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Absolute frequency measurement of the neutral 40Ca optical frequency standard at 657 nm based on microkelvin atoms



G Wilpers, Christopher W. Oates, Scott A. Diddams, A Bartels, Tara M. Fortier, Windell Oskay, James C. Bergquist, Steven R. Jefferts, Thomas P. Heavner, Thomas E. Parker, Leo W. Hollberg


We report an absolute frequency measurement of the optical clock transition at 657 nm in 40Ca with a relative uncertainty of 7.5x10-15, the most accurate frequency measurement of a neutral atom optical transition to date. Relative instabilities of 2x10-15 in 10 s of averaging time and 2x10-16 in 1000 s were demonstrated by measuring relative to the single-ion 199Hg+ optical frequency standard at NIST by means of a fs-laser-based frequency comb. The Hg+ frequency was simultaneously calibrated relative to the NIST Cs fountain via the NIST time scale for the absolute frequency measurement. The absolute frequency value of (455 986 240 494 135.8+/-3.4) Hz was measured by stabilizing a diode laser system to a thermal ensemble of about 106 atoms cooled in two stages to a temperature of 10 u K. A short cooling/spectroscopy cycle time of 25 ms led to a fractional frequency instability of ~2x10-16 for an averaging time of 2000 s.


atomic clocks, high resolution spectroscopy, optical frequency standards


Wilpers, G. , Oates, C. , Diddams, S. , Bartels, A. , Fortier, T. , Oskay, W. , Bergquist, J. , Jefferts, S. , Heavner, T. , Parker, T. and Hollberg, L. (2007), Absolute frequency measurement of the neutral <sup>40</sup>Ca optical frequency standard at 657 nm based on microkelvin atoms, Metrologia, [online], (Accessed April 21, 2024)
Created March 14, 2007, Updated February 17, 2017