Long-term measurement of the Sr transition frequency

New atomic clocks are constantly being developed worldwide for the future redefinition of the second, the SI unit of time. By using an optical transition, these clocks allow greater accuracy than caesium atomic clocks with their microwave transition. Today, some of these optical transitions are already used for the secondary realization of the unit second. By comparing the transition frequencies of these optical clocks at different institutes, it is possible to check their consistency, which is an important step towards validating them. The International Committee for Weights and Measures (CIPM) compiles all data available worldwide at regular intervals to check their consistency. Based on these findings, the frequencies of the secondary realizations with their uncertainties are then redefined. Due to their low uncertainty, PTB’s latest frequency measurements will considerably contribute to this redefinition. Due to the length of the dataset, statistical contributions to the measurement uncertainty hardly play any role anymore. This measurement uncertainty is limited by the systematic uncertainties of PTB’s caesium fountain clocks, which are among the most accurate in the world.

The frequency value observed is in very good agreement with earlier measurements performed at PTB and with the recommended transition frequency of the strontium transition.

In combination with results of other research groups, these new measurements were also used to test Einstein’s equivalence principle according to which atomic transition frequencies are independent of position and velocity. Since caesium and strontium clocks, with their atomic transitions in the microwave and the optical frequency ranges, respectively, are based on very different physical systems, a change in the frequency ratio of the two clocks could suggest a violation of the equivalence principle.

The frequency data were investigated as regards a drift over time and an annual modulation – where the latter could be caused by the annual modulation of the sun’s gravitational potential on Earth possibly influencing atomic parameters. The analysis of the new measurement data showed that there is a stricter limit for the coupling of the proton-to-electron mass ratio to a gravitational potential. Moreover, this analysis confirms the previously determined limits for a drift over time of this mass ratio.