# Properties of “hard light”

According to this, and this, we now have the ability to make "light molecules", which, from my admittedly layman understanding, have something to do with rubidium atoms, vacuum, and temperatures near absolute zero to do science magic that I can't rightly understand. What I get from it is that the photons use all their energy on a superconducting lattice, the become entangled with it (or something) and start to interact with each other, and generally behaving like particles with mass.

My question is, what would the physical/chemical properties of a material made from these "light molecules" be? Would it have chemical properties? I'm sorry if my if the above details weren't of a good enough quality to answer the question.

• Hi. It's better to incorporate concepts embedded in your links related to your question into your question rather than to simply 'dump' links on us. It makes for self-contained questions that are of greater interest to the community and its outside readers. – Gert Nov 24 '15 at 3:49
• @Gert I edited to add a slightly better "summing up" of the articles, but the links are still there. Is this ok? – Desolationgame Nov 24 '15 at 4:01

So, it's probably more boring than you're thinking.

What has happened is: within a certain speck of substance, if you fire two photons in, they come out together.

Now, light is known to behave differently in substances, with the clearest example being refraction: light "slows down" in certain substances like water and glass, behaving a little bit as if it had a "mass" (in that it goes slower than $c$, which is the speed of all massless objects).

Unfortunately, such "linear" media are not going to allow two photons, which don't interact with each other naturally, to interact with each other. Instead, you need the entrance of a photon to discourage entrances of other nearby photons.

So that's what they did: they looked at a magnetically trapped gas of Rubidium atoms. The setup is described here, for example. The key thing about these Rubidium atoms is that they are big alkali metals, with one valence electron in the outer shell. (You sometimes see similar experiments done with cesium.) It turns out that if that one outer electron gets into an excited state (a "Rydberg" state!), the whole atom gets into this really interesting situation where it pushes away other Rydberg-state Rubidium atoms. If you cool it down, then the Rydberg states live longer than the motions of individual atoms, and rather than "pushing away" other Rydberg states they just stop those states from existing among their nearest neighbors. So this really precise balance of strongly interacting conditions was what the researchers were looking for.

So they make this ultracold dot of gas in a magnetic trap, then fire some photons into it. The photons are around the frequency that you need to create excitations in the gas, so the photons interact really strongly with the Rydberg-state atoms in the gas. So in quantum mechanics, if you have the rough energy scales to excite a field, that field can sometimes have effects on you even if you don't excite it: and that's just what these photons are presumably doing. The Rydberg states strongly discourage other nearby Rydberg states, and what they found is that the photons fired into here seem to therefore stick together, as there is a sort of repulsion for them to be apart from each other and therefore an attraction when they are both localized on top of the same Rubidium atoms in the gas.

In all likelihood, the "chemical properties" for any such substance are heavily dependent on a lot of little particulars about the gas that you're using to get the photons to interact with each other, and these "properties" become totally irrelevant when the photons leave the substance together and start being ordinary photons again. We can guess, since the photons are still bosons, that the general behavior is to create a Bose-Einstein condensate as they all "stick together", but that's about as far as I'm willing to stick my neck out.