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arXiv:2006.01306 · Physical Review Research

Optical atomic clock comparison through turbulent air

Martha I. Bodine, Jean-Daniel Deschênes, Isaac H. Khader, William C. Swann, Holly Leopardi, K. Beloy, Tobias Bothwell, Samuel M. Brewer, Sarah Bromley, Jwo-Sy Chen, Scott A. Diddams, Robert Fasano, Tara M. Fortier, Youssef S. Hassan, David B. Hume, Dhruv Kedar, Colin J. Kennedy, Amanda Koepke, David R. Leibrandt, Andrew D. Ludlow, William F. McGrew, William R. Milner, Daniele Nicolodi, Eric Glenn Oelker, T.E. Parker, John Martin Robinson, Stefania Romisch, Stefan Alaric Schäffer, Jeff A. Sherman, Lindsay Sonderhouse, Jian Yao, Jun Ye, Xiaogang Zhang, Nathan R. Newbury, Laura C. Sinclair

Published Sep 11, 2020• 28 citations

Abstract

The authors compare two state-of-the-art, ytterbium and strontium, optical atomic clocks across a 1.5-km open-air path between two laboratories. The ratio measured across the open air is in agreement with the simultaneous ratio measurement obtained using a conventional noise-cancelled fiber link.

DOI: 10.1103/physrevresearch.2.033395

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Atomic clock frequency ratios with fractional uncertainty

\leq 3.2 \times 10^{-18}

We report high-precision frequency ratio measurements between optical atomic clocks based on

^{27}

Al

^+

,

^{171}

Yb, and

^{87}

Sr. With total fractional uncertainties at or below

3.2 \times 10^{-18}

, these measurements meet an important milestone criterion for redefinition of the second in the International System of Units. Discrepancies in

^{87}

Sr ratios at approximately

1\times10^{-16}

and the Al

^+

/Yb ratio at

1.6\times10^{-17}

in fractional units compared to our previous measurements underscore the importance of repeated, high-precision comparisons by different laboratories. A key innovation in this work is the use of a common ultrastable reference delivered to all clocks via a 3.6 km phase-stabilized fiber link between two institutions. Derived from a cryogenic single-crystal silicon cavity, this reference improves comparison stability by a factor of 2 to 3 over previous systems, with an optical lattice clock ratio achieving a fractional instability of

1.3 \times 10^{-16}

at 1 second. By enabling faster comparisons, this stability will improve sensitivity to non-white noise processes and other underlying limits of state-of-the-art optical frequency standards.

We report high-precision frequency ratio measurements between optical atomic clocks based on ^{27}Al^{+}, ^{171}Yb, and ^{87}Sr. With total fractional uncertainties at or below 3.2×10^{-18}, these measurements meet an important milestone criterion for redefinition of the second in the International System of Units. Discrepancies in ^{87}Sr ratios at approximately 1×10^{-16} and the Al^{+}/Yb ratio at 1.6×10^{-17} in fractional units compared to our previous measurements underscore the importance of repeated, high-precision comparisons by different laboratories. A key upgrade from our previous work is the use of a common ultrastable reference delivered to all clocks via a 3.6-km phase-stabilized fiber link between two institutions, enabling better accuracy and stability in frequency transfer. Derived from a cryogenic single-crystal silicon cavity, this reference improves comparison stability by a factor of 2-3 over previous systems, with an optical lattice clock ratio achieving a fractional instability of 1.3×10^{-16} at 1 s. Identifying individual clock stabilities with a three-corner-hat measurement, we demonstrate the most stable optical lattice and trapped-ion clocks used in a multispecies comparison. By enabling faster comparisons, this stability will improve sensitivity to nonwhite noise processes and other underlying limits of state-of-the-art optical frequency standards.

We demonstrate a 30-km-long optical fiber link for frequency comparison between two strontium optical lattice clocks being developed at Riken and the University of Tokyo. We use a transfer laser at 1397 nm which is twice the wavelength of the clock transition of strontium clocks. The link stability is estimated to be 1×1017for an averaging time τ = 1 s, which is in good agreement with the theoretical limit calculated from the fiber noise spectrum. We discuss a remote clock comparison with stability of 10 (τ/s) by synchronously operating the two distant clocks.