Measurement of Berry's phase using an atom interferometer

We report on the demonstration of Berry's phase in an atomic state interact ing with a laser field. We draw an analogy between this system and that of a spin interacting with a directionally varying magnetic field. This allows us to identify an effective magnetic quantum number for the atom-light sys tem that governs the maximum Berry phase the atomic state can acquire. We r ealize two systems that have different effective magnetic quantum numbers, and use a recently developed atom interferometer to make measurements of Be rry's phase. [S1050-2947(99)51309-7].