  Quantum-mechanical correlations of interacting fermions result in the
emergence of exotic phases. Magnetic phases naturally arise in the
Mott-insulator regime of the Fermi-Hubbard model, where charges are localized
and the spin degree of freedom remains. In this regime the occurrence of
phenomena such as resonating valence bonds, frustrated magnetism, and spin
liquids are predicted. Quantum systems with engineered Hamiltonians can be used
as simulators of such spin physics to provide insights beyond the capabilities
of analytical methods and classical computers. To be useful, methods for the
preparation of intricate many-body spin states and access to relevant
observables are required. Here we show the quantum simulation of magnetism in
the Mott-insulator regime with a linear quantum dot array. We characterize a
Heisenberg chain of four spins, dial in homogeneous exchange couplings, and
probe the low-energy antiferromagnetic eigenstate with singlet-triplet
correlation measurements. The methods and control presented here open new
opportunities for the simulation of quantum magnetism benefiting from the
flexibility in tuning and layout of gate-defined quantum dot arrays.
