Reacción a "The first thorium-229-based nuclear clocks are unveiled"

The press release clearly outlines the significance of these papers: they describe the first nuclear clock—an optical clock based on a nuclear transition. To me, the most extraordinary aspect is the ability to control a nuclear transition using a laser and then use that transition to stabilize the laser itself; this has led to significant technical implications and developments.

The papers are of excellent quality and achieve a goal first proposed in 2003: using the transition between an excited state of the 229Th nucleus and its ground state to keep time—much like atomic and optical clocks do, though those rely on an atom's outermost electron. This clock, however, would measure the nuclear transition and count seconds based on the frequency (defined as the number of oscillations per second). Because it relies on a nucleus—which is more isolated than an atomic electron—it would be more robust against external disturbances. This is a unique case, as nuclear transitions typically involve energies far too high to be driven by a laser.

Driving this nuclear transition—which involves an unusually low energy level for a nucleus—required a laser emitting at a wavelength of around 148 nm, a capability that did not exist until recently. Consequently, the technology had to be developed over several years, with different research groups proposing various approaches. Measuring the transition energy has taken decades; two years ago, a measurement was achieved by embedding 229Th into a CaF2 crystal during an experiment at ISOLDE/CERN. The key difference now is that the transition is not merely observed—detecting its fluorescence was a major achievement in itself—but the signal from the crystal is actually used to correct the laser radiation being measured, thereby enabling the conversion of frequency into a time standard. There is still room for improvement in achieving greater stability and reproducibility; however, given that this involves a nucleus and a low-energy excited state, the precision achieved could offer enhanced sensitivity for studying, for instance, the origin and properties of dark matter.

[Regarding potential limitations] As I understand it, the clock's stability—which is the factor that can cause frequency measurements to vary or fluctuate over time—can currently be improved.