A Non-Perturbative Framework for the study of Open Quantum Systems

Conventional adiabatic elimination techniques treat the system as unitary when computing reduced Lindbladians, adding the resultant induced decoherence channels to the intrinsic ones ad hoc. Besides compromising quantitative accuracy, these approaches fail to capture even qualitative aspects, such as Zeno and Purcell effects, that are rooted in interactions between decoherence channels acting on the same system. Here, we present a new dissipative coarse-graining (DISCO) framework that includes all decoherence channels for both the system and the environment from the outset. DISCO surpasses the accuracy of conventional adiabatic elimination methods in all parameter regimes, supported by extensive numerics. Further, using Pade resummation techniques, it can describe dynamics even in deeply non-adiabatic (slow bath) and strongly coupled regimes. Through pole–residue analysis of the DISCO propagator, we reinterpret the Markovian limit, clarify its validity and connections to adiabaticity for both fast and slow baths, and show that it retains diagnostic value even in non-Markovian regimes. Our results also shed light on the implications of the Born approximation in Redfield and Nakajima-Zwanzig descriptions, and provide a unified tool for the study of non-unitary physics.