Abstract
We express the density matrix for the N-qubit symmetric state or spin-j state (j = N/2) in terms of the well-known Fano statistical tensor parameters. Employing the multi-axial representation, where the spin-j density matrix is shown to be characterized by j(2j + 1) axes and 2j real scalars, we enumerate the number of invariants constructed out of these axes and scalars. We calculate these invariants explicitly in the particular case of the pure and mixed spin-1 state.
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Sirsi, S., Adiga, V. SU(2) invariants of symmetric qubit states. J Russ Laser Res 32, 495–501 (2011). https://doi.org/10.1007/s10946-011-9239-6
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DOI: https://doi.org/10.1007/s10946-011-9239-6