Spontaneous formation and optical manipulation of extended polariton condensates

Cavity exciton-polaritons ^1,2 (polaritons) are bosonic quasi-particles offering a unique solid-state system for investigating interacting condensates ^3-10 . Up to now, disorder-induced localization and short lifetimes ^4,6,11 have prevented the establishment of long-range off-diagonal order ¹² needed for any quantum manipulation of the condensate wavefunction. In this work, using a wire microcavity with polariton lifetimes much longer than in previous samples, we show that polariton condensates can propagate over macroscopic distances outside the excitation area, while preserving their spontaneous spatial coherence. An extended condensate wavefunction builds up with a degree of spatial coherence larger than 50% over distances 50 times the polariton de Broglie wavelength. The expansion of the condensate is shown to be governed by the repulsive potential induced by photogenerated excitons within the excitation area. The control of this local potential offers a new and versatile method to manipulate extended polariton condensates. As an illustration, we demonstrate synchronization of extended condensates by controlled tunnel coupling ^13,14 and localization of condensates in a trap with optically controlled dimensions.