Promising advances of quantum information technologies rely on the physical phenomenon of entanglement, a particular form of correlations exhibited by composite systems. After being created, the entanglement typically decreases due to unavoidable external noises. To fight with this undesirable effect, scientists have developed quaint methods to maintain correlations such as error correction and entanglement concentration protocols, etc. However, the nature can hardly be deceived, so if the noise level exceeds some fundamental limit, then the entanglement is lost without any chance to be resuscitated by local operations. In our paper [S.N. Filippov and M. Ziman, Bipartite entanglement-annihilating maps: Necessary and sufficient conditions, Phys. Rev. A 88, 032316 (2013)], we find the fundamental limit of such entanglement annihilation for any type of physical noisy evolution. The obtained result is crucial for determining the ultimate length of quantum communication lines.
Links to the paper: http://pra.aps.org/abstract/PRA/v88/i3/e032316 (restricted access), http://arxiv.org/abs/1306.6525 (free access)
Abstract: We fully characterize bipartite entanglement-annihilating (EA) channels that destroy entanglement of any state shared by subsystems and, thus, should be avoided in any entanglement-enabled experiment. Our approach relies on extending the problem to EA positive maps, the cone of which remains invariant under concatenation with partially positive maps. Due to this invariancy, positive EA maps adopt a well characterization and their intersection with completely positive trace-preserving maps results in the set of EA channels. In addition to a general description, we also provide sufficient operational criteria revealing EA channels. They have a clear physical meaning since the processes involved contain stages of classical information transfer for subsystems. We demonstrate the applicability of derived criteria for local and global depolarizing noises, and specify corresponding noise levels beyond which any initial state becomes disentangled after passing the channel. The robustness of some entangled states is discussed.