Imaging the decay of quantized vortex rings to decipher quantum dissipation

TL;DR

This study visualizes quantized vortex rings in superfluid helium-4 to validate a recent theory of mutual friction, clarifying quantum dissipation mechanisms and advancing understanding of two-fluid quantum systems.

Contribution

First experimental visualization of vortex ring decay in superfluid helium-4 confirming a recent coupled motion theory of superfluid and normal fluid components.

Findings

Vortex rings spontaneously shrink and accelerate as predicted by the theory.

Experimental data supports the coupled motion model over previous descriptions.

Results have implications for understanding quantum dissipation in various two-fluid systems.

Abstract

Like many quantum fluids, superfluid helium-4 (He II) can be considered as a mixture of two miscible fluid components: an inviscid superfluid and a viscous normal fluid consisting of thermal quasiparticles [1]. A mutual friction between the two fluids can emerge due to quasiparticles scattering off quantized vortex lines in the superfluid [2]. This quantum dissipation mechanism is the key for understanding various fascinating behaviors of the two-fluid system [3,4]. However, due to the lack of experimental data for guidance, modeling the mutual friction between individual vortices and the normal fluid remains an unsettled topic despite decades of research [5-10]. Here we report an experiment where we visualize the motion of quantized vortex rings in He II by decorating them with solidified deuterium tracer particles. By examining how the rings spontaneously shrink and accelerate, we provide unequivocal evidences showing that only a recent theory [9] which accounts for the coupled motion of the two fluids with a self-consistent local friction can reproduce the observed ring dynamics. Our work eliminates long-standing ambiguities in our theoretical description of the vortex dynamics in He II, which will have a far-reaching impact since similar mutual friction concept has been adopted for a wide variety of quantum two-fluid systems, including atomic Bose-Einstein condensates (BECs) [11,12], superfluid neutron stars [13-15], and gravity-mapped holographic superfluid [16,17].