  We study the quantitative characterization of the performance of qubit
measurements in this paper. In particular, the back-action evading nature of
quantum nondemolition (QND) readout of qubits is fully quantified by quantum
trace distance. Only computational basis states are necessary to be taken into
consideration. Most importantly, we propose an experimentally efficient method
to evaluate the QND fidelity based on the classical trace distance, which uses
an experimental scheme with two consecutive measurements. The three key
quantifiers of a measurement, i.e., QND fidelity, readout fidelity, and
projectivity, can be derived directly from the same experimental scheme.
Besides, we present the relationships among these three factors. Theoretical
simulation results for the dispersive readout of superconducting qubits show
the validity of the proposed QND fidelity. Efficient quantification of
measurement performance is of practical significance for the diagnosis,
performance improvement, and design of measurement apparatuses.
