Build up of off-diagonal long-range order in microcavity   exciton-polaritons across the parametric threshold
  We report an experimental study of the spontaneous spatial and temporal coherence of polariton condensates generated in the optical parametric oscillator configuration, below and at the parametric threshold, and as a function of condensate area. Above the threshold we obtain very long coherence times (up to 3 ns) and a spatial coherence extending over the entire condensate (40 \mu m). The very long coherence time and its dependence on condensate area and pump power reflect the suppression of polariton-polariton interactions by an effect equivalent to motional narrowing. 
R. Spano, J. Cuadra, C. Lingg, D. Sanvitto, M. D. Martin, P. R.   Eastham, M. van der Poel, J. M. Hvam and L. Vina
cond-mat.mes-hall cond-mat.mtrl-sci