First uncertainty evaluation of the cesium fountain primary frequency standard NMIJ-F2

Abstract

We report the first uncertainty evaluation of NMIJ-F2, the second atomic fountain primary frequency standard at the National Metrology Institute of Japan. To improve the frequency stability, we increase the number of detected atoms to 9 × 105 using high-power cooling laser beams for vapor-loaded optical molasses and optical pumping into the Zeeman sublevel m F = 0. We also employ an ultra-stable cryogenic sapphire oscillator as a local oscillator to prevent the degradation of frequency stability due to the Dick effect. After correcting the collisional frequency shift by alternating atom densities, the frequency stability typically reaches 2.5 × 10−13(τ/s)−1/2. Its value is 1.9 × 10−16 after 20 days of measurement. Type B uncertainty is typically evaluated at 4.7 × 10−16; the largest contribution is from a distributed cavity phase shift, followed by a microwave leakage shift. In long-term comparison, the frequency of NMIJ-F2 is found to be consistent with that of the other primary and secondary frequency standards within the uncertainty.