[beautifulfreak]

What would be cool is a circulating superfluid, say inside a hollow torus, that could be made frictional on command at a chosen spot on the interior surface of the torus. Imagine holding the torus like a steering wheel. Inside, the superfluid is rapidly circulating. Say when a laser zaps a region where fluid and container touch, the superfluid becomes slightly frictional in that spot. One would feel the steering wheel yank in the direction of flow. So long as energy could be added back to the superfluid to accelerate its circulation, it could be extracted again and again with laser zaps to do work. (How many ways can a superfluid circulate inside a hollow torus?)

[eesmith]

Why would it be cool?

We can do something like that already with current flowing in a circle in a superconductor. Resistance corresponds to friction. In the worst case you get a quench and all of the stored angular momentum results in a torque that breaks your machine.

[beautifulfreak]

Because it would do work in a novel way. It could be used to levitate a spaceship, for example.

[eesmith]

No, it wouldn't. It would cause a torque as the momentum from the fluid is transferred to the torus. It would not cause a lift. There's no overall momentum change.

It's basically a gyroscope, albeit with quantized vorticies.

An external magnetic field could be used to levitate an object. But that's not novel.