the three unknown quantities, $\theta_{13}$ plays a critical role in defining the future experimental program on neutrino oscillation. It is known that the CP-violating effect is proportional to $$J = \sin 2\theta_{12} \sin 2\theta_{23} \sin 2\theta_{13} \cos\theta_{13} \sin\delta_{\rm CP} \approx 0.9 \sin2\theta_{13} \sin\delta_{\rm CP} \,.$$ Resolution of the mass hierarchy problem also relies on the size of $\theta_{13}$. If it is too small, current technologies may not be able to determine $\delta_{\rm CP}$ and the mass hierarchy.

The mixing angle $\theta_{13}$ can be measured by accelerator-based or reactor-based experiments. However, the appearance probability of $\nu_{\mu} \to \nu_e$ in an accelerator neutrino experiment also depends on the yet unknown $\delta_{\rm CP}$ and the mass hierarchy. Hence, this type of experiments can only provide evidence for a non-zero $\theta_{13}$ but cannot measure its value unambiguously. On the other hand, reactor-based experiments can unambiguously determine $\theta_{13}$ via measuring the survival probability of the electron antineutrino $\overline{\nu}_{e}$ at short distance (${\cal O}(km)$) from the reactors. In the three-neutrino framework, the survival probability is given by $$\label{eq:psurv}
P = 1 - \cos^4\theta_{13}\sin^2 2\theta_{12}\sin^2\Delta_{21} - \sin^2 2\theta_{13}\sin^2
\Delta_{ee}\,,$$ where $\sin^2\Delta_{ee} \equiv \cos^2\theta_{12}\sin^2
\Delta_{31}+\sin^2\theta_{12}\sin^2{\Delta_{32}}\,$ and $\Delta_{ji}\equiv 1.267 {\Delta}m^2_{ji} L/E$. ${\Delta}m^2_{ji}$ is the mass-squared difference in eV$^2$, $E$ 