Frequency ratio of Yb and Sr clocks with 5 x 10-17 uncertainty at 150 seconds averaging time

Transition frequencies of atoms and ions are among the most accurately accessible quantities in nature, playing important roles in pushing the frontiers of science by testing fundamental laws of physics, in addition to a wide range of applications such as satellite navigation systems. Atomic clocks based on optical transitions approach uncertainties of 10(-18) (refs 1-3), where full frequency descriptions are far beyond the reach of the SI second. Direct measurements of the frequency ratios of such super clocks, on the other hand, are not subject to this limitation(4-8). They can verify consistency and overall accuracy for an ensemble of super clocks, an essential step towards a redefinition of the second(9). Here we report a measurement that finds the frequency ratio of neutral ytterbium and strontium clocks to be R = 1.207507039343337749(55), with a fractional uncertainty of 4.6 x 10(-17) and a measurement instability as low as 4 x 10(-16) (tau/s)(-1/2).