Next-generation optical atomic clocks could measure the gravitational distortion of space-time across the Earth’s surface more precisely than current methods, reports a paper published online this week in Nature. These clocks can be used to detect gravitational waves, test general relativity, and search for dark matter.

The passage of time is not absolute; it depends on the given frame of reference. As a result, clock measurements are sensitive to relative velocity, acceleration, and gravity potential - clocks atop mountains tick faster than those at ground level because of increased gravity potential. A common reference surface is required to compare clocks at different points in a gravity field. On Earth, this is the geoid - the surface of equal potential that best fits the global-mean sea level - which is currently determined from height measurements by the Global Navigation Satellite System and a geoid model to factor in gravity. Both are currently limited by uncertainties of several centimetres, which could be reduced by using atomic clocks.

Atomic clocks are based on measurements of specific atomic transitions at optical frequencies. The next generation of atomic clocks will be so sensitive to the relativistic effects of gravity that they could be used as geopotential probes.

William McGrew and colleagues characterize two ytterbium optical lattice clocks according to three fundamental benchmarks. They report, in units of the clock frequency, a systematic uncertainty of 1.4 × 10^-18, a measurement instability of 3.2 × 10^-19 and, through repeated local frequency comparisons, a reproducibility yielding a frequency difference between the clocks of the order of 10-19. Such performance would allow geoid determinations with less than one centimetre uncertainty, outperforming current techniques.