Comment on "first accuracy evaluation of NIST-F2"

TL;DR

This paper critiques the systematic error treatment in the 2014 NIST-F2 accuracy evaluation, highlighting the significance of microwave lensing shifts, alignment issues, and differences in background gas collision effects for cold atoms.

Contribution

It clarifies the treatment of microwave lensing shifts, emphasizes proper fountain alignment, and discusses the distinct collision effects for cold versus room-temperature atoms.

Findings

Microwave lensing shift is non-zero even without applied microwave field.

Incorrect treatment of microwave lensing contributed to systematic error.

Room-temperature collision data do not directly apply to cold atom clocks.

Abstract

We discuss the treatment of the systematic frequency shifts due to microwave lensing and distributed cavity phase in "First accuracy evaluation of NIST-F2" 2014 Metrologia 51 174-182. We explain that the microwave lensing frequency shift is generally non-zero and finite in the limit of no applied microwave field. This systematic error was incorrectly treated and we find that it contributes a significant frequency offset. Accounting for this shift implies that the measured microwave amplitude dependence (e.g due to microwave leakage) is comparable to the total reported inaccuracy. We also discuss the importance of vertically aligning the fountain perpendicular to the axis of the cavity feeds, when the cavity has only two independent feeds. Finally, we note that background gas collisions have a different behavior for cold clock atoms than for clock atoms at room-temperature, and therefore room temperature measurements do not directly apply to laser-cooled clocks.