  With reference to the vacuum induced Berry phase (VIBP) obtained in the
interaction of a spin-1/2 particle with quantized irradiation field under
rotating-wave approximation (RWA), we present completely different treatment
for the VIBP by a fully quantum mechanical treatment beyond the RWA, which
gives a new definition of the VIBP and indicates the validity of RWA to be
relevant not only to the energy conservation and comparison of characteristic
parameters, but also to more subtle physics, such as the geometric property of
the state evolution. Our result is of conceptual importance and also of
significant relevance to quantum information processing.
