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Double Slit Interference Question

Discussion in 'Cardiff City Forum' started by Steve Lewis, 30 Jun 2020.

  1. Steve Lewis

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    I have a really straightforward (I think) question regarding the effect(s) of obtaining the which-path information of photons that have travelled through a double slit and are hitting a photosensitive screen. In the thought experiment, all the photons arriving at the screen have entangled twins which simultaneously arrive at which-path detectors that are either ON or OFF. The set up is super-simple but I can’t find this particular configuration - so not sure if it would actually work. Main Question or Discussion Point A laser provides a constant stream of photons which pass through a double-slit. The photon stream emerging from each slit then passes through a crystal which splits each photon into coherent entangled pairs. One photon from each pair heads towards a photosensitive screen and the entangled twin travels toward one of two detectors (one for the left slit, one for the right slit). When the detectors are switched to the ON position, the which-path information for each photon (and its entangled twin) can be obtained. In this case, all photons behave as particles and do not interfere with one another. The pattern formed on the photosensitive screen does not exhibit a wave-interference pattern (no light and dark bands). When the detectors are switched to the OFF position the which-path information for each photon (and its entangled twin) is completely lost. In this case, all photons behave as waves and interfere with one another. The pattern formed on the photosensitive screen exhibits light and dark bands (standard wave-interference pattern).

    So basically, assuming 100% conversion of photons to entangled pairs, can an apparatus such as this be constructed/operate such that someone can, in real time, and only by observing the photosensitive screen, know whether the detectors are in the ON or OFF position? :shrug:
     
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  2. bluethrough

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    Yes:thumbup:
     
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  3. stantys tattoos

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    :unease:
     
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  4. Steve Lewis

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    Thanks so much but I believe it to be no however I have a bunch of questions, if you can stick with me. I’ll start with two:

    1) There's an article that refers to “idler” and “signal” photons. Are these fundamentally different in any way, or is it just shorthand so we don’t have to say “the photon that reaches the interference screen” and “the photon that reaches the which-path detectors”?

    2) My thought experiment uses a screen that you can just look at to observe the interference pattern but the the article refers to “detector D0, which can be scanned by a step motor along its x-axis for the observation of interference fringes”, so let’s use a detector like D0 for my system. What kind of graph would be generated if we removed the half-silvered mirrors, and let all the idler photons be detected by detectors that would definitely provide the which-path information for every single idler photon?
     
  5. Worse

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  6. Steve Lewis

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    I wonder if they respond with anything
    Capture.JPG
     
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  7. bluethrough

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    Obviously there are times when us scientific minds don't agree, doesn't mean either of us is wrong though.

    1) Idler and signal are different but have the same polarisation

    2)My thoughts too
     
  8. Wave_Maker

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    Copied straight from physics forum.com
     
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  9. lardy

    lardy biggest fucking hypocrite on the internet
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    Now @Feedback can't copy his answer from there too. He won't be happy about that.
     
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  10. Steve Lewis

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    The secret was surely already out, I can barely spell fisics.
     
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  11. Arfur Ap Llewellyn Europe

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    :agree:
     
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  12. Colonel Cardiffi

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    Well it's my understanding that in this case the photosensitive screen is itself the detector. I'm not 100% I've understood the question though.

    This topic fascinates me, there are some truly weird (and impossible) things going on at the quantum level, in fact the double slit experiment has led physicists, in all seriousness, to state that the moon is not there when you're not looking at it :hehe:

    The problem I have is that I would love to be able to delve further into the topic but I'm just not smart enough, you really have to know advanced mathematical concepts to get a proper grasp of it and it's just beyond me so I'll never truly understand it.

    I remember Jim Al-Khalili created a two part documentary for the BBC called "The Secrets of Quantum Physics" which was a great explanation of some of the more interesting and peculiar facets of quantum mechanics for the layman.

    I'm sure I uploaded it for someone on here before (was it you @Travis Bickle?) I'll see if I can find it. :thumbup:
     
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  13. Jimmytaff

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    What did the nerds have to say about Uncle Albert?:hehe:
     
  14. Colonel Cardiffi

    Colonel Cardiffi C'mon CITY!
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    Nevermind, they're on YouTube. Well worth a look for anyone with a slight interest in the topic.



     
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  15. Wave_Maker

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    Without engineering to bring it into the real world, physics is just philosophical BS
     
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  16. Steve Lewis

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    Naff all, it's still waiting to be approved before it's visible to the public. Stringent policies over there, I'm surprised Annis hasn't got it on his board to make sure everything posted is in his agreement.
     
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  17. Spanker

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  18. Travis Bickle

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    Yeah I watched those and his three part series about chemistry and the elements, both series were really interesting and Jim’s presentation was spot on:thumbup: I think they may still be on Catchup on BBC4
     
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  19. Colonel Cardiffi

    Colonel Cardiffi C'mon CITY!
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    Yeah, I've watched those as well. For some reason he has a weird habit of pronouncing 'hydrogen' as 'haaarrdrogen' :hehe:
     
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  20. Travis Bickle

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    Yeah I noticed that :hehe: he’s one of those blokes that I’d love to meet :thumbup:
     
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