  Probing the transport of fluids within confined domains is important in many
areas including material science, catalysis, food science, and cell biology.
The diffusion propagator fully characterizes the diffusion process, which is
highly sensitive to the confining boundaries as well as the structure within
enclosed pores. While magnetic resonance has been used extensively to observe
various features of the diffusion process, its full characterization has been
elusive. Here, we address this challenge by employing a special sequence of
magnetic field gradient pulses for measuring the diffusion propagator, which
allows for `listening to the drum' and determining not only the pore's shape
but also diffusive dynamics within it.
