Feedback control of the fluorescence light squeezing

We consider a trapped two-level atom stimulated by a coherent monochromatic laser and we study how to enhance the squeezing of the fluorescence light in the presence of a Wiseman-Milburn feedback mechanism, based on the homodyne detection of a fraction of the emitted light. We analyze the effect of the control parameters on the properties of the spectrum of the light and we also discuss the relation between light squeezing and atomic states, which qualitatively changes when the control is present. We also show that feedback can produce sub-natural line-narrowing of the fluorescence light, without the presence of squeezing in the stimulating light. The whole problem is tackled inside the framework of quantum trajectory theory.