  We prove the equivalence of an arbitrary single-mode Gaussian quantum channel
and a newly defined fiducial channel preceded by a phase shift and followed by
a Gaussian unitary operation. This equivalence implies that the
energy-constrained classical capacity of any single-mode Gaussian channel can
be calculated based on this fiducial channel, which is furthermore simply
realizable with a beam splitter, two identical single-mode squeezers, and a
two-mode squeezer. In a large domain of parameters, we also provide an
analytical expression for the Gaussian classical capacity, exploiting its
additivity, and prove that the classical capacity cannot exceed it by more than
1/ln2 bits.
