Recent improvements in NIST-F1 and a resulting accuracy

Abstract—Over the last several years, we have made many im-provements to NIST-F1 (a laser-cooled cesium fountain primary frequency standard) resulting in nearly a factor of 2 reduction in the uncertainty in the realization of the SI second at the National Institute of Standards and Technology. We recently submitted an evaluation with a combined standard fractional uncertainty of 0 61 10 15 to the Bureau International des Poids et Mesures (BIPM). The total fractional uncertainty of the evaluation (including dead time and time transfer contributions) was 0 88 10 15. This is the smallest uncertainty in a frequency standard yet submitted to the BIPM. Index Terms—Atomic clocks, cesium, standards, time measure-ment. I