From atomic to nuclear optical frequency standards
The perspectives for the increase in the accuracy of optical frequency standards by means of the development of "nuclear clocks", a novel frequency standard based on the nuclear transition to the long-living isomer nuclear state of thorium-229 with energy ~7.6 eV are discussed. Theoretical estimations give a possible accuracy 10 <sup>-20</sup> that allows wide scope of applications for a frequency standard, from satellite navigation systems to experimental verification of the principles of the general theory of relativity.