Optical atomic clock measurements at the mHz level

Time (or its inverse, frequency) is the most precisely measured physical quantity and therefore is exploited in many fundamental investigations of nature. The advent of atomic clocks based on optical transitions has led to 10-100 times improvement in our timekeeping capabilities, with some measurements of optical frequencies reaching the mHz level [1,2]. Timekeeping precision at 1 part in 10¹⁸ enables new timing applications in relativistic geodesy, enhanced Earth- and space-based navigation and telescopy, and new tests of physics beyond the standard model. Here, we describe the development of two optical lattice clocks, both using spin-polarized, ultracold atomic ytterbium, and we discuss their operation at 10^-18 instability as well as measurements toward the goal of 10^-18 uncertainty.