Geometric phase for mixed states: a differential geometric approach S. Chaturvedi, E. Ercolessi, G. Marmo, G. Morandi, N. Mukunda and R. Simon Eur. Phys. J. C, 35 3 (2004) 413-423 Published online: 18 May 2004 DOI: 10.1140/epjc/s2004-01814-5