  We quantify the usefulness of a bipartite quantum state in the
ancilla-assisted channel discrimination of arbitrary quantum channels, formally
defining a worst-case-scenario channel discrimination power for bipartite
quantum states. We show that such a quantifier is deeply connected with the
operator Schmidt decomposition of the state. We compute the channel
discrimination power exactly for pure states, and provide upper and lower
bounds for general mixed states. We show that highly entangled states can
outperform any state that passes the realignment criterion for separability.
Furthermore, while also unentangled states can be used in ancilla-assisted
channel discrimination, we show that the channel discrimination power of a
state is bounded by its quantum discord.
