Observation of Superfluorescence from a Spontaneous Coherence of   Excitons in ZnTe Crystal: Evidence for Bose-Einstein Condensation of   Excitons?
  Superfluorescence (SF) is the emission from a dense coherent system in population inversion, formed from an initially incoherent ensemble. This is characterised by an induction time (t_D) for the spontaneous development of the macroscopic quantum coherence. Here we report detailed observation of SF on ultrafast timescale from a quantum ensemble of coherent excitons in highly excited intrinsic bulk ZnTe single crystal at 5 K, showing a characteristic t_D from 40 ps to 10 ps, quantum noise and fluctuations, and quantum beating and ringing. From this clear observation of SF from a spontaneous coherence of excitons we infer that this is indicative of the formation of BEC of excitons on an ultrafast timescale. 
D. C. Dai and A. P. Monkman
physics.optics cond-mat.quant-gas