photon passsed through, the interefence patten does not form" is sloppy and in error.

PeterDonis is correct. If you place polarizers in front of each slit, their relative orientation controls whether there is interference or not. If they are parallel, there IS interference.

But if they are perpendicular, there is NO interference. It is possible in this case to determine whether the photons went through one slit or the other (even if you do not attempt to determine that). So of course there is no interference, and the light gets through.