  Based on the experimental evidence that impurities contribute to the
dissipation properties of solid-state open quantum systems, we provide here a
description in terms of nonlinear quantum Langevin equations of the role played
by two-level systems in the dynamics of a bosonic degree of freedom. Our
starting point is represented by the description of the system/environment
coupling in terms of coupling to two separate reservoirs, modelling the
interaction with external bosonic modes and two level systems, respectively.
Furthermore, we show how this model represents a specific example of a class of
open quantum systems that can be described by nonlinear quantum Langevin
equations. Our analysis offers a potential explanation of the parametric
effects recently observed in circuit-QED cavity optomechanics experiments.
