Review Reports

Round 1

Reviewer 1 Report

Please see the attached file.

Comments for author File: Comments.pdf

Author Response

Thank you for the great work done. Your comments and wishes are very important to us. We are sending a revised version of the article taking into account your comments.

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Manuscript ID: universe-950906

 

Title: Comparison of gravitational and light frequency shifts in rubidium atomic clock

 

Authors: Alexey Baranov, Sergey Ermak, Roman Lozov, Vladimir Semenov

 

The manuscript deals with the geomagnetic field influence on the onboard rubidium atomic clocks of navigation satellites. Moreover, the research results of the orientation errors influence on the rubidium atomic clocks frequency located in a rotating external magnetic field are presented.

 

As a general remark I suggest authors a revision of the English language by a native speaker.

 

In particular, the following indication are strongly suggested:

 

In the Abstract:

At rows 12-13: “which simulating the 12 geomagnetic field influence on the onboard rubidium atomic clocks of navigation satellites” – this sentence is not clear.

 

Paragraph Introduction:

Row 35 – between word clock and relative I think a is is necessary? The atomic clock is relative? Sentence not clear

 

Row 39 – If the formula is well known authors should provide a reference as far as possible.

 

Rows 53 to 58 – Authors should add a table.

 

Row 66 – Authors mean dependencies?

 

Paragraph 2. 2. Orientational frequency shift investigation.

Rows 73 to 77. This device must be represented in a Figure to help the reader.

 

Rows 91-92 – Figure 1 – The Physical Units of the Axis labels in general should be indicated in brackets: Ex.: (s) or (°).

 

Row 93 – I think that the word “which” can be eliminated – (which obtained in the experiment).

 

Rows 117 to 121 – I think that also in this case authors should add a Figure representing the device.

 

Rows 125-126 – In the Figure 2 as for the figure 1 authors should indicate the physical units in brackets (Time (s)).

 

Row 150 – Figure 3 – as for Figure 1 and 2 in general the physical units of the axis are indicated in brackets.

 

The manuscript needs moderate revisions and table and figures should be added to improve the readability of this work.

 

 

Author Response

Thank you for the great work done. Your comments and wishes are very important to us. We are sending a revised version of the article taking into account your comments.

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

I have reviewed the article by A Baranov et al. entitled “Comparison of gravitational and light frequency shifts in rubidium atomic clock.”  Briefly, this manuscript notes that in using atomic clocks to test predictions of General Relativity in space, researchers need to fully understand how the space environment could confound the interpretation of results.  In the present case, the researchers are concerned with orbital variations in the magnetic field experienced by an onboard atomic clock, and how those can change a rubidium (Rb) atomic clock’s light shift. While I fully agree with the general premise of the authors: tests of fundamental physics in space require thorough knowledge of confounding effects; I am, unfortunately, unconvinced by the authors’ claim that they have measured the magnetic field dependence of the tensor light shift. I thereby do not recommend publication of their article in your journal.

The most significant concern I have relates to Fig.1 in their paper.  Presumably, defining the z-axis of the coordinate system as along the optical axis, the authors change the orientation of the magnetic field from along the z-axis, to perpendicular to the z-axis, and then finally along the z-axis but in a direction reversed from its original direction.  (This is not stated clearly in the manuscript, and I infer that this is what was done.)  The authors note a frequency change in the 0-0 hyperfine transition frequency (i.e., Fig. 1), and presume that this frequency change must be due to the light shift.  That presumption, however, is not without argument:

1. The authors do not provide an indication of the orientation of the B-component of their microwave field (i.e., the electromagnetic field’s magnetic-field component). Presumably, when the dc magnetic field has a rotation angle of 0 or 180^o, the microwave field’s B-component is parallel to the atoms’ quantization axis. In this case, the microwave field drives Δm_F = 0 transitions between the ground-state hyperfine levels.  Changing the orientation of the quantization axis relative to the microwave field’s B-component implies that the field will generate Δm_F = ±1 transitions.  One could certainly imagine that this variation would give rise to a clock frequency shift due to the different magnetic field sensitivities of the coupled Zeeman sub-states.
2. The magnetic field variation could also give rise to a change in the observed hyperfine transition resonant frequency as a consequence of the position shift or inhomogeneous light shift.

In addition to the above major concern, there were a number of questions/issues I had that the authors can hopefully address in future manuscripts.

1. L77 – What were the resonance cell dimensions (length and diameter if cylindrical cell).
2. L79 – How were the microwaves introduced into the system: microwave cavity, microwave horn?
3. L96 – The authors refer to 0.01% changes causing light-shift effects, but I believe these are 0.1% changes.
4. L106 – The authors state that a field of 6 A/m is “almost two magnitude orders less than the field strength on the planet’s surface.” However, 6 A/m corresponds to 75 mG and the Earth’s magnetic field near the surface is 300 mG.  What am I missing?
5. L114 – The authors state that they employed a Frequency Electronics Inc. atomic clock in their experiments. It would be useful if the provided a model number for the reader.

Comments for author File: Comments.pdf

Author Response

Thank you for the great work done. Your comments and wishes are very important to us. We are sending a revised version of the article taking into account your comments.

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors did a good job in improving the manuscript.

Author Response

The authors thank the reviewer for the comment.

Reviewer 3 Report

First, let me say that in this revised submission the authors have added sufficient material to make the paper much more readable and informative.  Nevertheless, at the present time I still hold reservations about the publishablility of the article.

My main problem with this paper is the inference by the authors that the change in 0-0 hyperfine transition frequency with magnetic field orientation is due to the tensor component of the light shift effect.  It MIGHT be due to the tensor component, but just because it might be does not mean it IS. If the authors want to maintain this position, then I suggest one of the two following courses:

1) Compute the tensor component of the light-shift under the authors' experimental conditions, and then show agreement between theory and experiment. 

Or

2) Consider all other mechanisms that could produce a sensitivity of the 0-0 transition frequency to magnetic field orientation, and make a strong argument that none of these other possibilities is consistent with the data. In particular, if the change in 0-0 frequency is related to the light-shift effect, wouldn't the amplitude of the curve shown in Fig. 2 be proportional to the light intensity?

Alternatively, the authors could simply state that they have measured a sensitivity of the 0-0 resonance frequency to magnetic field orientation, and that the mechanism driving this behavior could be due to one of several mechanisms without arguing that one is necessarily more likely than the others.

Finally, I have a few minor questions/comments:

1) Near line 79, the authors write "[T]he antenna was oriented in such a way as to excite 0-0 resonant transitions in the variants of coaxial and orthogonal orientation..." Does this mean that the orientation of the horn was rotated with the orientation of the magnetic field?  If yes, this point should be carefully clarified and emphasized.

2) Near line 117, the authors write"[A]s the experiments shown with a laboratory rubidium atomic clock, the orientational frequency shift value does not depend on the strength of the external magnetic field..." I did not see this discussed explicitly in the paper.  Is this from separate measurements not reported here?  If yes, then a statement to that effect would be useful to the reader.

3) In the caption to Fig. 5, the authors write "...at a light intensity of 100 microwatts/cm^2..."  If this data is from a commercial clock, enclosed in the manufacturer's housing, how do the authors know the light intensity?

Author Response

We thank the reviewer for useful comments (answers to comments in the attached file).

Author Response File: Author Response.pdf

Round 3

Reviewer 3 Report

This paper is much improved over previous versions, and I recommend publication after the authors have had a chance to consider two recommendations.

1. Near line 82, it might be worth noting that the orientation of the microwave horn excited both pi and sigma transitions, which could be observed; further, while the magnitude of the pi and sigma transitions changed as the magnetic field orientation varied, the overall spectrum of pi and sigma transitions remained unaltered.
2. Near line 93 I would suggest the addition of a sentence like the following: "Separate experiments [Ref.] showed that the measured frequency shifts were proportional to the light intensity and independent of the magnetic-field magnitude."  The results to that effect in the authors' previous work adds considerable credibility to the claim that the authors are measuring the tensor component of the light shift.

Author Response

Thanks for the suggestions and recommendations!

 

Please see the attachment.

Author Response File: Author Response.pdf