Quantum master equation for dephasing of a two-level system with an initial correlation

Exact quantum master equation is derived for dephasing of a two-level system, which is an homogeneous time-convolutionless equation even though there is an initial correlation between a two-level system and a thermal reservoir. The result is compared with the quantum master equation derived by means of the projection operator method. Furthermore, the effects of the initial correlation on the dephasing process are examined.