  We investigate the dynamics of charge--states coherence in a degenerate
double--dot Aharonov--Bohm interferometer with finite interdot Coulomb
interactions. The quantum coherence of the charge states is found to be
sensitive to the transport setup configurations, involving both the
single--electron impurity channels and the Coulomb--assisted ones. We
numerically demonstrate the emergence of a complete coherence between the two
charge states, with the relative phase being continuously controllable through
the magnetic flux. Remarkably, a fully coherent charge qubit arises at the
double--dots electron pair tunneling resonance condition, where the chemical
potential of one electrode is tuned at the center between a single--electron
impurity channel and the related Coulomb--assisted channel. This pure quantum
state of charge qubit could be \emph{experimentally located} at the
current--voltage characteristic turnover position, where differential
conductance sign changes. We further elaborate the underlying mechanism for
both the real--time and the stationary charge--states coherences in the
double--dot systems of study.
