  Distant quasars are unique tracers to study the formation of the earliest
supermassive black holes (SMBHs) and the history of cosmic reionization.
Despite extensive efforts, only two quasars have been found at $z\ge7.5$, due
to a combination of their low spatial density and the high contamination rate
in quasar selection. We report the discovery of a luminous quasar at $z=7.642$,
J0313$-$1806, the most distant quasar yet known. This quasar has a bolometric
luminosity of $3.6\times10^{13} L_\odot$. Deep spectroscopic observations
reveal a SMBH with a mass of $(1.6\pm0.4) \times10^9M_\odot$ in this quasar.
The existence of such a massive SMBH just $\sim$670 million years after the Big
Bang challenges significantly theoretical models of SMBH growth. In addition,
the quasar spectrum exhibits strong broad absorption line (BAL) features in CIV
and SiIV, with a maximum velocity close to 20% of the speed of light. The
relativistic BAL features, combined with a strongly blueshifted CIV emission
line, indicate that there is a strong active galactic nucleus (AGN) driven
outflow in this system. ALMA observations detect the dust continuum and [CII]
emission from the quasar host galaxy, yielding an accurate redshift of $7.6423
\pm 0.0013$ and suggesting that the quasar is hosted by an intensely
star-forming galaxy, with a star formation rate of $\rm\sim 200 ~M_\odot
~yr^{-1}$ and a dust mass of $\sim7\times10^7~M_\odot$. Followup observations
of this reionization-era BAL quasar will provide a powerful probe of the
effects of AGN feedback on the growth of the earliest massive galaxies.
