Notes on Group Invariants and Positivity of Density Matrices and Superoperators
Mark Byrd, Navin Khaneja · arXiv (Cornell University) · 2003
In this paper, we construct a distinguished class of unitary invariants, the Casimir invariants, in terms of the generalized coherence vector representation of the density operator. Using a tensor product basis, we show how to extract local information about the density operator and the n-positivity of maps from density operators to density operators (superoperators). We then discuss some applications and implications.