NICT achieves a world first ― Optical lattice clock referenced to maintain national standard time enabling synchronization with UTC

On June 9, the National Institute of Information and Communications Technology (NICT) announced that it had achieved a world first by using an optical lattice clock as a reference to maintain national standard time.

Coordinated Universal Time (UTC) is provided by the Bureau International des Poids et Mesures (BIPM). Each country's standard time is generated and maintained by referencing and synchronizing to UTC with a time difference added (plus nine hours in the case of Japan). UTC is determined as a form of numerical data by collecting data from atomic clocks worldwide and taking their weighted average. Unfortunately, this method means data is delayed by more than half a month.

Therefore, it is impossible to provide time that is perfectly synchronized with UTC. Institutions that generate standard time, such as NICT, instead operate their own atomic clocks to generate time as close to UTC as possible and then provide time in a form that allows the time difference with UTC to be determined later as required.

The time provided by NICT is called Japanese Standard Time. It is generated using hydrogen maser atomic clocks and cesium atomic clocks and is provided to society through various means, including standard radio waves and NTP, the internet time protocol.

However, NICT has now developed a strontium optical lattice clock that is expected to achieve even higher accuracy by utilizing optical transition frequencies. This clock significantly contributes to time keeping around the world, including by calibrating the length of single UTC seconds.

By adjusting the tick width of Japanese Standard Time with reference to an optical lattice clock, NICT succeeded in generating a more accurate time. The institute demonstrated that the time difference from Japan Standard Time to UTC can be reduced to less than a quarter of the conventional difference. This is the first time an optical lattice clock has been used for national standard time.

Japan Standard Time has been generated in a stable manner since 2006 by combining results from a hydrogen maser atomic clock and about 18 cesium atomic clocks. However, the oscillating frequency of these commercial atomic clocks in the microwave region fluctuates at the 15th digit even when averaged over so many devices.

As a result, the time difference from UTC widens to more than ten nanoseconds across a few months, requiring that the frequency of Japan Standard Time be adjusted by referring to the time difference data published more than half a month later from BIPM.

However, NICT's strontium optical lattice clock can accurately measure the degree to which the Japanese standard time is incorrect by 16 digits. NICT has been evaluating the validity of the tick width of standard time using this clock at least once a week since June 2021. It has also continuously adjusted the frequency of standard time once or twice a week since August of the same year and has shown it can suppress variations of standard time relative to UTC more than previous methods.

This optical clock has made remarkable progress. An international committee dealing with time and frequency standards is now considering a "redefinition of the second" by 2030, in which the definition of the second would be changed from the current transition of cesium atoms in the microwave region to the transition frequency in the optical region of atoms.

Maintaining the accuracy of standard time through an optical lattice clock is one of the desirable conditions required for this redefinition. NICT's achievement is the first demonstration that has met this requirement and will provide momentum for the redefinition of the second.