Reconfigurable quantum fluid molecules of bound states in the continuum

Topological bound states in the continuum are confined wave-mechanical objects that offer advantageous ways to enhance light–matter interactions in compact photonic devices. In particular, their large quality factor in the strong-coupling regime has recently enabled the demonstration of Bose–Einstein condensation of bound-state-in-the-continuum polaritons. Here we show that polariton condensation into a negative-mass bound state in the continuum exhibits interaction-induced state confinement, opening opportunities for optically reprogrammable molecular arrays of quantum fluids of light. We exploit this optical-trapping mechanism to demonstrate that such artificial molecular complexes show hybridization into macroscopic modes with unusual topological charge multiplicity. Additionally, we demonstrate the scalability of our technique by constructing extended mono- and diatomic chains of bound-state-in-the-continuum polariton fluids that display non-Hermitian band formation and the opening of a minigap. Our findings offer insights into large-scale, reprogrammable, driven, dissipative many-body systems in the strong-coupling regime. Bound states in the continuum are topological states with useful symmetry protection properties. An experiment now shows how to use them to form macroscopically coherent complexes of polariton condensates.