  The complex internal atomic structure involved in radiative transitions has
an effect on the spectrum of fluctuations (noise) of the transmitted light. A
degenerate transition has different properties in this respect than a pure
two-level transition. We investigate these variations by studying a certain
transition between two degenerate atomic levels for different choices of the
polarization state of the driving laser. For circular polarization,
corresponding to the textbook two-level atom case, the optical spectrum shows
the characteristic Mollow triplet for strong laser drive, while the
corresponding noise spectrum exhibits squeezing in some frequency ranges. For a
linearly polarized drive, corresponding to the case of a multilevel system,
additional features appear in both optical and noise spectra. These differences
are more pronounced in the regime of a weakly driven transition: whereas the
two-level case essentially exhibits elastic scattering, the multilevel case has
extra noise terms related to spontaneous Raman transitions. We also discuss the
possibility to experimentally observe these predicted differences for the
commonly encountered case where the laser drive has excess noise in its phase
quadrature.
