  Artificial lattices are a powerful tool to reveal and study key interaction
mechanisms between constituents of condensed matter systems, such as
antiferromagnetic interactions leading to geometric frustration. Frustration in
magnetic materials is evidenced in their spin degree of freedom, whereas in
many-body quantum systems, such as Bose-Einstein condensates in optical
lattices, coupled lasers, and polariton condensates that are described by a
complex order parameter, frustration is scarcely explored in controllable
settings. Here, we realize a radically new platform to study geometric
frustration using a macroscopic quantum fluid of light based on
exciton-polariton condensates. We design pump configurations which drive
condensates into an all-optical triangular vortex lattice with each cell
possessing a vortex state of charge l = +-1. The orbital angular momentum (OAM)
of each vortex replaces the role of spin angular momentum in frustrated matter.
Our measurements and mean-field calculations reveal that individual pairs of
coupled condensates spontaneously arrange their OAM antiparallel, implying a
form of artificial orbital antiferromagnetism, with geometric frustration
appearing in the triangular graph of three condensates. Furthermore, we
demonstrate emergence of frustrated phases of driven-dissipative large-scale
vortex matter, and open a route towards constructing ordered vortex arrays with
nearly arbitrary configurations without directly phase-imprinting or stirring
the polariton fluid.
