  Observations of the most distant bright quasars imply that billion solar mass
supermassive black holes (SMBH) have to be assembled within the first eight
hundred million years. Under our standard galaxy formation scenario such fast
growth implies large gas densities providing sustained accretion at critical or
supercritical rates onto an initial black hole seed. It has been a long
standing question whether and how such high black hole accretion rates can be
achieved and sustained at the centers of early galaxies. Here we use our new
cosmological hydrodynamic simulation (MassiveBlack) covering a volume (0.75
\Gpc)^3 appropriate for studying the rare first quasars to show that steady
high density cold gas flows responsible for assembling the first galaxies
produce the high gas densities that lead to sustained critical accretion rates
and hence rapid growth commensurate with the existence of ~10^9 solar mass
black holes as early as z~7. We find that under these conditions quasar
feedback is not effective at stopping the cold gas from penetrating the central
regions and hence cannot quench the accretion until the host galaxy reaches
M_halo > 10^{12} solar masses. This cold-flow driven scenario for the formation
of quasars implies that they should be ubiquitous in galaxies in the early
universe and that major (proto)galaxy mergers are not a requirement for
efficient fuel supply and growth, particularly for the earliest SMBHs.
