Tuesday, October 9, 2012

1210.2119 (Stefano Pirandola)

Restoring broken entanglement by injecting separable correlations    [PDF]

Stefano Pirandola
The distribution of entanglement is central in many protocols of quantum information and computation. However it is also known to be a very fragile process when loss and noise come into play. The inevitable interaction of the quantum systems with the external environment induces effects of decoherence which may be so strong to destroy any input entanglement, a phenomenon known as "entanglement breaking". Here we study this catastrophic process in a correlated-noise environment showing how the presence of classical-type correlations can restore the distribution of entanglement. In particular, we consider a Gaussian environment whose thermal noise is strong enough to break the entanglement of two bosonic modes of the electromagnetic field. In this scenario, we show that the injection of separable correlations from the same environment is able to reactivate the broken entanglement. This paradoxical effect happens both in schemes of direct distribution, where a third party (Charlie) broadcasts entangled states to remote parties (Alice and Bob), and in schemes of indirect distribution which are based on the protocol of entanglement swapping, whose theory is here generalized. Furthermore, the amount of entanglement activated by the injection can be large enough to be distilled using one-way distillation protocols. As a result, entanglement distribution and its distillation are still possible in the presence of entanglement-breaking channels, as long as a sufficient amount of separable correlations can be identified in the environment. These findings pose foundamental questions about the intimate interplay between local and nonlocal correlations, and suggest new perspectives for quantum repeaters in the presence of memory channels and correlated-noise environments, such as those characterized by non-Markovian dynamics.
View original: http://arxiv.org/abs/1210.2119

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