Geometric phase for mixed states: a differential geometric approachS. Chaturvedi, E. Ercolessi, G. Marmo, G. Morandi, N. Mukunda and R. SimonEur. Phys. J. C, 35 3 (2004) 413-423DOI: https://doi.org/10.1140/epjc/s2004-01814-5