Among the formulations of the theory of quantum measurements in continuous
time, quantum trajectory theory is very suitable for the introduction of
measurement based feedback and closed loop control of quantum systems. In this
paper we present such a construction in the concrete case of a two-level atom
stimulated by a coherent, monochromatic laser. In particular, we show how fast
feedback \`a la Wiseman and Milburn can be introduced in the formulation of the
theory. Then, the spectrum of the free fluorescence light is studied and
typical quantum phenomena, squeezing and sub-natural line-narrowing, are
presented.Comment: 10 pages, 3 figures. Submitted to proceedings of PhysCon0
