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A pump beam excites a non-linear crystal which produces two entangled photons with perpendicular polarization, namely in the state $|HV>+|VH>$. Are there examples where one of the photons was emitted slightly earlier than the other? Or in other words, is there a measurable distribution in the relative time of detection at equidistant detectors from the source?

I would think that according to quantum mechanics the emission of the two photons can never occur exactly at the same instant. Is this correct?

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    $\begingroup$ The photons are emitted at the same time; see for instance journals.aps.org/prl/pdf/10.1103/PhysRevLett.25.84 You may also be aware of the Hong Ou Mandel experiment on two-photon interference using parametric down conversion. A much more comprehensive and detailed explanation of time resolution in two-photon interference experiments in general can be accessed at link.springer.com/article/10.1007%2Fs00340-003-1337-x which I would also recommend. $\endgroup$
    – jayann
    Sep 26, 2014 at 17:02
  • $\begingroup$ Great references! I will look at them more closely. Basically, from a glance at the articles, it comes down to that the two photons will not be entangled if they are not produced with the correct sum of their frequencies and momenta to the initial beam photon. More precisely, they are not entangled if we have detectors which can discern a time, frequency or momentum difference. $\endgroup$
    – Jasper
    Sep 26, 2014 at 18:24

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