A New Constraint on Reionization from Evolution of the Ly$\alpha$
  Luminosity Function at $z\sim6-7$ Probed by a Deep Census of $z=7.0$
  Ly$\alpha$ Emitter Candidates to 0.3 $L^*$
  We detect 20 $z=7.0$ Ly$\alpha$ emitter (LAE) candidates to $L({\rm
Ly}\alpha) \geq 2 \times 10^{42}$ erg s$^{-1}$ or $0.3$ $L^*_{z=7}$ and in
$6.1\times 10^5$ Mpc$^3$ volume in the Subaru Deep Field and the
Subaru/XMM-Newton Deep Survey field by 82 and 37 hours of Subaru Suprime-Cam
narrowband NB973 and reddest optical $y$-band imaging. We compare their
Ly$\alpha$ and UV luminosity functions (LFs) and densities and Ly$\alpha$
equivalent widths (EWs) to those of $z=5.7$, 6.6 and 7.3 LAEs from previous
Suprime-Cam surveys. The Ly$\alpha$ LF (density) rapidly declines by a factor
of $\times$1.5 (1.9) in $L({\rm Ly}\alpha)$ at $z=5.7-6.6$ (160 Myr),
$\times$1.5 (1.6) at $z=6.6-7.0$ (60 Myr) at the faint end and $\times$2.0
(3.8) at $z=7.0-7.3$ (40 Myr). Also, in addition to the systematic decrease in
EW at $z=5.7-6.6$ previously found, 2/3 of the $z=7.0$ LAEs detected in the UV
continuum exhibit lower EWs than the $z=6.6$ ones. Moreover, while the UV LF
and density do not evolve at $z=5.7-6.6$, they modestly decline at $z=6.6-7.0$,
implying galaxy evolution contributing to the decline of the Ly$\alpha$ LF.
Comparison of the $z=7.0$ Ly$\alpha$ LF to the one predicted by an LAE
evolution model further reveals that galaxy evolution alone cannot explain all
the decline of Ly$\alpha$ LF. If we attribute the discrepancy to Ly$\alpha$
attenuation by neutral hydrogen, the intergalactic medium transmission of
Ly$\alpha$ photons at $z=7.0$ would be $T_{{\rm Ly}\alpha}^{\rm IGM} \leq
0.6-0.7$. It is lower (higher) than $T_{{\rm Ly}\alpha}^{\rm IGM}$ at $z=6.6$
(7.3) derived by previous studies, suggesting rapid increase in neutral
fraction at $z > 6$.
