Coherent effects and optically oriented atoms

There are several aspects of the manifestation of coherent effects, both in the optical orientation of atoms and in the use of optically oriented atoms for studies into new 

The orientation of atoms by way of their optical pumping is a noncoherent process involving spontaneous emission. However, it is possible to transfer angular momentum from light to atoms in a coherent fashion. This possibility is based on off-resonant interaction between circularly polarized light and atoms, giving rise to light-induced 

2 Coherent effects with optically pumped atoms 

Optical pumping produces a nonequUibrium distribution of atoms among the magnetic (Zeeman) or hyperfine-structure (HFS) sublevels. The relationships between the atomic sublevels are defined by the diagonal elements of the density matrix of the state (see Section 2.5.2). However, the phases of the wave functions of the sublevels can be interrelated, so that the off-diagonal elements of the density matrix are not equal to zero, which makes the Zeeman levels coherent. This gives rise to new effects that cannot be described within the framework of populated levels. 

A still more interesting effect, referred to as coherent population trapping, arises in three-level systems, when an atom is prepared in a state that is a superposition of two states, 1 and 2 (Fig. 4.7(a))  

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