Geometry along evolution of mixed quantum states

The metric underlying the mixed state geometric phase in unitary and nonunitary evolution [Phys. Rev. Lett. 85, 2845 (2000); 93, 080405 (2004)] is delineated. An explicit form for the line element is derived and shown to be related to an averaged energy dispersion in the case of unitary evolution. The line element is measurable in interferometry involving nearby internal states. Explicit geodesics are found in the single qubit case. It is shown how the Bures line element can be obtained by extending our approach to arbitrary decompositions of density operators. The proposed metric is applied to a generic magnetic system in a thermal state.