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.Building networks of spin-1/2 objects with adjustable interactions represents a versatile approach for quantum simulation of model Hamiltonians [1, 2] because it provides direct experimental access to quantum emergent phenomena, such as topologically generated gapped excitations [3], spin liquids [4] and anyon excitations [5]. However, the precise control of spin interactions and integration beyond a few spins, while maintaining the ability to address individual spins, remains notoriously challenging [6]. Atomically engineered spin networks on surfaces, such as coupled atomic dimers, chains [7,8], ladders [9] and arrays [10], provide a bottom-up realization of tailored spin systems, by using STM to position and address individual atoms [9,11]. Atoms with large spin S generally exhibit strong magnetocrystalline anisotropy that results in Ising-like interactions [4,12]. In contrast, quantum fluctuations scale in proportion to 1/S, so they are maximal for the smallest possible spin, S = 1/2 [4].Spins interact via exchange and dipolar interactions. At the scale of a few coupled spins, shortrange exchange coupling can give rise to magnetic ordering such as magnetic bistability [9,13] and

Taking advantage of nuclear spins for electronic structure analysis, magnetic resonance imaging, and quantum devices hinges on knowledge and control of the surrounding atomic-scale environment. We measured and manipulated the hyperfine interaction of individual iron and titanium atoms placed on a magnesium oxide surface by using spin-polarized scanning tunneling microscopy in combination with single-atom electron spin resonance. Using atom manipulation to move single atoms, we found that the hyperfine interaction strongly depended on the binding configuration of the atom. We could extract atom- and position-dependent information about the electronic ground state, the state mixing with neighboring atoms, and properties of the nuclear spin. Thus, the hyperfine spectrum becomes a powerful probe of the chemical environment of individual atoms and nanostructures.

Shrinking spintronic devices to the nanoscale ultimately requires localized control of individual atomic magnetic moments. At these length scales, the exchange interaction plays important roles, such as in the stabilization of spin-quantization axes, the production of spin frustration, and creation of magnetic ordering. Here, we demonstrate the precise control of the exchange bias experienced by a single atom on a surface, covering an energy range of four orders of magnitude. The exchange interaction is continuously tunable from milli-eV to micro-eV by adjusting the separation between a spin-1/2 atom on a surface and the magnetic tip of a scanning tunneling microscope (STM). We seamlessly combine inelastic electron tunneling spectroscopy (IETS) and electron spin resonance (ESR) to map out the different energy scales. This control of exchange bias over a wide span of energies provides versatile control of spin states, with applications ranging from precise tuning of quantum state properties, to strong exchange bias for local spin doping. In addition we show that a time-varying exchange interaction generates a localized AC magnetic field that resonantly drives the surface spin. The static and dynamic control of the exchange interaction at the atomic-scale provides a new tool to tune the quantum states of coupled-spin systems.Exchange interaction between magnetic atoms gives rise to exotic forms of quantum magnetism such as quantum spin liquids [1], and spin transport in magnetic insulators [2,3]. It is also of great technological importance in tailoring magnetic devices [4][5][6][7][8]. For instance, in magnetic read heads, a ferromagnetic layer is "biased" to a specific magnetization direction by strong exchange coupling to an antiferromagnetic layer [4]. Weaker exchange interaction also plays an important role, in the spin dynamics of quantum magnets [9], in giant magnetoresistance devices [10], and in magnetic phases of coupled spins that depend on next-nearest-neighbor interactions such as spin chains [11] and spin glasses [12].