2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/217505With growing success in experimental implementations it is critical to identify a "gold standard" for quantum information processing, a single measure of distance that can be used to compare and contrast different experiments. We enumerate a set of criteria such a distance measure must satisfy to be both experimentally and theoretically meaningful. We then assess a wide range of possible measures against these criteria, before making a recommendation as to the best measures to use in characterizing quantum information processing.15 pages; this version in line with published versionQuantum Physicstext