Item

Abstract

Recent theoretical works have proposed atomic clocks based on narrow optical
transitions in highly charged ions. The most interesting candidates for
searches of new physics are those which occur at rare orbital crossings where
the shell structure of the periodic table is reordered. There are only three
such crossings expected to be accessible in highly charged ions, and hitherto
none have been observed as both experiment and theory have proven difficult. In
this work we observe an orbital crossing in highly charged ions for the first
time, in a system chosen to be tractable from both sides: Pr⁹⁺. We present
electron beam ion trap measurements of its spectra, including the
inter-configuration lines that reveal the sought-after crossing. The proposed
nHz-wide clock line, found to be at 452.334(1) nm, proceeds through hyperfine
admixture of its upper state with an E2-decaying level. With state-of-the-art
calculations we show that it has a very high sensitivity to new physics and
extremely low sensitivity to external perturbations, making it a unique
candidate for proposed precision studies.