  We propose a new model of the spherical symmetric quantum black hole in the
reduced phase space formulation. We deparametrize gravity by coupling to the
Gaussian dust which provides the material coordinates. The foliation by dust
coordinates covers both the interior and exterior of the black hole. After the
spherical symmetry reduction, our model is a 1+1 dimensional field theory
containing infinitely many degrees of freedom. The effective dynamics of the
quantum black hole is generated by an improved physical Hamiltonian ${\bf
H}_\Delta$ where the holonomy correction is implemented by the
$\bar{\mu}$-scheme regularization with a Planckian area scale $\Delta$. The
effective dynamics recovers the semiclassical Schwarzschild geometry at low
curvature regime and resolves the black hole singularity with Planckian
curvature. Our model predicts that the evolution of the black hole at late time
reaches the charged Nariai geometry ${\rm dS}_2\times S^2$ with Planckian
radii. The Nariai geometry is stable under linear perturbations but may be
unstable by nonperturbative quantum effects. Our model suggests the existence
of quantum tunneling of the Nariai geometry and a scenario of
black-hole-to-white-hole transition. During the transition, the linear
perturbations exhibit chaotic dynamics with Lyapunov exponent $\lambda=2\pi
T_{\rm dS}\sim \Delta^{-1/2}$ relating to the Hawking temperature $T_{\rm dS}$
of ${\rm dS}_2$. In addition, the Nariai geometry in our model provides an
interesting example of Wheeler's bag of gold and contains infinitely many
infrared soft modes with zero energy density. These infrared modes span a
Hilbert space carrying a representation of 1-dimensional spatial
diffeomorphisms (or the Witt/Virasoro algebra) which are conserved charges of
the effective dynamics by ${\bf H}_\Delta$.
