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Physical Review Letters

Inseparable Two Spin- 12 Density Matrices Can Be Distilled to a Singlet Form

Michał Horodecki, Paweł Horodecki, Ryszard Horodecki

Published Jan 27, 1997• 415 citations
Publisher

Abstract

A quantum system is called inseparable if its density matrix cannot be written as a mixture of product states. In this Letter we apply the separability criterion, local filtering, and Bennett et al. distillation protocol [Phys. Rev. Lett. 76, 722 (1996)] to show that any inseparable $2\ifmmode\times\else\texttimes\fi{}2$ system represents the entanglement which, however small, can be distilled to a singlet form.

DOI
10.1103/physrevlett.78.574

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A quantum system is called inseparable if its density matrix cannot be written as a mixture of product states. In this Letter we apply the separability criterion, local filtering, and Bennett et al. distillation protocol [Phys. Rev. Lett. 76, 722 (1996)] to show that any inseparable 2\ifmmode×\else\texttimes\fi22\ifmmode\times\else\texttimes\fi{}22\ifmmode×\else\texttimes\fi2 system represents the entanglement which, however small, can be distilled to a singlet form.
A quantum system is called separable if its density matrix can be written as a mixture of product states. In this paper we apply the separability criterion as well as the distillability criterion to show that any initial inseparable $2\ifmmode\times\else\texttimes\fi{}2$ system retains its inseparability after its transmission through a noisy channel which consists of a set of harmonic oscillators. Hence the final state of the system can be distilled to a singlet form. For higher dimensions, however, this result is no longer true in general. The distillability of a state with higher dimensions after its transmission through the noisy channel is also discussed in this paper.
We apply the inseparability criterion for 2×22 \times 22×2 systems, local filtering and Bennett et al. purification protocol [Phys. Rev. Lett. {\bf 76}, 722 (1996)] to show how to distill {\it any} inseparable 2×22\times 22×2 system. The extended protocol is illustrated geometrically by means of the state parameters in the Hilbert-Schmidt space.
We analyze the problem of distillation of entanglement of mixed states in higher-dimensional compound systems. Employing the positive maps method [M. Horodecki et al., Phys. Lett. A 223, 1 (1996)] we introduce and analyze a criterion of separability that relates the structures of the total density matrix and its reductions. We show that any state violating the criterion can be distilled by suitable generalization of the two-qubit protocol that distills any inseparable two-qubit state. In particular, this means that any state $\ensuremath{\varrho}$ of two N-level systems with $〈{\ensuremath{\psi}}_{+}|\ensuremath{\varrho}|{\ensuremath{\psi}}_{+}〉>1/N$ can be distilled $({\ensuremath{\psi}}_{+}$ is the singlet state generalized to higher dimension). The criterion also singles out all the states that can be distilled by a class of protocols. The proof involves construction of the family of states that are invariant under transformation $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\varrho}}U\ensuremath{\bigotimes}{U}^{*}\ensuremath{\varrho}{U}^{\ifmmode\dagger\else\textdagger\fi{}}\ensuremath{\bigotimes}{U}^{*\ifmmode\dagger\else\textdagger\fi{}},$ where U is a unitary transformation and the asterisk denotes complex conjugation. The states are related to the depolarizing channel generalized to the nonbinary case.
A quantum system consisting of two subsystems is separable if its density matrix can be written as $\ensuremath{\rho}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ensuremath{\Sigma}{A}^{}{w}_{A}{\ensuremath{\rho}}_{A}^{\ensuremath{'}}\ensuremath{\bigotimes}{\ensuremath{\rho}}_{A}^{\ensuremath{'}\ensuremath{'}},$ where ${\ensuremath{\rho}}_{A}^{\ensuremath{'}}$ and ${\ensuremath{\rho}}_{A}^{\ensuremath{'}\ensuremath{'}}$ are density matrices for the two subsystems, and the positive weights wA{w}_{A}wA​ satisfy $\ensuremath{\Sigma}{w}_{A}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1$. In this Letter, it is proved that a necessary condition for separability is that a matrix, obtained by partial transposition of \ensuremath{\rho}, has only non-negative eigenvalues. Some examples show that this criterion is more sensitive than Bell's inequality for detecting quantum inseparability.
We analyse the problem of distillation of entanglement of mixed states in higher dimensional compound systems. Employing the positive maps method [M. Horodecki et al., Phys. Lett. A 223 1 (1996)] we introduce and analyse a criterion of separability which relates the structures of the total density matrix and its reductions. We show that any state violating the criterion can be distilled by suitable generalization of the two-qubit protocol which distills any inseparable two-qubit state. Conversely, all the states which can be distilled by such a protocol must violate the criterion. The proof involves construction of the family of states which are invariant under transformation ϱ→U⊗U∗ϱU†⊗U∗†\varrho\to U\otimes U^*\varrho U^\dagger\otimes U^{*\dagger}ϱ→U⊗U∗ϱU†⊗U∗† where UUU is a unitary transformation and star denotes complex conjugation. The states are related to the depolarizing channel generalized to non-binary case.