The ticking of thorium nuclear optical clocks

In a perspective article recurrently published in the National Science Review, Dr Xin Tong (Innovation Academy for Precision Measurement Science and Technology, Chinese Academy of Sciences) and his colleagues delve into the exciting prospects and challenges surrounding the development of the thorium-229 (²²⁹Th) nuclear optical clock.

Time and frequency are the most precisely measurable physical quantities. Currently, atomic clocks, especially atomic optical clocks, set the standard for accuracy. However, the ²²⁹Th nuclear optical clock could potentially outperform them all.

²²⁹Th is unique among all known nuclides as it allows for precise laser manipulation of nuclear quantum states. This has led to the concept of the thorium nuclear optical clock. Its superiority lies in the fact that the nucleus is much smaller than an atom, making it less vulnerable to external disturbances. Also, nuclear quantum states are well - separated, and extranuclear electrons shield against external electromagnetic fields. All these factors suggest it could achieve a very high-precision time and frequency standard.

The research journey began around half a century ago. Scientists first identified the low-lying excited nuclear state of ²²⁹Th, which laid the groundwork for further studies. Since then, there have been significant milestones. In 2024, direct laser excitation of the ²²⁹Th nuclear transition was achieved. Different research teams, such as those from Technische Universität (TU) Wien, University of California at Los Angeles (UCLA), and Joint Institute for Laboratory Astrophysics (JILA) have conducted experiments using various materials like doped crystals and thin films. These experiments have gradually improved the understanding and measurement capabilities related to the nuclear transitions.

Despite these remarkable achievements, numerous challenges remain. Nuclear transitions in solid - state environments are highly sensitive to temperature-related changes. The scarcity of ²²⁹Th isotope, the difficulty in developing a specific high-power, narrow-linewidth laser, the incomplete understanding of interaction mechanisms, and the lack of closed-loop manipulation are all major obstacles.

Nevertheless, overcoming these challenges is essential. The successful realization of the thorium nuclear clock would revolutionize timekeeping and open new frontiers in fundamental physics research. It could lead to a paradigm shift in optical clock systems, from relying on electronic to nuclear transitions, and provide deeper insights into the fundamental laws of the universe.

---

---