Berry phase of a resonant state

Abstract

We derive closed analytical expressions for the complex Berry phase of an open quantum system in a state which is a superposition of resonant states and evolves irreversibly due to the spontaneous decay of the metastable states. The codimension of an accidental degeneracy of resonances and the geometry of the energy hypersurfaces close to a crossing of resonances differ significantly from those of bound states. We discuss some of the consequences of these differences for the geometric phase factors. For example, instead of a diabolical point singularity there is a continuous closed line of singularities formally equivalent to a continuous distribution of `magnetic' charge on a diabolical circle, there are different classes of topologically inequivalent non-trivial closed paths in parameter space, the topological invariant associated with the sum of the geometric phases, dilations of the wavefunction due to the imaginary part of the Berry phase and others.