  Precision spectroscopy of atomic systems is an invaluable tool for the
advancement of our understanding of fundamental interactions and symmetries.
Recently, highly charged ions (HCI) have been proposed for sensitive tests of
physics beyond the Standard Model and as candidates for high-accuracy atomic
clocks. However, the implementation of these ideas has been hindered by the
parts-per-million level spectroscopic accuracies achieved to date. Here, we
cool a trapped HCI to the lowest reported temperatures, and introduce coherent
laser spectroscopy on HCI with an eight orders of magnitude leap in precision.
We probe the forbidden optical transition in $^{40}$Ar$^{13+}$ at 441 nm using
quantum-logic spectroscopy and measure both its excited-state lifetime and
$g$-factor. Our work ultimately unlocks the potential of HCI, a large,
ubiquitous atomic class, for quantum information processing, novel frequency
standards, and highly sensitive tests of fundamental physics, such as searching
for dark matter candidates or violations of fundamental symmetries.
