We consider a general unitary operator acting on two qubits in a product state. We find the conditions such that the state of the qubits after the action is as entangled as possible. We also consider the possibility of using ancilla qubits to increase the entanglement.
@article{arxiv.quant-ph/0011050,
title = {Optimal Creation of Entanglement Using a Two--Qubit Gate},
author = {B. Kraus and J. I. Cirac},
journal= {arXiv preprint arXiv:quant-ph/0011050},
year = {2009}
}