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Fig. 1. Quantum mechanics restricts the rotational motion of a superfluid to quantized vortex filaments or loops (red). The radius of the vortex-core region is approximately 10 nm in the B-phase of helium-3. (a) When a "seed" vortex loop is injected into a rotating sample of helium-3, it can grow in length by taking energy from the normal fluid. (More than one loop is injected in the experiment of Finne et al., but for clarity only one is shown in these schematic diagrams.) For temperatures above 0.6 Tc, where Tc is the superfluid transition temperature, the vortex line expands (b) and eventually forms a straight line that is parallel to the axis of rotation. If the temperature is less than 0.6 Tc, the result is turbulence (c). The transition is thought to be due to the growth of Kelvin waves of wavelength λ on the vortices (d).