Like the creation and annihilation operators a i and a i † † also found in quantum field theory, the creation and annihilation operators S i + and S i − − act on bosons in multi-particle states. While a i and a i † † allows us to determine whether a particle was created or destroyed in a system, the spin operators S i + and S i − − allow us to determine how. A photon can become both a positron and electron and vice versa. Because of the anti-symmetric statistics, a particle of spin 1 2 obeys the Pauli-Exclusion Rule. Two particles can exist in the same state if and only if the spin of the particle is opposite.