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 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 the exploration of quantum many-body systems based on networks of spin-1/2 atoms on surfaces.
Bibliographical noteFunding Information:
We thank Bruce Melior for expert technical assistance and T. Greber for discussions. We gratefully acknowledge financial support from the Office of Naval Research. Y. B., P. W., T. C., and A. J. H. acknowledge support from IBS-R027-D1. W. P. thanks the Natural Sciences and Engineering Research Council of Canada for fellowship support. F. D. N. appreciates support from the Swiss National Science Foundation under Project No. PZ00P2_167965. A. F. acknowledges Consejo Nacional de Investigaciones Científicas y Técnicas (PIP11220150100327) and Fondo para la Investigación Científica y Tecnológica (PICT-2012-2866). J. F.-R. and J. L. L. thank Marie Curie-Initial Training Networks (ITN) program through Grant No. 607904-SPINOGRAPH and Fundação para a Ciência e a Tecnologia (FCT), under the project “PTDC/FIS-NAN/4662/2014.”
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