  The quantum Zeno effect (QZE) is the striking prediction that the decay of
any unstable quantum state can be inhibited by sufficiently frequent
observations (measurements). The consensus opinion has upheld the QZE as a
general feature of quantum mechanics, which should lead to the inhibition of
any decay. The claim of QZE generality hinges on the assumption that successive
observations can in principle be made at time intervals too short for the
system to change appreciably. However, this assumption and the generality of
the QZE have scarcely been investigated thus far. We have addressed these
issues by showing that (i) the QZE is principally unattainable in radiative or
radioactive decay, because the required measurement rates would cause the
system to disintegrate; (ii) decay acceleration by frequent measurements (the
anti-Zeno effect -- AZE) is much more ubiquitous than its inhibition. The AZE
is shown to be observable as the enhancement of tunneling rates (e.g., for
atoms trapped in ramped-up potentials or in current-swept Josephson junctions),
fluorescence rates (e.g., for Rydberg atoms perturbed by noisy optical fields)
and photon depolarization rates (in randomly modulated Pockels cells).
