  We experimentally demonstrate a new dynamic fashion of quantum Zeno effect in
nuclear magnetic resonance systems. The frequent measurements are implemented
through quantum entanglement between the target qubit(s) and the measuring
qubit, which dynamically results from the unitary evolution of duration
$\tau_{m}$ due to dispersive-coupling. Experimental results testify the
presence of "the critical measurement time effect", that is, the quantum Zeno
effect does not occur when $\tau_{m}$ takes the some critical values, even if
the measurements are frequent enough. Moreover, we provide a first experimental
demonstration of an entanglement preservation mechanism based on such dynamic
quantum Zeno effect.
