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Peres–Horodecki criterion

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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 2×2 and 2×3 dimensional cases the condition is also sufficient. It is used to decide the separability of mixed states, where the Schmidt decomposition does not apply. The theorem was discovered in 1996 by Asher Peres[1] and the Horodecki family (Michał, Paweł, and Ryszard)[2]

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

Definition

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If we have a general state which acts on Hilbert space of

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 .

Example

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Consider this 2-qubit family of Werner states:

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

Its density matrix is

and the partial transpose

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

Demonstration

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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 systems

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The Peres–Horodecki criterion has been extended to continuous variable systems. Rajiah Simon[3] 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.[4] for a seemingly different but essentially equivalent approach). It was later found [5] 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 [6][7] or by using entropic measures.[8][9]

Symmetric systems

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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 [10]

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

Moreover, bipartite symmetric PPT can be written as

where are probabilities and fulfill and However, for a subsystem larger than a qubit, are not necessarily physical pure density matrices since they can have negative eigenvalues. In this case, even entangled states can be written as a mixture of tensor products of single-party aphysical states, very similar to the form of separable states. In the qubit case, are physical density matrices, which is consistent with the fact that for two qubits all PPT states are separable.

The concept of such pseudomixtures has been extended to non-symmetric states and to the multipartite case, by the definition of pseudoseparable states[11]

where is the number of subsystems and fulfill and The single subsystem aphysical states are just states that live on the higher dimensional equivalent of the Bloch sphere even for systems that are larger than a qubit. Separable states are the subset of the set of speudoseparable states, while for qubits the two sets coincide with each other. For systems larger than qubits, such quantum states can be entangled, and in this case they can have PPT or non-PPT bipartitions.

References

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  1. ^ Peres, Asher (August 19, 1996). "Separability Criterion for Density Matrices". Physical Review Letters. 77 (8): 1413–1415. arXiv:quant-ph/9604005. doi:10.1103/PhysRevLett.77.1413. PMID 10063072. S2CID 5246518.
  2. ^ Horodecki, Michał; Horodecki, Paweł; Horodecki, Ryszard (1996). "Separability of mixed states: necessary and sufficient conditions". Physics Letters A. 223 (1–2): 1–8. arXiv:quant-ph/9605038. doi:10.1016/S0375-9601(96)00706-2. S2CID 10580997.
  3. ^ 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. S2CID 11664720.
  4. ^ 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. S2CID 9948874.
  5. ^ 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. S2CID 20897950.
  6. ^ 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. S2CID 28595936.
  7. ^ 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. S2CID 43756465.
  8. ^ 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. S2CID 10523704.
  9. ^ 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. S2CID 7149863.
  10. ^ 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. PMID 19518768. S2CID 43527866.
  11. ^ Vitagliano, Giuseppe; Gühne, Otfried; Tóth, Géza (2024). "su(d)-squeezing and many-body entanglement geometry in finite-dimensional systems". arXiv:2406.13338 [quant-ph].