  One of the milestones of quantum mechanics is Bohr's complementarity
principle. It states that a single quantum can exhibit a particle-like
\emph{or} a wave-like behaviour, but never both at the same time. These are
mutually exclusive and complementary aspects of the quantum system. This means
that we need distinct experimental arrangements in order to measure the
particle or the wave nature of a physical system. One of the most known
representations of this principle is the single-photon Mach-Zehnder
interferometer. When the interferometer is closed an interference pattern is
observed (wave aspect of the quantum) while if it is open, the quantum behaves
like a particle. Here, using a molecular quantum information processor and
employing nuclear magnetic resonant (NMR) techniques, we analyze the quantum
version of this principle by means of an interferometer that is in a quantum
superposition of being closed and open, and confirm that we can indeed measure
both aspects of the system with the same experimental apparatus. More
specifically, we observe with a single apparatus the interference between the
particle and the wave aspects of a quantum system.
