The authors report on measurements of the transmission spectrum of a single atom stored in a strongly focused light field. They investigate the role of additional magnetic fields as well as of the dipole trap depth and find that large trap depth are causing rapid dephasing of rf qubit transitions due to inhomogeneous light shifts including vector and tensor contributions. The optical transmission spectrum gives a good indication for parameters (B-field, trap depth) promising long coherence times.

The manuscript is sound, clearly written for the most part and gives a good overview of the authors system (an atom trapped in an optical tweezer). What is missing in this article, however, is an account of the literature with immediate relevance. 

Let us without completeness name the following publications: 

Kim et al., Phys. Rev. Lett. 111, 243004 and more publications from this group; Neuzner et. al., Phys. Rev. A 92, 053842, 2015; Barret et. al., NJP 20, 073030, 2018.

Since the results of the present manuscript are directly and specifically tied to the authors system potential publication would require an effort to identify the additional physical insight which is gained form the author’s efforts with respect to the already published material. We cannot recommend publication at this point, but working out the more general scheme could make the article publishable.

Minor comments: 
1- The dipole trap has linear polarization perpendicular to the quantization axis (so balanced sigma+/-), to avoid the vectorial light shifts. Would there be an advantage to have it parallel to the B field, i.e. Pi- polarized? 
2- There must be an error of an order of magnitude in the B-field axis of Fig. 3 since the maximum is around 14 Gauss = 1.4 mT, and in the text the maximum reported value is 144 uT. 
3- In part IV to test the discrimination of bright and dark states during state detection, it is not clear if they drive the atom with the probe beam along the quantization axis, or from the side with the "state redout beam" (Fig. 2). If they drive it from the side, which polarization is used? In that case the sigma- polarization is not defined, and the coupling should be lower and differ from their previous characterization (Fig. 3).