Highly-precise comparison of Hg and Sr optical lattice clocks
- a step towards redefinition of time standard -

As part of the JST Targeted Basic Research Program, a team of researchers at the University of Tokyo and RIKEN have developed a mercury-based optical lattice clock and measured the mercury clock frequency using the recently developed strontium-based optical lattice clock as a reference. The research team achieved a fractional frequency uncertainty, which is smaller than the realization of the current definition of SI(International System of Unit) second, i.e., the SI limit.

Optical lattice clocks are currently being developed all over the world aiming at nearly 1000 fold improvement in uncertainty over the International Atomic Time (TAI) based on microwave cesium clocks. One major challenge in reducing the uncertainty of optical lattice clocks is to eliminate the influence of electromagnetic waves radiated from the surrounding walls at room temperature (blackbody radiation), which perturbs the transition frequency of atoms.

To address this issue, researchers have developed an optical lattice clock based on mercury atoms, which are at least 10 times less sensitive to the blackbody radiation compared to other candidates for optical lattice clocks such as strontium and ytterbium atoms. Direct optical frequency comparison between mercury clock operated in room temperature and cryo-strontium clock has yielded a frequency ratio with the fractional uncertainty of 8x10^-17, which is beyond the SI limit.

Determination of the frequency ratio with the uncertainty beyond the realization of the current definition of a second is essential for a step towards the redefinition of the second. Furthermore, such a highly-precise clock comparison can be a probe for new physics by investigating the constancy of the physical constant. For example, high-precision frequency comparison of clocks consisted of different atomic species can be used to investigate temporal variation of the fine structure constant.