Coherent surface spin manipulation: Spin-based quantum information processing requires coherent spin manipulation. Yang et al. demonstrate coherent control of surface titanium and iron atom spins on a magnesium oxide surface with a magnetic scanning tunneling microscope tip. Arbitrary sequences of fast electrical pulses delivered to the top induced large electric fields. These fields drove metal-atom movement, which then modulated the tip-atom exchange interaction to create an oscillating effective magnetic field. Advanced spin-control protocols such as Ramsey fringes and Hahn spin echoes revealed quantum dynamics, such as coherent oscillations in a titanium atom dimer assembled on the surface with the tip. Science , this issue p. 509

Magnetism at low dimensions is a thriving field of research with exciting opportunities in technology. This Colloquium focuses on the properties of 1D magnetic systems on solid surfaces. From the emulation of 1D quantum phases to the potential realization of Majorana edge states, spin chains are unique systems to study. The advent of scanning tunneling microscope (STM) based techniques has permitted us to engineer spin chains in an atom-by-atom fashion via atom manipulation and to access their spin states on the ultimate atomic scale. Here the current state of research on spin correlations and dynamics of atomic spin chains as studied by the STM is presented. After a brief review of the main properties of spin chains on solid surfaces, spin chains are classified according to the coupling of their magnetic moments with the holding substrate. This classification scheme takes into account that the nature and lifetimes of the spin-chain excitations intrinsically depend on the holding substrate. Interest is shown of using insulating layers on metals, which generally results in an increase in the spin state’s lifetimes such that their quantized nature gets evident and they are individually accessible. Next shown is the use of semiconductor substrates promising additional control through the tunable electron density via doping. When the coupling to the substrate is increased for spin chains on metals, the substrate conduction electron mediated interactions can lead to emergent exotic phases of the coupled spin chain-substrate conduction electron system. A particularly interesting example is furnished by superconductors. Magnetic impurities induce states in the superconducting gap. Because of the extended nature of the spin chain, the in-gap states develop into bands that can lead to the emergence of 1D topological superconductivity and consequently to the appearance of Majorana edge states. Finally, an outlook is given on the use of spin chains in spintronics, quantum communication, quantum computing, quantum simulations, and quantum sensors.

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.