The procedure to obtain an even number of atoms. (a) The atoms are prepared in the internal state ∣ 1 〉 at first. (b) With the resonance condition of the odd-allowed transition fulfilled, one atom is allowed to transition from state ∣ 1 〉 to ∣ 2 〉 and then be pumped to an untrapped state. (c) The resonance condition is no longer fulfilled as the number of atoms has changed from odd to even. (d) The evolution of transition probability of the atoms in the Fock states ∣ 21 , 0 〉 (odd) and ∣ 20 , 0 〉 (even) with ν = 0.01 , γ = 200 π , g = 100 π , and T=0.01.