Generalized Hanbury Brown-Twiss effect for Stokes parameters

The classic experiments by Hanbury Brown and Twiss (HBT) were concerned with the correlation of intensity fluctuations at two different positions in a wave field. We generalize the HBT effect that occurs in random electromagnetic beams by examining its polarization-resolved version. This leads naturally to the concept of correlations of fluctuations of the four Stokes parameters. We calculate the correlations of such "Stokes fluctuations" for the case of Gaussian statistics. When the two points of observation coincide, these correlations reduce to "Stokes scintillations." Our work reveals a new layer of complexity in random beams by showing that the HBT effect and the scintillation coefficient are just two of many correlations that are present. We illustrate that, in general, the fluctuations of the various Stokes parameters are all correlated by studying beams and sources with different polarization states.