  We prove that the entangling capacity of a two-qubit unitary operator without
local ancillas, both with and without the restriction to initial product
states, as quantified by the maximum attainable concurrence, is directly
related to the distinguishability of a closely related pair of two-qubit
unitary operators. These operators are the original operator transformed into
its canonical form and the adjoint of this canonical form. The
distinguishability of these operators is quantified by the minimum overlap of
the output states over all possible input probe states. The entangling capacity
of the original unitary operator is therefore directly related to the degree of
non-Hermiticity of its canonical form, as quantified in an operationally
satisfactory manner in terms of the extent to which it can be distinguished, by
measurement, from its adjoint. Furthermore, the maximum entropy of
entanglement, again without local ancillas, that a given two-qubit unitary
operator can generate, is found to be closely related to the classical
capacities of certain quantum channels.
