Mixed-State Entanglement and Distillation: Is there a “Bound” Entanglement in Nature?

@article{Horodecki1998MixedStateEA,
  title={Mixed-State Entanglement and Distillation: Is there a “Bound” Entanglement in Nature?},
  author={Michal Horodecki and Paweł Horodecki and Ryszard Horodecki},
  journal={Physical Review Letters},
  year={1998},
  volume={80},
  pages={5239-5242}
}
It is shown that if a mixed state can be distilled to the singlet form it must violate partial transposition criterion [A. Peres, Phys. Rev. Lett. 76, 1413 (1996)]. It implies that there are two qualitatively different types of entanglement: ``free'' entanglement which is distillable, and ``bound'' entanglement which cannot be brought to the singlet form useful for quantum communication purposes. A possible physical meaning of the result is discussed. 
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