  Driven two-level system is a useful model to describe many quantum objects,
particularly in quantum information processing. However, the exact master
equation for such a system is barely explored. Making use of the Feynman-Vernon
influence functional theory, we derive an exact non-Markovian master equation
for the driven two-level system and show the lost feature in the perturbative
treatment for this system. The perturbative treatment leads to the
time-convolutionless (TCL) and the Nakajima-Zwanzig (NZ) master equations. So
to this end, we derive the time-convolutionless (TCL) and the Nakajima-Zwanzig
(NZ) master equations for the system and compare the dynamics given by the
three master equations. We find the validity condition for the TCL and NZ
master equations. Based on the exact non-Markovian master equation, we analyze
the regime of validity for the secular approximation in the
time-convolutionless master equation and discuss the leading corrections of the
nonsecular terms to the quantum dynamics, significant effects are found in the
dynamics of the driven system.
