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

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