Abstract:

Whether or not  entanglement is necessary pre-­requisite for quantum information protocols had been debated ever since the first experiments on NMR quantum computing, which were performed successfully  with separable systems.  There were several hints that entanglement is sufficient in such applications but that it is not absolutely necessary.  The first was that the entanglement in a Werner state [1] vanishes discontinuously as the state is gradually tuned towards more mixedness.  Yu and Eberly [2] found another example of a discontinuous disappearance of entanglement.  In a seminal paper Ollivier and Zurek [3] introduced quantum discord as a measure for quantum correlations, which does not show such discontinuities.  Quantum discord can be seen as a measure for the entanglement, which can be extracted from a separable system in a mixed state.  Along this line, three experiments recently demonstrated distributing entanglement with separable states [4,5,6].

References:

1.  R.F. Werner, Phys. Rev. A 40, 4277 (1989)

2.  H. Ollivier, W.H. Zurek, Phys. Rev. Lett. 88, 017901 (2002)

3.  T. Yu and J.H. Eberly, Phys. Rev. Lett. 93, 140404 (2004)

4.  A. Fedrizzi et al., Phys. Rev. Lett. 111, 230504 (2013)

5.  C.E. Vollmer et al., Phys. Rev. Lett. 111, 230505 (2013)

6.  Ch. Peuntinger et al., Phys. Rev. Lett. 111, 230506 (2013)