Entangling efficiency of linear-optical quantum gates

We propose a measure of the nonclassicality of quantum gates which is particularly suitable for probabilistic devices. This measure enables to compare deterministic devices which prepare entangled states with a low amount of entanglement with probabilistic devices which generate highly entangled states but which fail sometimes. We provide examples demonstrating the advantages of this measure over the so-far employed entangling power. The experimentally determined entangling efficiency of a tunable linear-optical controlled-phase gate is presented to highlight the practical aspects of this measure.