# Tag Info

Assuming $x$ is $n$ bits, here's a simple procedure: take $n$ ancilla qubits, all prepared in $|0\rangle$. Do a transversal controlled-not (i.e. bit by bit controlled-not) from the register with $|x\rangle$ to the ancilla register. THis means that if you started wuth $$\sum_xa_x|x\rangle,$$ you now have $$\sum_xa_x|x\rangle|x\rangle.$$ Next, find a bit ...