Assisted Cloning and Orthogonal Complementing of an Arbitrary Unknown Two-qubit Entangled State via Positive Operator-valued Measure

By using positive operator-valued measure (POVM) instead of the more commonly used collective unitary operation in the quantum teleportation process, Zhan's scheme (Zhan in Phys. Lett. A 336: 317, 2005) is generalized to treat an arbitrary (not a special class of) unknown two-qubit entangled state. Generally speaking, conditioned on Victor's classical message, Alice can create either a copy or an orthogonal-complement copy of the original state. However, it is shown that Alice's perfect clone can be realized with higher probability or even in a deterministic manner via an appropriate unitary operation provided that the state to be cloned is chosen from six special ensembles.