  The Berry phase acquired by an electromagnetic field undergoing an adiabatic
and cyclic evolution in phase space is a purely quantum-mechanical effect of
the field. However, this phase is usually accompanied by a dynamical
contribution and cannot be manifested in any light-beam interference experiment
because it is independent of the field state. We here show that such a phase
can be produced using an atom coupled to a quantized field and driven by a
slowly changing classical field, and it is manifested in atomic Ramsey
interference oscillations. We also show how this effect may be applied to
one-step implementation of multiqubit geometric phase gates, which is
impossible by previous geometric methods. The effects of dissipation and
fluctuations in the parameters of the pump field on the Berry phase and
visibility of the Ramsey interference fringes are analyzed.
