In combination with the magnetic field, pairs of counter-propagating circularly-polarized laser beams are sent in along three orthogonal axes, for a total of six MOT beams (there are exceptions to this, but a minimum of six beams is required to make a 3D MOT). The beams are red-detuned from the formula_15 transition by an amount formula_16 such that formula_17, or equivalently, formula_18, where formula_19 is the frequency of the laser beams and formula_20 is the frequency of the transition. The beams must be circularly polarized to ensure that photon absorption can only occur for certain transitions between the ground state formula_21 and the sublevels of the excited state formula_22, where formula_23. In other words, the circularly-polarized beams enforce selection rules on the allowed electric dipole transitions between states.