  We propose and investigate a novel method for the controlled coupling of two
Josephson charge qubits by means of a variable electrostatic transformer. The
value of the coupling capacitance is given by the discretized curvature of the
lowest energy band of a Josephson junction, which can be positive, negative, or
zero. We calculate the charging diagram of the two-qubit system that reflects
the transition from positive to negative through vanishing coupling. We also
discuss how to construct a phase gate making use of the controllable coupling.
