  We address the problem of unambiguous comparison of a pair of unknown qudit
unitary channels. Using the framework of process positive operator valued
measures (PPOVM) we characterize all solutions and identify the optimal ones.
We prove that the entanglement is the key ingredient in designing the optimal
experiment for comparison of unitary channels. Without entanglement the
optimality can not be achieved. The proposed scheme is also experimentally
feasible.
