Physics: Comparing atomic clocks with record accuracy

Three atomic clocks, based on aluminium, strontium and ytterbium, have been compared with record accuracy, reports a study published in Nature this week. The findings are an important step towards a more precise redefinition of the second.

The accuracy of atomic clocks makes them excellent instruments for timekeeping and other precision measurements. This is because atoms emit and absorb photons at specific frequencies, which are largely unaffected by environmental factors. Clocks operating at frequencies with an accuracy of 18 digits have been demonstrated previously, and outperform caesium clocks — currently used in the definition of a second. However, in order to produce a more accurate definition, comparisons between these clocks are required. To date, the highest reported accuracy for measurements of the frequency ratio between optical clocks that use different atomic species has reached a measurement of uncertainty at the level of 17-digit accuracy.

David Hume and colleagues from the BACON collaboration operated a network of three atomic clocks, which were physically housed in buildings in different parts of Boulder, Colorado, and compared their respective frequency ratios between November 2017 and June 2018. The authors found that the precision of the measurements they obtained when comparing the clocks was in the range of 18 digits. These measurements are the first reported frequency ratios with uncertainties below 17 digits. The findings lay the groundwork for the development of networks of mobile, airborne and remote optical clocks, which could be used to test theories beyond the Standard Model and lead to the redefinition of the second.

After the embargo ends, the full paper will be available at: https://www.nature.com/articles/s41586-021-03253-4