Entangled photons in double slit experiment

  • #1

Main Question or Discussion Point

Hi,
imagine setup with source of entangled photons (A and B). Photon A travels to double slit so that either wave or particle pattern can be observed, photon B hits detector either before (setup 1) or after (setup 2) photon A has hit screen. Question:

Will setup 1 show interference pattern but setup 2 have it destroyed because photon B has lost its wave function (which should cause photon A to loose its wave function as well)?

Thnx,
Beefs
 

Answers and Replies

  • #3
DrChinese
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There is no difference in the result due to time ordering. So the first setup will show the same result as the second setup.

Please note that these type setups are sensitive to a number of somewhat complicated details. You must jump through some special hoops to get entangled photons to evidence self-interference. Else there is no direct interference pattern.
 
  • #4
DevilsAvocado
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You must jump through some special hoops to get entangled photons to evidence self-interference.
Woowaah, it is possible use entangled photons in a double-slit?? :bugeye:
 
  • #5
DrChinese
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Woowaah, it is possible use entangled photons in a double-slit?? :bugeye:
No*

*Yes**

**Maybe.

The issue relates to the exact setup. Generally, we know plain ol' entangled ones don't make an interference pattern. If they did, you could either send FTL signals or determine which slit information. So assuming that can't happen, the explanation must be that you can't have interference when there is complete (in some sense?) entanglement.
 
  • #6
DevilsAvocado
Gold Member
751
91
No*

*Yes**

**Maybe.
:smile:

The issue relates to the exact setup. Generally, we know plain ol' entangled ones don't make an interference pattern. If they did, you could either send FTL signals or determine which slit information. So assuming that can't happen, the explanation must be that you can't have interference when there is complete (in some sense?) entanglement.
Yes, FTL seems to be the "ultimate barrier".

Ron Garret has a nice explanation why this must be (close to?) impossible:

  • If entangled photons do produce interference, we could use it to tell which-path in a double-slit, which is forbidden by QM.

  • If entangled photons do not produce interference, we could use it to send FTL signals, by utilizing a quantum eraser in one end (= interference in the other).
The answer to this "Catch-22" seems to be that a quantum eraser do indeed 'free' the other photon to produce interference, but this interference is a 'two-path-mixture' of interference patterns resulting in random noise, and the only way to filter out one path is by classical channels, i.e. exactly as in the 'ordinary' Delayed Choice Quantum Eraser.

So maybe the consequence of this is that; the more entanglement we have, for stricter which-path, the more noise we will have in a double-slit... and the less entanglement we have, for weaker which-path, the less noise we will have in a double-slit... or something like that...
 

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