  Quantum spin networks having engineered geometries and interactions are
eagerly pursued for quantum simulation and access to emergent quantum phenomena
such as spin liquids. Spin-1/2 centers are particularly desirable because they
readily manifest coherent quantum fluctuations. Here we introduce a
controllable spin-1/2 architecture consisting of titanium atoms on a magnesium
oxide surface. We tailor the spin interactions by atomic-precision positioning
using a scanning tunneling microscope (STM), and subsequently perform electron
spin resonance (ESR) on individual atoms to drive transitions into and out of
quantum eigenstates of the coupled-spin system. Interactions between the atoms
are mapped over a range of distances extending from highly anisotropic dipole
coupling, to strong exchange coupling. The local magnetic field of the magnetic
STM tip serves to precisely tune the superposition states of a pair of spins.
The precise control of the spin-spin interactions and ability to probe the
states of the coupled-spin network by addressing individual spins will enable
exploration of quantum many-body systems based on networks of spin-1/2 atoms on
surfaces.
