The thermal state |I(t)i represents the effects of the initial correlation of the system and reservoir. Because, if the initial
state |ρT(0)i is given by the decoupled one |ρ(0)i|ρRi with |ρ(0)i given by |ρ(0)i = h1R|ρT(0)i, i.e., |ρT(0)i = |ρ(0)i|ρRi,
then |I(t)i vanishes, since Q|ρ(0)i|ρRi = 0. We assume that the system and reservoir are in the thermal equilibrium state
at the initial time t = 0, i.e., |ρT(0)i = |ρTEi.
We now consider the case that the quantal system is interacting so weakly with its heat reservoir that we can use the
lowest Born approximation, and expand Eq. (2.14) up to the second order in powers of the system–reservoir interaction.
Then, it reduces to