  In the tensor network representation, a deformed $Z_{2}$ topological ground
state wave function is proposed and its norm can be exactly mapped to the
two-dimensional solvable Ashkin-Teller (AT) model. Then the topological (toric
code) phase with anyonic excitations corresponds to the partial order phase of
the AT model, and possible topological phase transitions are precisely
determined. With the electric-magnetic self-duality, a novel gapless Coulomb
state with quasi-long-range order is obtained via a quantum Kosterlitz-Thouless
phase transition. The corresponding ground state is a condensate of pairs of
logarithmically confined electric charges and magnetic fluxes, and the scaling
behavior of various anyon correlations can be exactly derived, revealing the
effective interaction between anyons and their condensation. Deformations away
from the self-duality drive the Coulomb state into either the gapped Higgs
phase or confining phase.
