Bad Qubits — Learn Quantum by Doing

Multiple choice

Per this lesson, why must every eigenvalue

\lambda

of a unitary operator satisfy

|\lambda| = 1

Because unitaries are always diagonal with real entries
Because a unitary preserves norms, so taking the norm of $U|u\rangle = \lambda|u\rangle$ forces $|\lambda|^2 = 1$
Because $\lambda$ must equal the dimension of the Hilbert space
Because all eigenvalues of every matrix have modulus 1