The above implies that for eigenstates, C | ψ ψ ⟩ ⟩ = | ψ ψ ¯ ¯ ⟩ ⟩ = ± ± | ψ ψ ⟩ ⟩ . Since antiparticles and particles have charges of opposite sign, only states with all quantum charges equal to zero, such as the photon and particle–antiparticle bound states like the neutral pion, η or positronium, are eigenstates of C .