With the advent of optical clocks, a worldwide exploration of ultraprecise and accurate frequency ratio measurements has now begun in earnest. Eight optical transitions in atoms and ions are currently recognized as secondary standards for the SI second, with numerous other candidates under development or proposed. Many of these have demonstrated systematic uncertainty and stability beyond the primary Cesium standards. How do we confirm the performance of these optical standards? How will we redefine the second with these systems all improving
rapidly? How do we compare clocks here to other clocks around the world? The most immediate foothold we have to start answering these questions is the measurement of frequency ratios between optical clocks. These measurements will be critical in the eventual redefinition of the SI second. They also allow us to realize applications of optical clocks including relativistic geodesy and tests of fundamental physics. I will focus on a series of measurements taken at NIST/JILA over the last
15 years between optical clocks of several different atomic species and consider what the next steps in this campaign may entail.