Professor Hidetoshi Katori and his research group have succeeded for the first time in developing a compact and robust ultra-high precision optical lattice clock with a device volume of 250-liter, one quarter the size of conventional clocks. Since the system of the optical lattice clock is equivalent to an error of one second in ten billion years, it is attracting attention as a strong candidate for redefining the SI second, which is based on the current definition of the second as a cesium atomic clock. This time, the volume of the device has been successfully downsized by a quarter, from 920 litres to 250 litres. This is expected to enable its use not only as a time standard, but also in various research fields and applications, such as relativistic geodesy. This research was carried out in collaboration with RIKEN, Shimadzu Corporation and JEOL Ltd.