Intrinsic decoherence mechanisms in the microcavity polariton condensate

The recent observations of polariton condensation in semiconductor microcavities have provided a new, solid state system for the study of Bose-Einstein Condensate (BEC) phenomena (J.Kasprzak et al, Nature 2006). The Bose-condensed state exhibits characteristic properties including massive occupation of the ground state and long range spatial coherence, which due to the very small mass of polaritons (10-5 of the free electron mass) occur at high critical temperatures of 20-30K. We investigated first and second order temporal coherence of polariton BEC, which are fundamental properties of systems undergoing transition into high density macroscopically occupied state. For the first time very long coherence times up to ~150-200 ps, much longer than polariton lifetime (~1.5 ps), are observed for both g1 and g2 -correlation functions. We demonstrate that the phase coherence time of polariton BEC is intrinsically limited by the combined effect of number fluctuations and interactions in the coherent state. The results are explained quantitatively by a quantum-optical model which takes into account interactions, fluctuations, and gain and loss in the system.