Geometric phases for nondegenerate and degenerate mixed states

Abstract

The issue of phase holonomy of both nondegenerate and degenerate mixed state undergoing unitary evolution was considered. Starting with the nondegenerate case, it was shown that the usual procedure of subtracting the dynamical phase from the total phase to yield the geometric phase, does not hold for mixed states. To this end, an expression for the geometric phase that was gauge invariant was furnished.