Geodetic measurements and quantitative evaluation for reduced gravitational redshift uncertainty of NICT optical frequency standards

The frequency accuracy of optical atomic clocks has dramatically increased over the past 15 years, improving by more than two orders of magnitude from 16 digits of precision to 18 or even 19 digits of precision. Since around 2015 researchers from around the world began to consider a redefinition of the second that uses optical atomic clocks. Since then, the development of optical atomic clocks has progressed and the recent results demonstrated to detect the frequency change with 18 digits of precision.

The National Institute of Information and Communications Technology (NICT) has developed the Sr optical lattice clock and optical ion clocks employing In+ and Ca+, as well as a Sr optical lattice clock that provides calibration data to BIPM. On the other hand, the centimeter-level uncertainty of site elevation has caused 10^-18-level frequency uncertainties of optical frequency standards. Therefore, it is significantly important to understand frequency changes caused by solid-earth tides that often range from 10 to 20 cm in amplitude, by oceanic tidal loading, crustal deformations due to earthquakes, and ground movements with groundwater changes for the stable operation of optical atomic clocks.

NICT, in collaboration with partners including the Geospatial Information Authority of Japan (GSI), has begun observations and data analysis to evaluate how these effects interact with optical atomic clocks. Since early 2021, NICT and GSI have been jointly conducting leveling surveys and relative gravimeter observations at NICT’s headquarters in Koganei. These observations reduce the contribution of gravitational redshift to the total uncertainty of the NICT-Sr1 optical lattice clock has been reduced to the 10^-19 level.

With the support of National Institute of Polar Research (NIPR), absolute gravity measurements were performed in the August 2019 and May 2022 to evaluate the effects of  the 2011 March 11 Tohoku megaquake on coseismic vertical crustal movement. The obtained absolute gravity change between the two periods was -43.8 μGal. This matches the trend of GNSS result obtained by GSI, which show a vertical movement of up to 31.5 mm from August 2019 to May 2022, equivalent to about -10 μGal gravity change, even though the values do not agree precisely.

We have introduced a Micro-g LaCoste's gPhoneX gravimeter for continuous gravity measurements near by the optical clocks in the end of 2021. The preliminary results over seven months detects stable gravity change due to solid-earth tide with about 22 μGal precision. In addition, we have started to investigate the temporal variation of the ground water level at Koganei. We are also monitoring vertical crustal movements by geodetic GNSS measurements. We will investigate uncertainties of optical clocks due to vertical movements caused by geodetic phenomena using continuous gravity and GNSS measurements.