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arXiv:2003.00694 · Quantum Information Processing

The decompositions of Werner and isotropic states

Ma-Cheng Yang, Jun-Li Li, Cong-Feng Qiao

Published Aug 1, 2021• 5 citations
arXivHTMLPDF
DOI
10.1007/s11128-021-03193-y

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Isotropic States in Discrete Phase Space

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The present methods for obtaining the optimal Lewenestein–Sanpera decomposition of a mixed state are difficult to handle analytically. We provide a simple analytical expression for the optimal Lewenstein–Sanpera decomposition by using semi-definite programming. In particular, we obtain the optimal Lewenstein–Sanpera decomposition for some examples such as: the Bell decomposable state, the iso-concurrence state, the generic two-qubit state in the Wootters basis, the 2⊗3 Bell decomposable state, the d⊗d Werner and isotropic states, a one parameter 3⊗3 state, and finally a multi-partite isotropic state.
We present a new method of constructing sets of states with known entanglement of formation. The method realizes the optimal decomposition families of states. We also derive the additivity of the entanglement of formation and thus calculate the entanglement cost. These results are used to find out the undistillable entanglement of some states. We illustrate our method by investigating the two-qubit state, the separable state, the maximally correlated state, the isotropic state and the Werner state. We also discuss the realization of Werner state in experiment.
It is shown that for a given bipartite density matrix and by choosing a suitable separable set (instead of a product set) on the separable–entangled boundary, the optimal Lewenstein–Sanpera (LS) decomposition (with respect to an arbitrary separable set) can be obtained via a direct optimization procedure for a generic entangled density matrix. On the basis of this, we obtain the optimal LS decomposition for some bipartite systems such as 2 ⊗ 2 and 2 ⊗ 3 Bell decomposable (BD) states, a generic two qubit state in Wootters basis, iso-concurrence decomposable states, states obtained from BD states via one-parameter and three-parameter local operations and classical communications (LOCC), d ⊗ d Werner and isotropic states and a one-parameter 3 ⊗ 3 state. We also obtain the optimal decomposition for multi-partite isotropic states. It is shown that in all 2 ⊗ 2 systems considered here the average concurrence of the decomposition is equal to the concurrence. We also show that for some 2 ⊗ 3 Bell decomposable states, the average concurrence of the decomposition is equal to the lower bound of the concurrence of the state presented recently in Lozinski et al (2003 Preprint quant-ph/0302144), so an exact expression for concurrence of these states is obtained. It is also shown that for a d ⊗ d isotropic state where decomposition leads to a separable and an entangled pure state, the average I-concurrence of the decomposition is equal to the I-concurrence of the state.