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How can the Hanbury Brown and Twiss effect (photon bunching) be explained if photons don't interact in free space?

To explain it with the influence of the two photons on the two detectors simultaneously would seem strange to me, because of the fact that "if the source consists of a single atom which can only emit one photon at a time, simultaneous detection in two closely spaced detectors is clearly impossible" (same Wiki under the "Quantum interpretation" heading).

Comment: The question is not about uniting or dividing two photons, which is possible only on the atomic level. The question is about synchronizing two photons from different sources like the sun.

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  • $\begingroup$ In my answer to Gaining some intuition: Photon bunching I explain how photon bunching arises from interference (not interaction!) of many phase-independent waves. Also, I derive how a big number of single-photon-emitters by superposition of their emissions create a field with thermal light statistics (and hence bunching). $\endgroup$
    – A. P.
    Commented Oct 19, 2017 at 22:22

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There's no contradiction.

Without considering high-energy physics, photons don't interact in free space in the sense that they pass through each other intact. Here's a very recent review article: http://www.nature.com/nphoton/journal/v8/n9/full/nphoton.2014.192.html

Free space photons can of course lead to interference patterns and the HBT effect is still in some sense due to interference (although it's more subtle as a 2nd order coherence effect).

More accurately, it's the Bose statistics of photons that leads to the bunching.

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    $\begingroup$ the correct statement should be :"show interference patterns" . The concept "interfere" can confuse that an interaction takes place, whereas the phenomenon is all about relative phases and observed coherence. $\endgroup$
    – anna v
    Commented Sep 5, 2014 at 5:22
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To make one thing perfectly clear: photons do interact with each other. That interaction is simply very weak for low energy photons, but non-trivial at very high energies.

Photon-photon interaction does, of course, not play any measurable role in optical experiments with visible light. As for the Hanbury and Twiss effect... IMHO it's trivial and classic. Any bandwidth limited signal shows self-correlation. Do the experiment with perfectly white light (rather than with light limited to a 2:1 wavelength interval), and the "effect" disappears.

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