Off-diagonal geometric phase in polarized neutron interferometry

The concept of a Hamiltonian-independent geometric phase which depends only on the path of the evolution in parameter space is well established in a broad spectrum of physical phenomena. Recently it has been extended to the so-called 'off-diagonal' geometric phase, which is associated with the off-diagonal matrix elements of the evolution. Here we present a polarized neutron interferometer experiment which for the first time allows an explicit observation and quantitative determination of off-diagonal geometric phase shifts.