Characterization of the Positivity of the Density Matrix in Terms of the Coherence Vector Representation

Details Characterization of the Positivity of the Density Matrix in Terms of the Coherence Vector Representation Characterization of the Positivity of the Density Matrix in Terms of the Coherence Vector Representation

2003-02-03

arxiv.org/abs/quant-ph/0302024v2

PRA 68, 062322 (2003)

Physics

Quantum Physics

significantly rewritten and submitted for publication

Scientific paper

10.1103/PhysRevA.68.062322

A parameterization of the density operator, a coherence vector representation, which uses a basis of orthogonal, traceless, Hermitian matrices is discussed. Using this parameterization we find the region of permissible vectors which represent a density operator. The inequalities which specify the region are shown to involve the Casimir invariants of the group. In particular cases, this allows the determination of degeneracies in the spectrum of the operator. The identification of the Casimir invariants also provides a method of constructing quantities which are invariant under {\it local} unitary operations. Several examples are given which illustrate the constraints provided by the positivity requirements and the utility of the coherence vector parameterization.

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