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.