Kinematic approach to the mixed state geometric phase in nonunitary   evolution
  A kinematic approach to the geometric phase for mixed quantal states in nonunitary evolution is proposed. This phase is manifestly gauge invariant and can be experimentally tested in interferometry. It leads to well-known results when the evolution is unitary. 
D.M. Tong, E. Sj\"oqvist, L.C. Kwek, and C.H. Oh