Visualization and characterization of quantum fluid flows

Long-range quantum order underlies a number of related physical phenomena including superfluidity, superconductivity, the Higgs mechanism, Bose-Einstein condensates, and spin systems. While superfluidity in Helium-4 was one of the earliest discovered of these, it is not the best understood, owing to locally-strong interactions which make theoretical progress difficult, and a lack of local experimental probes. Our group discovered that micron-sized hydrogen particles, and most recently fluorescent nanoparticles, may be used to label quantized vortices in flows of superfluid helium.  Particles not on vortices trace the motion of the normal component of the superfluid. This diagnostic tool has given us a new perspective on an old subject. By directly observing and tracking these particles, we have confirmed the two-fluid model, observed vortex rings and reconnection, characterized thermal counterflows, observed Kelvin waves, and taken local observations of the very peculiar nature of quantum turbulence.  One of many surprising observations is the existence of power law tails in the probability distribution of velocity for these flows.  That was predicted by our group and verified as stemming from the reconnection of quantized vortices. We conclude that quantum turbulence is dominated by reconnection and vortex ring collapse, making turbulence in a quantum liquid distinct from classical turbulence of a Newtonian fluid.