Peres–Horodecki criterion

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The Peres–Horodecki criterion is a necessary condition, for the joint density matrix \rho of two quantum mechanical systems A and B, 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 \rho which acts on \mathcal{H}_A \otimes \mathcal{H}_B

\rho = \sum_{ijkl} p^{ij}_{kl} |i\rangle \langle j | \otimes |k\rangle \langle l|

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

\rho^{T_B} := I \otimes T (\rho) = \sum_{ijkl} p^{ij} _{kl} |i\rangle \langle j | \otimes (|k\rangle \langle l|)^T = \sum_{ijkl} p^{ij} _{kl} |i\rangle \langle j | \otimes |l\rangle \langle k|

Note that the partial in the name implies that only part of the state is transposed. More precisely,  I \otimes T (\rho) 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:

\rho = \begin{pmatrix} A_{11} & A_{12} & \dots & A_{1n} \\ A_{21} & A_{22} & & \\ \vdots & & \ddots & \\ A_{n1} & & & A_{nn} \end{pmatrix}

Where n = \dim \mathcal{H}_A, and each block is a square matrix of dimension m = \dim \mathcal{H}_B. Then the partial transpose is

\rho^{T_B} = \begin{pmatrix} A_{11}^T & A_{12}^T & \dots & A_{1n}^T \\ A_{21}^T & A_{22}^T & & \\ \vdots & & \ddots & \\ A_{n1}^T & & & A_{nn}^T \end{pmatrix}

The criterion states that if \rho\;\! is separable, \rho^{T_B} has non-negative eigenvalues. In other words, if \rho^{T_B} has a negative eigenvalue, \rho\;\! is guaranteed to be entangled. If the eigenvalues are non-negative, and the dimension is larger than 6, the test is inconclusive.

The result is independent of the party that was transposed, because \rho^{T_A} = (\rho^{T_B})^T.


Consider this 2-qubit family of Werner states:

\rho = p |\Psi^-\rangle \langle \Psi^-| + (1-p) \frac{I}{4}

It can be regarded as the convex combination of |\Psi^-\rangle, a maximally entangled state, and identity, the maximally mixed state.

Its density matrix is

\rho = \frac{1}{4}\begin{pmatrix}
1-p & 0 & 0 & 0\\
0 & p+1 & -2p & 0\\
0 & -2p & p+1 & 0 \\
0 & 0 & 0 & 1-p\end{pmatrix}

and the partial transpose

\rho^{T_B} = \frac{1}{4}\begin{pmatrix}
1-p & 0 & 0 & -2p\\
0 & p+1 & 0 & 0\\
0 & 0 & p+1 & 0 \\
-2p & 0 & 0 & 1-p\end{pmatrix}

Its least eigenvalue is (1-3p)/4. Therefore, the state is entangled for  p > 1/3 .


If ρ is separable, it can be written as

 \rho = \sum p_i \rho^A_i \otimes \rho^B_i

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

\rho^{T_B} = I \otimes T (\rho) = \sum p_i \rho^A_i \otimes (\rho^B_i)^T

As the transposition map preserves eigenvalues, the spectrum of \rho^{T_B} is the same as the spectrum of \rho\;\!, and in particular \rho^{T_B} must still 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 I \otimes \Lambda (\rho) being positive for all positive maps Λ is a necessary and sufficient condition for the separability of ρ, where Λ maps  B(\mathcal{H}_B) to B(\mathcal{H}_A)

Furthermore, every positive map from  B(\mathcal{H}_B) to B(\mathcal{H}_A) can be decomposed into a sum of completely positive and completely copositive maps, when \textrm{dim}(\mathcal{H}_B) = 2 and \textrm{dim}(\mathcal{H}_A) = 2\;\textrm{or}\;3. In other words, every such map Λ can be written as

\Lambda = \Lambda _1 + \Lambda _2 \circ T,

where \Lambda_1 and \Lambda_2 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 \rho^{T_B} is positive, I \otimes \Lambda (\rho) is positive for any Λ. Thus we conclude that the Peres–Horodecki criterion is also sufficient for separability when \textrm{dim}(\mathcal{H}_A \otimes \mathcal{H}_B) \le 6 .

In higher dimensions, however, there exists 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.