  We demonstrate that the multipoles associated with the density matrix are
truly observable quantities that can be unambiguously determined from intensity
moments. Given their correct transformation properties, these multipoles are
the natural variables to deal with a number of problems in the quantum domain.
In the case of polarization, the moments are measured after the light has
passed through two quarter-wave plates, one half-wave plate, and a polarizing
beam splitter for specific values of the angles of the waveplates. For more
general two-mode problems, equivalent measurements can be performed.
