Mixed-state entanglement and distillation: is there a ``bound''   entanglement in nature?
  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. Possible physical meaning of the result is discussed. 
Michal Horodecki, Pawel Horodecki and Ryszard Horodecki