Creating Spin Switches and Junctions on Surfaces

Inspired by the work of Hirjibehedin \textit{et al}, (\textit{Science} \textbf{317} 1199) creating Heisenberg spin chains on an insulating surface, we examine geometries in which excitations down a spin chain are either blocked or transmitted depending on the state of a central junction, made from a spin dimer. The dimer state can be controlled by excitations down an additional chain, creating a spin switch. In addition to the technological applications of such a switch, the theoretical language developed has application to certain quantum computation schemes.