2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/160693We derive the exact time-evolution for a general quantum system under the influence of pure phase-noise and demonstrate that for a Gaussian initial state of the bath, the exact result can be obtained also within a perturbative time-local master equation approach already in second order of the system-bath coupling strength. We reveal that this equivalence holds if the initial state of the bath can be mapped to a Gaussian phase-space distribution function. Moreover, we discuss the relation to the standard Bloch-Redfield approach.7 pages; revised version; to appear in Chem. PhysQuantum PhysicsStatistical Mechanicstext