Title: Almost-idempotent quantum channels and approximate $C^*$-algebras

Abstract: Let $\Phi$ be a unital completely positive map on the space of operators on some Hilbert space. We assume that $\Phi$ is almost idempotent, namely, $\|\Phi^2-\Phi\|_{\mathrm{cb}} \le\eta$, and construct a corresponding "$\varepsilon$-$C^*$ algebra" for $\varepsilon=O(\eta)$. This type of structure has the axioms of a unital $C^*$ algebra but the associativity and other axioms involving the multiplication and the unit hold up to $\varepsilon$. We further prove that any finite-dimensional $\varepsilon$-$C^*$ algebra is $O(\varepsilon)$-isomorphic to a genuine $C^*$ algebra. These bounds are universal, i.e.\ do not depend on the dimensionality or other parameters.