Past exam of the computer science course of the University of Oxford / 2020 / Quantum information / 1 / d / Solution

The map sends $|0⟩$ to $|0⟩$, $|1⟩$ to $|2⟩$, and $|2⟩$ to $\lambda |1⟩$. These output columns are mutually orthogonal, with squared norms $1,1,|\lambda {|}^2$. ConsequentlyandNecessity follows from preserving the norm of $|2⟩$; sufficiency follows because the three columns are then an orthonormal basis. Merely requiring $\lambda \ne 0$ would establish invertibility but not unitarity. This is the weighted permutation unitarity criterion.