Conserved photon current
Margaret Hawton · Physical Review A · 2024
A conserved photon current is derived from the commutation relations satisfied by the electromagnetic four-potential and field tensor operators. The density is found to be a sum over positive and negative frequency terms, both of which contribute a positive number density and propagate in a common direction. Discrete positive and negative frequency excitations are both identified as photons. The photon number, equal to the spatial integral of the photon density, is conserved in the absence of sources and sinks.