Concept of a thorium-229 based nuclear optical clock.
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,[1] 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 10−19 level.[2]
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 8.355733554021(8) eV,[3][4][5] this corresponds to a wavelength of 148.382182883 nm in the vacuum ultraviolet region, making it accessible to laser excitation.[6][7]
^Tiedau, J.; Okhapkin, M. V.; Zhang, K.; Thielking, J.; Zitzer, G.; Peik, E.; et al. (29 April 2024). "Laser Excitation of the Th-229 Nucleus"(PDF). Physical Review Letters. 132 (18) 182501: 182501. Bibcode:2024PhRvL.132r2501T. doi:10.1103/PhysRevLett.132.182501. PMID38759160. The nuclear resonance for the Th4+ ions in Th:CaF2 is measured at the wavelength 148.3821(5) nm, frequency 2020.409(7) THz, and the fluorescence lifetime in the crystal is 630(15) s, corresponding to an isomer half-life of 1740(50) s for a nucleus isolated in vacuum.
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