Solution
= Solution
The <spectral norm> is the <operator norm> induced by the <Euclidean norm>:
$$
\|A\|=\sup_{v\ne0}\frac{\|Av\|_2}{\|v\|_2}
=\sqrt{\lambda_{\max}(A^\dagger A)}.
$$
The <Pauli operators> $X_1$ and $Z_2$ act on different <qubits>, and each is a <unitary operator>. Their product is therefore unitary and preserves the norm of every vector. Hence
$$
\boxed{\|X_1Z_2\|=1.}
$$