We show that the tensor rank of tensor product of two three-qubit W states is
not less than eight. Combining this result with the recent result of M.
Christandl, A. K. Jensen, and J. Zuiddam that the tensor rank of tensor product
of two three-qubit W states is at most eight, we deduce that the tensor rank of
tensor product of two three-qubit W states is eight. We also construct the
upper bound of the tensor rank of tensor product of many three-qubit W states.Comment: 10 pages, accepted by Linear Algebra and Application
