DECi-hertz Interferometer Gravitational wave Observatory (DECIGO) is the future Japanese space gravitational wave antenna. DECIGO is expected to open a new window of observation for gravitational wave astronomy especially between 0.1 Hz and 10 Hz, revealing various mysteries of the universe such as dark energy, formation mechanism of supermassive black holes, and inflation of the universe. The pre-conceptual design of DECIGO consists of three drag-free spacecraft, whose relative displacements are measured by a differential Fabry-Perot Michelson interferometer. We plan to launch two missions, DECIGO pathfinder and pre-DECIGO first and finally DECIGO in 2024.

DECi-hertz Interferometer Gravitational wave Observatory (DECIGO) is the future Japanese space gravitational wave antenna. It aims at detecting various kinds of gravitational waves between 1 mHz and 100 Hz frequently enough to open a new window of observation for gravitational wave astronomy. The pre-conceptual design of DECIGO consists of three drag-free satellites, 1000 km apart from each other, whose relative displacements are measured by a Fabry–Perot Michelson interferometer. We plan to launch DECIGO in 2024 after a long and intense development phase, including two pathfinder missions for verification of required technologies.

We demonstrate a precision frequency measurement using a phase-stabilized 120-km

We performed an absolute frequency measurement of the 1 S 0 -3 P 0 transition in 87 Sr with a fractional uncertainty of 1.2 × 10 −15 , which is less than one third that of our previous measurement. A caesium fountain atomic clock was used as a transfer oscillator to reduce the uncertainty of the link between a strontium optical lattice clock and the SI second. The absolute value of the transition frequency is 429 228 004 229 873.56(49) Hz.Recently, some optical clocks have reached the 10 −18 level 1, 2 in both uncertainty and stability, and these values surpass the caesium fountain microwave primary standards used to realise the SI unit of time. The high-performance of the optical clocks means that the scientific community is discussing a re-definition of the second. Therefore, there is a need for the metrology community to make a strenuous effort to determine the absolute frequencies of the optical frequency standards in relation to the current primary frequency standards, so that the length of one second remains unchanged after the re-definition.At the National Metrology Institute of Japan (NMIJ) we have developed atomic clocks based on optical transitions in an ensemble of neutral atoms trapped in Stark-shift-free optical lattices, 3-5 which are called optical lattice clocks. 6 In 2014, we measured the frequency of the 1 S 0 -3 P 0 clock transition in 87 Sr. 5 At that time the uncertainty of the absolute frequency (3.7 × 10 −15 ) was mainly limited by the uncertainty of a comparison with NMIJ coordinated ‡ These two authors contributed equally to this work.