  Dispersive coupling based on the Rabi model with large detuning is widely
used for quantum nondemolition (QND) qubit readout in quantum computation.
However, the measurement speed and fidelity are usually significantly limited
by the Purcell effects, i.e.: Purcell decay, critical photon numbers, and
qubit-dependent Kerr nonlinearity. To avoid these effects, we propose how to
realize an ideal QND readout of a gradiometric flux qubit with a tunable gap
via its non-perturbative dispersive coupling (NPDC) to a frequency-tunable
measurement resonator. We show that this NPDC-based readout mechanism is free
of dipole-field interactions, and that the qubit-QND measurement is not
deteriorated by intracavity photons. Both qubit-readout speed and fidelity can
\emph{avoid the Purcell limitations}. Moreover, NPDC can be conveniently turned
on and off via an external control flux. We show how to extend this proposal to
a multi-qubit architecture for a joint qubit readout.
