Point Form Electrodynamics and the Construction of Conserved Current   Operators
  A general procedure for constructing conserved electromagnetic current operators, for both finite and infinite degree of freedom systems, is given. A four-momentum operator consisting of matter, photon, and electromagnetic interactions is assumed to be polynomial in photon creation and annihilation operators. Commutators of the four-momentum operator with the four-vector potential operator at the space-time point zero give the electromagnetic field tensor and current operator at the space-time point zero. In this construction there are no equations of motion to be satisfied and field operators at an arbitrary space-time point-defined as the four-momentum translates of the corresponding operators at the space-time point zero-are not local operators. Several examples are given to show how the construction is carried out. 
W. H. Klink