  We propose a scheme to realize a quantum non-demolition (QND) measurement of
a superconducting flux qubit by a Josephson bifurcation amplifier. Our scheme
can implement a perfect QND measurement for a qubit subject to a variable
magnetic bias. Measurement back-action induced qubit relaxation can be
suppressed and hence the QND fidelity is expected to be high over a wide range
of bias conditions.
