A nuclear clock is a notional clock that would use the frequency of a nuclear transition as its reference frequency, in the same manner as an atomic clock uses the frequency of an electronic transition in an atom's shell. Such a clock is expected to be more accurate than the best current atomic clocks, with a fractional instability at the 10−19 level.
The thorium-229m transition
The only nuclear transition suitable as a reference frequency is the gamma decay of thorium-229m, a nuclear isomer of thorium-229 and the lowest-energy nuclear isomer known. The transition is expected to be in the vacuum ultraviolet, which would make it accessible to laser excitation.
In 2016, the first direct detection of the isomeric transition has constrained its energy to be between 6.3 and 18.3 eV.