Non-canonical quantization of electromagnetic fields and the meaning of   $Z_3$
  Non-canonical quantization is based on certain reducible representations of canonical commutation relations. Relativistic formalism for electromagnetic non-canonical quantum fields is introduced. Unitary representations of the Poincar\'e group at the level of fields and states are explicitly given. Multi-photon and coherent states are introduced. Statistics of photons in a coherent state is Poissonian if an appropriately defined thermodynamic limit is performed. Radiation fields having a correct $S$ matrix are constructed. The $S$ matrix is given by a non-canonical coherent-state displacement operator, a fact automatically eliminating the infrared catastrophe. This, together with earlier results on elimination of vacuum and ultraviolet infinities, suggests that non-canonical quantization leads to finite field theories. Renormalization constant $Z_3$ is found as a parameter related to wave functions of non-canonical vacua. 
Marek Czachor