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Can someone explain this

  1. Jun 17, 2012 #1
    Can someone explain this......

    Before I completely misinterpret what this experiment is saying, could someone explain it in terms that a lay person would understand.


    Is it in any way refuting the standard interpretation of the double slit experiment, in which knowing the path of the particle, collapses the wave function?
  2. jcsd
  3. Jun 17, 2012 #2


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    Re: Can someone explain this......

    No, it does not mean that. The authors also explicitly state that their results are in accordance with standard qm.

    What happens is in a nutshell (and as simplified as I can manage without falsifying it) the following: For an interference pattern to appear you basically need two indistinguishable ways to reach the same final event. For the double slit, the final event is the detection at the screen and the two ways are the two slits. Basically the two slits act like mutually coherent point sources and the direction between the final detection point and these two slits fixes the wave vector (in easy terms roughly identified by the direction the photon is traveling) of the light arriving from each of the slits. Basically these must be different for every detection point which leads to some different phase difference between both possible ways the light could take at each point and therefore to an interference pattern.

    Now in the present experiment they use a slightly different beam consisting of two peaks. Entanglement allows them to monitor which peak goes to which slit, thereby giving which way information. However these peaks are somewhat different. If you do the math, you will find out that if you measure the wavevector of the idler photon, you do not instantly get a well defined wave vector for the signal photon like in usual experiments concerning entanglement, but you will get a double peak structure allowing two different wave vectors. This is a consequence of the double peak shaped pulse shape the experiment starts with (check figure 6 of the original publication - it is available for free).

    Now remember that in the standard double slit the two slits lead to two possible paths to the final detection spot with the light having different wave vectors for each of these two possible paths. In the present experiment you already have two different allowed wave vectors present within that single beam already as laid out above. If you now do the math for the present experiment, you will find out that these two wave vectors are already two different indistinguishable ways that suffice to create an interference pattern. It should be noted that the interference pattern is seen in coincidence counting, so it is a little bit different from a standard double slit experiment anyway. One could in principle construct a similar experiment for an experiment without coincidence counting, but I suppose the visibility of the pattern might be lousy.
  4. Jun 18, 2012 #3
    Re: Can someone explain this......

    Interesting experiment Fiziqs. We are getting both: which-way and interference pattern.

    I guess - this does not refute the standard interpretation because we are using two photons.

    Looks like - one photon is being used to get which-way, the other photon is being used to re-construct/create the interference pattern, after it has been destroyed.
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