$\mathbb{Z}_2$ topological insulator analog for vortices in an interacting bosonic quantum fluid

Abstract

$\mathbb{Z}_2$ topological insulators for photons and in general bosons cannot be strictly implemented because of the lack of symmetry-protected pseudospins. We show that the required protection can be provided by the real-space topological excitation of an interacting quantum fluid: quantum vortex. We consider a Bose-Einstein Condensate at the $\Gamma$ point of the Brillouin zone of a quantum valley Hall system based on two staggered honeycomb lattices. We demonstrate the existence of a coupling between the winding number of a vortex and the valley of the bulk Bloch band. This leads to chiral vortex propagation at the zigzag interface between two regions of inverted staggering, where the winding-valley coupling provides true topological protection against backscattering, contrary to the interface states of the non-interacting Hamiltonian. This configuration is an analog of a $\mathbb{Z}_2$ topological insulator for quantum vortices.