Engineering the Hamiltonian of coupled spin-1/2 atoms on a surface

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 and entanglement. Here we introduce a controllable spin-1/2 architecture consisting of 3d transition metal 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 entangled states. 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 tunes the energy states of any selected atom, and serves to precisely adjust the level of entanglement of a pair of spins. The precise control of the spin-spin interactions and ability to probe the entangled states on individual spins demonstrated here will enable exploration of quantum many-body systems based on networks of spin-1/2 atoms on surfaces.