Entanglement capacity of two-qubit unitary operator for rank two mixed states

The entanglement capacity of two-qubit unitary operator acting on rank two mixed states in concurrence is discussed. The condition of perfect entangler is the same as that acting on pure states and the entanglement capacity is the mixing parameter v ₁. For non-perfect entangler, the upper and lower bound of the entanglement capacity are given. Supported by the National Natural Science Foundation of China (Grant No. 60433050) and the Science Foundation of Xuzhou Normal University (Key Project) (Grant No. 06XLA05)