On the Entangling Power of Quantum Evolutions

We analyze the entangling capabilities of unitary transformations $U$ acting on a bipartite $d_1\times d_2$-dimensional quantum system. To this aim we introduce an entangling power measure $e(U)$ given by the mean linear entropy produced acting with $U$ on a given distribution of pure product states. This measure admits a natural interpretation in terms of quantum operations. For a uniform distribution explicit analytical results are obtained using group-theoretic arguments. The behaviour of the features of $e(U)$ as the subsystem dimensions $d_1$ and $d_2$ are varied is studied both analytically and numerically. The two-qubit case $d_1=d_2=2$ is argued to be peculiar.