A nuclear clock or nuclear optical clock is a notional atomic clock that would use the frequency of a nuclear transition as its reference frequency, instead of the atomic electron transition frequencies used by conventional atomic clocks. 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 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 8.355 733 554 021 (8) eV, this corresponds to a wavelength of 148.382 182 883 nm in the vacuum ultraviolet region, making it accessible to laser excitation.