 Eigenvalue mixing matrix of a unital quantum channel

Choose input eigenvectors $u_i$ and output eigenvectors $v_j$ for a unital quantum channel. Positivity makes the displayed entries nonnegative. Trace preservation makes every column sum one; unitality makes every row sum one. Thus $A$ is a doubly stochastic matrix and the output spectrum satisfies $s=Ar$. The mixing matrix need not be a matrix of squared overlaps of a single unitary, even though its row and column constraints are the same.