  Repeated measurements in quantum mechanics can freeze (the quantum Zeno
effect) or enhance (the quantum anti-Zeno effect) the time-evolution of a
quantum system. In this paper, we present a general treatment of the quantum
Zeno and anti-Zeno effects for arbitrary driven open quantum systems, assuming
only that the system-environment coupling is weak. In particular, we obtain a
general expression for the effective decay rate of a two-level system subjected
to arbitrary driving fields as well as periodic measurements. We demonstrate
that the driving fields change the decay rate, and hence the quantum Zeno and
anti-Zeno behavior, both qualitatively and quantitatively. We also extend our
results to systems consisting of more than one two-level system, as well as a
two-level system strongly coupled to an environment of harmonic oscillators, to
further illustrate the non-trivial effect of the driving fields on the quantum
Zeno and anti-Zeno effects.
