Summary we have seen that in quantum optics the em field is represented as a set of independent quantized harmonic oscillators of energy 1 hoa(t)a (t) (1.53) moreover, we express the vector potential and the electric field in terms of plane determined by the creation à and annihilation à operators waves whose amplitudes are quantized and eral description and quantization of em fields associated with the mode k of the field as - hc] (1.54) hop k 4(+) = e (f, t)+(f, t), (1.55) where h. c. stands for the hermitian conjugate of the first term in the bracket, and (- e 4(+) f, t) hox -en(t) ek. (1.56) 260v (f. i-(0', the constant ha/(2e0v) is called the quantum unit of electric strength