  We demonstrate a new versatile building block for optical quantum
technologies, based on an intracavity Rydberg-blockaded atomic ensemble acting
as a single two-level superatom. We coherently control its state and optically
detect it in a single shot with a $95\%$ efficiency. Crucially, we demonstrate
a superatom-state-dependent $\pi$ phase rotation on the light reflected from
the cavity. Together with the state manipulation and detection, it is a key
ingredient for implementing deterministic photonic entangling gates and for
generating highly non-classical light states.
