  We examine the physical significance of fidelity as a measure of similarity
for Gaussian states, by drawing a comparison with its classical counterpart. We
find that the relationship between these classical and quantum fidelities is
not straightforward, and in general does not seem to provide insight into the
physical significance of quantum fidelity. To avoid this ambiguity we propose
that the efficacy of quantum information protocols be characterized by
determining their transfer function and then calculating the fidelity
achievable for a hypothetical pure reference input state.
