Measuring technology has long been contributing to the development of science and technology. The global standard units of measurement originally comprised meters for distance, kilograms for weight, and seconds for time, based on the metric system that was created after the French Revolution. Today, these measurements are determined by the General Conference on Weights and Measures (CGPM) as the International System of Units (SI). There used to be actual artifacts, known as “prototypes” that served as the standards for both the meter and the kilogram. The second was also derived from the duration of the earth‘s rotation. As science advanced, more precise measurements were sought. In 2018, SI units came to be based on “physical constants” and “physical quantities” replacing the earlier prototypes that could be prone to deviation. For example, one meter is now defined as the distance that light travels in　1/299,792,458 s. The foundations of precision measurements, such as distance and electrical current, are based on duration and frequencies that are inextricably linked to the standard of “one second.”

Time is determined by counting the number of recurrences of a periodic phenomenon. An easy-to-understand example is an antique pendulum clock that measures time using the periodic motion of a pendulum. If this periodic motion deviates, the time measurement also becomes inaccurate. Hence, more precise periodic phenomena were sought. A quartz clock uses the oscillation period of a quartz crystal when a voltage is applied to it and which can achieve a relative precision of ten digits (an error of one second per 100 years). One second is defined as the time taken for the resonance frequency of the cesium to reach 9,192,631,770 oscillations. Currently, the most precise cesium clock has a relative precision of 15.5 digits, which deviates by just one second every 60 million years.

The optical lattice clock, which received the Honda Prize this year, was an invention designed to measure time to a super-high level of precision. The optical lattice clock also uses the specific resonant frequency of an atom; however, the mechanism eliminates the influence from the Doppler effect by stabilizing the atom without changing its natural frequency of oscillation. This enabled us to achieve a precision of 18 digits, equivalent to deviating one second in 30 billion years. It is estimated to be 13.8 billion years since the birth of the universe—so you can imagine how accurate this is.

First, atoms are cooled to close to absolute zero temperature by applying laser cooling. Then the ultracold atoms are trapped in an optical lattice formed by lasers tuned to the so-called “magic wavelength.” Until the optical lattice clock appeared, the single-ion optical clock was the strongest candidate for the next generation clock, in which only one ion is trapped to count its oscillations. To aim at 18-digit precision using a single ion one needs to repeat one-second-long measurement for one million times, which requires 10 days. However, the optical lattice clock can trap a million atoms in its lattice, and measuring them at the same time significantly reduces the measuring period.

Utilizing the super-high precision of the optical lattice clock enabled us to measure the dilation of time as altered by gravity and movement, as described by Einstein's Theory of Relativity. With 18-digit precision, it is possible to observe the dilation of time measured by two clocks with only one centimeter difference in altitude. The time dilation effect (detecting the altitude difference) were measured at the TOKYO SKYTREE, and at the University of Tokyo and RIKEN. It is believed that the optical lattice clock will be a quick and precise geodesic tool in a relativistic spacetime approach. It is also expected to have applications in searching for underground resources and crustal movements. For such practical implementations, the next challenge is to downsize optical lattice clocks for portability and to network them.

The definition of “one second” to date has been based on the cesium atomic clock, but soon this will be replaced by the optical lattice clock. Comparing the measurement of time by optical lattice clocks that use different atoms over an extended period could lead to test the constancy of physical constants, including the fine-structure constant. If this is achieved, the optical lattice clock would also have made a fundamental contribution to physics.