Unlocking Hidden Entanglement with Classical Information

Abstract

The notion of “hidden” entanglement is introduced, and it is shown that this is a property associated with every separable mixed quantum state of two subsystems. The hidden entanglement is explicitly quantified for a general class of separable mixed states of two $\mathrm{spin}-1/2$ particles, and a formula is derived giving the maximum amount of entanglement that can be hidden. The process of “unlocking” hidden entanglement with classical information is explained, and the number of bits required to unlock each ebit of entanglement is evaluated. It is argued that the entanglement-unlocking process can be seen as the converse of quantum cryptography schemes that use EPR pairs.