Superfluids, quantum vortices and their interaction with particles. Can we really use particles to study quantum vortex dynamics?

When a system composed of Bosons is cooled down, an amazing transition takes place. The system becomes superfluid and exhibits surprising physical properties. Unlike any classical fluid, at very low temperatures a superfluid is characterised by the fact that its viscosity vanishes. Even more strikingly, as it was first suggested by Onsager and Feynman in the 40's, due to quantum effects the circulation of the flow (a line integral of the velocity field around a closed path) is quantised. This unique property is a consequence of the fact this quantum system is described by a macroscopic wave function and such vortices appear as topological defects of the order parameter. Because of this reason, such vortices are known as quantum vortices. Quantum vortices exhibit a hydrodynamic behaviour and their dynamics is crucial to understand the smallest scales of what we know today as quantum turbulence.

For more than a decade, particles have been used to directly visualise quantum vortices. Such particles are typically several orders of magnitude larger than the vortex core size, but nevertheless it is believed that particles are trapped in quantum vortices and by following their trajectories, we can study the dynamics of quantum vortices. In this seminar, I will explain how such particles have been used to experimentally study quantum vortex dynamics, what is the physics that has been revealed and what are the possible issues of using such particles to track vortices. To explain the particle-vortex interaction I will used a simple self-consistent model based on the three-dimensional Gross-Pitaevskii equation coupled with classical particle dynamics. I will show numerically and theoretically the kind of interaction that arises between quantum vortices and particles and what are the consequences of it. Then, I will try to address the question of how well particles that are trapped inside vortices can be used to track vortex dynamics.