Tensor representation of spin States.

@article{Giraud2015TensorRO,
  title={Tensor representation of spin States.},
  author={Olivier Giraud and Daniel Braun and Dorian Baguette and T Bastin and J. Mart{\'i}n},
  journal={Physical review letters},
  year={2015},
  volume={114 8},
  pages={
          080401
        }
}
We propose a generalization of the Bloch sphere representation for arbitrary spin states. It provides a compact and elegant representation of spin density matrices in terms of tensors that share the most important properties of Bloch vectors. Our representation, based on covariant matrices introduced by Weinberg in the context of quantum field theory, allows for a simple parametrization of coherent spin states, and a straightforward transformation of density matrices under local unitary and… 
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