Conditions for a class of entanglement transformations

Suppose Alice and Bob jointly possess a pure state, |ψ\ra. Using local operations on their respective systems and classical communication it may be possible for Alice and Bob to transform |ψ\ra into another joint state |ϕ\ra. This Letter gives necessary and sufficient conditions for this process of entanglement transformation to be possible. These conditions reveal a partial ordering on the entangled states, and connect quantum entanglement to the algebraic theory of majorization. As a consequence, we find that there exist essentially different types of entanglement for bipartite quantum systems.