Timekeeping technology has advanced from the use of quartz movements to the precision of atomic clocks. By tracking electronic transitions of atoms at optical wavelengths, optical atomic clocks are accurate to within one part in 10¹⁸, measuring time with a fidelity that will lose less than a second over the life of the universe to date. An even more accurate clock could be built by using a nuclear transition instead of an electronic transition in the form of a nuclear optical clock expected to be less sensitive to external perturbations.

Previous research identified a nuclear transition suitable for a nuclear clock in the thorium-229 isotope but the transition frequency has only recently been determined with sufficient precision to allow its direct excitation Schematic of the magnetic micro-calorimeter. Source: Tomas Sikorsky et al.with narrow-band lasers, a prerequisite for the operation of an optical clock. Researchers have now documented the high-precision measurement of the ²²⁹Th transition that significantly narrows the spectral range and could pave the way for developing a nuclear optical clock.

A cryogenic magnetic microcalorimeter was used to monitor radiation emitted by a sample of uranium-233 as it decayed to ²²⁹Th. The method produced a precise estimate of the wavelength of ultraviolet (UV) light needed to measure oscillations of the isotope's nucleus. The transition energy was measured at 8.1 electronvolts, providing scope for direct laser excitation of the ²²⁹Th isomer. The finding indicates that a UV laser with a wavelength of 153.1 nm could be used to build a nuclear clock.

Scientists from Heidelberg University (Germany), Vienna University of Technology (Austria), Johannes Gutenberg University (Germany), Helmholtz Institute Mainz (Germany) and GSI Helmholtzzentrum für Schwerionenforschung mbH (Germany) contributed to this research, which is published in Physical Review Letters.