  We calculate the effect of quantum noise in supersonic transport of
Bose-Einstein condensates. When an obstacle obstructs the flow of atoms,
quantum fluctuations cause atoms to be scattered incoherently into random
directions. This suppresses the propagation of Cherenkov radiation, creating
quantum turbulence and a crescent of incoherent atoms around the obstacle. We
observe similar dynamics if the BEC is stirred by a laser beam: crescents of
incoherent atoms are emitted from the laser's turning-points. Finally, we
investigate supersonic flow through a disordered potential, and find that the
quantum fluctuations generate an accumulation of incoherent atoms as the
condensate enters the disorder.
