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B Double slit with one detector

  1. Jun 27, 2017 #1
    In the double slit experiment , if one place only one detector around one slit, while leaving the other slit unchecked, the interference pattern vanishes. Does this show entanglement?
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  3. Jun 27, 2017 #2


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    No, why would it?
  4. Jun 28, 2017 #3
    If the detector did not click, one will know that the particle passed through the other slit.
  5. Jun 28, 2017 #4


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    What other particle? The photon source emits single photons, not entangled pairs.
  6. Jun 28, 2017 #5
    But that is only observing that a single particle goes through one slit rather than another - nothing spooky about that, is there?

    I am a complete newbie when it comes to reading up on QT, but If I have the history correct, the first proposal for what later was to be called entanglement was the EPR thought experiment in 1935, which stipulated a radioactive particle which decays into two equal-mass particles; this led Schrödinger to propose entanglement; which in general involves pairs or groups of particles with a common origin & with quantum states that can't be described separately, even if they are far apart; or so I read, though I don't claim to understand yet.

    Whereas with the double-slit experiment, for most of its history the point hasn't been to explore entanglement but rather wave vs. particle behaviors; i.e. that light behaved like a wave in showing interference (Young, 1801), and did the same even down to single photons as they accumulated (Taylor, 1909); and later that not only did closing a slit remove the interference effect, but so did putting detectors at the slits.

    A later variation of the double-slit apparently does involve entanglement; this is the "delayed choice quantum eraser" first presented in 1999. By coincidence, earlier today I was reading about a DIY home version you can do of this experiment, described in May 2007 in Scientific American; although I haven't read enough yet to know how similar or different this DIY version is from the original.
    Last edited: Jun 28, 2017
  7. Jun 28, 2017 #6


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    That experiment is using entanglement as a clever way of measuring the photon's position without absorbing it. Just because we use this technique in a double-slit experiment doesn't mean that the experiment is demonstrating entanglement.
  8. Jun 28, 2017 #7
    Having thought about it a bit more -

    I'm not at all clear that the distinction applies in this case. It's true that I'm not clear as to anything involving QT; but to me, "use" and "demonstrate" are ordinary words here; so we could as well be talking about hammering a nail as about QT when it comes to understanding the intent.

    Obviously there can be a distinction drawn between what an experiment's main point is - what is intended to show - versus the technical means used to demonstrate this point; however in this case, if I look at both the original study & Wikipedia's reporting on that study, entanglement seems to have been very much one of the aspects the authors wanted to demonstrate; and in fact they say as much in their conclusion.

    Some excerpts:

    Link to the original study is here; and this is the abstract – quite short; bold is mine:
    Similarly the link to the Wikipedia article is here; and this is the lead paragraph - bold is mine:
    And here is the beginning of the subsection in that article titled "Does delayed choice violate causality?":
    And back to the study itself again, here is the concluding paragraph; I have bolded the last sentence:
    That sentence seems to show that they wanted not merely to use entanglement, but to demonstrate something interesting about having used it. Which makes it part of the study's focus.
    Last edited: Jun 28, 2017
  9. Jun 28, 2017 #8


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    This is a great paper, and there are a number of PF members that are familiar with this and related variations. But I guess I didn't see a particular question.

    I suspect everyone here would agree with you that this experiment uses entanglement to demonstrate some very interesting elements of QM. One of those elements being issues around time ordering: in many cases, ordering of events has no impact on the outcomes. That is true even when it defies classical causal ordering.
  10. Jun 28, 2017 #9
    In response to the OP's original question, I said it was my understanding that most of the double-slit experiments historically have not been concerned with entanglement; I then mentioned the quantum eraser experiment as a possible exception - I had come across a mention of it but hadn't looked at it yet.

    @Nugatory then said, in response to my comment, that "Just because we use this technique in a double-slit experiment doesn't mean that the experiment is demonstrating entanglement." I interpreted this, perhaps mistakenly, as going against what I read in the study itself. Or not; I'm just trying to clarify so I understand as much as I can, bearing in mind that the technical details are over my head.
    Last edited: Jun 28, 2017
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