  Recent work has unveiled a new class of optical systems that can exhibit the
characteristic features of superfluidity. One such system relies on the
repulsive photon-photon interaction that is mediated by a thermal optical
nonlinearity and is therefore inherently nonlocal due to thermal diffusion.
Here we investigate how such a nonlocal interaction, which at a first
inspection would not be expected to lead to superfluid behavior, may be
tailored by acting upon the geometry of the photon fluid itself. Our models and
measurements show that restricting the laser profile and hence the photon fluid
to a strongly elliptical geometry modifies thermal diffusion along the major
beam axis and reduces the effective nonlocal interaction length by two orders
of magnitude. This in turn enables the system to display a characteristic trait
of superfluid flow: the nucleation of quantized vortices in the flow past an
extended physical obstacle. These results are general and apply to other
nonlocal fluids, such as dipolar Bose-Einstein condensates, and show that
"thermal" photon superfluids provide an exciting and novel experimental
environment for probing the nature of superfluidity, with applications to the
study of quantum turbulence and analogue gravity.
