How does the superfluid rotate?
MARTIN: Well, it has to have vortices inside. Now, these vortices, they cannot just suddenly pop up in the center of the superfluid. That would violate conservation of angular momentum in this superfluid. They have to come from the outside. So, at the rim of our puff of gas, we see some fluctuations that appear here and there. These are already the vortices and they sometimes, after a while, they make it inside. Some of them make it inside, and then some more, and then some more, and eventually we end up with a huge vortex lattice piercing through the superfluid core and that is the final state. This would happen, if you keep the rotation on, it would also still happen in a metastable way. If you stop the rotation after a while, the vortices would still remain there because, as a superfluid, it doesn’t even feel friction. So, at zero temperature, it would rotate forever. We are not at zero temperature so, eventually, if you stop the stirring, the normal cloud will spin down and it will
