  Quantum fluids of light are the photonic counterpart of Bose gases. They
currently attract increasing interest since they are versatile and highly
tunable systems for probing many-body physics quantum phenomena, such as
superfluidity. Superfluid flow of light has already been reported in
microcavity exciton-polariton condensates but clear observation of this
phenomenon in cavityless systems, that is, for propagating photon fluids,
remains elusive. In this thesis, we study the hydrodynamical properties of
light propagating close to resonance in hot rubidium vapors. Whereas photons in
air are not interacting with each other, the situation is different in rubidium
vapors, as an effective interaction between them, mediated by the atomic
ensemble, appears. The light behaves therefore as a fluid flowing in the plane
perpendicular to the optical axis. The primary purpose of this thesis was to
show that light in those systems can behave as a superfluid. The first step
toward the observation of superfluidity is to measure the dispersion relation
of small amplitude density waves travelling onto the propagating photon fluid.
I show that this dispersion exhibits a linear trend for small excitation
wave-vectors, which is, according to the Landau criterion, a sufficient
condition for guaranteeing superfluidity. I present then an all-optical defect
experiment - where the photon fluid flows against an optically induced obstacle
(namely, a local change of refractive index) - designed to measure the drag
force cancellation at the fluid/superfluid threshold.
