# Order of interference

yefeiwen
Hi everyone, I am reading some paper about quantum interference experiments. They mentioned some experiments are first-order interference, some are second-order interference, some are fourth-order interference. What do these mean? And why I never read anything about a "third-order" interference?
thanks
Ye

cesiumfrog
Is this by any chance the Bennett, et al, paper: PR A 77 023803 feb'08? Apparently the http://en.wikipedia.org/wiki/Degree_of_coherence" [Broken] (accidental pun?) have been used for decades (since Glauber?); not sure whether they have a simple physical interpretation.

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Baggio
Hi, I'm Raj Patel and I'm actually working with Anthony Bennett at Toshiba, we have a couple more papers on two-photon interference being published in APL and PRL (within the next week or so)... in the meantime http://arxiv.org/abs/0804.1897 , http://arxiv.org/abs/0803.3700. In the PRL paper I measure first and second-order coherence functions.

To answer your question the degree of interference, first-order coherence and second-order interference are considered the same thing. And second-order coherence and fourth-order interference are also considered the same thing.

First-order coherence is what you would measure if you shone light into a Michelson interferometer and observe interference fringes. Essentially this is a single-photon interference experiment - interference between the probability amplitudes of a photon taking a particular route through the interferometer. It is a measure of correlations between field amplitudes and it is second order in the field amplitudes.

Second order coherence basically describes correlations between field intensities between two points. It is fourth order in field amplitudes. A common test of how good a single photon source is to measure the second-order correlation function. This is basically achieved by firing photons at a beamsplitter and placing a detector equidistant from each output port of a 50:50 beamsplitter. If it's a perfect single photon source you will never measure simultaneous detection events.

Hope that clarifies things a bit.

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Demystifier