Activities of Time and Frequency Metrology at NICT: Optical and Microwave Frequency Standards and Their Remote Comparisons

Abstract

The National Institute of Information and Communications Technology (NICT) provides the national frequency standard of Japan as well as the Japan Standard Time (JST). Besides that, atomic frequency standards, including cesium fountains, a calcium ion clock, a strontium (Sr) lattice clock, and an indium ion clock, are also studied. This contribution briefly summarizes the current status of these frequency standards. We also study the technology to make remote comparison of these standards against others developed in external institutes. Sr lattice clock offers a convenient platform for this purpose because nearly ten Sr-based clocks are available worldwide including the one in NICT with the systematic uncertainty of 8.6 × 10⁻¹⁷. In 2011, a fiber link to the University of Tokyo was established with the baseline of 24 km, whereas that with a long baseline to PTB (Physikalisch-Technische Bundesanstalt, Germany) was realized in 2013 by a two-way satellite link using the carrier phase. The former revealed the gravitational redshift caused by the elevation difference of 56 m with uncertainty of 7 m. On the other hand, the latter for the first time realized direct comparison of two optical clocks in intercontinental scale.