The paper reports on measurements of the transition frequency of a strontium-87 lattice clock with a record uncertainty of 1.5 x 10-16, by using two local caesium fountain clocks. This result is an important contribution to the decision processes concerning a redefinition of the SI base unit second based on optical clocks. With data spanning three years of observation, the authors set more stringent limits to the coupling of fundamental constants to the gravitational potential.
Here is a link to the full paper text:
https://journals.aps.org/prresearch/abstract/10.1103/PhysRevResearch.2.033242
