  Following the discovery of Bose-Einstein condensation (BEC) in ultra cold
atoms [E. Gosta, Nobel Lectures in Physics (2001-2005), World Scientific
(2008)], there has been a huge experimental and theoretical push to try and
illuminate a superfluid state of Wannier-Mott excitons. Excitons in quantum
wells, generated by a laser pulse, typically diffuse only a few micrometers
from the spot they are created. However, Butov et al. and Snoke et al. reported
luminescence from indirect and direct excitons hundreds of micrometers away
from the laser excitation spot in double and single quantum well (QW)
structures at low temperatures. This luminescence appears as a ring around the
laser spot with the dark region between the spot and the ring. Developing the
theory of a free superflow of Bose-liquids we show that the macroscopic
luminesce rings and the dark state are signatures of the coherent superflow of
condensed excitons at temperatures below their Berezinskii-Kosterlitz-Thouless
(BKT) transition temperature. To further verify the dark excitonic superflow we
propose several keystone experiments, including interference of superflows from
two laser spots, vortex formation, scanning of moving dipole moments, and a
giant increase of the luminescence distance by applying one-dimensional
confinement potential. These experiments combined with our theory will open a
new avenue for creating and controlling superflow of coherent excitons on
nanoscale.
