The epoch of cosmic reionization marks the latest major phase transition of the universe, during which the intergalactic medium (IGM) was ionized by the first astrophysical objects, and the universe became transparent to UV photons. This period is the time when the first galaxies started to form and shine, and the time when the first supermassive black holes (SMBHs) started to accrete and grow. Cosmic reionization peaks at z~8.5 and ends at z~6. The major piece of evidence for the latter comes from the observations of high-redshift (z>=6) quasars. These quasars are among the most luminous objects known and serve as cosmological probes for studying the early universe. They are powered by radiation from matter accreting onto central SMBHs. Their spectra are encoded with rich information about the IGM state at high redshift, as well as their own properties.

We will carry out a series of programs to search for high-redshift quasars, measure their physical properties, estimate their contribution to cosmic reionization, characterize the growth of early SMBHs, and understand the galaxy-SMBH co-evolution at early epochs. First, we will search for high-redshift quasars based on large imaging datasets such as DESI imaging data, and obtain a highly complete, large quasar sample. With this sample, we will derive an accurate quasar luminosity function (QLF) at high redshift, and answer an important question: can the AGN/quasar population provide enough ionizing photons for cosmic reionization. With deep optical and near-IR spectroscopy, we will measure various physical properties of quasars, including SMBH masses and the mass function at high redshift. The latter will provide clues to understand the formation and growth of the earliest SMBHs, particularly seed BHs. We will also carry out mm/sub-mm and radio observations to study quasar host galaxies, including star formation, gas and dust properties, dynamics, and so on. We will probe the possible co-evolution between quasars and their hosts at the highest possible redshifts. Finally, we will explore some of the above aspects via theory and simulations, including the growth history of massive black holes, the cosmological evolution of the relationships between massive black holes and their host galaxies,  the nuclear activity triggering history, the evolution of massive black hole spins, and the properties of the host dark matter halos, and so on.