In a pioneering study led by Chuankun Zhang and Jun Ye, scientists have directly measured the frequency ratio between the Thorium-229 nuclear clock transition and the 87Sr atomic clock using a vacuum ultraviolet (VUV) frequency comb. By embedding Thorium-229 in a CaF2 crystal, they achieved precision spectroscopy of the nuclear transition, marking the first comparison of nuclear and atomic clocks. This breakthrough could revolutionize timekeeping and metrology, offering new avenues for fundamental physics exploration.
Timekeeping has reached extraordinary levels of precision, with optical atomic clocks, such as those based on Strontium-87 (87Sr), achieving fractional frequency uncertainties better than 1 × 10^-20. While these clocks have been instrumental in studying quantum effects under gravity and testing fundamental physics, the future of timekeeping might lie in nuclear clocks. Thorium-229 (229Th) is a uniquely promising candidate for this next-generation timekeeping, thanks to its low-energy nuclear transition—an ideal feature for high-precision metrology.
In this landmark study published in Nature, Chuankun Zhang and colleagues took a major step toward realizing a nuclear clock by directly linking the Thorium-229 nuclear transition to the well-established 87Sr atomic clock. Using a vacuum ultraviolet (VUV) frequency comb, the team directly excited the 229Th transition embedded in a CaF2 crystal. This setup allowed for the precise measurement of the frequency ratio between the two clocks, which is essential for developing future nuclear-based timekeeping devices.
One of the most remarkable features of the 229Th nuclear transition is its extreme insensitivity to external electromagnetic fields, making it ideal for use in portable, highly stable clocks. The transition occurs at a uniquely low energy, just 8.4 electron volts (eV), making it several orders of magnitude lower than typical nuclear transitions. This low energy allows for longer coherence times and higher accuracy, which could surpass even the most advanced atomic clocks.
The researchers achieved their breakthrough by utilizing a technique called high-harmonic generation to upconvert the fundamental frequency of a stabilized infrared frequency comb to the VUV range, targeting the 229Th transition. This frequency comb, stabilized by the JILA 87Sr optical lattice clock, provided a direct link between nuclear and atomic timekeeping systems. The team also measured nuclear quadrupole splittings, further refining their understanding of the 229mTh isomer’s intrinsic properties.
This groundbreaking work could have far-reaching implications. The ability to compare nuclear and atomic clocks opens new possibilities for testing fundamental physics, including searches for ultralight dark matter and time variations in fundamental constants such as the fine-structure constant. In addition, nuclear clocks could enable portable, highly precise timekeeping for space exploration, communication networks, and global positioning systems.
Conclusion: By directly linking the 229Th nuclear transition to the 87Sr atomic clock, this research marks a major milestone in precision timekeeping. The future development of nuclear clocks based on Thorium-229 could push the boundaries of what is possible in metrology, with implications for fundamental physics, portable clocks, and space-based applications. This research is a significant step toward a new era of precision timekeeping.
Source: Frequency ratio of the 229m Th nuclear isomeric transition and the 87 Sr atomic clock