New physics searches with highly charged ions

Highly charged ions (HCI) are promising candidates for novel optical clocks with applications in frequency metrology and tests of fundamental physics [1]. Typically, megakelvin-range temperatures needed to produce HCI hinder high-precision spectroscopy. To overcome this, we extract HCI from an electron beam ion trap (EBIT) and transfer them to a cryogenic linear Paul trap. There, single HCI are sympathetically cooled by laser-cooled Be⁺ ions down to millikelvin temperatures, thus enabling quantum logic state readout [2]. We demonstrated in this way an optical clock based on Ar¹³⁺, and determined its absolute frequency with sub-Hz uncertainty [3] against the Yb⁺ octupole ion clock at PTB [4]. Our techniques are readily applicable to many ions, e. g. Ca¹⁴⁺ [5] as well as Xe HCI [6]. Recently, we determined the isotope shift of a narrow M1 transition in stable even isotopes of Ca¹⁴⁺ with 150 mHz accuracy. We combine these results with available isotope-shift data of Ca⁺ [7] in a King plot which is sensitive to a new force that would couple electrons and neutrons [8,9]. In this way, we strengthen the constraints on the existence of such a hypothetical interaction by a factor of about ten as compared to previous studies [10]. We also estimate how far improved measurements of Ca isotope masses and isotope shifts of the Ca^{+ 2}S_1/2 - ²D_5/2 transition would enhance such constraints.