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Long-term measurement of the Sr transition frequency

Measurements with caesium fountain clocks also set new limits for fundamental tests

PTBnews 1.2021
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fundamental research

the redefinition of the SI unit second

Optical clocks such as the strontium lattice clock are used for the secondary realization of the SI unit second and are now being envisaged as potential candidates for the redefinition of the unit of time. To ensure a seamless transition, it is necessary to know the strontium transition frequency in the present system, which is determined by caesium atomic clocks. The two types of clocks were compared from 2017 to 2019. This comparison now allows this frequency to be determined in relative units with a record uncertainty of only 1.5 × 10–16. These data have yielded limits for a drift over time of the proton-to-electron mass ratio and a possible coupling to the gravitational potential of the sun.

Transition frequencies of PTB’s strontium lattice clock measured from 2010 to 2019. The black bars and gray arrays indicate the transition frequency and uncertainty for the Sr transition as a secondary realization of the unit second recommended by the CIPM at the time of the respective measurement.

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.


Christian Lisdat
Department 4.3
Quantum Optics and Unit of Length
Phone: +49 531 592-4320

Stefan Weyers
Department 4.4
Time and Frequency
Phone: +49 531 592-4410

Scientific publication

R. Schwarz, S. Dörscher, A. Al-Masoudi, E. Benkler, T. Legero, U. Sterr, S. Weyers, J. Rahm, B. Lipphardt, C. Lisdat: Long term measurement of the 87Sr clock frequency at the limit of primary Cs clocks, Phys. Rev. Research 2, 033242 (2020)

Opens external link in new windowDOI: 10.1103/PhysRevResearch.2.033242