Light-front gauge invariant formulation and electromagnetic duality

The gauge invariant formulation of Maxwell's equations and the electromagnetic duality transformations are given in the light-front (LF) variables. The novel formulation of the LF canonical quantization, which is based on the kinematic translation generator $P^{+}$ rather then on the Hamiltonian $P^{-}$, is proposed. This canonical quantization is applied for the free electromagnetic fields and for the fields generated by electric and magnetic external currents. The covariant form of photon propagators, which agrees with Schwinger's source theory, is achieved when the direct interaction of external currents is properly chosen. Applying the path integral formalism, the equivalent LF Lagrangian density, which depends on two Abelian gauge potentials, is proposed. Some remarks on the Dirac strings and LF non local structures are presented in the Appendix.