  Controlling strongly interacting many-body systems enables the creation of
tailored quantum matter, with properties transcending those based solely on
single particle physics. Atomic ensembles which are optically driven to a
Rydberg state provide many examples of this, such as atom-atom entanglement,
many-body Rabi oscillations, strong photon-photon interaction and spatial pair
correlations. In its most basic form, Rydberg quantum matter consists of an
isolated ensemble of strongly interacting atoms spatially confined to the
blockade volume - a so-called superatom. Here we demonstrate the controlled
creation and characterization of an isolated mesoscopic superatom by means of
accurate density engineering and excitation to Rydberg $p$-states. Its variable
size allows to investigate the transition from effective two-level physics for
strong confinement to many-body phenomena in extended systems. By monitoring
continuous laser-induced ionization we observe a strongly anti-bunched ion
emission under blockade conditions and extremely bunched ion emission under
off-resonant excitation. Our experimental setup enables in vivo measurements of
the superatom, yielding insight into both excitation statistics and dynamics.
We anticipate straightforward applications in quantum optics and quantum
information as well as future experiments on many-body physics.
