One news item, that nobody reported on last year is, that we now have three optical transitions measured to just 1e-16 (88Sr+), 1.8e-16 (87Sr) and 1.9e-16 (171Yb).
This means, that we now have three atom species that can be used for optical clocks, whose uncertainty is below what the realization of the second was, just a few years ago.
https://doi.org/10.1088/1681-7575/ade4d1
https://doi.org/10.1088/1681-7575/abc232
https://doi.org/10.1088/1681-7575/adb754
To explain why this is exciting to me: We have now optical clocks that have uncertainties in their realization in the order of 1e-17 to 1e-18, while the current definition only gives us 1e-16. I.e. our best clocks are limited in their performance by the definition of the second and we would have better clocks if we would use a different definition.
Measuring the frequencies of these optical clocks to high accuracy against the current definition is an important step towards this redefinition.
Attila Kinali