Lovász's theta function, Rényi's divergence and the sphere-packing bound

Lovász's bound to the capacity of a graph and the the sphere-packing bound to the probability of error in channel coding are given a unified presentation as information radii of the Csiszár type using the Rényi divergence in the classical-quantum setting. This brings together two results in coding theory that are usually considered as being of a very different nature, one being a 'combinatorial' result and the other being 'probabilistic'. In the context of quantum information theory, this difference disappears.