Life and Death of a photon

There was recently an article in Nature about how a team of French physicists managed to observe a single photon. The trick, of course, is to observe the photon without destroying it. Apparently, the trick is to use an interaction between atoms of rubidium and photons, that causes them to tick a little late. There are a few more details in this article (in French), but not much.

Now, I wonder how one can talk about “a single photon” for a particle that has been bouncing around and interacting with all sorts of particles. If a photon is absorbed, and then re-emitted, is this the “same” photon? At least, that’s the meaning I believe was given to the term “the same”. In any event, a photon with identical properties.

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