The time evolution of the atom inversion 〈 S z 〉 for a cascade three-level atom. In these pictures, the field is the coherent state with initial average photon number n ¯ = | α | 2 = 225 . (a) The atom is initially in a ground state with θ = π/2, φ = 0; (b) The atom is initially in superposition state of | 3 〉 and | 1 〉 , θ = π/4, φ = 0; (c) The atom is in a coherent superposition state with θ = π/3, φ = arcsin ( 6 / 3 ) .