A two-photon source based on spontaneous parametric down- conversion emits pairs

Question

A two-photon source based on spontaneous parametric down- conversion emits pairs of identical photons (a) If measure g using a standard Hanbury Brown and Twiss setup, what would be the smallest value of g2(0) that you can get for such source? (b) What would be the main causes of higher g(0) in experimental measurements for photon-pair sources based on spontaneous parametric processes? (c) If measure a Hong-Ou-Mandel effect, what coincidence rate do you expect at zero delay for an ideal source of identical photon pairs? d) Why is this coincidence rate usually higher in real experiments?

Explanation / Answer

a) In SPDC,the two photons can be released at the same time by splitting the pump photon. For such source, The smallest value of g2(0) = 0 as the two photons are ideally perfectly correlated.

b) The experimental value will be higher than 0 which is due to the emission of multiple photons at same time as well as accidental counts.

c) For zero delay the coincidence count must be zero as both the photons will be moving in the same direction in HOM experiment.

d) This is due to the imperfections in the optics used for experiment as well as detection errors.

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