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Michael Berry (physicist) showed that slowly carrying a nondegenerate energy eigenstate around a closed loop of external parameters can produce a geometric phase in addition to its dynamical phase. The geometric phase depends on the path through parameter space and survives after the ordinary energy-dependent contribution is removed. For a spin following a slowly varying magnetic field, it is proportional to the solid angle enclosed by the field direction, making the geometry measurable through interference.[ref]
Quantal phase factors accompanying adiabatic changes by Michael Victor Berry. Abstract and section 4, spinning particle in a slowly changing magnetic field
Portrait of Michael Berry (physicist)
. Vahram Mekhitarian, photograph; Jakob Gottfried, derivative crop. CC BY-SA 4.0, via Wikimedia Commons. Source..jpg){kind=link}