Experiment results demonstrate potential of compact, portable nuclear clocks

Scientists use atomic clocks to measure the “second,” the smallest standard unit of time, with great precision. These clocks use the natural oscillations of electrons in atoms, much in the same way that the pendulums in old grandfather clocks work. The quest for an even more precise timekeeper led to the discovery of nuclear clocks, which use the transitions of atomic nuclei instead of electrons to measure time.

An increasingly important contender for the development of ultra-precise nuclear optical clocks is the first excited nuclear state ²²⁹Isotope Th. Its long half-life of 10³ Its few seconds and low excitation energy of a few electron volts make it ideal for excitation by vacuum ultraviolet (VUV) lasers, providing a precise reference transition for nuclear clocks.

Nuclear clocks can also be used in compact solid-state metrology devices and in fundamental physics research. To explore the potential application of ²²⁹The isomer, it is essential to understand in detail its fundamental properties such as isomeric energy, half-life and excitation and decay dynamics.

Working in this direction, Assistant Professor Takahiro Hiraki of Okayama University, Japan, and his team, including Akihiro Yoshimi and Koji Yoshimura, have developed an experimental setup to efficiently assess the population of ²²⁹The isomeric state and detect its radiative decay.

In their study published in Nature Communications On July 16, 2024, they synthesized ²²⁹CaF transparent VUV doped with Th₂ crystals and have demonstrated their ability to control ²²⁹Population of the isomeric state using X-rays.

“Our group works on fundamental physics using atoms and lasers. To realize a solid-state nuclear clock using ²²⁹“It is necessary to control the excitation and deexcitation state of the nucleus. In this study, we succeeded in controlling the nuclear states using X-rays, which brings us one step closer to building a nuclear clock,” said Assistant Professor Hiraki while explaining the motivation for their study.

To study radiative decay (de-excitation), the team created an excitation from the ground state of ²²⁹The nucleus to an isomeric state, via the second excited state, using a resonant X-ray beam. They found that the doped ²²⁹The nucleus underwent radiative decay to its ground state, accompanied by the emission of a VUV photon.

One of the key findings was the rapid degradation of the isomeric state when exposed to X-ray irradiation and the “X-ray quenching” effect, which allowed the isomer to be depopulated on demand. The researchers believe that this controlled quenching could advance the development of the nuclear clock, as well as other potential applications, such as wearable gravity sensors and higher-precision GPS systems.

“When the nuclear clock under development is completed, it will allow us to test whether ‘physical constants’, especially fine-structure constants, which were previously thought to be unchanged, can vary over time,” said Assistant Professor Hiraki, emphasizing the potential of optical nuclear clocks. “If we observe a temporal variation of physical constants, this could lead to the elucidation of dark energy, one of the greatest mysteries of the universe.”