Title: Complexity and capacity bounds for quantum channels,
Abstract: We generalise some well-known graph parameters to operator systems by
considering their underlying quantum channels. In particular, we introduce the
quantum complexity as the dimension of the smallest co-domain Hilbert space a
quantum channel requires to realise a given operator system as its
non-commutative confusability graph. We describe quantum complexity as a
generalised minimum semidefinite rank and, in the case of a graph operator
system, as a quantum intersection number. The quantum complexity and a closely
related quantum version of orthogonal rank turn out to be upper bounds for the
Shannon zero-error capacity of a quantum channel, and we construct examples for
which these bounds beat the best previously known general upper bound for the
capacity of quantum channels, given by the quantum Lovász theta number.