Comment on "Frequency shifts in NIST Cs primary frequency standards due to transverse rf field gradients"

TL;DR

This paper critically examines the theoretical calculations of microwave lensing frequency shifts in NIST atomic fountain clocks, identifying fundamental issues and clarifying the relationship to photon recoil shifts.

Contribution

It highlights key problems in the NIST treatment of microwave lensing shifts and clarifies their connection to photon recoil effects.

Findings

NIST calculations underestimate microwave lensing shifts.

Fundamental issues in the NIST theoretical approach are identified.

Microwave lensing shifts smoothly transition to photon recoil shifts for large wave packets.

Abstract

We discuss the theoretical treatment of the microwave lensing frequency shift of the NIST-F1 and F2 atomic fountain clocks by Ashby et al. [Phy. Rev. A. 91, 033624 (2015)]. The shifts calculated by NIST are much smaller than the previously evaluated microwave lensing frequency shifts of other clocks contributing to International Atomic Time. We identify several fundamental problems in the NIST treatment and demonstrate that each significantly affects their results. We also show a smooth transition of microwave lensing frequency shifts to the photon recoil shift for large wave packets.