  Exotic phases of matter can emerge from strong correlations in quantum
many-body systems. Quantum gas microscopy affords the opportunity to study
these correlations with unprecedented detail. Here we report site-resolved
observations of antiferromagnetic correlations in a two-dimensional,
Hubbard-regime optical lattice and demonstrate the ability to measure the
spin-correlation function over any distance. We measure the in-situ
distributions of the particle density and magnetic correlations, extract
thermodynamic quantities from comparisons to theory, and observe statistically
significant correlations over three lattice sites. The temperatures that we
reach approach the limits of available numerical simulations. The direct access
to many-body physics at the single-particle level demonstrated by our results
will further our understanding of how the interplay of motion and magnetism
gives rise to new states of matter.
