# Bose-Einstein Condensate Photons

• B
https://www.livescience.com/10288-kind-light-created-physics-breakthrough.html

I was reading here that you can freeze photons.
What does it mean to freeze up a photon, are you slowing down it's motion, changing it's energy levels. Or are you changing the state of the particles around it and that then would cause it to change it's state?

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king vitamin
Gold Member
The photons aren't really "frozen," a work which usually is used to describe a solid state of matter (where particles form some regular lattice). A Bose-Einstein condensate (BEC) is more like a quantum liquid or gas depending on whether the BEC is approximately incompressible or not. And here I should say that when we call quantum phases "solid/liquid/gas" we are really making analogies with their classical definitions, and their quantum counterparts behave differently in general.

The reason why one gets a liquid-like state at low temperatures involves the Heisenberg uncertainty principle. At low temperature, the system will relax to its ground state, meaning a state of minimum kinetic energy and minimum interaction energy. Here we can model the system as have some short-range repulsive interaction between any pair of particles. In a classical system, one expects that all particles will simply have zero kinetic energy in their ground state, and one simply chooses the particle positions such that the interactions between them are minimized - this is usually some lattice, and the ground state is a solid.

In contrast, in quantum mechanics the Heisenberg uncertainty relation tells you that you cannot place the particles in definite positions without the particles having extremely large kinetic energy. If the repulsive interactions between particles is small enough, the system will prefer to have low kinetic energy and have the particle positions be uncertain across a large distance (the particles overlap, but the reduction in kinetic energy beats this interaction cost). Then the system looks like a liquid in that particle positions fluctuate so that on average the particle density is constant everywhere.

Everything I discussed above qualitatively describes massive bosons - usually for light BECs don't form because photons can just choose to annihilate themselves into the vacuum, so the low energy state is just a state with no photons. The technical achievement of these researchers was to engineer a system where photons have an effective mass and some weak repulsive interactions between them, so they could get BEC physics.

Okay thanks. Now in the scenario where all the photons get annihilated when cooled down does it also mean that when it is warmed back to the original temperature these photons would reemerge?

king vitamin