Coherence, long-range transport and nuclear polarization in a driven-dissipative dark exciton condensate
We report direct evidence for macroscopic coherence in a condensate of dark dipolar excitons in coupled quantum wells and show that its formation follows a non-equilibrium, driven-dissipative mechanism. The condensation transition is governed by gain-loss competition, in which the exceptionally long lifetime of dark excitons enables their dominance in mode selection. Condensate formation is revealed by photoluminescence darkening, changes in radiative recombination channels, and the emergence of long-range hydrodynamic transport manifested by propagation of density (sound) modes over millimeter-scale distances. The buildup of dark exciton density induces dynamic nuclear polarization, which closes the dark-bright exciton gap, \Delta, via the Overhauser field. This leads to nuclear spin polarization across the entire mesa, far beyond the optically excited region, and produces pronounced hysteresis behavior. At \Delta ~ 0 the gap is locked and the condensate loss are minimal, resulting in a second threshold manifested as a photoluminescence blueshift. Coherence is revealed through interference between incident and boundary-reflected exciton currents, producing spatial modulation of the photoluminescence from the radiative reservoir and enabling extraction of the condensate coherence length. These results establish dark excitons as a platform for coherent quantum fluids in a driven-dissipative, strongly interacting regime with electrical tunability, bridging the physics of polariton condensates and matter-like excitonic systems.