  Two different measures of operator entanglement of two-qubit gates, namely,
Schmidt strength and linear entropy, are studied. While these measures are
shown to have one-to-one relation between them for Schmidt number 2 class of
gates, no such relation exists for Schmidt number 4 class, implying that the
measures are inequivalent in general. Further, we establish a simple relation
between linear entropy and local invariants of two-qubit gates. The implication
of the relation is discussed.
