Active upstream motion in a quantum fluid of light arising from nonreciprocal interactions
Phys. Rev. A 114, 033506 – Published 8 September, 2026
DOI: https://doi.org/10.1103/lwyj-7m5f
Abstract
Upstream motion, a typical form of nonequilibrium dynamics, is generally exhibited by active matter that is closely correlated with nonreciprocal interactions. This classic picture is likely to be extended into the quantum regime, benefiting from recent advances that endowed the interactions of quantum entities with a nonreciprocal feature. In this work, we demonstrate the upstream motion of a swimmer—a narrow optical beam—in a quantum fluid of light. The swimmer undergoes nonreciprocal mutual interactions with the optical fluid, acquiring a counterintuitive recoil inverted relative to that in equilibrium systems. Experimental validation via an optical platform enabling nonreciprocal light-light interactions confirms the upstream motion. Our findings further reveal that the maximum upstream capability is achieved at moderate fluid-flow velocity and density. Our results unveil another quantum form of active behavior induced by nonreciprocal interactions, providing key inspiration for achieving the goal of designing novel quantum devices with active functionalities.
