  At the heart of a Bose-Einstein condensate lies its description as a single
giant matter wave. Such a Bose-Einstein condensate represents the most
"classical" form of a matter wave, just as an optical laser emits the most
classical form of an electromagnetic wave. Beneath this giant matter wave,
however, the discrete atoms represent a crucial granularity, i.e. a
quantization of this matter wave field. Here we show experimentally that this
quantization together with the cold collisions between atoms lead to a series
of collapses and revivals of the coherent matter wave field of a Bose-Einstein
condensate. We observe such collapses and revivals directly in the dynamical
evolution of a multiple matter wave interference pattern, and thereby
demonstrate a striking new behaviour of macroscopic quantum matter.
