  Recently remarkable progress in quantum technology has been witnessed. In
view of this it is important to investigate an open quantum system as a model
of such quantum devices. Quantum devices often require extreme conditions such
as very low temperature for the devices to operate. Dynamics can be
non-Markovian in such a situation in contrast with Markovian dynamics in high
temperature regime. This observation necessitates us to investigate a
non-Markovian open quantum system, both theoretically and experimentally. In
this paper, we report two important results: 1) Exact solution of a simple but
non-trivial theoretical model and 2) demonstration of this model by NMR
experiments, where non-Markovianity is continuously controllable. We observe
qualitative agreement between theory and experiment.
