Quantum codes do not increase fidelity against isotropic errors

J. Lacalle, L. M. Pozo-Coronado, A. L. Fonseca de Oliveira +1

Given an $m-$qubit ${\Phi }_0$ and an $(n,m)-$quantum code $\mathcal{C}$, let $\Phi$ be the $n-$qubit that results from the $\mathcal{C}-$encoding of ${\Phi }_0$. Suppose that the state $\Phi$ is af…