UNLOCKING HIDDEN ENTANGLEMENT WITH CLASSICAL INFORMATION

The notion of ''hidden'' entanglement is introduced, and it is shown t hat this is a property associated with every separable mixed quantum s tate of two subsystems. The hidden entanglement is explicitly quantifi ed for a general class of separable mixed states of two spin-1/2 parti cles, and a formula is derived giving the maximum amount of entangleme nt that can be hidden. The process of ''unlocking'' hidden entanglemen t with classical information is explained, and the number of bits requ ired to unlock each ebit of entanglement is evaluated. It is argued th at the entanglement-unlocking process can be seen as the converse of q uantum cryptography schemes that use EPR pairs.