An optical atomic clock based on a highly charged ion.

Optical atomic clocks are the most accurate measurement devices ever constructed and have found many applications in fundamental science and technology. The use of highly charged ions (HCI) as a new class of references for highest-accuracy clocks and precision tests of fundamental physics has long been motivated by their extreme atomic properties and reduced sensitivity to perturbations from external electric and magnetic fields compared with singly charged ions or neutral atoms. Here we present the realization of this new class of clocks, based on an optical magnetic-dipole transition in Ar.