Coulomb Crystallized Highly Charged Ions

Electric-dipole forbidden optical transitions in highly charged ions (HCI) are both insensitive to external perturbations and extremely sensitive to possible drifts of the fine structure constant 𝛼. Thus, cold, strongly localized HCI are of particular interest for bound-state QED studies (g-factor measurements), metrology (development of novel optical clocks) and the search for α variation. We have recently succeeded in the preparation of Coulomb crystallized HCI in a cryogenic linear Paul trap through sympathetic cooling with laser cooled 9Be+ ions. The HCI (40Ar13+) are produced in and extracted from an electron beam ion trap (EBIT), decelerated and pre-cooled by means of two pulsed drift tubes, and injected into the Paul trap. Subsequently, they are forced to perform an alternating motion along the trap axis, interacting multiple times with a preformed Be+ Coulomb crystal and thereby loosing enough energy to become implanted in and thermalizing with it. Various cooling configurations of large mixed-species crystals and fluids, over strings of few ions down to a single HCI cooled by a single Be+ ion - a prerequisite for future quantum logic spectroscopy at a potential 10−19 level accuracy have been investigated, and final temperatures below 250 mK determined.