Circulation in superfluid helium is quantized — each vortex carries exactly one quantum of circulation, κ = h/m₄. Multiquantum vortices, carrying 2κ or 3κ, are energetically unstable: they should split into single-quantum filaments almost immediately. This is textbook quantum fluid mechanics, confirmed by decades of experiment. A vortex ring in helium-4 carries one quantum. Period.
Xing and colleagues tracked frozen deuterium particles trapped on vortex rings in He II and found events where the particle kinematics are incompatible with single-quantum circulation. The speeds are too high. The trajectories require an effective circulation nκ with n > 1. Either these are multiquantum vortex rings that persist long enough to be observed, or they are tightly bundled single-quantum rings traveling together.
Vortex-filament simulations rule out the bundle explanation — closely spaced single-quantum rings disperse quickly. The particles should escape as the bundle separates. Instead, the particles remain trapped at high velocities, suggesting a deeper core potential that only a genuinely multiquantum core provides. The binding is too strong for the unstable explanation.
The textbook instability is real — multiquantum vortices in bulk superfluid do decay. But a vortex ring is not bulk superfluid. The ring geometry, the finite core size, and the particle loading may stabilize what the linear stability analysis says should collapse. The instability was derived for infinite straight vortices. The ring's curvature changes the problem.
What persists isn't what's supposed to persist. The theory that predicted rapid decay was correct about the mechanism but wrong about the geometry where it applies.