Highly-charged ions as a basis of optical atomic clockwork of   exceptional accuracy
  We propose a novel class of atomic clocks based on highly charged ions. We consider highly-forbidden laser-accessible transitions within the $4f^{12}$ ground-state configurations of highly charged ions. Our evaluation of systematic effects demonstrates that these transitions may be used for building exceptionally accurate atomic clocks which may compete in accuracy with recently proposed nuclear clock. 
Andrei Derevianko, V. A. Dzuba, and V. V. Flambaum
physics.atom-ph physics.optics quant-ph