  For a bosonic (fermionic) open system in a bath with many bosons (fermions)
modes, we derive the exact non-Markovian master equation in which the memory
effect of the bath is reflected in the time dependent decay rates. In this
approach, the reduced density operator is constructed from the formal solution
of the corresponding Heisenberg equations. As an application of the exact
master equation, we study the active probing of non-Markovianity of the quantum
dissipation of a single boson mode of electromagnetic (EM) field in a cavity
QED system. The non-Markovianity of the bath of the cavity is explicitly
reflected by the atomic decoherence factor.
