If it can help:

- Supersolid: it's a state of matter when a system is both organised like a solid but presents superfluid-like flow without viscosity.

- Driven-dissipative: it's a qualifier for systems which are dissipative (excitations of the systems dissipate energy as heat) and driven by external fields (in this case strong electromagnetic fields are keeping the supersolid in shape). In physics, "driven" usually refers to an external influx of energy or force kept on the system.

- non-equilibrium system is what we call systems where the physics cannot be fully described by a statistical analysis of the whole over long times. These systems have transitory behaviours which are often complex before they return to equilibrium (unless they are driven away from it).

Exciton-polariton: excitons and polaritons are what we call quasiparticles. They're not particles in the sense of an electron or photon, but instead they are excitations of the collective system which look like particles. Kind of like how waves on the ocean aren't one water molecule, but a bunch of them. An exciton has no charge (it's essentially an electron and a missing electron stuck together). When an exciton couples to electromagnetic waves (photons), it can make a special type of polariton (another quasiparticle) which we call an exciton-polariton.

- Photonic = made of photons

> a system is both organised like a solid but presents superfluid-like flow without viscosity

This is hard for me to grasp. For example, if it bumps hard into a hard solid surface, does it shear like, say, bent metal? Or is it itself 'hard' / rigid - if that is a reasonable descriptor for something at such small scales - itself, and - despite being able to flow, with no friction, smoothly - still remain 'locked in shape' at a structural level? My real question I guess is how does a collision work here?

Or does it go through the hard solid surface and emerge unscathed on the other side? Probably not or it will fall towards the center of Earth and bounce back and forth, at least until it loses its supersolidity, which I bet will happen pretty quickly.

So, about the excitons - this is a bit like, with semiconductors, people talk about the "holes" as though they were particles moving and interacting:

https://en.wikipedia.org/wiki/Electron_hole

but it's more like a way of describing a system.

Which also means that nobody is making a solid object from photons, it's all "behaves like a solid would if you look at our system a certain way". Right?

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Also - I don't see how the phrases "behaves like a solid" and "presents flow" can agree. Isn't the whole point of a solid that it _doesn't_ flow?

So there are two aspects to reconcile the solid and fluid parts together.

First, just like a superfluid, a supersolid can have a portion that flows and one that doesn't (the normal fraction).

The second part is that, as you rightfully mentioned, holes are just vacancies in the structure. Here the vacancies in the crystalline structure they made are able to flow along the solid. But as they flow, the structure retains its shape. And at low enough temperature, these excitations condense into a macroscopic quantum state which makes them a superfluid.

So in a nutshell, you have an organised structure like a solide, but a portion of that structure flows without breaking the overall structure.

Can we have lightsabers now?

Is Exciton-polariton what photosynthesis captures?

Thanks, appreciated

Not a single quantum in there. Could be even more advanced!

Photons, held by force-fields? Sounds Trekky.