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On Tripartite Pure-State Entanglement

Ashish V. Thapliyal

Published Nov 30, 1998• 0 citations
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Abstract

Two important subsets of tripartite pure states, namely {\\em triseparable} states (i.e. states such that the bipartite density matrices obtained by tracing out any party's subsystem are separable) and {\\em triorthogonal} states (i.e. states with a three-party Schmidt-like decomposition) are introduced. It is proved that any triseparable state is triorthogonal and vice-versa. The relevance of this result to quantifying tripartite pure-state entanglement is discussed. It is also proved that any purification of a bipartite bound entangled state with positive partial transpose is {\\em tri-inseparable}. This is a new necessary condition for such bound entanglement.

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arXiv:quant-ph/0109056•Sep 13, 2001•2 citations
Two important subsets of tripartite pure states, namely {\em triseparable} states (i.e. states such that the bipartite density matrices obtained by tracing out any party's subsystem are separable) and {\em triorthogonal} states (i.e. states with a three-party Schmidt-like decomposition) are introduced. It is proved that any triseparable state is triorthogonal and vice-versa. The relevance of this result to quantifying tripartite pure-state entanglement is discussed. It is also proved that any purification of a bipartite bound entangled state with positive partial transpose is {\em tri-inseparable}. This is a new necessary condition for such bound entanglement.
We show that pure states of multipartite quantum systems are multiseparable (i.e., give separable density matrices on tracing any party) if and only if they have a generalized Schmidt decomposition. Implications of this result for the quantification of multipartite pure-state entanglement are discussed. Further, as an application of the techniques used here, we show that any purification of a bipartite-bound entangled state having a positive partial transpose is tri-inseparable, i.e., has none of its three bipartite partial traces separable.
Abstract The relation between the distillability of entanglement of three bipartite reduced density matrices from a tripartite pure state has been studied in Hayashi and Chen [2011 Phys. Rev. A 84 012325]. We extend this result to the tripartite mixed state of rank at most three. In particular we show that if the state has two bipartite reduced density operators with rank two, then the third bipartite reduced density operator additionally having non positive partial transpose (non-PPT) is distillable. In contrast, we show that the tripartite PPT state with two reduced density operators of rank two is a three-qubit fully separable state. We obtain these facts by proving a conjectured matrix inequality for the bipartite matrix M with Schmidt rank at most three. This is one of the main results of this paper. We also prove it for some M with arbitrary Schmidt rank.
We study separability properties in a five-dimensional set of states of quantum systems composed of three subsystems of equal but arbitrary finite Hilbert space dimension. These are the states that can be written as linear combinations of permutation operators, or equivalently, commute with unitaries of the form $U\ensuremath{\bigotimes}U\ensuremath{\bigotimes}U.$ We compute explicitly the following subsets and their extreme points: (1) triseparable states, which are convex combinations of triple tensor products, (2) biseparable states, which are separable for a twofold partition of the system, and (3) states with positive partial transpose with respect to such a partition. Tripartite entanglement is investigated in terms of the relative entropy of tripartite entanglement and of the trace norm.
Abstract Multipartite entanglement has a much more complex structure than bipartite entanglement. A state that lacks generic multipartite entanglement is 2-producible, i.e. it can be written as a tensor product of at most 2-partite entangled states. Recently, it has been proved that a tripartite pure state is 2-producible if and only if the gap between the entanglement of purification and its lower bound vanishes. Here, we show that the entanglement of purification gap is insufficient to detect more than tripartite entanglement in 4-partite stabilizer states. We then generalize entanglement of purification to the multipartite cases, and demonstrate that a multipartite pure state is 2-producible if and only if all the generalized entanglement of purification gaps vanish. The generalized entanglement of purification gap quantifies the quantum communication cost for redistributing one part of the system to the others, and also relates to the local recoverability of a multipartite state and the relative entropy between that state and 2-producible states. Moreover, we calculate the generalized entanglement of purification for states satisfying the general Schmidt decomposition, which implies that 4-partite stabilizer states do not necessarily have a general Schmidt decomposition. Our results provide a quantitative characterization of multipartite entanglement in multipartite system, which will promote further investigations and understanding of multipartite entanglement.
The classification of the multipartite entanglement is an important problem in quantum information theory. We propose a class of two qubit mixed states σAB=p∣χ1⟩⟨χ1∣⊗ρ1+(1−p)∣χ2⟩⟨χ2∣⊗ρ2σ_{AB}= p|χ_{1}\rangle\langleχ_{1}|\otimesρ_{1}+(1-p)|χ_{2}\rangle\langleχ_{2}|\otimesρ_{2}σAB​=p∣χ1​⟩⟨χ1​∣⊗ρ1​+(1−p)∣χ2​⟩⟨χ2​∣⊗ρ2​, where ∣χ1⟩=α∣0⟩+β∣1⟩|χ_{1}\rangle=α|0\rangle+β|1\rangle∣χ1​⟩=α∣0⟩+β∣1⟩, ∣χ2⟩=β∣0⟩+(−1)nα∣1⟩|χ_{2}\rangle=β|0\rangle+(-1)^{n}α|1\rangle∣χ2​⟩=β∣0⟩+(−1)nα∣1⟩. We have shown that the state σABσ_{AB}σAB​ represent a classical state when nnn is odd while it represent a non-classical state when nnn is even. The purification of the state σABσ_{AB}σAB​ is studied and found that the purification is possible if the spectral decomposition of the density matrices ρ1ρ_{1}ρ1​ and ρ2ρ_{2}ρ2​ represent pure states. We have established a relationship between three tangle, which measures the amount of entanglement in three qubit system and the quantity ⟨χ1∣χ2⟩\langleχ_{1}|χ_{2}\rangle⟨χ1​∣χ2​⟩, which identifies whether the two qubit mixed state is classical or non-classical. The three qubit purified state is then classified as a separable or biseparable or W-type or GHZ-type state using the quantum correlation, measured by geometric discord, of its reduced two qubit density matrix.
We introduce algebriac sets in the products of complex projective spaces for multipartite mixed states, which are independent of their eigenvalues and only measure the "position" of their eigenvectors, as their non-local invariants (ie. remaining invariant after local untary transformations). These invariants are naturally arised from the physical consideration of checking multipartite mixed states by measuring them with multipartite separable pure states. The algebraic sets have to be the sum of linear subspaces if the multipartite mixed state is separable, and thus we give a new separability criterion of multipartite mixed states. A continuous family of 4-party mixed states, whose members are separable for any 2:2 cut and entangled for any 1:3 cut (thus bound entanglement if 4 parties are isolated), is constructed and studied from our invariants and separability criterion. Examples of LOCC-incomparable entangled tripartite pure states are given to show it is hopeless to characterize the entanglement properties of tripartite pure states by only using the eigenvalue speactra of their partial traces. We also prove that at least n2+n−1n^2+n-1n2+n−1 terms of separable pure states, which are "orthogonal" in some sence, are needed to write a generic pure state in HAn2⊗HBn2⊗HCn2H_A^{n^2} \otimes H_B^{n^2} \otimes H_C^{n^2}HAn2​⊗HBn2​⊗HCn2​ as a linear combination of them.