  In this paper, a simple geometric structure is shown, which underlies the
interaction Hamiltonian of quantum electrodynamics. Specifically, eight parts
of the interaction Hamiltonian, corresponding to eight basic Feynman diagrams,
are found derivable from two operators called fundamental interaction operators
(FIOs), with the help of a superoperator that describes vacuum fluctuations.
And, the two FIOs have the simple geometric meanings of mapping the state space
of an electron-positron pair to that of a photon and the reverse.
