Solution
= Solution
The columns are $i|+\rangle$ and $-i|-\rangle$. Their <norms> are one and their <inner product> is zero, since $|+\rangle,|-\rangle$ are <orthonormal>. Thus \b[$M_1$ is a <unitary matrix>]. Equivalently,
$$
M_1=iHZ,\qquad M_1^\dagger M_1=Z H^\dagger H Z=I,
$$
using the <Hadamard gate> and <Pauli Z gate>. The different column phases do not destroy <orthonormality>.