William F. McGrew, Xiaogang Zhang, Robert J. Fasano, Stefan A. Schaeffer, Kyle P. Beloy, Daniele Nicolodi, Roger C. Brown, N. Hinkley, G. Milani, Marco Schioppo, T. H. Yoon, Andrew D. Ludlow
The passage of time is tracked by counting oscillations of a suitable frequency reference (e.g., the number of revolutions of Earth around the sun or the number of swings of a pendulum of a grandfather clock). By referencing the oscillations arising from electronic transitions in atoms, frequency (and thus time) is the physical parameter that can be realized with the greatest precision, with the current generation of optical clocks reporting fractional performance below the 10-17 level. However, the theory of general relativity prescribes that the passage of time is not absolute, but impacted by an observer's reference frame. As a result, clocks exhibit sensitivity to velocity, acceleration, and gravity. This suggests that very precise clocks could, for example, serve as detectors of geopotential. Clock comparisons at the 1x10-18 level could provide 1 cm resolution, vastly outperforming state-of-the-art geodetic techniques. In two independent ytterbium optical lattice clocks, we demonstrate unprecedented levels in three fundamental benchmarks of clock performance. In units of the clock frequency, we report systematic uncertainty of 1.4x10-18, measurement instability of 3.2x10-19 and reproducibility characterized by ten blinded frequency comparisons, yielding a frequency difference of [-7±(5)stat±(8)sys]x10^-19^. If these clock comparisons were carried out over a long baseline across Earth's surface, they would be chiefly limited by the ability to map the relativistic distortion of space-time. While sensitivity to gravitational potentials degrades the performance of optical clocks as terrestrial standards of time, this same sensitivity can be used as an exquisite probe to explore geophysical phenomena, detect gravitational waves, test general relativity and search for dark matter.