  We construct a $q$-boson representation of the Zamolodchikov-Faddeev algebra
whose structure function is given by the stochastic $R$ matrix of
$U_q(A^{(1)}_n)$ introduced recently. The representation involves quantum
dilogarithm type infinite products in the $n(n-1)/2$-fold tensor product of
$q$-bosons. It leads to a matrix product formula of the stationary
probabilities in the $U_q(A_n^{(1)})$-zero range process on a one-dimensional
periodic lattice.
