Locking Classical Correlations in Quantum States

We show that there exist bipartite quantum states which contain a large locked classical correlation that is unlocked by a disproportionately small amount of classical communication. In particular, there are [Formula presented]-qubit states for which a one-bit message doubles the optimal classical mutual information between measurement results on the subsystems, from [Formula presented] bits to [Formula presented] bits. This phenomenon is impossible classically. However, states exhibiting this behavior need not be entangled. We study the range of states exhibiting this phenomenon and bound its magnitude. © 2004 The American Physical Society.