Optical Clocks Contribute to Steering International Atomic Time

Optical clocks, which have seen rapid progress in the last 20 years, have evaluated the “one second” tick of International Atomic Time (TAI). The resulting data was provided to the Bureau International des Poids et Mesures (BIPM), which used the data to adjust the TAI tick rate.

To maintain the reliability of TAI, BIPM collects data from more than 400 atomic clocks operated around the world. The accuracy of the ticking rate has been maintained by the calibrations provided by frequency standards. While microwave standards have long been in charge of calibrations for adjusting tick rate, in the future it is expected that optical clocks will also serve to steer TAI.

Strontium-87 optical lattice clock at the National Institute of Information and Communications Technology. Courtesy of National Institute of Information and Communications Technology.

Two laboratories — the National Institute of Information and Communications Technology (NICT) in Tokyo and LNE-SYRTE in Paris — performed calibrations for the evaluation of TAI using optical lattice clocks. The calibrations from the two laboratories corresponded and were consistent with those provided by state-of-the-art microwave standards.

Researchers at NICT and LNE-SYRTE operated their strontium optical lattice clocks independently from Dec. 2 to 12, 2018, and evaluated the mean frequency of local hydrogen masers (HMs) with reference to the lattice clock. The HMs were linked to TAI by BIPM, allowing the researchers at NICT and LNE-SYRTE to connect the lattice clocks to TAI. Calibration of the mean TAI scale was performed over the 10-day interval using the strontium optical lattice clocks.

The laboratories conducted independent evaluations and demonstrated consistent results: 0.84(71)E-15 and 0.74(74)E-15 at NICT and LNE-SYRTE, respectively. State-of-the-art primary frequency standards in PTB and SYRTE were also in operation during the 10-day evaluation, and their calibrations were also consistent with the results, indicating the validity of using optical clocks to provide a reference to steer the tick rate of TAI. According to the researchers, this is the first time that optical clocks have contributed to steering TAI in real time.

Once a year, BIPM reviews TAI with reference to the calibration results reported from laboratories and corrects Coordinated Universal Time (UTC), a timescale on which national standard times, information networks, and international financial systems depend. The calibrations provided by NICT and LNE-SYRTE this year also contributed to the computation of this yearly, more accurate timescale — referred to as TT(BIPM). TT(BIPM2018) was published by BIPM on Feb. 1, 2019.

The results could contribute to the future redefinition of the second. Optical clocks, including the lattice clocks at NICT and LNE-SYRTE, have already surpassed the state-of-the-art primary frequency standards based on cesium, the researchers said. Time and frequency metrologists have initiated a discussion toward the change of the definition of the Standard International (SI) second, which could occur as early as 2026.

The researchers expect that more laboratories with optical clocks will contribute to the generation of TAI by providing their own evaluation results to BIPM. Reliable calibrations made by NICT, LNE-SYRTE, and other laboratories will allow BIPM to foresee possible maintenance requirements for UTC based on the new optical definition of the second. Calibration data from various laboratories will also provide information to help BIPM determine the absolute frequency of the strontium optical clock transition, which could turn out to be the frequency that defines the new SI second.