The National Institute of Advanced Industrial Science and Technology (AIST) and Yokohama National University (Yokohama National University) jointly announced on November 3 that they have achieved the world’s first long-term high operating rate operation of “optical lattice clocks”.

The results were produced by Takumi Kobayashi, Senior Researcher, Time Standards Research Group, Physical Measurement Standards Research Division, AIST, Daisuke Akamatsu, Senior Researcher, Kazumoto Hosaka, Deputy Research Director, Masami Yasuda, Research Group Leader, AIST. Hajime Inaba, Research Group Leader, Optical Frequency Measurement Research Group, Optical Frequency Measurement Research Group, Research Institute for Metrology Standards, Research Institute, Masato Wada, Senior Researcher, Tomoya Suzuyama, Research Center for Measurement Standards, AIST, Professor Hongfeng Rai, Yokohama National University, By a joint research team of Dr. Yusuke Kui and others. Details were published in the academic journal “Metrologia”.

As Japan’s national metrological standard organization, AIST Metrological Standards Center develops various “measurement standards” (measurement standards). There was a major global change in its metrological standards in May 2019. The four definitions of the International System of Units (SI), which are the units of mass, current, thermodynamic temperature, and amount of substance, kilogram (kg), ampere (A), Kelvin (K), and mole (mol), are simultaneously defined. It was revised.

With this revision of the definition, the unit of time, seconds (s), has become the unit required to realize other SI basic units (m, kg, A, K, kd), and the time standard has become a particularly important standard. It is said that it became.

Seconds are currently defined using frequencies in the microwave region that resonate with cesium atoms (approximately 9.2 GHz). Using light with a frequency 4 to 5 orders of magnitude higher than this microwave allows for further subdivision of 1 second, improving the accuracy of the clock. For this reason, research on atomic clocks (optical clocks) that use light, such as optical lattice clocks, has been promoted around the world.

So far, eight types of optical clocks, including optical lattice clocks, have been recommended as secondary representations of seconds (candidates for the definition of new seconds) at the Meter Convention-related conference held at the International Bureau of Weights and Measures headquartered in France. However, it has not yet been concluded which watch is most suitable.

Nevertheless, the time is steadily approaching when switching from atomic clocks to optical clocks, and at the recent Meter Convention-related conference, conditions such as required accuracy for redefining seconds were set concretely. One of the conditions was to improve the accuracy of the International Atomic Time by the contribution of optical clocks. It is required to continuously operate high-precision clocks and supply them to society as a time standard.

However, in order to accurately control the state of the reference atom, it is very difficult for an optical clock that uses multiple extremely high-precision lasers to operate continuously for a long period of time. At present, the occupancy rate was only about 80% in 25 days.

Against this background, AIST succeeded in developing the ytterbium optical lattice clock in 2009, ahead of the rest of the world. In the first place, the optical lattice clock is a technology originating in Japan that was proposed by Associate Professor Hidetoshi Katori (at that time) of the Graduate School of Engineering, the University of Tokyo in 2001.

By capturing a large number of atoms in space with laser light, it is possible to measure the frequencies of those atoms at the same time, so it is possible to measure the accurate time based on the frequencies of the atoms. 18 digits (100K) for a cesium atomic clock (cesium atomic fountain primary frequency standard) that realizes the current definition of 1 second with an accuracy of 15 to 16 digits (1000 trillion to 1/1000). 1) Improvement to the number of units has been demonstrated.

The key to the stable operation of optical lattice clocks is the development of the world’s highest level “optical frequency comb” with extremely low frequency noise. An optical frequency comb is a wide-band, comb-like (comb) spectrum of light output from an ultrashort pulsed laser called a “mode-locked laser.” Since the teeth of each comb are lined up at regular frequency intervals, they can be used to measure the frequency of light as a measure of frequency.

The optical frequency comb operates stably for a long period of time and can be applied to control the laser frequency with high accuracy. In 2018, in order to solve the instability of the laser frequency, which had been a problem so far, the ytterbium optical lattice clock was developed, which adopted a method of stabilizing the laser by applying an optical frequency comb. And it has succeeded in driving for more than 60 hours in a few months.

There are multiple lasers used in ytterbium optical lattice clocks. Lasers for ittelbium atomic deceleration with a wavelength of 399 nm, lasers for atomic cooling (399 nm, 556 nm), lasers for optical lattices (759 nm), clock lasers (578 nm), etc. In order for the optical lattice clock to continue to operate properly, it is essential to keep controlling the frequencies and powers of all these lasers with extremely high precision.

The ytterbium optical lattice clock incorporates a number of feedback controls such as frequency lock, but the problem is that the conventional control method is controlled by external factors such as slight changes in temperature and atmospheric pressure, vibration and acoustic noise. It was sometimes interrupted. This is the reason why the continuous operation time of the optical lattice clock has been shortened. Therefore, when operating the optical lattice clock, it was necessary to have a person in the laboratory who could handle the restart of the interrupted device.

Therefore, in this development, in order to enable unmanned continuous operation, the frequency auto-relock function of each laser was developed and introduced into the control system. With this function, even if the frequency lock of each laser is interrupted for some reason, the abnormality is detected instantly. Since the frequency can be automatically returned to the original frequency, the operation of the optical lattice clock can be continued, and as a result, unmanned operation is realized.

  • Optical lattice clock

    Conceptual diagram of the frequency auto-relock function of the ytterbium optical lattice clock developed this time. The frequency auto-relock function eliminates the interruption of laser frequency control and realizes unmanned operation. In addition, if a problem occurs in the operation of the optical lattice clock, the person in charge is automatically notified of the abnormality by e-mail. The system allows remote monitoring of the status of the optical lattice clock from outside the laboratory and partial remote control (Source: AIST website).

The operation of the improved ytterbium optical lattice clock that introduces these functions will start in October 2019. The occupancy rate for the six months (185 days) until March 2020 was 80.3%. Looking at the operating status in about one month, it is said that in some months the operating rate exceeded 90% and high operating rate operation was realized, and it was also successful to observe the fluctuation of the time frequency national standard frequency in real time. ..

  • Optical lattice clock

    An example of high operating rate operation data for a ytterbium optical lattice clock (measured for 25 days). The vertical axis of the graph above shows the difference (6.8 seconds average) between the relative frequency of the ytterbium optical lattice clock and the relative frequency of the time frequency national standard UTC (NMIJ). The part that looks like a white streak is the missing part of the data. The graph below is daily (Source: AIST website)

The 185 days set a new record, and it can be said that it is the most stable optical clock in the world at the moment. In addition, the realization of high operating rate operation has greatly improved the accuracy of long-distance comparison of clocks performed via artificial satellites, and realized monitoring of the International Atomic Time with 16-digit accuracy.

The unmanned operation of the optical lattice clock has been realized by this development, and the optical lattice clock has a high operating rate, similar to the cesium atomic fountain primary frequency standard, which is currently contributing significantly to the operation of the International Atomic Time. It is said that it was shown that it can continue to contribute to the International Atomic Time. In the future, it is expected that discussions on redefining seconds will be further accelerated at conferences related to the Meter Convention.

In addition, since time and frequency can be measured most accurately among all measured quantities, the clock frequency can be changed to a clock frequency due to a very small environmental disturbance (electromagnetic field, gravitational field) by improving the accuracy of the clock and improving the comparison technology of the clock. The effect can be observed. By using an accurate time frequency source that can operate at a high operating rate for a long period of time to verify the homeostasis of basic physical constants and the theory of relativity, it is expected to contribute to the development of basic science.