A nuclear clock or nuclear optical 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 by a factor of about 10, with an achievable accuracy approaching the 10−19 level.
The only nuclear state suitable for the development of a nuclear clock using existing technology is thorium-229m, a nuclear isomer of thorium-229 and the lowest-energy nuclear isomer known. With an energy of about 8 eV,
the corresponding ground-state transition is expected to be in the vacuum ultraviolet wavelength region around 150 nm, which would make it accessible to laser excitation. A comprehensive review can be found in reference.