which have been rotated u03c0 2 radians this aspect of the present invention is the basis for faraday rotation optical filters transitions from the ground state to higher energy excited states in cs 455 nm and rb 780 nm are known in the art the faraday effect also works for transitions between excited states if the population can be pumped into the first excited state from the ground state the right and left circularly polarized light beams travel through the atomic vapor at slightly different velocities the effect of this difference on a linearly polarized beam of light which may be considered to be a superposition of two equal amplitude right and left circularly polarized beams is the rotation of its plane of polarization because of this rotation the linearly polarized light is effectively blocked by the cross polarizers 98 110 a typical extinction ratio of 10 5 or higher is partially transmitted in the absence of the axial magnetic field the magnitude of rotation and consequently the transmission depend upon the number density of atoms in the vapor of the potassium vapor cell 102 the transition cross section the interaction length and the strength of the axial field provided by helmholtz coils 106 a simplified energy level diagram 140 illustrating the excitation process for potassium is shown in fig2 the excitation process is twofold first the population is optically excited from the 4s.