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arXiv:quant-ph/0203084 · Journal of Physics A Mathematical and General

The reduction of the closest disentangled states

Satoshi Ishizaka

Published Sep 10, 2002• 5 citations
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Abstract

We study the closest disentangled state to a given entangled state in any system (multi-party with any dimension). We obtain the set of equations the closest disentangled state must satisfy, and show that its reduction is strongly related to the extremal condition of the local filtering on each party. Although the equations we obtain are not still tractable, we find some sufficient conditions for which the closest disentangled state has the same reduction as the given entangled state. Further, we suggest a prescription to obtain a tight upper bound of the relative entropy of entanglement in two-qubit systems.

DOI
10.1088/0305-4470/35/38/310

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We study the closest disentangled state to a given entangled state in any system (multi-party with any dimension). We obtain the set of equations the closest disentangled state must satisfy, and show that its reduction is strongly related to the extremal condition of the local filtering on each party. Although the equations we obtain are not still tractable, we find some sufficient conditions for which the closest disentangled state has the same reduction as the given entangled state. Further, we suggest a prescription to obtain a tight upper bound of the relative entropy of entanglement in two-qubit systems.
We show that, for a given entangled state in any system (multi-party with any dimension), the closest disentangled state always has the same reduction as the corresponding reduction of the given entangled state. This implies that any party cannot distinguish these two state without any classical communications. Furthermore, we determined 12 parameters in the Hilbert-Schmidt representation of the closest disentangled state in two qubits, and the minimization problem of the relative entropy of entanglement is reduced to the problem minimizing three parameters.
The separable state closest to a given entangled state in the relative entropy measure is called the closest disentangled state. We provide an analytical formula connecting the entangled states and the closest disentangled state in two qubits. Using this formula, when any disentangled state $\ensuremath{\sigma}$ located at the entangle-disentangle boundary is given, entangled states to which $\ensuremath{\sigma}$ is closest can be obtained analytically. Further, this formula naturally defines the direction normal to the boundary surface. The direction is uniquely determined by $\ensuremath{\sigma}$ in almost all cases.
The long-standing problem of finding a closed formula for the relative entropy of entanglement (REE) for two qubits is addressed. A compact-form solution to the inverse problem, which characterizes an entangled state for a given closest separable state, is obtained. Analysis of the formula for a large class of entangled states strongly suggests that a compact analytical solution of the original problem, which corresponds to finding the closest separable state for a given entangled state, can be given only in some special cases. A few applications of the compact-form formula are given to show additivity of the REE, to relate the REE with the Rains upper bound for distillable entanglement, and to show that a Bell state does not have a unique closest separable state.
The relative entropy of entanglement of a mixed state σ for a bipartite quantum system can be defined as the minimum of the quantum relative entropy over the set of completely disentangled states. Vedral et al. [Phys. Rev. A 57(1998)1619] have recently proposed a numerical method to obtain the relative entropy of entanglement E re for two-qubit systems. This letter shows that the convex programming method can be applied to calculate E re of two-qubit systems analytically, and discusses the conditions under which the method can be adopted.