Quantum entanglement of unitary operators on bi-partite systems
  We study the entanglement of unitary operators on $d_1\times d_2$ quantum systems. This quantity is closely related to the entangling power of the associated quantum evolutions. The entanglement of a class of unitary operators is quantified by the concept of concurrence. 
X. Wang, P. Zanardi (Institute for Scientific Interchange (ISI)   Foundation)