Solution
= Solution
For $n\ge2$, the <Pauli X gate> and <Pauli Z gate> act on different <qubits> in $X_0Z_1$, so they commute. Their product is a <Hermitian matrix> and squares to the <identity operator>. Its <eigenvalues> have modulus one; equivalently it is a <unitary operator>. Therefore \b[its <spectral norm> is]
$$
\boxed{\|X_0Z_1\|=1.}
$$
The $n\ge2$ qualification is necessary because the qubit labelled one does not exist for $n=1$.