Currently, the second is defined by a microwave frequency (about 9.2 GHz) that resonates cesium atoms and is realized by cesium atomic fountain clocks with an accuracy of 16 digits. Optical lattice clocks using light (approximately 500 THz), which has a higher frequency than microwaves, can further improve the accuracy of time by one to two orders of magnitude, and are considered as a candidate for redefining the second. If the accuracy of atomic clocks improves to this level, there is a possibility that changes in the fundamental physical constants can be detected. Fundamental physical constants are considered to be constant and invariant, and even if they change, the change is extremely small and will not cause any inconvenience to daily life such as clock malfunctions. On the other hand, changes in the fundamental physical constants are an interesting subject of research in physics, for example, there are theoretical suggestions that changes might be caused by dark matter.

It has been suggested that dark matter, the true nature of which is unknown, exists in the universe with a mass approximately five times greater than that of ordinary matter such as atoms. The study of the nature of dark matter has been an interesting subject of research in physics, and dark matter candidates have been proposed in a wide range of masses, and search experiments have been conducted using various instruments such as telescopes, artificial satellites, particle detectors, and laser interferometers. The ultralight dark matter focused on in this study is one of the candidates that have been vigorously studied from the viewpoint of the structure formation of the universe. In quantum mechanics, electrons, protons, neutrons, and photons are considered to be both particles and waves. In ultralight dark matter, the wave nature is more pronounced than the particle nature. If ultralight dark matter interacts with ordinary matter, the fundamental physical constants, such as the fine structure constant and the electron mass, are expected to oscillate periodically.

Atomic clocks have recently attracted much attention in dark matter research because of their ability to detect periodic variations in these fundamental physical constants with high sensitivity. The use of the highest precision atomic clocks, such as optical lattice clocks and cesium fountain atomic clocks, is useful for increasing the detection sensitivity of ultralight dark matter. Periodic oscillations of the fine structure constants have been searched for from the frequency ratio of two optical lattice clocks, but there has been no report of a search combining an optical lattice clock and a cesium fountain atomic clock. This combination is sensitive to periodic variations in electron mass that are hard to detect by a search using only an optical lattice clock. Since the Cs fountain atomic clock is noisier than the optical lattice clock, it is important to run both clocks simultaneously at high availability for a long period of time (e.g., 10 days or more) to take advantage of this combination. However, optical lattice clocks are very complex devices, making long-term operation difficult for many institutions.

Researchers at AIST, in collaboration with Yokohama National University, used two high-precision atomic clocks, an ytterbium optical lattice clock and a cesium fountain atomic clock, to search for dark matter, which exists in large amounts in the universe but whose true nature remains unknown.

AIST contributes to International Atomic Time, the international standard time, by operating the cesium fountain atomic clock, which realizes the definition of the unit of time "second," and the ytterbium optical lattice clock, one of the candidates for the redefinition of the second, at high operating rates for a long time. The frequency of an atomic clock is determined by fundamental physical constants such as the fine structure constant and electron mass, and the constant and invariant nature of the fundamental physical constants guarantees the accuracy of the atomic clock. On the other hand, it can be said that an atomic clock is an experimental device to verify whether the fundamental physical constants are truly constant and invariant.

Recently, ultra-light dark matter, which is more than 20 orders of magnitude lighter than the electron mass (about 9 × 10^-31 kg), has been proposed as a candidate for dark matter. This very light dark matter behaves as a wave, not a particle. If dark matter waves interact with ordinary matter such as atoms, it is theoretically predicted that the fundamental physical constants will oscillate periodically, which in turn will cause periodic oscillations in the frequency of atomic clocks. In this study, we searched for such periodic oscillations in the frequency ratio data of the ytterbium optical lattice clock and the cesium fountain atomic clock. The results show that there is no such interaction between ultra-light dark matter in the mass range 10^-58 kg to 10^-56 kg and electrons, or if such an interaction exists, its strength is very weak. This result contributes to fundamental physics aimed at understanding dark matter.

Details of this study were published in Physical Review Letters on December 7, 2022.

Journal: Physical Review Letters
Title: Search for Ultralight Dark Matter from Long-Term Frequency Comparisons of Optical and Microwave Atomic Clocks
Authors: Takumi Kobayashi, Akifumi Takamizawa, Daisuke Akamatsu, Akio Kawasaki, Akiko Nishiyama, Kazumoto Hosaka, Yusuke Hisai, Masato Wada, Hajime Inaba, Takehiko Tanabe, Masami Yasuda
DOI: 10.1103/PhysRevLett.129.241301