  Singly quantized vortices have been already observed in many systems
including the superfluid helium, Bose Einstein condensates of dilute atomic
gases, and condensates of exciton polaritons in the solid state. Two
dimensional superfluids carrying spin are expected to demonstrate a different
type of elementary excitations referred to as half quantum vortices
characterized by a pi rotation of the phase and a pi rotation of the
polarization vector when circumventing the vortex core. We detect half quantum
vortices in an exciton-polariton condensate by means of polarization resolved
interferometry, real space spectroscopy and phase imaging. Half quantum
vortices coexist with single quantum vortices in our sample.
