Vector beams are inhomogeneously polarized optical fields with nonseparable, quantum-like correlations between their polarisation and spatial components, and hold tremendous promise for classical and quantum communication across various channels, e.g. the atmosphere, underwater, and in optical fibre. Here we show that by exploiting their quantum-like features by virtue of the nonseparability of the field, the decay of both the polarisation and spatial components can be studied in tandem. In particular, we invoke the principle of channel state duality to show that the degree of nonseparability of any vector mode is purely determined by that of a maximally nonseparable one, which we confirm using orbital angular momentum (OAM) as an example for topological charges of l = 1 and l = 10 in a turbulent atmosphere. A consequence is that the well-known cylindrical vector vortex beams are sufficient to predict the behaviour of all vector OAM states through the channel, and find that the rate of decay in vector quality decreases with increasing OAM value, even though the spread in OAM is opposite, increasing with OAM. Our approach offers a fast and easy probe of noisy channels, while at the same time revealing the power of quantum tools applied to classical light.
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