Direct observation of room-temperature exciton condensation

Room-temperature condensation of pure excitons would provide a solid-state analogue of superconducting phase coherence, enabling collective quantum transport under ambient conditions. Yet directly verified phase-coherent pure-exciton condensation at 300 K has remained elusive, owing to a binding–lifetime dilemma—the densities required for degeneracy must still allow thermalisation before recombination. Here we leverage spin-forbidden dark excitons and, using plasmon-enhanced interferometric microscopy, directly image their first-order spatial coherence despite their optical inactivity. Using smooth nanoscale spacing-graded Stark confinement with off-axis injection, we demonstrate a quasi-equilibrium Berezinskii-Kosterlitz-Thouless (BKT)-type condensation of dark excitons in monolayer tungsten diselenide at 300 K and ambient pressure. Above a sharp degeneracy threshold, coherence extends beyond 250 thermal de Broglie wavelengths and exhibits algebraic order with a critical exponent consistent with the universal BKT criterion. This room-temperature phase-coherent exciton condensate establishes an accessible nanophotonic platform for phase-resolved studies of correlated quantum phenomena under ambient conditions.