  It was predicted that frequently repeated measurements on an unstable quantum
state may alter the decay rate of the state. This is called the quantum Zeno
effect (QZE) or the anti-Zeno effect (AZE), depending on whether the decay is
suppressed or enhanced. In conventional theories of the QZE and AZE, effects of
measurements are simply described by the projection postulate, assuming that
each measurement is an instantaneous and ideal one. However, real measurements
are not instantaneous and ideal. For the QZE and AZE by such general
measurements, interesting and surprising features have recently been revealed,
which we review in this article. The results are based on the quantum
measurement theory, which is also reviewed briefly. As a typical model, we
consider a continuous measurement of the decay of an excited atom by a
photodetector that detects a photon emitted from the atom upon decay. This
measurement is an indirect negative-result one, for which the curiosity of the
QZE and AZE is emphasized. It is shown that the form factor is renormalized as
a backaction of the measurement, through which the decay dynamics is modified.
In a special case of the flat response, where the detector responds to every
photon mode with an identical response time, results of the conventional
theories are reproduced qualitatively. However, drastic differences emerge in
general cases where the detector responds only to limited photon modes. For
example, against predictions of the conventional theories, the QZE or AZE may
take place even for states that exactly follow the exponential decay law. We
also discuss relation to the cavity quantum electrodynamics.
