Geometry of mixed states for a q-bit and the quantum Fisher information tensor

After a review of the pure state case, we discuss from a geometrical point of view the meaning of the quantum Fisher metric in the case of mixed states for a two-level system, i.e. for a q-bit, by examining the structure of the fiber bundle of states, whose base space can be identified with a co-adjoint orbit of the unitary group. We show that the Fisher Information metric coincides with the one induced by the metric of the manifold of SU(2), i.e. the 3-dimensional sphere $S^3$, when the mixing coefficients are varied. We define the notion of Fisher Tensor and show that its anti-symmetric part is intrinsically related to the Kostant Kirillov Souriau symplectic form that is naturally defined on co-adjoint orbits, while the symmetric part is nontrivially again represented by the Fubini Study metric on the space of mixed states, weighted by the mixing coefficients.