  Cosmic (hydrogen) reionization marks one of the major phase transitions of
the universe at redshift z >= 6. During this epoch, hydrogen atoms in the
intergalactic medium (IGM) were ionized by Lyman continuum (LyC) photons.
However, it remains challenging to identify the major sources of the LyC
photons responsible for reionization. In particular, individual contributions
of quasars (or active galactic nuclei, AGNs) and galaxies are still under
debate. Here we construct the far-ultraviolet (far-UV) luminosity function for
type 1 quasars at z >= 6 that spans 10 magnitudes (-19 < M_UV < -29),
conclusively showing that quasars made a negligible contribution to
reionization. We mainly search for quasars in the low-luminosity range of M_UV
> -23 mag that is critical to determine quasars' total LyC photon production
but has been barely explored previously. We find that the quasar population can
only provide less than 7% (95% confidence level) of the total photons needed to
keep the universe ionized at z = 6.0 - 6.6. Our result suggests that galaxies,
presumably low-luminosity star-forming systems, are the major sources of
hydrogen reionization.
