  Macroscopically ordered arrays of vortices in quantum liquids, such as
superconductors, He-II, and atom Bose-Einstein Condensates (BEC), demonstrate
macroscopic coherence in flowing superfluids [1-4]. Despite of the rich variety
of systems where quantum liquids reveal macroscopic ordering, experimental
observation of a macroscopically ordered electronic state in semiconductors has
remained a challenging unexplored problem. A system of excitons is a promising
candidate for the realization of macroscopic ordering in a quantum liquid in
semiconductors. An exciton is a bound pair of an electron and a hole. At low
densities, it is a Bose quasi-particle. At low temperatures, of the order of a
few Kelvins, excitons can form a quantum liquid, i.e., a statistically
degenerate Bose gas and eventually BEC [5-9]. Here, we report the experimental
observation of a macroscopically ordered state in an exciton system.
