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ENTANGLEMENT WITNESSES CONSTRUCTED BY PERMUTATION PAIRS
侯晋川, 王文丽
数学物理学报(英文版). 2021 (3):
843-874.
DOI: 10.1007/s10473-021-0313-z
For n≥3, we construct a class {Wn,π1,π2} of n2×n2 hermitian matrices by the permutation pairs and show that, for a pair {π1,π2} of permutations on (1,2,…,n), Wn,π1,π2 is an entanglement witness of the n⊗n system if {π1,π2} has the property (C). Recall that a pair {π1,π2} of permutations of (1,2,…,n) has the property (C) if, for each i, one can obtain a permutation of (1,…,i−1,i+1,…,n) from (π1(1),…,π1(i−1),π1(i+1),…,π1(n)) and (π2(1),…,π2(i−1),π2(i+1),…,π2(n)). We further prove that Wn,π1,π2 is not comparable with Wn,π, which is the entanglement witness constructed from a single permutation π; Wn,π1,π2 is decomposable if π1π2=id or π21=π22=id. For the low dimensional cases n∈{3,4}, we give a sufficient and necessary condition on π1,π2 for Wn,π1,π2 to be an entanglement witness. We also show that, for n∈{3,4}, Wn,π1,π2 is decomposable if and only if π1π2=id or π21=π22=id; W3,π1,π2 is optimal if and only if (π1,π2)=(π,π2), where π=(2,3,1). As applications, some entanglement criteria for states and some decomposability criteria for positive maps are established.
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