Probing Superfluidity with Quantum Vortex Necklaces

Abstract: We present a method for measuring the superfluid fraction of a Bose-Einstein condensate (BEC) without relying on external perturbations or imposed optical lattices. Our approach leverages the intrinsic rotation of vortex necklaces in one component of a binary superfluid mixture, where the vortex cores act as effective potential wells for the second component. The rotation of the vortex necklace transfers angular momentum to the latter, enabling a direct determination of its effective moment of inertia. Comparing this value with its classical counterpart allows us to extract the superfluid fraction, which we find to be precisely bracketed by the Leggett bounds. By increasing intercomponent interactions, the second component undergoes a crossover from a delocalized and fully superfluid state to an insulating state consisting of a regular array of localized density peaks. Furthermore, the dynamical instability of vortex necklaces provides a natural framework for investigating superfluidity in dynamically evolving and disordered landscapes.