Linking measures for macroscopic quantum states via photon–spin mapping

We review and compare several measures that identify quantum states that are “macroscopically quantum”. These measures were initially formulated either for photonic systems or for spin ensembles. Here, we compare them through a simple model which maps photonic states to spin ensembles. On one hand, we reveal problems for some spin measures to handle correctly photonic states that typically are considered to be macroscopically quantum. On the other hand, we find significant similarities between other measures even though they were differently motivated.