  An open question in quantum complexity theory is whether or not the class
$\operatorname{MIP}^{co}$, consisting of languages that can be efficiently
verified using interacting provers sharing quantum resources according to the
quantum commuting model, coincides with the class $coRE$ of languages with
recursively enumerable complement. We introduce the notion of a qc-modulus,
which encodes approximations to quantum commuting correlations, and show that
the existence of a computable qc-modulus gives a negative answer to a natural
variant of the aforementioned question.
