The last few years have seen a revolution in ultra-accurate measurements of optical atomic transitions using highly coherent laser light. By gently holding a single atomic ion using an electro-dynamic trapping field, we can approach as close as possible the ideal situation of an isolated and unperturbed quantum system. Our team has realized an optical frequency reference at 445 THz (674 nm) based on a single atomic ion of strontium. In this talk, we will describe our results that include the resolution of spectral features at the 4 Hz level (1 part in 10 14 ) together with the evaluated accuracies of this system that exceed by an order of magnitude the best current realizations of the SI second. At this level of accuracy, it is possible to measure the distortion of local time due to Earth’s gravitational field by changes of the clock height at the sub-meter level. Some comments will be made as to what we expect these improvements to yield in terms of sensitive tests of relativity and the ultimate limits of measuring time in the laboratory.