Peres–Horodecki criterion

The Peres–Horodecki criterion is a necessary condition, for the joint density matrix of two quantum mechanical systems and , to be separable. It is also called the PPT criterion, for positive partial transpose. In the 2x2 and 2x3 dimensional cases the condition is also sufficient. It is used to decide the separability of mixed states, where the Schmidt decomposition does not apply.

In higher dimensions, the test is inconclusive, and one should supplement it with more advanced tests, such as those based on entanglement witnesses.


If we have a general state   which acts on  


Its partial transpose (with respect to the B party) is defined as


Note that the partial in the name implies that only part of the state is transposed. More precisely,   is the identity map applied to the A party and the transposition map applied to the B party.

This definition can be seen more clearly if we write the state as a block matrix:


Where  , and each block is a square matrix of dimension  . Then the partial transpose is


The criterion states that if   is separable then all the eigenvalues of   are non-negative. In other words, if   has a negative eigenvalue,   is guaranteed to be entangled. The converse of these statements is true if and only if the dimension of the product space is   or  .

The result is independent of the party that was transposed, because  .


Consider this 2-qubit family of Werner states:


It can be regarded as the convex combination of  , a maximally entangled state, and identity, the maximally mixed state.

Its density matrix is


and the partial transpose


Its least eigenvalue is  . Therefore, the state is entangled for  .


If ρ is separable, it can be written as


In this case, the effect of the partial transposition is trivial:


As the transposition map preserves eigenvalues, the spectrum of   is the same as the spectrum of  , and in particular   must still be positive semidefinite. Thus   must also be positive semidefinite. This proves the necessity of the PPT criterion.

Showing that being PPT is also sufficient for the 2 X 2 and 3 X 2 (equivalently 2 X 3) cases is more involved. It was shown by the Horodeckis that for every entangled state there exists an entanglement witness. This is a result of geometric nature and invokes the Hahn–Banach theorem (see reference below).

From the existence of entanglement witnesses, one can show that   being positive for all positive maps Λ is a necessary and sufficient condition for the separability of ρ, where Λ maps   to  

Furthermore, every positive map from   to   can be decomposed into a sum of completely positive and completely copositive maps, when   and  . In other words, every such map Λ can be written as


where   and   are completely positive and T is the transposition map. This follows from the Størmer-Woronowicz theorem.

Loosely speaking, the transposition map is therefore the only one that can generate negative eigenvalues in these dimensions. So if   is positive,   is positive for any Λ. Thus we conclude that the Peres–Horodecki criterion is also sufficient for separability when  .

In higher dimensions, however, there exist maps that can't be decomposed in this fashion, and the criterion is no longer sufficient. Consequently, there are entangled states which have a positive partial transpose. Such states have the interesting property that they are bound entangled, i.e. they can not be distilled for quantum communication purposes.

Continuous variable systemsEdit

The Peres–Horodecki criterion has been extended to continuous variable systems. Simon [1] formulated a particular version of the PPT criterion in terms of the second-order moments of canonical operators and showed that it is necessary and sufficient for  -mode Gaussian states (see Ref.[2] for a seemingly different but essentially equivalent approach). It was later found [3] that Simon's condition is also necessary and sufficient for  -mode Gaussian states, but no longer sufficient for  -mode Gaussian states. Simon's condition can be generalized by taking into account the higher order moments of canonical operators [4][5] or by using entropic measures.[6][7]

Symmetric systemsEdit

For symmetric states of bipartite systems, the positivity of the partial transpose of the density matrix is related to the sign of certain two-body correlations. Here, symmetry means that


holds, where   is the flip or swap operator exchanging the two parties   and  . A full basis of the symmetric subspace is of the form   with   and   Here for   and     must hold, where   is the dimension of the two parties.

It can be shown that for such states,   has a positive partial transpose if and only if [8]


holds for all operators   Hence, if   holds for some   then the state possesses non-PPT entanglement.


  1. ^ Simon, R. (2000). "Peres-Horodecki Separability Criterion for Continuous Variable Systems". Physical Review Letters. 84 (12): 2726–2729. arXiv:quant-ph/9909044. Bibcode:2000PhRvL..84.2726S. doi:10.1103/PhysRevLett.84.2726. PMID 11017310.
  2. ^ Duan, Lu-Ming; Giedke, G.; Cirac, J. I.; Zoller, P. (2000). "Inseparability Criterion for Continuous Variable Systems". Physical Review Letters. 84 (12): 2722–2725. arXiv:quant-ph/9908056. Bibcode:2000PhRvL..84.2722D. doi:10.1103/PhysRevLett.84.2722. PMID 11017309.
  3. ^ Werner, R. F.; Wolf, M. M. (2001). "Bound Entangled Gaussian States". Physical Review Letters. 86 (16): 3658–3661. arXiv:quant-ph/0009118. Bibcode:2001PhRvL..86.3658W. doi:10.1103/PhysRevLett.86.3658. PMID 11328047.
  4. ^ Shchukin, E.; Vogel, W. (2005). "Inseparability Criteria for Continuous Bipartite Quantum States". Physical Review Letters. 95 (23): 230502. arXiv:quant-ph/0508132. Bibcode:2005PhRvL..95w0502S. doi:10.1103/PhysRevLett.95.230502. PMID 16384285.
  5. ^ Hillery, Mark; Zubairy, M. Suhail (2006). "Entanglement Conditions for Two-Mode States". Physical Review Letters. 96 (5): 050503. arXiv:quant-ph/0507168. Bibcode:2006PhRvL..96e0503H. doi:10.1103/PhysRevLett.96.050503. PMID 16486912.
  6. ^ Walborn, S.; Taketani, B.; Salles, A.; Toscano, F.; de Matos Filho, R. (2009). "Entropic Entanglement Criteria for Continuous Variables". Physical Review Letters. 103 (16): 160505. arXiv:0909.0147. Bibcode:2009PhRvL.103p0505W. doi:10.1103/PhysRevLett.103.160505. PMID 19905682.
  7. ^ Yichen Huang (October 2013). "Entanglement Detection: Complexity and Shannon Entropic Criteria". IEEE Transactions on Information Theory. 59 (10): 6774–6778. doi:10.1109/TIT.2013.2257936.
  8. ^ Tóth, Géza; Gühne, Otfried (May 1, 2009). "Entanglement and Permutational Symmetry". Physical Review Letters. 102 (17): 170503. arXiv:0812.4453. doi:10.1103/PhysRevLett.102.170503.