If you think of photons, you can maybe ponder why do we not have a complex form made light. For example. If you’ve other particles such as electrons, atoms, they form complex structures, they form liquids, they form solids. All the world around us is made matter which is basically quarks and leptons. But why we cannot create similar things with light? Light is a boson, so in principle we could attain a fluid of light or even superfluid of light or maybe even a solid of light. So why that’s not been possible so far?
So one reason is that photons don’t interact: the cross-section between the interaction between two photons is very tiny and optical frequencies in principle the photons could interact via creation of electron-positron pairs but in practice it doesn’t happen beca the process is extremely unlikely. So photons don’t interact, and this is one of the limitations why we cannot complex structures.
Another limitation is that the photons are massless, the mass of photons is zero. For any complex structure that’s liquid or solid to be built, there is a contest between kinetic and potential energy, so if photons not have a mass and they don’t interact, we don’t have this notion and we cannot more complex forms made of photons. However, this can be overcome nowadays, so we can cheat the nature. We can give the photons a mass artificially and we can give photons interactions. So how can we do that?
So to give a photon a mass it’s sufficient to capture the photons in one direction. If you can mirrors such that the photons won’t escape exterior the mirrors, the motion will be frozen in one direction, it’ll be possible to move in only one direction perpendicular to those mirrors, then effectively in these two other directions it’ll imply that the photons are massive. This mass will be finite but it’ll be very small, so it’ll be in order of 10-9 of typical atoms. This is actually quite ful beca if the masses are light, then any quantum effect would show up at very high temperatures, even up to room temperatures.
So we can fix the problem of the mass by capturing the photons between two mirrors. The problem of interactions between photons can also be solved if you couple the photons to something of a matter type beca if the photons will annihilate and , let’s say, an exciton or if they couple to some internal degrees of freedom interior an atom, for example, then via that other object the photons will effectively interact. So this was possible to attain either in semiconductor structures with electron-hole pairs, so excitons, or atoms placed in cavities interaction between photon could be achieved via coupling to this internal atomic degrees of freedom.
So you could solve these two problems, the mass and the interaction between photons. Beca that was possible in experiments to be done, we could create photons interact interior that cavity and in experiment we were able to actually a fluid of light, so interacting photons with a mass which is very similar to any other fluid and in specific beca the mass is so light, it was possible to attain quantum effect such as, for example, superfluidity at very high temperatures.
So presently can stand back a tiny and ask what’s superfluidity. Superfluidity is a state of matter, it’s a new state of matter microscopic no of particles, a very large no of particles form one object, they behave collectively, they behave together beca they occupy the same state. Now, beca each atom isn’t an individual entity, if the atoms (or photons in our case) start flowing, they’ll flow without any disturbance. A well-known example is liquid helium they can flow up in a container and escape the container: if they flow and they meet an obstacle, they’ll not scatter. If we see distant away the obstacle, it’ll see as if our superfluid isn’t having any resistance to anything, it doesn’t look anything.
So that was possible to be d for photons coupling them to electron-hole pairs in semiconductors forming polaritons. Once the quantum fluid was d, once the superfluid was d, it allowed injection of quite an fascinating object: we could keep vortices interior such a superfluid, we could inject persistent currents into the superfluid, observation of the frictionless flow was possible. Our photons were doing a very similar thing to what liquid helium or electron-hole pairs in semiconductors, flowing without resistance, not seeing any obstacle on the way and supporting persistent current, supporting vortices.
So then we went a step further in experiments in various groups looking at the transition between a normal state and the superfluid state in the case of these interacting photons. It was possible to look that the transition between a normal state and the superfluid state very similar to the matter particles is happening via creation and annihilation of vortex-anti-vortex pairs, so binding and unbinding those pairs, in the same spirit as Kosterlitz-Thouless transition. So we already had our quantum fluid which presently can be d as a laboratory to examine quantum turbulence, to examine conduct of the vortices, conduct of the solitons, but presently we wanted to go one step further. Now we’ve a fluid, we’ve a superfluid, why can’t we a solid out of light?
And that’s a very new area of research at the moment. We can do that, we can a potential for light. There are various ways of doing that technologically: one can keep different pieces of a different material with a different refractive index on the top of our mirrors giving effective potentials for light or can keep surface acoustic waves, so modulate the energy of photons via external soundwaves. Instead of having two-dimensional structure one can create one-dimensional lattices out of semiconductor. So there are various ways of creating potentials for photons, and if presently you space a photon which has a mass and interactions interior a potential, then you’ll something that’s just the same as in the case of solid.
Why’d we be interested in doing that? As I said, the photons have a very light mass, so the quantum effects are happening at very high temperatures. So presently by creating these artificial latices for photons, we can attempt to model, to quantum simulate some other materials which are much more complicated to study than photons. Moreover, this is already quite an active field in the area of ultracold atoms but ultracold atoms have their limitations. Firstly, they’ve to be cooled to extremely low temperatures, and here, with photons, we can be nearly at the room temperature: that’s one advantage of that system. Another advantage is that the mirrors are never ideal, so there always will be some leakage of photons coming out of these cavities, and by capturing the photons and measuring their energy, their momentum and their position, we know everything about the system inside. So we know not only the ground state, we know also all the excitation spectrum of the system which can be resolved in space. So the quantity of information which is given in that system is by distant much larger than the information which can be drawn other platforms, the ultracold atoms, for example.
At the moment the photons interior those lattices are still in a so-called semiclassical level which means that there is a quite no of photons, of polaritons on a one lattices’ side. So presently the goal of the field, the open question of the field which is mainly for experimentalists to solve is how to create those latices tight sufficient so that only one or a few photons can be placed in these lattices so we can examine strongly correlated states with this platform. However, even without that tight confinement already these photons in lattices or polaritons in lattices have been d to see at topological states.
In particular, you can imagine that if you a in lattice which is very similar to graphene, a honeycomb structure, then what happens is that these materials have very fascinating properties. They’ve a linear dispersion, the so-called Dirac Cone, and the same was possible to attain in the case of photons. And now, beca one can also engineer a spin-orbit coupling in the case of this photonic systems, when we space this photonic system interior a magnetic field, it’s possible to open a gap in the spectrum and then the edge states which will be interior the gap. What it means is that if you’ve your lattice and you begin a current which would flow of these photonic particles, if you can attain topologically protected state, if would imply that if you inject the current in one direction even if it meets some other obstacle, some imperfection at the edge, it’d just always low around in one direction, if will presently flow in the other direction, there will be number scattering back. So these topological protected states are quite an interest in the community for encoding the information, let’s say, in the direction of that flow.
So basically theses photonic quantum fluids not only authorize the realization of superfluidity and the study of the superfluid property such as vortices, quantum turbulence. It’s presently emotional into the area of quantum simulations they can be placed in lattices and simulate hopefully strongly correlated stated soon but topological states already.
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