  Quantum fluids of light are an emerging platform for energy efficient signal
processing, ultra-sensitive interferometry and quantum simulators at elevated
temperatures. Here we demonstrate the optical control of the topological
excitations induced in a large polariton condensate, realising the bosonic
analog of a long Josephson junction and reporting the first observation of
bosonic Josephson vortices. When a phase difference is imposed at the
boundaries of the condensate, two extended regions become separated by a sharp
$\pi$-slippage of the phase and a solitonic depletion of the density, forming
an insulating barrier with a suppressed order parameter. The superfluid
behavior, that is a smooth phase gradient across the system instead of the
sharp phase jump, is recovered at higher polariton densities and it is mediated
by the nucleation of Josephson vortices within the barrier. Our results
contribute to the understanding of dissipation and stability of elementary
excitations in macroscopic quantum systems.
