RocketRoo has contributed another interesting comment to the post on non-local realism of last April. A long time has past since then, and as this comment is more of a new post than a comment, I am taking the liberty to repost it here.
UC Riverside physicists have apparently created the first observed diatomic positronium molecule.
I suppose if I write Pi = (e+e-) for positronium [has to be capital pi, since lower case 'pi' is a meson = (quark-antiquark) pair], then what they have seen is Pi2. Their formal paper has appeared in the Sept. 13 issue of Nature.
This is interesting for another reason having to do with entanglement and coherence; the subjects of this blog thread.
Positronium is basically unstable, and when it decays by falling into itself (like falling down a set of quantum stairs) it usually gives off 1,2,3,… photons (depending on the number of stairs). The most common decay channel is 2 photons. John Wheeler (he of the so-called "delayed-choice" interferometer, amongst other things) suggested in c.1945 that these photons should have complemetary polarizations. In fact, they were the first entangled photons produced in the lab c.1949 by Wu and Shaknov at Columbia Univ. In today's lingo, they are type-II entangled.
Because of the annihilation energy involved, however, these are gamma-ray photons. So, we have the odd situation where it is "easier" to produce entangled gamma-photons than coherent gamma-photons! That's where the Pi2 comes in. The diatomic form occurs on a silica (sand) substrate. One goal is to get enough of these groupings on the substrate to form a BEC (see Chaotic light sources comments). That, it seems, would allow one to have more than one source emitting simultaneously and therefore phase-coherently. Voilà! The gamma-ray laser.
From this I can't tell how what the binding orbitals are, how the diatoms bind to the substrate or what temperatures apply. Perhaps someone who takes a look at the Nature paper when it comes out, can report on that.
Credits: RocketRoo
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