  We report the realization of a synthetic magnetic field for photons and
polaritons in a honeycomb lattice of coupled semiconductor micropillars. A
strong synthetic field is induced in both the s and p orbital bands by
engineering a uniaxial hopping gradient in the lattice, giving rise to the
formation of Landau levels at the Dirac points. We provide direct evidence of
the sublattice symmetry breaking of the lowest-order Landau level wavefunction,
a distinctive feature of synthetic magnetic fields. Our realization implements
helical edge states in the gap between n=0 and n=1 Landau levels,
experimentally demonstrating a novel way of engineering propagating edge states
in photonic lattices. In light of recent advances in the enhancement of
polariton-polariton nonlinearities, the Landau levels reported here are
promising for the study of the interplay between pseudomagnetism and
interactions in a photonic system.
