  Vortices are topological objects representing the circular motion of a fluid.
With their additional degree of freedom, the 'vorticity', they have been widely
investigated in many physical systems and different materials for fundamental
interest and for applications in data storage and information processing.
Vortices have also been observed in non-equilibrium exciton-polariton
condensates in planar semiconductor microcavities. There they appear
spontaneously or can be created and pinned in space using ring-shaped optical
excitation profiles. However, using the vortex state for information processing
not only requires creation of a vortex but also efficient control over the
vortex after its creation. Here we demonstrate a simple approach to control and
switch a localized polariton vortex between opposite states. In our scheme,
both the optical control of vorticity and its detection through the orbital
angular momentum of the emitted light are implemented in a robust and practical
manner.
