An electrical current can set a quantum fluid in motion. Our new paper in Physical Review B explains how this drag emerges from microscopic interactions between a Bose–Einstein condensate and a moving reservoir of particles.

The key process is scattering: the reservoir exchanges energy and momentum with the condensate while leaving its particle number unchanged. We derive an equation for the coherent condensate wave in which these collisions generate a drag potential. The drag strength follows from the reservoir’s density fluctuations and the condensate’s density profile, without introducing an independent phenomenological drag coefficient.

We apply the theory to an exciton–polariton condensate, a quantum fluid of light and matter, interacting with a drifting electron gas. An electron current flowing alongside the condensate increases its momentum; a current flowing against it slows the condensate down. Our simulations reproduce these shifts in the momentum distribution, in agreement with earlier experiments on electrically controlled polariton flow.

The work also connects this electrical drag to energy damping, a reservoir interaction familiar from the theory of atomic Bose gases. Both involve scattering that changes the motion of a coherent field without exchanging particles with it. Polaritons therefore offer a route to testing the same momentum-transfer mechanism in a different experimental setting.

This work brings together collaborators at the University of Maryland, the University of Pittsburgh, and the University of Otago.

Paper: Hassan Alnatah, Shuang Liang, Shouvik Mukherjee, Qi Yao, Ashton S. Bradley, and David W. Snoke, Microscopic theory of drag in a Bose condensate interacting with a moving reservoir, Physical Review B 114, 194514 (2026). Published 26 October 2026. Read the open-access preprint.