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Consider we have a [itex]\Lambda[/itex] type three level system, the one upper level is denoted by 3, and the two lower levels are denoted by 1 and 2. Assuming that the transitions between 1[itex]\leftrightarrow[/itex]3 and 2[itex]\leftrightarrow[/itex]3 are dipole alowed, while the transition between two lower states 1 and 2 is dipole forbidden. As we know the real and imaginary part of ρ

_{31}describe the dispersion and absorption of the external light in 1[itex]\leftrightarrow[/itex]3 channel. But what does the ρ_{12}mean?
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