Reconstructing the density matrix of a spin s through Stern-Gerlach measurements:: II

The density matrix of a spin s is fixed uniquely if the probabilites to obtain the value s upon measuring (n) over right arrow . (s) over right arrow are known for 4s(s + 1) appropriately chosen directions (n) over right arrow in space. These numbers are just the expectation values of the density operator in coherent spin stares, and they are easily obtained in an experiment carried out with a Stern-Gerlach apparatus. Furthermore, the experimental data is non-redundant, and it can be inverted explicitly which allows for a parametrization of the statistical operator by the 4s(s + 1) positive parameters.