  Off-diagonal geometric phases acquired in the evolution of a spin-1/2 system
have been investigated by means of a polarized neutron interferometer. Final
counts with and without polarization analysis enable us to observe
simultaneously the off-diagonal and diagonal geometric phases in two detectors.
We have quantitatively measured the off-diagonal geometric phase for noncyclic
evolutions, confirming the theoretical predictions. We discuss the significance
of our experiment in terms of geometric phases (both diagonal and off-diagonal)
and in terms of the quantum erasing phenomenon.
