Polaron transformed canonically consistent quantum master equation

A central challenge in the theory of open quantum systems is the development of theoretical frameworks capable of accurately describing large, strongly interacting quantum many-body systems in the regime of strong system–bath interaction. In this work, we take a step toward this goal by formulating a polaron-transformed version of the canonically consistent quantum master equation (CCQME) [T. Becker et al., Phys. Rev. Lett. 129, 200403 (2022)]. The CCQME extends beyond standard weak-coupling approaches while retaining the same numerical complexity as conventional quantum master equations, thereby enabling the treatment of large quantum systems. The polaron transformation further enhances the accessible system–bath interaction strengths, allowing us to move from moderate to ultra-strong interaction regimes. We present a unified and transparent derivation of these two approaches and combine them to obtain the polaron-transformed CCQME. Applying our method to the paradigmatic spin–boson model, we find excellent agreement with numerically exact time-evolving matrix product operator simulations. Finally, we predict an initial-state-independent slowing down of thermalization in the strong-coupling regime of the spin-boson model.