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

We report an absolute frequency measurement of the optical clock transition at 657 nm in 40Ca with a relative uncertainty of 7.5 x 10-15 , one of the most accurate frequency measurements of a neutral atom optical transition to date. The frequency (455986240494135.8 ± 3.4) Hz was measured by stabilizing a diode laser system to a spectroscopic signal derived from an ensemble of 106 atoms cooled in two stages to a temperature of 10 flK. The measurement used a femtosecond-laser-based frequency comb to compare the Ca transition frequency with that of the single-ion 199Hg+ optical frequency standard at NIST. The Hg+ frequency was simultaneously calibrated relative to the NIST Cs fountain via the NIST time scale to yield an absolute value for the Ca transition frequency. The relative fractional instability between the two o~tical standards was 2 x 10-15 for 10 s of averagmg tIme and 2 x 10-1 for 2000 s