= Quantum erasure correction
{title2=$m<d$}
Errors on a known subset of $m$ <qubits> are correctable by a distance-$d$ code whenever $m<d$. Every product $E_a^\dagger E_b$ of errors supported on that subset still has support in the same subset, so its compression is scalar. The <Knill--Laflamme condition> therefore supplies one recovery for every channel on that subset. Equivalently, erasing the subset leaks no logical information to it: if a reference is maximally entangled with the encoded logical system, its joint <density operator> with the erased subsystem is a <product state>.
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