The subject itself is fascinating: to improve the definition of the second using much more precise and reproducible references than the current one, i.e., the cesium microwave transition that we use in all countries. There are hundreds of these classical cesium atomic clocks, and their stability reaches the sixteenth decimal place, but it is practically impossible to improve it further because of the natural limitation of microwaves: nine billion oscillations per second.

In order to get the sixteen decimal places, one of these oscillations must be ‘divided’ with a millionth of accuracy, so to speak. On the other hand, optical clocks oscillate about one hundred thousand times faster than cesium clocks. Thus, it is enough to divide a single oscillation by a thousand, and the reference is already improved by a factor of a hundred over the current one. Of course, for this to work well, the laboratories have to work in an extraordinarily accurate and reproducible way, since the atoms and their optical transitions can be affected by indefinite perturbations if care is not taken. To give an example: the height at which the clock is installed has to be known and can be compared with other clocks in other countries with a centimeter of accuracy in relation to sea level (geodetic), as this height influences the frequency of oscillations due to Einstein's theory of relativity.

The work presents the most comprehensive international comparison and shows that in a few years the technical problems will be solved: all the optical clocks of the various institutes in the different countries must be stable, comparable and reproducible to eighteen decimal places. When this happens, an international agreement will be reached for all to use a new system of optical frequency references, with more variety of atoms and ions on the one hand but also more intercomparability and safety reserve in accuracy.

The clocks must all keep pace, run continuously at each site, and be perfectly synchronized. Although the present work has seen some disparities that could be improved, the fact that several clocks at different sites have managed to demonstrate seventeen and even eighteen decimal places of accuracy tells us that we are well on the way to that goal, and that if things are done perfectly atoms and ions serve to mark time more accurately than they do today.

The technical and scientific advantages will be very great, and it is already possible to glimpse how the quantum optics techniques used in these clocks will also benefit quantum computing, communication and cryptography. For science, measuring time with such precision is becoming a new way of discovering whether physics as we know it now is incomplete: for example, whether there are particles that are only hypothetical at the moment and that no particle accelerator fundable by the largest empire will be able to synthesize in the next five decades, or whether dark matter oscillates in time or space....

In short: the precision of the new optical clocks is being analyzed in overwhelming depth and detail, but that's what it takes to make the redefinition of the second a hundred times better than it is today. Scientists working in this field have to eliminate the smallest inaccuracies in their clocks for us all to enjoy the immense advantages this will bring. And when it comes to eighteen decimal places, no excuses can be used to get out of the way: the figures are unrelenting, and their comparisons are beyond dispute. This work demonstrates that time measurement is in the hands of experts who have developed techniques of unparalleled accuracy in the world of science.

In 2030, the General Conference on Weights and Measures is expected to approve a new definition of the second based on optical frequencies, which will achieve unprecedented measurement accuracy thanks to the development of optical clocks. This new definition will enable significant improvements in satellite navigation, power grids, computing and communications, and financial transactions. It will also influence improvements in the performance of the other units of the International System and will lay the groundwork for advances in fundamental fields of physics.

The research presented in this article is essential because for the first time a large-scale comparison of optical clocks has been carried out, it has demonstrated the technical feasibility of operating an international network of high-precision optical clocks, it has allowed cross-validations between clocks, making it possible to detect and correct systematic errors, and it has made it possible to verify the measurement uncertainties involved. This comparison not only improves confidence in the accuracy of optical clocks and their interconnections, but is also a key element in the transition to a new definition of the second, based on more accurate optical standards than the current ones based on the cesium atom.