In two-level system, with external field applied, why there is no permant dipole moment? In classical point of view, dipole moment is coming from displacement of positive and negative charge. In quantum case, inside the atom, even no external field, there is certain probability for the electron be anywhere around the nucleus. Hence, the displacement should induced a dispole moment, but text says there is no such dipole moment, why? By the way, if we consider the density matrix, what does the off-diagonal element refer to? How the off-diagonal element correspons to the induced dipole moment (with external field applied)?