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 a reduction in the uncertainty by nearly a factor of 2 in the realization of the SI second at NIST. We recently submitted an evaluation with a combined standard fractional uncertainty of 0.61 × 10^-15 to the BIPM (Bureau International des Poids et Mesures). 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.