Hong-Ou-Mandel interference

Why do photons pair up?

http://www.sciencedaily.com/releases/2015/04/150422121959.htm

What the article is saying is that if the incoming photons are different (have a different frequency) there are four different outcomes - both photons transmitted, both reflected or one transmitted and one reflected. Since they impinge on the beam splitting plate from different sides in the latter two cases (that's two cases when they emerge on the side, one for each side) they emerge together which is what the article means by "bunched up".

When the photons are identical (have the same frequency and polarization) then the wavefunctions destructively interfere in the two cases when they emerge on different sides. So they can only emerge together. The other possibilities are ruled out as the wavefunction is zero and there is some energy which has to be conserved so that particular out state is suppressed.

Originally posted by DeepThought
What the article is saying is that if the incoming photons are different (have a different frequency) there are four different outcomes - both photons transmitted, both reflected or one transmitted and one reflected. Since they impinge on the beam splitting plate from different sides in the latter two cases (that's two cases when they emerge on the side ...[text shortened]... o and there is some energy which has to be conserved so that particular out state is suppressed.

So they are not allowed to destroy each other as if one were positive matter and the other anti-matter.
That brings to mind a question: Is there such a thing as an anti-photon? That is, one that if meeting its normal type WILL destroy each other?

Originally posted by sonhouse
So they are not allowed to destroy each other as if one were positive matter and the other anti-matter.
That brings to mind a question: Is there such a thing as an anti-photon? That is, one that if meeting its normal type WILL destroy each other?

This is not related to matter or anti-matter but to quantum statistics.

To answer your question: yes and no. There is an anti-photon, but it is indistinguishable from a photon. There are annihilation reactions involving photons but they require very high energy. For example, if you consider the electron-positron annihilation it can also occur in reverse, with two photons colliding and creating an electron-positron pair.