  Four-body interaction plays an important role in many-body systems, and it
can exhibit interesting phase transition behaviors. Historically it was the
need to efficiently simulate quantum systems that lead the idea of a quantum
computer. In this Letter, we report the experimental demonstration of a
four-body interaction in a four- qubit nuclear magnetic resonance quantum
information processor. The strongly modulating pulse is used to implement spin
selective excitation. The results show a good agreement between theory and
experiment.
