1. Materials Sciences Division, E. O. Lawrence Berkeley National Laboratory;
2. Department of Physics, University of California at Berkeley, Berkeley, California 94720, USA
3. Institute of Solid State Physics, Russian Academy of Sciences, 142432 Chernogolovka, Russia
4. Department of Electrical and Computer Engineering, University of California, Santa Barbara, California 93106, USA

Correspondence to: L. V. Butov^1,2 Correspondence and requests for materials should be addressed to L.V.B. (e-mail: Email: lvbutov@lbl.gov).

There is a rich variety of quantum liquids—such as superconductors, liquid helium and atom Bose–Einstein condensates—that exhibit macroscopic coherence in the form of ordered arrays of vortices^{1, 2, 3, 4}. Experimental observation of a macroscopically ordered electronic state in semiconductors has, however, remained a challenging and relatively unexplored problem. A promising approach for the realization of such a state is to use excitons, bound pairs of electrons and holes that can form in semiconductor systems. At low densities, excitons are Bose-particles⁵, and at low temperatures, of the order of a few kelvin, excitons can form a quantum liquid—that is, a statistically degenerate Bose gas or even a Bose–Einstein condensate^{5, 6, 7}. Here we report photoluminescence measurements of a quasi-two-dimensional exciton gas in GaAs/AlGaAs coupled quantum wells and the observation of a macroscopically ordered exciton state. Our spatially resolved measurements reveal fragmentation of the ring-shaped emission pattern into circular structures that form periodic arrays over lengths up to 1 mm.