A Single-Ion Nuclear Clock for Metrology at the 19th Decimal Place
  The 7.6(5) eV nuclear magnetic-dipole transition in a single 229Th3+ ion may provide the foundation for an optical clock of superb accuracy. A virtual clock transition composed of stretched states within the 5F5/2 electronic ground level of both nuclear ground and isomeric manifolds is proposed. It is shown to offer unprecedented systematic shift suppression, allowing for clock performance with a total fractional inaccuracy approaching 1 x 10^-19. 
C. J. Campbell, A. G. Radnaev, A. Kuzmich, V. A. Dzuba, V. V.   Flambaum, and A. Derevianko
physics.atom-ph