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## Main Question or Discussion Point

For two level system , let denotes the ground state as 1 and exctied state as 2, for writing the office off-diagonal matrix element for the density operator, shall it be

[tex]\rho_{12} = |2\rangle\langle 1|[/tex]

and

[tex]\rho_{21} = |1\rangle\langle 2|[/tex]

???

[tex]\rho_{12} = |2\rangle\langle 1|[/tex]

and

[tex]\rho_{21} = |1\rangle\langle 2|[/tex]

???