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Frequency Ratio Measurements with 18-Digit Accuracy Using a Network of Optical Clocks



Kyle Beloy, Martha I. Bodine, Tobias B. Bothwell, Samuel M. Brewer, Sarah L. Bromley, Jwo-Sy Chen, Jean-Daniel Deschenes, Scott Diddams, Robert J. Fasano, Tara Fortier, Youssef Hassan, David Hume, Dhruv Kedar, Colin J. Kennedy, Isaac Kader, Amanda Koepke, David Leibrandt, Holly Leopardi, Andrew Ludlow, Will McGrew, William Milner, Daniele Nicolodi, Eric Oelker, Tom Parker, John M. Robinson, Stefania Romisch, Stefan A. Schaeffer, Jeffrey Sherman, Laura Sinclair, Lindsay I. Sonderhouse, William C. Swann, Jian Yao, Jun Ye, Xiaogang Zhang


Atomic clocks occupy a unique position in measurement science, exhibiting higher accuracy than any other measurement standard and underpinning six out of seven base units in the SI system. By exploiting higher resonance frequencies, optical atomic clocks now achieve greater stability and lower frequency uncertainty than existing primary standards. Here, we report frequency ratios of the 27-Al+, 171-Yb and 87-Sr optical clocks in Boulder, Colorado, measured across an optical network spanned by both fiber and free-space links. These ratios of remotely located clocks exhibit measurement uncertainties between 6x10^-18 and 8x10^-18, making them the most accurate reported measurements of frequency ratios to date. This represents a critical step towards redefinition of the SI second and future applications such as relativistic geodesy and tests of fundamental physics.


atomic clock, frequency comb, frequency metrology, frequency transfer, optical lattice, optical network, trapped ion


Beloy, K. , Bodine, M. , Bothwell, T. , Brewer, S. , Bromley, S. , Chen, J. , Deschenes, J. , Diddams, S. , Fasano, R. , Fortier, T. , Hassan, Y. , Hume, D. , Kedar, D. , Kennedy, C. , Kader, I. , Koepke, A. , Leibrandt, D. , Leopardi, H. , Ludlow, A. , McGrew, W. , Milner, W. , Nicolodi, D. , Oelker, E. , Parker, T. , Robinson, J. , Romisch, S. , Schaeffer, S. , Sherman, J. , Sinclair, L. , Sonderhouse, L. , Swann, W. , Yao, J. , Ye, J. and Zhang, X. (2021), Frequency Ratio Measurements with 18-Digit Accuracy Using a Network of Optical Clocks, Nature, [online],, (Accessed May 30, 2024)


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Created March 24, 2021, Updated July 27, 2022