Can Two Photons Really Form Bound States?

[Claustral]

With great interest I read an article about a paper where scientists were able to create two photon bound states ("molecules of light").

http://physicsworld.com/cws/article/news/2013/sep/26/physicists-create-molecules-of-light

I was quite astonished since light normally does not self-interact (apart from Delbrück scattering, which is strictly speaking no direct interaction). Of course, in this experiment the interaction between the two photons is not direct either, but mediated via the interaction of the photons with the electrons in the ultracold Rubidium atoms. Still I wonder, how such a two photon bound state is described theoretically and how far one can go with the analogue of a normal molecule: Can one define an average distance between these two photons? It sounds at least strange, since photons are normally not localizable, are they? Are these states photons at all or merely mixed photon-electron states? How long can this bound state in principle live? Are there energy levels such as a electron bound to a nucleus has them?

 

[UltrafastPED]

This is a coherent interaction between a highly excited atom (Rydberg state) and the coherent forward scattering of the follow-on photon.

It is a fine example of very "non-linear optics", and depends upon the states of the matter. It is probably best regarded as a new kind of pseudo-particle.

You can read the Nature article abstract here: http://www.nature.com/nature/journal/v502/n7469/full/nature12512.html

 

[Claustral]

Thanks for the answer. I got access to the original article and it helps a lot.

 

[meBigGuy]

When I read this it seemed like what they are describing is 2 photons with the appearance of being bound, not actually bound. Their progress through the medium is controlled by the medium in such a way that they exit together, but then there is no "bound" characteristic other than their proximity.

Am I missing something obvious?

 

[TumblingDice]

When I read this it seemed like what they are describing is 2 photons with the appearance of being bound, not actually bound. Their progress through the medium is controlled by the medium in such a way that they exit together, but then there is no "bound" characteristic other than their proximity.

Am I missing something obvious?

It reads that way to me, too. The the first photon parties with more than one atom at a time, creating the state and slowing the speed of the photon. Then they seem to suggest that the second photon stays behind the first because the state offers a different index of refraction than the surrounding cloud. Are they suggesting a lower index of refraction is fooling photon #2 into following as closely as possible to slower photon #1?