Normal Forms and Tensor Ranks of Pure States of Four Qubits
arXiv:quant-ph/0612184
Abstract
We examine the SLOCC classification of the (non-normalized) pure states of four qubits obtained by F. Verstraete et al. The rigorous proofs of their basic results are provided and necessary corrections implemented. We use Invariant Theory to solve the problem of equivalence of pure states under SLOCC transformations of determinant 1 and qubit permutations. As a byproduct, we produce a new set of generators for the invariants of the Weyl group of type F_4. We complete the determination of the tensor ranks of 4-qubit pure states initiated by J.-L. Brylinski. As a result we obtain a simple algorithm for computing these ranks. We obtain also a very simple classification of pure states of rank at most 3.
40 pages, 8 tables, 4 figures. Table 4 and the treatment of examples in section 4 have been corrected. The paper will appear, as an invited chapter, in the forthcoming book "Linear Algebra Research Advances" by Nova Science Publishers, Inc. Comments are welcome