  The discovery of luminous quasars at redshifts up to 7.5 demonstrates the
existence of several billion M_sun supermassive black holes (SMBHs) less than a
billion years after the Big Bang. They are accompanied by intense star
formation in their host galaxies, pinpointing sites of massive galaxy assembly
in the early universe, while their absorption spectra reveal an increasing
neutral intergalactic medium (IGM) at the epoch of reionization. Extrapolating
from the rapid evolution of the quasar density at z=5-7, we expect that there
is only one luminous quasar powered by a billion M_sun SMBH in the entire
observable universe at z~9. In the next decade, new wide-field, deep
near-infrared (NIR) sky surveys will push the redshift frontier to the first
luminous quasars at z~9-10; the combination with new deep X-ray surveys will
probe fainter quasar populations that trace earlier phases of SMBH growth. The
identification of these record-breaking quasars, and the measurements of their
BH masses and accretion properties require sensitive spectroscopic observations
with next generation of ground-based and space telescopes at NIR wavelengths.
High-resolution integral-field spectroscopy at NIR, and observations at
millimeter and radio wavelengths, will together provide a panchromatic view of
the quasar host galaxies and their galactic environment at cosmic dawn,
connecting SMBH growth with the rise of the earliest massive galaxies.
Systematic surveys and multiwavelength follow-up observations of the earliest
luminous quasars will strongly constrain the seeding and growth of the first
SMBHs in the universe, and provide the best lines of sight to study the history
of reionization.
