Optical orientation of atoms and nuclei

Here, for simplicity s sake, the degeneracy of the quantum levels of a two-level system in a real atom is not taken into accoimt. Consideration of the degeneracy of levels due to the angular momentum of an atom in a given quantum state (s,p,d...) and the polarization of radiation (linear, circular, or natural) is essential (see, for example. Foot 2004). This effect is fundamental for the optical orientation of atoms and nuclei (Chapter 4) and the laser cooling of atoms by the polarization gradient method (Chapter 5). 

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