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In Quantum Information processing, we can extract entanglement from $n$-copies of a weakly entangled state to produce a fully or highly entangled states in $d$-dimensions, using the known distillation protocols.

However, there are states from which no entanglement can be extracted out using the known distillation protocols for no value of $n$. Such states contain bound entanglement.

For qubits, PPT is a necessary and sufficient criterion for separability. In higher dimensions, i.e qudits, there are states which satisfy PPT but are entangled and such states contain bound i.e non-distillable entanglement. I am not sure but I guess there are states which are NPT but bound entangled.

What is the deep physical reason behind the existence of bound entanglement?

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    $\begingroup$ What is your question? $\endgroup$ Jul 20 '20 at 1:01
  • $\begingroup$ Whether there is any deep physical reason for existence of bound entanglement or not $\endgroup$ Jul 20 '20 at 4:32
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    $\begingroup$ "deep physical reason" is a bit subjective. Can you be more specific? $\endgroup$
    – glS
    Jul 21 '20 at 15:52
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    $\begingroup$ "I am not sure but I guess there are states which are NPT but bound entangled." -- That's one of the big open questions in quantum information theory. $\endgroup$ Jul 21 '20 at 19:37