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I Delayed choice - Bohr's experiment with a Camera

  1. Jul 25, 2016 #1
    I have some questions and ideas regarding Bohr's double slit experiment which he extended with a Camera as an illustration of delayed choice. I read about it 40 years ago but can't find a reference to it, I'm hoping it wasn't apocryphal but it's consistent with Q-phys as I know it.

    The experiment layout.
    Imagine a diamond shape, 1 metre each side. The entangled photons are emitted at the 'south' angle, mirrors at east and west and the 'result' screen at north. The east mirror is a half-mirror with a 2 metre extension leading to a potential 'observation point', in this case a camera.

    My memory tells me as Bohr ran the slit experiment with the camera:
    1) With film - No interference
    2) No film - Interference.

    Can someone confirm (or deny) this before I go further, as, well I'm sure you know why.
  2. jcsd
  3. Jul 25, 2016 #2


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  4. Jul 25, 2016 #3
    Thank Strilanc,
    Is there any chance someone could point to the simplest setup diagram that involves the 'Delayed choice' phenomenon. The simplest ones I've seen involves 20 odd mirrors.
  5. Jul 25, 2016 #4


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    The rule is: if the possibility exists to determine which-slit information, there will be no interference. So generally, the presence (or absence) of film is not the issue.

    Also: entangled photons do not, while entangled, produce interference patterns. The reason for this is complicated to explain, but the easiest explanation is that they are not coherent.
  6. Jul 25, 2016 #5
    Thx DrChinese, the experiment I am misremembering was to be an illustration of a different thought I have.
    Is there a bare-bones 'Delayed choice' real or thought experiment you know of that I could use for explanation?
  7. Jul 25, 2016 #6


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    These are the diagrams I made to try to make it simple to understand.

    The setup:


    And the result (well, not exactly, since the above setup uses beam splitters instead of some kind of manual input for the choice and the labels for cases differ, but whatever):


    And an equivalent quantum circuit that works on qubits, which is more abstract but avoids a lot of the vagueries of optics:

  8. Jul 25, 2016 #7


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    Not sure. There are some great delayed choice experiments that have been performed. However, in the quantum world, there is no apparent difference in the experimental context between one with delayed choice and one without. For example, you can entangle photons after they have been detected.

    So without knowing the specific point you are after, it is hard for me to say. A lot of experiments have been done in recent years that touch on elements you mention.

    Strilanc: sorry to jump over your excellent post.
  9. Jul 25, 2016 #8
    I will think on, thanks.
  10. Jul 25, 2016 #9
    thanks Strilanc, one last question. Is it preferable that I post 'thanks' messages (i did not see it mentioned in the rules), I imagine you get lots of unnecessary alerts.
  11. Jul 26, 2016 #10


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    That's what the "like" button is for. :cool:
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