  In a homogeneous two-dimensional system at non-zero temperature, although
there can be no ordering of infinite range, a superfluid phase is predicted for
a Bose liquid. The stabilization of phase in this superfluid regime is achieved
by the formation of bound vortex-antivortex pairs. It is believed that several
different systems share this common behaviour, when the parameter describing
their ordered state has two degrees of freedom, and the theory has been tested
for some of them. However, there has been no direct experimental observation of
the phase stabilization mechanism by a bound pair. Here we present an
experimental technique that can identify a single vortex-antivortex pair in a
two-dimensional exciton polariton condensate. The pair is generated by the
inhomogeneous pumping spot profile, and is revealed in the time-integrated
phase maps acquired using Michelson interferometry, which show that the
condensate phase is only locally disturbed. Numerical modelling based on open
dissipative Gross-Pitaevskii equation suggests that the pair evolution is quite
different in this non-equilibrium system compared to atomic condensates. Our
results demonstrate that the exciton polariton condensate is a unique system
for studying two-dimensional superfluidity in a previously inaccessible regime.
