Macroscopic coherence of a single exciton state in an organic quantum wire

Macroscopic quantum coherence has been observed in some many-body systems including superconductors, quantum liquids(1) and cold atom condensates(2), but never for a single quasi-particle state. In an ideal semiconductor, excitons (electron-hole pairs bound by the Coulomb interaction) can, in principle, exist as delocalized plane waves extending over the entire volume. However, any kind of disorder prevents long-range spatial coherence from emerging. There has been evidence for the formation of macroscopic coherent states only in condensate phases such as in the case of microcavity polaritons condensation(3,4) or in a dense quasi-two-dimensional exciton gas(5). It is unclear however, whether in this latter case the observations are really related to macroscopic coherence(6). Here, we show that a single exciton state in an individual ordered conjugated polymer chain(7,8), shows macroscopic quantum spatial coherence reaching tens of micrometres, limited by the chain length. The spatial coherence of the k=0 exciton state is demonstrated by selecting two spatially separated emitting regions of the chain and observing their interference.