We can define the annihilation operators for circularly polarized transverse photons: a ^ ^ k , L = 1 2 ( a ^ ^ k , 1 − − i a ^ ^ k , 2 ) , a ^ ^ k , R = 1 2 ( a ^ ^ k , 1 + i a ^ ^ k , 2 ) , with polarization unit vectors e ( k , L ) = 1 2 [ e ( k , 1 ) + i e ( k , 2 ) ] , e ( k , R ) = 1 2 [ e ( k , 1 ) − − i e ( k , 2 ) ] .