Figure 2

Progress in the relative accuracy of atomic clocks [27, 28, 49, 52–80]. Cs microwave clocks have steadily improved since the emergence of laser-cooled fountain clocks in the early 1990s [81–83], but two distinct types of optical clock currently compete at a fractional frequency uncertainty of approximately \(1\times 10^{-18}\): trapped-ion clocks (Yb⁺ [27], Al⁺ [49], Hg⁺ [69], Sr⁺ [71], Ca⁺ [84]) and optical lattice clocks (Sr [79], Yb [78], Hg [77], Cd [85])