  The geometric phase has been proposed as a candidate for noise resilient
coherent manipulation of fragile quantum systems. Since it is determined only
by the path of the quantum state, the presence of noise fluctuations affects
the geometric phase in a different way than the dynamical phase. We have
experimentally tested the robustness of Berry's geometric phase for spin-1/2
particles in a cyclically varying magnetic field. Using trapped polarized
ultra-cold neutrons it is demonstrated that the geometric phase contributions
to dephasing due to adiabatic field fluctuations vanish for long evolution
times.
