  Quantum logic gates can perform calculations much more efficiently than their
classical counterparts. However, the level of control needed to obtain a
reliable quantum operation is correspondingly higher. In order to evaluate the
performance of experimental quantum gates, it is therefore necessary to
identify the essential features that indicate quantum coherent operation. In
this paper, we show that an efficient characterization of an experimental
device can be obtained by investigating the classical logic operations on a
pair of complementary basis sets. It is then possible to obtain reliable
predictions about the quantum coherent operations of the gate such as
entanglement generation and Bell state discrimination even without performing
these operations directly.
