Physics & Computer Science
In an open system, the geometric phase should be described by a distribution. We show that a geometric phase distribution for open system dynamics is in general ambiguous, but the imposition of reasonable physical constraints on the environment and its coupling with the system yields a unique geometric phase distribution that applies even for mixed states, nonunitary dynamics, and noncyclic evolutions.
Marzlin, K.-P.; Ghose, Shohini; and Sanders, Barry C., "Geometric Phase Distributions for Open Quantum Systems" (2004). Physics and Computer Science Faculty Publications. 64.