Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How would MWI explain delayed choice quantum eraser?

  1. Feb 28, 2015 #1
    How would MWI explain the delayed choice quantum eraser?


    I don't see how this can be explained with MWI.

    If the idler photon hits detector 1 or 2, an interference pattern can be seen on the screen.
    If the idler photon hits detector 3 or 4, NO interference pattern is seen.

    But the screen is hit BEFORE the idler photon hits the detector.

    Can someone please explain this in detail according to the MWI explanation?
  2. jcsd
  3. Mar 1, 2015 #2


    User Avatar
    2017 Award

    Staff: Mentor

    In the same way every experiment works in MWI: let the wave-function evolve.
    You'll see that after the experiment is done (at the time where you check correlations between the detectors) all squared amplitudes match the observed frequencies (within statistical fluctuations of course). That is easier than collapse interpretations because you don't have to ask where which collapse happens.
    If you find a detailed description of the experiment with collapses, remove the collapses and you get a detailed description in MWI. I don't have time now to write down everything, sorry, and I'm sure others did that before anyway.
  4. Mar 1, 2015 #3


    User Avatar

    Staff: Mentor

    The interference pattern is not seen either way, if by "seen" you mean that it can be observed before (or after, for that matter) the idler has been detected. An essential aspect of Kim's experiment is that there is no screen for the signal photons to strike and form an interference pattern. Instead there is a detector that is moved from one point to another, and at each point we see how many photons strike at the same time that an idler photon goes one way or another - the interference pattern only shows up in the after-the-fact correlation of idler photon detections and detections of the corresponding signal photon at various points.

    The MWI explanation is that every entangled pair gives rise to two worlds, one in which the idler photon hits detectors one or two (another split) and the signal photon is more likely to end up in some places than others (with further multiplicity of worlds); and one in which the idler hits detectors three or four and the signal photon is more likely to end up in different places. Pick one of these worlds, then repeat for the next pair, and eventually you'll find yourself in a world in which a large number of detections have been made, all consistent with interference when the idler went one way and no interference when the idler went the other way.
    Last edited: Mar 2, 2015
  5. Mar 2, 2015 #4


    User Avatar
    Science Advisor

  6. Mar 5, 2015 #5
    Actually it's four worlds, |D1> |D2> |D3> and |D4> :biggrin: There are two which end up in separate "interference histories" and two which end up in "non-interference histories". The two "interference histories" show opposite peaks and troughs so they add up to a non-interference pattern. The two "non-interference histories" are extremely similar but are staggered by the slit spacing which is negligible. So as you say, a screen at D0 would definitely show a non-interference pattern.
  7. Mar 5, 2015 #6
    It's important (I think) to realise that the interference patterns are not created by the two-slits! They, in fact, merely allow the laser to pump the down-converter at two small places. The down-conversions are independent events. The interference patterns are not normal interference at all, they are an example of "ghost" interference. What happens is this. The two paths effectively catch some photons and miss others. Thus they create a double aperture for the idler photons. The angle of the idler photon is mirrored by the signal photon, hence the (encrypted) interference patters are those of the double aperture, not of the original slits. I believe this accounts for why the interference patters are so crude compared to the nice clear stripes that a proper Young's Slit set-up can give.
  8. Jul 26, 2015 #7
    This is incorrect. The pump is in-phase across the BBO so it behaves exactly like a two slit source of entangled photons even though it is not physically two slits. The aperture argument is wrong.
  9. Oct 6, 2015 #8
    this helped me to understand, in a non-mathematical sense, what the delayed choice quantum data was showing , observation of an event entangles the observer and the observed.
    from wikipedia

    Everett's Ph.D. work provided such an alternative interpretation. Everett stated that for a composite system – for example a subject (the "observer" or measuring apparatus) observing an object (the "observed" system, such as a particle) – the statement that either the observer or the observed has a well-defined state is meaningless; in modern parlance, the observer and the observed have become entangled; we can only specify the state of one relative to the other, i.e., the state of the observer and the observed are correlated after the observation is made. This led Everett to derive from the unitary, deterministic dynamics alone (i.e., without assuming wavefunction collapse) the notion of a relativity of states.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook