A molecular device that contains the smallest possible atomistic switching unit is reported online in Nature Nanotechnology this week. The switch is based on the transfer of a single proton and could be useful in the development of molecular electronics. Molecular electronics use molecules as components in electrical circuitry, and the fabrication of switches is a key requirement. Willi Auwarter and colleagues built a molecular switch using a porphyrin molecule that was anchored to a silver surface. Porphyrins are flat organic molecules that have a central cavity that can incorporate two hydrogen atoms. These hydrogen atoms can be flipped between two states using electrons from the tip of a scanning tunnelling microscope (STM). Moreover, by removing one of the hydrogen atoms with the STM tip, a four-level conductance switch can be created in which the remaining proton hops between four different positions within the cavity of the porphyrin molecule. The authors report that the overall size of the porphyrin molecule is unaffected by the switching action and the molecule could be connected to other species or components without altering the performance of the switch. This should allow more intricate electronic devices to be developed.