Sr + single ion optical clock

The development of modern optical atomic clocks was made possible by a number of important scientific milestones. Of particular importance are laser cooling, atom trapping, quantum state manipulation, and laser frequency stabilization. The development of self-referenced frequency combs based on femto-second lasers was another milestone, one that now provides a convenient method of comparing an optical frequency standard with microwave clocks that define the SI second, and with optical standards operated using different atomic systems. These advances have in turn motivated further developments in optical clocks, especially towards a better understanding and control of the systematic shifts. The current best optical clocks have evaluated uncertainties that are from one to two orders of magnitude lower than state-of-the-art cesium fountain clocks [1, 2].