Observable Dirac-type singularities in Berry's phase and the monopole

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The physical reality and observability of 2nπBerry phases, as opposed to the usually considered modulo 2πtopological phases is demonstrated with the help of computer simulation of a model adiabatic evolution whose parameters are varied along a closed loop in the parameter space. Using the analogy of Berry's phase with the Dirac monopole, it is concluded that an interferometer loop taken around a magnetic monopole of strength n/2 yields an observable 2nπphase shift, where n is an integer. An experiment to observe the effect is proposed.