The emission of electromagnetic radiation from a quantum system interacting with an external noise: A general result

We compute the spectrum of emitted radiation by a generic quantum system interacting with an external classic noise. Our motivation is to understand this phenomenon within the framework of collapse models. However the computation is general and applies practically to any situation where a quantum system interacts with a noise. The computation is carried out at a perturbative level. This poses problems concerning the correct way of performing the analysis, as repeatedly discussed in the literature. We will clarify also this issue.