Sufficient and Necessary Conditions of Entanglement Transformations Between Mixed States

Based on a new measure of entanglement for finite-dimensional bi-particle pure states, we give sufficient and necessary conditions that a bi-particle mixed state rho can be transformed into another mixed state sigma by local operations and classical communication (LOCC). This result can be regarded as a generalization of Nielsen's theorem (Nielsen, Phys. Rev. Lett. 83: 436, 1999). However, we find that it is more difficult to determine the entanglement transformations between mixed states than to do between pure ones.