  The difficulty of an optimization task in quantum information science depends
on the proper mathematical expression of the physical target. Here we
demonstrate the power of optimization functionals targeting an arbitrary
perfect two-qubit entangler, creating a maximally-entangled state out of some
initial product state. For two quantum information platforms of current
interest, nitrogen vacancy centers in diamond and superconducting Josephson
junctions, we show that an arbitrary perfect entangler can be reached faster
and with higher fidelity than specific two-qubit gates or local equivalence
classes of two-qubit gates. Our results are obtained with two independent
optimization approaches, underlining the crucial role of the optimization
target.
