The Tensor Rank of the Tripartite State $\ketW^\otimes n$. Yu, N., Chitambar, E., Guo, C., & Duan, R. Physical Review A - Atomic, Molecular, and Optical Physics, 81(1):3-5, 10, 2009.
The Tensor Rank of the Tripartite State $\ketW^\otimes n$ [pdf]Paper  The Tensor Rank of the Tripartite State $\ketW^\otimes n$ [link]Website  doi  abstract   bibtex   
Tensor rank refers to the number of product states needed to express a given multipartite quantum state. Its non-additivity as an entanglement measure has recently been observed. In this note, we estimate the tensor rank of multiple copies of the tripartite state $\ketW=\tfrac1\sqrt3(\ket100+\ket010+\ket001)$. Both an upper bound and a lower bound of this rank are derived. In particular, it is proven that the tensor rank of $\ketW^\otimes 2$ is seven, thus resolving a previously open problem. Some implications of this result are discussed in terms of transformation rates between $\ketW^\otimes n$ and multiple copies of the state $\ketGHZ=\tfrac1\sqrt2(\ket000+\ket111)$.

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