Positivity of theN×Ndensity matrix expressed in terms of polarization operators
S Kryszewski, M Zachciał · Journal of Physics A Mathematical and General · 2006
We use polarization operators known from the quantum theory of angular momentum to expand the ( N × N )-dimensional density operators. Thereby we construct generalized Bloch vectors representing density matrices. We study their properties and derive positivity conditions for any N . We also apply the procedure to study Bloch vector space for a qubit and a qutrit.