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

The papers represent a significant breakthrough in a very new field, given that it has only been five years since the idea of a nuclear clock began to take shape. Both papers are clearly of a very high standard.

A very important point is that the project carried out by the Beijing team has, in a very short space of time, caught up with the pioneering teams in Braunschweig, Vienna and Boulder.

Although the accuracy of the nuclear clock does not yet match that of the best optical atomic clocks, its sensitivity to certain hypothetical forms of dark matter is almost ten thousand times greater than that of the latter. In general, the clock based on the thorium-229 (Th-229) nucleus is a unique case in physics: it is like placing a different oil tanker on each arm of a set of scales, and finding that their weights coincide to within one kilogram. Such is the balance of energies that determines the frequency of the nuclear clock in this truly unique and fascinating case. The ‘kilogram’ in this analogy can already be measured using the nuclear clock to an accuracy of thirteen or fourteen decimal places. The consequence is that, should a new particle or interaction emerge, or should there be temporal fluctuations in the physics we know, this balance would be disrupted sufficiently for us to be able to measure it. The new clock will therefore enable us to explore these frontiers of known physics in greater depth. The Vienna-Braunschweig collaboration is already carrying out an initial analysis of this possibility. This has enabled them to rule out almost three orders of magnitude in the possible range of interactions due to scalar bosons, which are hypothetical particles postulated as dark matter.

The experiments are compact. Although they rely on advanced laser systems, these require far fewer research resources than the large accelerator and detector facilities also used for these purposes. Of course, the nuclear clock does not cover all the types of dark matter that have been hypothesised, but neither do its ‘bigger brothers’ at high-energy centres.

By achieving greater precision than previous experiments, the new results confirm and expand upon what has been known for the past few years and demonstrate that the nuclear clock is now capable of ruling out the range of plausibility for certain hypotheses regarding the nature of dark matter. High-precision experiments such as these make it possible to test various hypotheses that would lead to deviations from what has been observed.

[As for possible limitations] It is logical that, in such a short space of time, certain aspects of the experimental set-ups have not yet been studied. These are the first nuclear clocks in the sense accepted by the very exclusive community of ‘atomic clockmakers’, who specialise in comparing their results internationally to the eighteenth decimal place. For example, the temperature stability of the calcium fluoride crystals containing the thorium-229 isotope can be technically improved. The purity and transparency of the crystals will also need to be improved. But the results demonstrated by both groups are already of a remarkable standard. In a few years’ time, we will see successive advances in this fascinating field: weighing up the balance between two fundamental interactions—the strong and the electroweak—to study or rule out the presence of possible factors hitherto unknown.