  Quasars at cosmic dawn provide powerful probes of the formation and growth of
the earliest supermassive black holes (SMBHs) in the universe, their
connections to galaxy and structure formation, and the evolution of the
intergalactic medium (IGM) at the epoch of reionization (EoR). Hundreds of
quasars have been discovered in the first billion years of cosmic history, with
the quasar redshift frontier extended to z~7.6. Observations of quasars at
cosmic dawn show that: (1) The number density of luminous quasars declines
exponentially at z>5, suggesting that the earliest quasars emerge at z~10; the
lack of strong evolution in their average spectral energy distribution
indicates a rapid buildup of the AGN environment. (2) Billion-solar-mass BHs
already exist at z>7.5; they must form and grow in less than 700 Myr, by a
combination of massive early BH seeds with highly efficient and sustained
accretion. (3) The rapid quasar growth is accompanied by strong star formation
and feedback activity in their host galaxies, which show diverse morphological
and kinetic properties, with typical dynamical mass of lower than that implied
by the local BH/galaxy scaling relations. (4) HI absorption in quasar spectra
probes the tail end of cosmic reionization at z~5.3-6, and indicates the EoR
midpoint at 6.9 < z < 7.6 with large spatial fluctuations in IGM ionization.
Observations of heavy element absorption lines suggest that the circumgalactic
medium also experiences evolution in its ionization structure and metal
enrichment during the EoR.
