  We report the experimental investigation and theoretical modeling of a
rotating polariton superfluid relying on an innovative method for the injection
of angular momentum. This novel, multi-pump injection method uses four coherent
lasers arranged in a square, resonantly creating four polariton populations
propagating inwards. The control available over the direction of propagation of
the superflows allows injecting a controllable non-quantized amount of optical
angular momentum. When the density at the center is low enough to neglect
polariton-polariton interactions, optical singularities, associated to an
interference pattern, are visible in the phase. In the superfluid regime
resulting from the strong nonlinear polariton-polariton interaction, the
interference pattern disappears and only vortices with the same sign are
persisting in the system. Remarkably the number of vortices inside the
superfluid region can be controlled by controlling the angular momentum
injected by the pumps.
