An alkali atom has a single outer electron that interacts with incoming photons of the right wavelength (for alkalies it's in the visible & IR range). If there is an external magnetic field, the electron has a well defined quantization axis and the the incoming light can be separated into three components: linearly polarized and left/right-handed circular polarized parts. The interaction strength between the different atomic energy levels (levels described by different quantum numbers) and each of the polarization components is relatively straightforward to calculate (with the appropriate Clebsch-Gordan coefficients and 6-j symbols).
What is then the situation if there is no external magnetic field to provide the quantization axis? Is "polarization" meaningful from the atoms point of view? How one sets out to calculate the interaction strength for the different polarizations in the laboratory frame?