Recent improvements in NIST-F1 and a resulting accuracy of δf/f=0.61×10-15

Over the last several years, we have made many improvements 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.