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].
Starting from the Pauli current we obtain the decomposition of the nonrelativistic local velocity in two parts: one parallel and the other orthogonal to the impulse. The former is recognized as the “classical” part, that is, the velocity of the center-of-mass, and the latter the “quantum” one, that is, the velocity of the motion in the center-of-mass frame (namely, the internal “spin motion” or zitterbewegung), Inserting the complete expression of the velocity into the kinetic energy term of the classical nonrelativistic (i.e. Newtonian) Lagrangian, we straightforwardly get the appearance of the so-called quantum potential associated, as it is known, to the Madelung fluid. In such a way, the quantum mechanical behavior of particles appears to be strictly correlated to the existence of spin and zitterbewegung.