A field-biased HPZ master equation and its Markovian limit

We present a first-principles derivation of a non-equilibrium quantum master equation for a continuously driven open quantum system interacting with a structured electromagnetic environment. Starting from a driven Caldeira-Leggett model in which an external classical field couples simultaneously to the system and reservoir degrees of freedom, we proceed without assuming that the standard equilibrium fluctuation-dissipation theorem holds. The bath statistics acquire explicit dependence on the two-time autocorrelation function of the applied field, leading to drive-biased noise correlations and intrinsically non-Markovian dynamics. By eliminating the reservoir exactly at the operator level, we obtain a driven quantum generalized Langevin equation whose noise and dissipation kernels depend on two independent times. Exploiting the Gaussian nature of the driven bath, we derive a modified Hu-Paz-Zhang master equation in which the diffusion coefficients and coherent forces inherit explicit memory of the external field. We demonstrate that the physically observable oscillation frequency remains encoded in the homogeneous Green's function of the Langevin equation, while the drive-induced corrections manifest exclusively through modified diffusion and drift terms. Our results provide a unified microscopic framework for understanding field-biased fluctuation relations with direct relevance to cavity and circuit quantum electrodynamics experiments operating far from equilibrium.