  The effect of the anti-rotating terms on the short-time evolution and the
quantum Zeno (QZE) and anti-Zeno (AQZE) effects is studied for a two-level
system coupled to a bosonic environment. A unitary transformation and
perturbation theory are used to obtain the electron self-energy, energy shift
and the enhanced QZE or the AQZE, simultaneously. The calculated Zeno time
depends on the atomic transition frequency sensitively. When the atomic
transition frequency is smaller than the central frequency of the spectrum of
boson environment, the Zeno time is prolonged and the anti-rotating terms
enhance the QZE; when it is larger than that the Zeno time is reduced and the
anti-rotating terms enhance the AQZE.
