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