  Despite significant theoretical efforts devoted to studying the interaction
between quantized light modes and matter, the so-called ultra-strong coupling
regime still presents significant challenges for theoretical treatments and
prevents the use of many common approximations. Here we demonstrate an approach
that can describe the dynamics of hybrid quantum systems in any regime of
interaction for an arbitrary electromagnetic (EM) environment. We extend a
previous method developed for few-mode quantization of arbitrary systems to the
case of ultrastrong light-matter coupling, and show that even such systems can
be treated using a Lindblad master equation where decay operators act only on
the photonic modes by ensuring that the effective spectral density of the EM
environment is sufficiently suppressed at negative frequencies. We demonstrate
the validity of our framework and show that it outperforms current
state-of-the-art master equations for a simple model system, and then study a
realistic nanoplasmonic setup where existing approaches cannot be applied.
