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Abstract

Highly charged ions (HCIs) are excellent candidates for next-generation frequency standards,
as they feature a much reduced susceptibility to external perturbations, and are
proposed to stringently test physics beyond the Standard Model of particle physics. These
applications require spectroscopy of HCIs with an accuracy on the level of state-of-theart
trapped-ion frequency standards. In order to overcome the current limitations due to
motional frequency shifts, an efficient suppression of trap-induced heating rates is essential.
To that end, a new cryogenic Paul trap experiment, CryPTEx-II, was developed and
commissioned within this work. It consists of a novel superconducting ion trap uniquely
combining a radio-frequency (rf) cavity and a Paul trap, and a low-vibration cryogenic
supply to cool the trap to temperatures of 4.15K while decoupling external vibrations.
The cavity features an electric quadrupole mode at 34.52MHz with a quality factor of
~ 2.3 × 10⁵, which allows for stable confinement of ions in ultralow-noise rf potentials
and is expected to result in strongly suppressed motional heating rates. Commissioning
experiments comprised the operation of the cavity as a quadrupole mass filter to focus
HCIs through the cavity as well as the first trapping and laser cooling of ⁹Be⁺ ions. These
constitute the successful proof-of-principle operation of this unique apparatus, which will
be used for high-precision experiments with HCIs in the future.