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 five beams is required to make a 3D MOT). The beams are red-detuned from the J = 0 → → J = 1 transition by an amount δ δ such that δ δ ≡ ≡ ν ν 0 − − ν ν L > 0 , or equivalently, ν ν L = ν ν 0 − − δ δ , where ν ν L is the frequency of the laser beams and ν ν 0 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 | 0 , 0 ⟩ ⟩ and the sublevels of the excited state | 1 , m J ⟩ ⟩ , where m J = − − 1 , 0 , 1 . In other words, the circularly-polarized beams enforce selection rules on the allowed electric dipole transitions between states.