Unlocking Hidden Entanglement with Classical Information
Oliver Cohen · Physical Review Letters · 1998
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 $\mathrm{spin}\ensuremath{-}1/2$ 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.