An optical clock generates a frequency reference in the form of light stabilized to an atomic transition frequency in the optical frequency range. The two types of optical clocks with the highest performance are optical lattice clocks
that confine neutral atoms in the standing wave field of intense laser beams, and ion clocks that typically confine a single ion in an electric field. Within the field of time and frequency metrology, these clocks are most accurately described as “optical frequency standards”, because they only generate a reference frequency and do not track or display the progression of time.
Conceptual diagram of atoms trapped in the “lattice” of an optical lattice clock
Optical lattice clock
The optical lattice clock was first proposed in 2001 by KATORI Hidetoshi, then an associate professor at the University of Tokyo’s Graduate School of Engineering. Today, optical lattice clocks developed at six institutions across the world, including NICT, are internationally recognized as Secondary Frequency Standards and deemed capable of calibrating the duration of the one-second interval of Coordinated Universal Time (UTC)
. NICT was the second institution to obtain this recognition for its optical lattice clock, following the Paris Observatory, and began contributing calibration data to BIPM in December 2018. Since August 2021, NICT has submitted a calibration for each month, an important contribution to maintaining the accuracy of UTC.
Coordinated Universal Time (UTC)
UTC is the internationally accepted global standard time, and all countries including Japan synchronize their national standard time with it. It is calculated by the International Bureau of Weights and Measures (BIPM)
by taking the average of more than 400 atomic clocks operated at metrology and astronomy institutes around the world, and then calibrating the frequency of this average by the measurements of cesium Primary Frequency Standards and Secondary Frequency Standards based on other atoms. A leap second adjustment is added to the result. The data for each month is published around the 10th day of the following month in the form of a time difference between UTC and the standard time generated by each institute, listed for the exact moment of UTC midnight (9:00 JST) every five days.
Redefinition of the SI second
In the International System of Units (SI), one second is defined by setting the frequency of a specific microwave transition of the cesium atom to be exactly 9 192 631 770 oscillations per second (Hz). Recent developments make it possible to probe transitions in the optical range with much greater accuracy, but translating this into an increased accuracy of the duration of the SI second requires a change in its definition. For this it is essential that the optical clocks implementing the new definition demonstrate their capability to maintain the one-second interval of UTC, the reference time of international society, with the same reliability as the current cesium clocks. A similarly important criterion is that institutions generating national standard time should be able to confirm its accuracy by operating their own optical clocks. The supreme decision-making body for the definition of units is the international General Conference on Weights and Measures (CGPM), which meets once in approximately every four years. Its next meeting in the fall of 2022 will determine further steps towards a redefinition of the second, and the redefinition itself is now widely expected for the meeting planned in 2030.
International Bureau of Weights and Measures (BIPM)
BIPM is a permanent institution created by the countries that signed the Metre Convention of 1875, the international treaty that forms the basis for international agreement on units of measurement. It carries out work on metrological standards as directed by the General Conference of Weights and Measures (CGPM), which consists of representatives of the signatory states, and under the supervision of the International Committee of Weights and Measures (CIPM). Located in Sèvres, a suburb of Paris, it plays a major role in the field of time and frequency by calculating UTC, which all nations refer to when determining their national standard time.
Hydrogen maser atomic clock
Cesium atomic clock
A clock that obtains a stable frequency by probing cesium atoms with a resonant microwave frequency. Although these clocks implement the definition of the second, there are technical limitations to their accuracy. Commercial devices are typically accurate to 13 digits.
Hydrogen maser atomic clock
A clock based on a transition of atomic hydrogen in the microwave region. It has a very low frequency noise for a microwave clock, but systematic frequency shifts that are difficult to control cause gradual frequency changes over long periods of time.
The Network Time Protocol (NTP) is used to distribute accurate time over the internet and other networks. NICT operates a dedicated server that responds to time queries using this protocol and is capable of handling over a million requests per second.
Calibration of the UTC one-second interval by an optical lattice clock
Optical frequency comb
A pulsed laser with a spectrum of comb-like lines equally spaced in the domain of optical frequencies. By comparing one of the optical comb lines to an optical signal and the pulse repetition rate to an electronic signal, the frequency comb performs precise measurements across the different frequency regions.
Consultative Committee for Time and Frequency (CCTF)
The CCTF is a subordinate body of the International Committee of Weights and Measures, created to handle technical issues related to time and frequency standards. It consists of members with appropriate research experience from metrology institutes worldwide. Japan is represented by members from both NICT and the National Institute of Advanced Industrial Science and Technology (AIST).
Executive Order of the President of the United States
Executive Order 13905, 2020/02/12:
“Strengthening National Resilience Through Responsible Use of Positioning, Navigation, and Timing Services”