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Detecting entangled photons

  1. Sep 21, 2013 #1
    Hello all,
    I have been searching and trying to find out how entanglement is detected experimentally, but I have a few sticking points. Initially I just want to know the principles behind the experiments. In summary this is how I understand it so far.
    1. Entangled photons produced eg in pairs.
    2. One member of a pair goes to a detector(A) the second member goes to a different detector(B)
    3. Random results detected at A and at B.
    4. Results correlate when compared on a photon by photon basis, in other words when members of each separate pair are compared.

    I think I'm okay with points with points 1,2,and 3. but have been trying to find out how point 4 is achieved experimentally.
    Am I right in assuming that some sort of coincidence detector is used?
    Are other methods used?

    I would also like to see, in more detail, how the experiments are set up. In my searches I have found some nice photographs of the equipment but these have not been helpful. Schematics would be more useful but I haven't found any except for those used to test Bell's theorem.
    Thank you to anyone who can help.
  2. jcsd
  3. Sep 21, 2013 #2
    Coincidence counting is the only method yet. The photons are correlated by time of emission .......(and then adjusted for path lenght etc).

    See if the below help:

    http://en.m.wikipedia.org/wiki/Coincidence_counting_(physics [Broken])



    Not a physicist

    Only like 1 in a trillion photon gets entangled when you try to do it
    Via SPDC
    Last edited by a moderator: May 6, 2017
  4. Sep 21, 2013 #3
    That was very helpful San K. So it seems that coincidence techniques are used in all methods tried so far. I can't think of any other way it can be done.
    Now I need to search for more details of the actual experiments. Thank you for your post.
  5. Sep 21, 2013 #4


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    Hi, Dadface.

    I'd like to point out that point 2) actually is like this:

    2. One member of a pair goes to a polarizer and then a detector(A) the second member goes to a different polarizer and then a detector(B) (my additions bolded)

    Have you looked at

    EDIT: By the way, if you are interested in entanglement you could also have a look at this PF thread (from May 2013) about a pretty recent entanglement experiment done in a new way.
    Last edited by a moderator: May 6, 2017
  6. Sep 21, 2013 #5
    Thank you DennisN. I'm sort of familiar with the spontaneous parametric down conversion method but is it not so that a polarisation state can be revealed upon detection without additional polarisers. I am interested in the wider principles of entanglement and as far as I understand it any extra polarisers are needed for specific investigations such as Bell tests.
    I will look at the things you suggested. Thank you again.
  7. Sep 21, 2013 #6


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    I am also interested in this subject.
    can we have a protocol which can give us the neumann entropy of the global state when pairs of electrons are send to Bob and Alice?
  8. Sep 22, 2013 #7
    I have done some more searching and as I see it so far the following are necessary requirements:
    . The photon source
    . Detector systems incorporating polaroid filters (different angles of polarisation investigated for Bell tests).
    . Coincidence counter/detector.

    There seem to be loads of schematics showing how the apparatus is set up for Bell tests but not so many for other tests. For example what is the simplest system that can be set up to prove that entanglement occurs? I must be searching in the wrong places.

    DennisN. I have looked at the other things you suggested. Thank you
  9. Sep 23, 2013 #8


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    Hi again, Dadface!

    I think you got it :smile:. This is the basic equipment needed to investigate polarization entanglement of photons.

    I'm not sure what you are looking for. Are you thinking about other types of entanglement (e.g. spin entanglement?). Anyway, please note that you will always have to compare measurements of at least two particles to evaluate any entanglement.

    Excellent, you're welcome.

    I just remembered that I have some other links I know which might be useful, but I have to check my resources, so I will get back to you regarding this...
  10. Sep 23, 2013 #9
    Hello DennisN Basically I wanted to see labelled diagrams of how the entanglement detecting apparatus is set up. When I wrote my previous post I had found diagrams for Bell test experiments but not for other schemes. I am mainly interested in how coincidence is achieved.
    I think I now understand it for short range schemes and I have just found and started looking at investigations using large spatial separations and with one of each photon pair travelling freely through the air. It seems that the biggest separation so far achieved is 143km. Bigger separations are being considered.
    It seems incredible that coincidence can be achieved with these new generation experiments. I have found some information on how it is done but I need to go through it.Thank you again.
  11. Sep 23, 2013 #10


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  12. Sep 23, 2013 #11


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    :biggrin: That was exactly one of the links I was thinking about, but I did not remember it before when I was posting, thanks Dr Chinese!

    I also remember a certain nice pdf document I've saved, which I believe was for students who were going to do entanglement experiments. I will see if I can find it (I have too many science links in my stash, and they are badly organized at the moment, haha).

    In the meantime you could also check out QuantumLab (uni-erlangen.de), there might be some more info there.
  13. Sep 23, 2013 #12


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    Here is another great one:


    Both read together will give Dadface some additional insight (if he is not already there).

  14. Sep 23, 2013 #13


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    Hi again, the additional links I was thinking about before may have been these (but my memory may fail me, I browsed through them and did not see much info about coincidence counting, but I post them anyway :wink:):

    From this laboratory page (University of Rochester):

    Entanglement and Bell's Inequalities;

    Quantum B Lab Presentation 2012 (p.10 has at least pictures of coincidence counters :smile:)

    George Gehring lectures: Lecture 1, Lecture 2, Lecture 3

    If I remember more, I will post it here.
  15. Sep 24, 2013 #14
    Thanks DrChinese and DennisN. I think I'm okay now with my original enquiry.The links look great. Some of the parts seem to be suitable for my present level of understanding and I look forward to reading through them properly.
    Thanks again.
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