  The discovery of luminous quasars at redshift z ~ 6 indicates the presence of
supermassive black holes (SMBHs) of mass ~10^9 Msun when the Universe was less
than one billion years old. This finding presents several challenges for
theoretical models. Here, we present the first multi-scale simulations that,
together with a self-regulated model for the SMBH growth, produce a luminous
quasar at z ~ 6.5 in the LCDM paradigm. We follow the hierarchical assembly
history of the most massive halo in a ~ 3 Gpc^3 volume, and find that this halo
of ~ 8x 10^{12} Msun forming at z ~ 6.5 after several major mergers is able to
reproduce a number of observed properties of SDSS J1148+5251, the most distant
quasar detected at z =6.42 (Fan et al. 2003). Moreover, the SMBHs grow through
gas accretion below the Eddington limit in a self-regulated manner owing to
feedback. We find that the progenitors experience significant star formation
(up to 10^4 Msun/yr) preceding the major quasar phase such that the stellar
mass of the quasar host reaches 10^{12} Msun at z ~ 6.5, consistent with
observations of significant metal enrichment in SDSS J1148+5251. Our results
provide a viable formation mechanism for z ~ 6 quasars in the standard LCDM
cosmology, and demonstrate a common, merger-driven origin for the rarest
quasars and the fundamental SMBH-host correlation in a hierarchical
Universe.(Abridged)
