In the case of systems composed of $\backslash ;\; m>2$ subsystems the definition of '''separable and entangled states''' is richer than in the bipartite case. Indeed, in the multipartite case, apart from '''fully separable''' and '''fully entangled''' states, there also exists the notion of '''partial separability'''.
== Full and partial separability ==
The definitions of '''fully separable''' and '''fully entangled''' multipartite states naturally generalizes that of separable and entangled states in the bipartite case, as follows.
'''Definition [Full $\backslash ;\; m$-partite separability ($\backslash ;\; m$-separability) of $\backslash ;\; m$ systems]:''' The state $\backslash ;\; \backslash varrho\_\{A\_1\backslash ldots\; A\_m\}$ of $\backslash ;\; m$ subsystems $\backslash ;\; A\_1,\; \backslash ldots,\; A\_m$ with Hilbert space $\backslash ;\; \backslash mathcal\{H\}\_\{A\_1\; \backslash ldots\; A\_m\}=\backslash mathcal\{H\}\_\{A\_1\}\backslash otimes\backslash ldots\backslash otimes\; \backslash mathcal\{H\}\_\{A\_m\}$ is '''fully separable''' if and only if it can be written in the form
:$\backslash ;\; \backslash varrho\_\{A\_1\backslash ldots\; A\_m\}\; =\; \backslash sum\_\{i=1\}^k\; p\_i\; \backslash varrho\_\{A\_1\}^i\; \backslash otimes\; \backslash ldots\; \backslash otimes\; \backslash varrho\_\{A\_m\}^i.$
Correspondingly, the state $\backslash ;\; \backslash varrho\_\{A\_1\backslash ldots\; A\_m\}$ is '''fully entangled''' if it cannot be written in the above form.
As in the bipartite case, the set of $\backslash ;\; m$-separable states is ''convex'' and ''closed'' with respect to trace norm, and separability is preserved under $\backslash ;\; m$-separable operations $\backslash ;\; \backslash sum\_i\backslash Omega\_i^1\backslash otimes\backslash ldots\backslash otimes\backslash Omega\_i^n$ which are a straightforward generalization of the bipartite ones:
:$\backslash ;\; \backslash varrho\_\{A\_1\backslash ldots\; A\_m\}\backslash to\; \backslash frac\{\backslash sum\_i\backslash Omega\_i^1\backslash otimes\backslash ldots\backslash otimes\backslash Omega\_i^n\backslash varrho\_\{A\_1\backslash ldots\; A\_m\}\; (\backslash Omega\_i^1\backslash otimes\backslash ldots\backslash otimes\backslash Omega\_i^n)^\backslash dagger\}\{Tr[\backslash sum\_i\; \backslash Omega\_i^1\backslash otimes\backslash ldots\backslash otimes\backslash Omega\_i^n\backslash varrho\_\{A\_1\backslash ldots\; A\_m\}\; (\backslash Omega\_i^1\backslash otimes\backslash ldots\backslash otimes\backslash Omega\_i^n)^\backslash dagger]\}\; .$
As mentioned above, though, in the multipartite setting we also have different notions of '''partial separability'''.
'''Definition [separability with respect to partitions]:''' The state $\backslash ;\; \backslash varrho\_\{A\_1\backslash ldots\; A\_m\}$ of $\backslash ;\; m$ subsystems $\backslash ;\; A\_1,\; \backslash ldots,\; A\_m$ is '''separable with respect to a given partition''' $\backslash ;\; \backslash \{I\_1,\; \backslash ldots,\; I\_k\backslash \}$, where $\backslash ;\; I\_i$ are disjoint subsets of the indices $\backslash ;\; I=\backslash \{1,\; \backslash ldots,\; m\backslash \},\; \backslash cup\_\{j=1\}^k\; I\_j\; =\; I$, if and only if it can be written
:$\backslash ;\; \backslash varrho\_\{A\_1\backslash ldots\; A\_m\}\; =\; \backslash sum\_\{i=1\}^N\; p\_i\; \backslash varrho\_1^i\; \backslash otimes\; \backslash ldots\; \backslash otimes\; \backslash varrho\_k^i.$
'''Definition [semiseparability]:''' The state $\backslash ;\; \backslash varrho\_\{A\_1\backslash ldots\; A\_m\}$ is '''semiseparable''' if and only if it is '''separable under all $\backslash ;\; 1$-$\backslash ;\; (m-1)$ partitions''', $\backslash ;\; \backslash big\backslash \{I\_1=\backslash \{k\backslash \},\; I\_2=\backslash \{1,\backslash ldots,k-1,k+1,\backslash ldots,m\backslash \}\backslash big\backslash \},\; 1\backslash leq\; k\; \backslash leq\; m$.
'''Definition [s-particle entanglement]:''' An $\backslash ;\; m$-particle system can have at most '''$\backslash ;\; s$-particle entanglement''' if it is a mixture of all states such that each of them is separable with respect to some partition $\backslash ;\; \backslash \{I\_1,\backslash ldots,I\_k\backslash \}$, where all sets of indices $\backslash ;\; I\_k$ have cardinality $\backslash ;\; N\backslash leq\; s$.
== Separability characterization and criteria==
=== Pure states ===
'''Definition [Full $\backslash ;\; m$-partite separability]:''' The pure state $\backslash ;\; |\backslash Psi\_\{A\_1\backslash ldots\; A\_m\}\backslash rangle$ of $\backslash ;\; m$ subsystems $\backslash ;\; A\_1,\; \backslash ldots,\; A\_m$ is '''fully $\backslash ;\; m$-partite separable''' if and only if it can be written
:$\backslash ;\; |\backslash Psi\_\{A\_1\backslash ldots\; A\_m\}\backslash rangle\; =\; |\backslash psi\_\{A\_1\}\backslash rangle\; \backslash otimes\; \backslash ldots\; \backslash otimes\; |\backslash psi\_\{A\_m\}\backslash rangle.$
In order to check this, it is enough to compute reduced density matrices of elementary subsystems and see whether they are pure. However, this cannot be done so easily in the multipartite case, as only rarely multipartite pure states admit the ''generalized Schmidt Decomposition'' $\backslash ;\; |\backslash Psi\_\{A\_1\backslash ldots\; A\_m\}\backslash rangle\; =\; \backslash sum\_\{i=1\}^\{min\backslash \{d\_\{A\_1\},\backslash ldots,\backslash \}d\_\{A\_m\}\}a\_i\; |e\_\{A\_1\}^i\backslash rangle\; \backslash otimes\; \backslash ldots\; \backslash otimes\; |e\_\{A\_m\}^i\backslash rangle$. A multipartite state admits generalized Schmidt decomposition if, tracing out any subsystem, the rest is in a fully separable state.
Thus, in general the entanglement of a pure state is described by the spectra of the reduced density matrices of all bipartite partitions: the state is '''genuinely $\backslash ;\; m$-partite entangled''' if and only if all bipartite partitions produce mixed reduced density matrices.
=== Mixed states ===
In the multipartite case there is no simple necessary and sufficient condition for separability like the one given by the PPT criterion for the $\backslash ;\; 2\backslash otimes2$ and $\backslash ;\; 2\backslash otimes3$ cases.
However, many separability criteria used in the bipartite setting can be generalized to the multipartite case.
* '''Positive but not completely positive (PnCP) maps and entanglement witnesses'''
The characterization of separability in terms of positive but not completely positive maps can be naturally generalized from the bipartite case, as follows.
Any ''positive but not completely positive (PnCP) map'' $\backslash ;\; \backslash Lambda\_\{A\_2\backslash ldots\; A\_m\}:\backslash mathcal\{B\}(\backslash mathcal\{H\}\_\{A\_2\backslash ldots\; A\_m\})\; \backslash to\; \backslash mathcal\{B\}(\backslash mathcal\{H\}\_\{A\_1\})$ provides a nontrivial necessary separability criterion in the form:
:$\backslash ;\; (I\_\{A\_1\}\backslash otimes\; \backslash Lambda\_\{A\_2\backslash ldots\; A\_m\})[\backslash varrho\_\{A\_1\backslash ldots\; A\_m\}]\; \backslash geq\; 0\; ,$
where $\backslash ;\; I\_\{A\_1\}$ is the identity acting on the first subsystem $\backslash ;\; \backslash mathcal\{H\}\_\{A\_1\}$.
The state $\backslash ;\; \backslash varrho\_\{A\_1\backslash ldots\; A\_m\}$ is ''separable'' if and only if the above condition is satisfied for all PnCP maps $\backslash ;\; \backslash Lambda\_\{A\_2\backslash ldots\; A\_m\}:\backslash mathcal\{B\}(\backslash mathcal\{H\}\_\{A\_2\backslash ldots\; A\_m\})\; \backslash to\; \backslash mathcal\{B\}(\backslash mathcal\{H\}\_\{A\_1\})$.
The definition of ''entanglement witness'' and the ''Choi-Jamiolkowski isomorphism'' that links PnCP maps to entanglement witnesses in the bipartite case can also be generalized to the multipartite setting.
We therefore get a separability condition from ''entanglement witnesses'' for multipartite states: the state $\backslash ;\; \backslash varrho\_\{A\_1\backslash ldots\; A\_m\}$ is ''separable'' if it has non-negative mean value $\backslash ;\; Tr(W\backslash varrho\_\{A\_1\backslash ldots\; A\_m\})\; \backslash geq\; 0$ for all entanglement witnesses $\backslash ;\; W$. Correspondingly, the ''entanglement'' of $\backslash ;\; \backslash varrho\_\{A\_1\backslash ldots\; A\_m\}$ is ''detected by the witness'' $\backslash ;\; W$ if and only if $\backslash ;\; Tr(W\backslash varrho\_\{A\_1\backslash ldots\; A\_m\})\; <\; 0$.
The above description provides a full characterization of $\backslash ;\; m$-separability of $\backslash ;\; m$-partite systems.
* '''Range criterion'''
The range criterion can also be immediately generalized from the bipartite to the multipartite case. In the latter case the range of $\backslash ;\; \backslash varrho\_\{A\_1\backslash ldots\; A\_m\}$ must be spanned by the vectors $\backslash ;\; \backslash \{|\backslash phi\_\{A\_1\}\backslash rangle,\; \backslash ldots,\; |\backslash phi\_\{\; A\_m\}\backslash rangle\backslash \}$, while the range of $\backslash ;\; \backslash varrho\_\{A\_1\backslash ldots\; A\_m\}^\{T\_\{A\_\{k\_1\}\backslash ldots\; A\_\{k\_l\}\}\}$ partially transposed with respect to the subset $\backslash ;\; \backslash \{A\_\{k\_1\}\backslash ldots\; A\_\{k\_l\}\backslash \}\; \backslash subset\; \backslash \{A\_1\backslash ldots\; A\_m\backslash \}$ must be spanned by the products of these vectors where those with indices $\backslash ;\; k\_1,\; \backslash ldots,\; k\_l$ are complex conjugated. If the state $\backslash ;\; \backslash varrho\_\{A\_1\backslash ldots\; A\_m\}$ is ''separable'', then all such partial transposes must lead to matrices with non-negative spectrum, i.e. all the matrices $\backslash ;\; \backslash varrho\_\{A\_1\backslash ldots\; A\_m\}^\{T\_\{A\_\{k\_1\}\backslash ldots\; A\_\{k\_l\}\}\}$ should be states themselves.
* '''Realignment criteria'''
The realignment criteria from the bipartite case are generalized to '''permutational criteria''' in the multipartite setting: if the state $\backslash ;\; \backslash varrho\_\{A\_1\backslash ldots\; A\_m\}$ is ''separable'', then the matrix $\backslash ;\; [R\_\backslash pi(\backslash varrho\_\{A\_1\backslash ldots\; A\_m\})]\_\{i\_1j\_1,i\_2j\_2,\backslash ldots,i\_nj\_n\}\backslash equiv\backslash varrho\_\{\backslash pi(i\_1j\_1,i\_2j\_2,\backslash ldots,i\_nj\_n)\}$, obtained from the original state via permutation $\backslash ;\; \backslash pi$ of matrix indices in product basis, satisfies $\backslash ;\; ||R\_\backslash pi(\backslash varrho\_\{A\_1\backslash ldots\; A\_m\})]||\_\{Tr\}\backslash leq1$.
*'''Contraction criterion'''
Finally, the contraction criterion generalizes immediately from the bipartite to the multipartite case.
== Multipartite entanglement measures ==
Many of the ''axiomatic entanglement measures'' for bipartite states, such as ''relative entropy of entanglement'', ''robustness of entanglement'' and ''squashed entanglement'', can be generalized to the multipartite setting.
The ''relative entropy of entanglement'', for example, can be generalized to the multipartite case by taking a suitable set in place of the set of bipartite separable states. One can take the set of fully separable states, even though with this choice the measure will not distinguish between truly multipartite entanglement and several instances of bipartite entanglement, such as $\backslash ;\; EPR\_\{AB\}\backslash otimes\; EPR\_\{CD\}$. In order to analyze truly multipartite entanglement one has to consider the set of states containing no more than $\backslash ;\; k$-particle entanglement.
In the case of ''squashed entanglement'', its multipartite version can be obtained by simply replacing the mutual information of the bipartite system with its generalization for multipartite systems, i.e.
$\backslash ;\; I(A\_1\; :\; \backslash ldots\; :\; A\_N)\; =\; S(A\_1)\; +\; \backslash ldots\; +\; S(A\_N)\; -\; S(A\_1\; \backslash ldots\; A\_N)$.
However, in the multipartite setting many more parameters are needed to describe the entanglement of the states, and therefore many new entanglement measures have been constructed, especially for pure multipartite states.
=== Multipartite entanglement measures for pure states ===
In the multipartite setting there are entanglement measures that simply are functions of sums of bipartite entanglement measures, as, for instance, the '''global entanglement''', which is given by the sum of concurrences between one qubit and all others. For these multipartite entanglement measures the ''monotonicity under LOCC'' is simply inherited from the bipartite measures.
But there are also entanglement measures that were constructed specifically for multipartite states, as the following.
*'''Tangle'''
The first multipartite entanglement measure that is neither a direct generalization nor an easy combination of bipartite measures was introduced by Coffman ''et al.'' and called '''tangle'''.
'''Definition [tangle]:'''
:$\backslash ;\; \backslash tau(A\; :\; B\; :\; C)\; =\; \backslash tau(A\; :\; BC)\; -\; \backslash tau(AB)\; -\; \backslash tau(AC)\; ,$
where the $\backslash ;\; 2$-tangles on the right-hand-side are the squares of ''concurrence''.
'''Properties of ''tangle'':''' it is permutationally invariant; it vanishes on all states that are separable under any cut; it is nonzero, for example, on the GHZ-state; it can be thought to be zero for states that are $\backslash ;\; 3$-entangled (i.e. that are not product with respect to any cut) as, for instance, the W-state.
Moreover, there might be the possibility to obtain a good generalization of the ''tangle'' for multiqubit systems by means of hyperdeterminant.
*'''Schmidt measure'''
This was one of the first entanglement measures constructed specifically for multipartite states.
'''Definition [Schmidt measure]:''' The minimum of $\backslash ;\; \backslash log\; r$, where $\backslash ;\; r$ is the number of terms in an expansion of the state in product basis.
This measure is zero if and only if the state is fully product; therefore, it cannot distinguish between truly multipartite entanglement and bipartite entanglement, but it may nevertheless be useful in many contexts.
*'''Measures based on normal forms'''
This is an interesting class of multipartite entanglement measures obtained in the context of classification of states.
Namely, one considers any homogeneous function of the state: if it is invariant under SLOCC (stochastic LOCC) operations with determinant equal to 1, then it is an ''entanglement monotone in the strong sense'', i.e. it satisfies the condition of strong monotonicity.
*'''Measures based on hyperdeterminant'''
It was proved by Miyake that ''hyperdeterminants'' are entanglement monotones and they describe truly multipartite entanglement in the sense that states such as products of $\backslash ;\; EPR$'s have zero entanglement.
In particular ''concurrence'' and ''tangle'' are special cases of ''hyperdeterminant''. Indeed for two qubits ''concurrence'' is simply the modulus of the determinant, which is the hyperdeterminant of first order; whereas the ''tangle'' is the hyperdeterminant of second order, i.e. a function of tensors with three indices.
*'''Geometric measure'''
'''Definition [geometric measure]:'''
:$\backslash ;\; E\_g\; =\; 1\; -\; \backslash Lambda^k[\backslash psi]\; ,$
where $\backslash ;\; \backslash Lambda^k[\backslash psi]\; =\; sup\_\{\backslash phi\; \backslash in\; S\_k\}|\backslash langle\; \backslash psi|\backslash phi\backslash rangle|^2$, with $\backslash ;\; S\_k$ the set of $\backslash ;\; k$-separable states.
This measure belongs to a family of entanglement measures defined by Barnum and Linden, and it is the multipartite generalization of the Shimony measure.
*'''Localisable entanglement'''
This entanglement measure is a generalization of the ''entanglement of assistance'' and was constructed in the context of ''spin chains''.
Namely, one chooses two spins and performs LOCC operations that aim at obtaining the largest possible bipartite entanglement between them (measured according to a chosen entanglement measure for two bipartite states).
== Related papers ==
* R. Horodecki, P. Horodecki, M. Horodecki, K. Horodecki, ''Quantum entanglement'', e-print {{Arxiv|number=quant-ph/0702225}}.
*R. Horodecki, ''Phys. Lett. A'' '''187''', 145 (1994)
*D. A. Meyer, N. R. Wallach, ''Global entanglement in multipartite systems'' e-print {{Arxiv|number=quant-ph/0108104}}(2001)
*V. Coffman ''et al.'', ''Phys. Rev. A'' '''61''', 052306 (2000)
*A. Miyake, ''Phys. Rev. A'' '''67''', 012108 (2003)
*Eisert, Briegel, ''Phys. Rev. A'' '''64''', 022306 (2001)
*F. Verstraete ''et al.'', ''Phys. Rev. A'' '''68''', 012103 (2003)
*A. Miyake, ''Int. J. Quant. Info.'' '''2''', 65 (2004)
*H. Barnum, N. Linden, ''J. Phys. A'' '''34''',6787 (2001)
*F. Verstraete ''et al.'', ''Phys. Rev. Lett.'' '''92''', 027901 (2004)
Category:Handbook of Quantum Information

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