= Single-local-gate Bell readout information
{title2=$I=1-h_2((1+n_z)/2)$}
For four uniformly distributed <Bell states>, measure one <qubit> along <Bloch vector> $\boldsymbol n$ and the other along $z$. The sign within each $\Phi^\pm$ or $\Psi^\pm$ pair is invisible and the displayed <mutual information> is at most one bit. A local <Hadamard gate> before a computational detector changes $n_z$ from one to zero and destroys the remaining information. An entangling Bell decoder falls outside this product-measurement restriction and recovers two bits.
Back to article page
