Measuring the geometric component of the transition probability in a two-level system

We describe the measurement of a component of the nonadiabatic transition probability in a two-level system that depends only on the path through parameter space followed by the Hamiltonian, and not on how fast the path is traversed (M. V. Berry, Proc. R. Soc. London 430, 405 (1990)). We performed the measurement by sweeping a radio-frequency field through the Zeeman resonance of carbon-13 in a static magnetic field and measuring the transition probability {ital P} at the end of each sweep. We found that, for appropriately chosen radio-frequency sweep forms, a factor of {ital P} is independent of the duration of the sweep, in accordance with the theory of Berry.