  In this Letter we show that the strong coupling between a disordered set of
molecular emitters and surface plasmons leads to the formation of spatially
coherent hybrid states extended on acroscopic distances. Young type
interferometric experiments performed on a system of J-aggregated dyes spread
on a silver layer evidence the coherent emission from different molecular
emitters separated by several microns. The coherence is absent in systems in
the weak coupling regime demonstrating the key role of the hybridization of the
molecules with the plasmon.
