Optical clock comparison for Lorentz symmetry testing

Questioning basic assumptions about the structure of space and time has greatly enhanced our understanding of nature. State-of-the-art atomic clocks 1 – 3 make it possible to precisely test fundamental symmetry properties of spacetime and search for physics beyond the standard model at low energies of just a few electronvolts 4 . Modern tests of Einstein’s theory of relativity try to measure so-far-undetected violations of Lorentz symmetry 5 ; accurately comparing the frequencies of optical clocks is a promising route to further improving such tests 6 . Here we experimentally demonstrate agreement between two single-ion optical clocks at the 10 −18 level, directly validating their uncertainty budgets, over a six-month comparison period. The ytterbium ions of the two clocks are confined in separate ion traps with quantization axes aligned along non-parallel directions. Hypothetical Lorentz symmetry violations 5 – 7 would lead to periodic modulations of the frequency offset as the Earth rotates and orbits the Sun. From the absence of such modulations at the 10 −19 level we deduce stringent limits of the order of 10 −21 on Lorentz symmetry violation parameters for electrons, improving previous limits 8 – 10 by two orders of magnitude. Such levels of precision will be essential for low-energy tests of future quantum gravity theories describing dynamics at the Planck scale 4 , which are expected to predict the magnitude of residual symmetry violations. Agreement between two single-ion clocks is demonstrated experimentally at the 10 −18 level over a six-month period, confirming a key postulate of Einstein’s theory of relativity with hundredfold-improved precision.