  We have realized a quantum walk in momentum space with a rubidium spinor
Bose-Einstein condensate by applying a periodic kicking potential as a walk
operator and a resonant microwave pulse as a coin toss operator. The generated
quantum walks appear to be stable for up to ten steps and then quickly transit
to classical walks due to spontaneous emissions induced by laser beams of the
walk operator. We investigate these quantum to classical walk transitions by
introducing well controlled spontaneous emissions with an external light source
during quantum walks. Our findings demonstrate a scheme to control the
robustness of the quantum walks and can also be applied to other cold atom
experiments involving spontaneous emissions.
