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Quantum Entangled Photons

  1. Sep 24, 2005 #1
    What are Quantum Entangled Photons and how are they produced?
  2. jcsd
  3. Sep 24, 2005 #2

    entangled photons can be produced by a process called "parametric down conversion". You shine with a blue laser on a BBO crystal. In the crystal, one blue photon (about 400nm wavelength) is split up into two red photons (800nm wavelength). This process is called "parametric down conversion"

    Have a look here:
    You have two (red) cones, the intersection of the two cones yields two points where you put ur detectors. There you can detect entangled photons.



    And here some papers (you need subscription, but universities will probably will access to them):

    D. Dehlinger and M. W. Mitchell, "Entangled photons, nonlocality, and Bell inequalities in the undergraduate laboratory," Am. J. Phys. 70, 903-910 (2002).

    D. Dehlinger and M. W. Mitchell, "Entangled photon apparatus for the undergraduate laboratory," Am. J. Phys. 70, 898-902 (2002).

    Or here for free:
    Last edited by a moderator: May 2, 2017
  4. Sep 24, 2005 #3
    Noticed this comment in the first referenced link given. Is this part of how they get single pairs of photons for this experiment? For the experiment to be useful we of course cannot have more then one photon being split in two at a time for the experiment to work. I’ve always wondered how they get a laser (made to generate lots of light) to “slow down” to sending photons slow enough to be considered “one at time”. Is it because of the odds of 1 in 1000000, that the subject photons coming out of the BBO are separated in time enough that no two pair come close enough in time to interfere with the testing?
    Or can they actually tune the “Pump Laser” to send out one proton a time. Such action is needed in double slit experiments to show individual photons, one at a time, can create the patterns of light and dark bands. I.e. - Without other photons getting involved in making the interference.

    Does anyone know how “Pump Lasers” are set & verified to produce photons “one at a time” for experiments like these?
  5. Sep 24, 2005 #4
    Cheers Edgardo for the links that really was a big help. It does however raises the question of whats happening inside the crystal to 'split' the original photon into two entangled photons ... hmmmmm ... i think i need to find out more about these non-linear crystals.
  6. Sep 24, 2005 #5


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    I don't know if a more explicit microscopic picture is actually known - if somebody has some pointers to that, I'm interested too. What I know about it is simply that one "quantizes" the classical non-linear constitutive equation (in classical non-linear optics, with a macroscopic polarization vector P which is a non-linear function of the E-field). Classically, you then simply have mode couplings between different frequencies, and in the corresponding quantum picture, this corresponds then of course to vertices between the photons of the different modes (frequencies).
  7. Sep 25, 2005 #6
    Last edited: Sep 25, 2005
  8. Sep 26, 2005 #7


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    The output angle of the pair is dependent on the frequencies of the light being produced, which actually occurs over a small spectrum. Also, the output only produces superposition of state where there is overlap between the two output cones (elsewhere, the pairs are entangled but their polarization state is known to be either vertical or horizontal). I think once this is factored in, the resulting number of pairs ends up very managable.
  9. Sep 27, 2005 #8
    I asked a colleague and from what he told me I understand the following:
    The photon rate at the points where you measure is about 10.000 photons per second. That is, the time between two photons is 0.1ms.
    Now you could say, that's quite fast, how would you detect those two photons independently?

    For this purpose, so-called single photon detectors which are quite fast are used. They can "scan" with 5MHz, that is 5 million photons per second could still be detected independently. So the detectors are always "waiting" for the next photon.

    Note: After a photon is detected, the detector isn't able to measure another photon immediately, it needs to "recover" for a time, which is called "dead time". But this is also in the order of microseconds.

    So unless your intensity is too high (which is not the case for the red downconverted light) you can be sure that no "two photon pairs come close enough".

    Actually the pump laser should be as powerful as possible, to produce much red downconverted light (ok, not so powerful that it destroys the BBO crystal).

    In the double slit experiment where you see only one photon at a time one could use two polarizers that can be rotated with respect to each other. Thus, by setting e.g. an angle of 89° (I don't know what the real angle in the experiments is), a low intensity is achieved.

    Quote from http://www.fas.harvard.edu/~scdirof...tonInterference/SinglePhotonInterference.html
    "The polarized light from the laser is attenuated by two rotatable Polaroid filters which allow one to adjust the intensity down to barely visible."

    Or one could use neutral density filters:
    Quote from
    "The neutral density filters are mounted in a black felt-lined holder between the laser and the slits. They attenuate the laser light by a great factor, making the mean distance between single photons quite large, on the order of a km."
    Last edited: Sep 27, 2005
  10. Sep 29, 2005 #9
    Thanks Edgardo just what I needed on how they do the experiments.

    I also noticed an additional comment that the pairs of photons are all entangled regardless of where they show up in the cones.
    But only those pairs that are inside the overlap of the cones are also in “superposition”.
    In QM “entanglement” and “superposition” are the same thing aren’t they?
    How can QM define superposition as different from entanglement?

  11. Sep 29, 2005 #10


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    Really it is just a conservation issue which leads to the entanglement. All the pairs that come out of the BBO crystals are entangled AND according operate within the HUP (i.e. anything you learn about one applies to the other). But the ones in the overlapping cones are polarization entangled too. That is because they are a mixture of two different streams (coming from 2 crystals with orthogonal polarizations). Outside of the overlapping cones, they are not entangled as to polarization. So the question is: what do you already know about the photon pair?
  12. Sep 29, 2005 #11
    I think the entangled photons from other sources maybe helpful to understand it. David has just provided a reference in the thread "Original Data from Bell's Experiment" in the forum.
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