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The study of quantum vortex dynamics in He II holds great promise to refinequantum-fluid models. Bose-Einstein condensates, neutron stars or evensuperconductors exhibit quantum vortices, whose interactions are a key elementof dissipation in these systems. These quantum objects have their velocitycirculation around their core quantized and, in He II, a core as thin as ahelium atom. They have been observed experimentally by indirect means, such assecond sound attenuation or electron bubble imprints on photo sensitivematerial, and for the last twenty years, decorating cryogenic flows withparticles has proved to be a powerful method to study these vortices. However,in these recent particle visualization observations, experimental stability,initial condition, stationarity and reproducibility are elusive or fragmentedand 2d dynamical analysis are performed although most of the considered flowsare inherently 3d. Here we show that we are able to visualize these vortices inthe canonical and higher symmetry case of a stationary rotating superfluidbucket. Using direct visualization, we quantitatively verify Feynman's rulelinking the resulting quantum vortex density to the imposed rotational speed.Our statistically meaningful results demonstrate that decorated quantumvortices behave as Feynman predicted. It follows that hydrogen flakes are goodtracers of quantum vortices for stationary cases in He II. The observed vortexlattices are analogous to Abrikosov lattices found in superconductors andBose-Einstein condensates. Moreover, these lattices aligned with the rotationaxis can play the role of a well-defined and controlled initial condition fordynamical cases. We make the most of this stable configuration by observingcollective wave mode propagation along quantum vortices and quantum vortexinteractions in rotating He II. These results provide a new experimentalbaseline for models to evolve towards a better description of allquantum-fluids.

Direct visualization of the quantum vortex density law in rotating 4He