The P k dynamic shows a structure where an individual P k is coupled to all other microscopic polarizations due to the Coulomb interaction V k . Therefore, the transition amplitude P k is collectively modified by the presence of other transition amplitudes. Only if one sets V k to zero, one finds isolated transitions within each k state that follow exactly the same dynamics as the optical Bloch equations predict. Therefore, already the Coulomb interaction among P k produces a new solid-state effect compared with optical transitions in simple atoms.