  In this paper, a formulation, which is completely established on a quantum
ground, is presented for basic contents of quantum electrodynamics (QED). This
is done by moving away, from the fundamental level, the assumption that the
spin space of bare photons should (effectively) possess the same properties as
those of free photons observed experimentally. Within this formulation, bare
photons with zero momentum can not be neglected when constructing the photon
field; and an explicit expression for the related part of the photon field is
derived. When a local gauge transformation is performed on the electron field,
this expression predicts a change that turns out to be equal to what the gauge
symmetry requires for the gauge field. This gives an explicit mechanism, by
which the photon field may change under gauge transformations in QED.
