Indigenisation of the Quantum Clock: An Indispensable Tool for Modern Technologies

Round 1

Reviewer 1 Report

Comments for author File: Comments.pdf

Author Response

Please see the attached file to find our reply to each of your comments. The corresponding changes in the manuscript are done using track-change mode in the revised version as suggested by the Editor. 

Author Response File: Author Response.pdf

Reviewer 2 Report

Dear Editor and Authors,

The present manuscript gives an insightful review of the technology of optical clocks, starting from the old microwave clocks to the most recent portable clocks with transferable time and frequency standards. With this information, the purpose of this review-manuscript is to present a roadmap of 10 years to stablishing the most stat-of-the-art optical clock technology in India, as a collaboration of research groups and National institutions related with these technologies.

The overall historic and most recent advances have been clearly explained, not only in the international landscape, but also at the Indian research groups. The necessary level of detail was also given to the roadmap, however, the justification of the choice of time periods in each milestone time-period is not provided, ie., why is expected for the setting of experiments being 4 years, for example. This reasoning can included throughout section 3 without increasing it significantly. Overall, this is a minor improvement, and I would accept this manuscript with minor corrections.   

Some other minor corrections/suggestions:

Line 47: quantum nature of particle is vague. Please elaborate. “…allowed due (reason). Furthermore, these transitions, having ..., are

Line 49: “…excite. Therefore,”

Line 53 fractional accuracy is the Allan deviation?  if so please use one term only and without redundancies. If not then no need to define the sigma. The review is intended to a wide audience and terminology should be made as simple as possible.

Line 89: “...ion, repeated spectroscopic”

Line 127: “Within the country of India, most”

Line 141 “...clocks need to be operated in a real-time network mode [46].”

Line 193 “here one can also refer that quantum logic with optical clocks have already been made with highly charged ions http://www.nature.com/articles/s41586-020-1959-8”

Table 2 Would be useful to include another column with the main ref. for each measurement.

Line 269-272 replace the strange symbols with & or simple "and"

Lines 273-275 -confusing sentence.

Line 297 – “Figure 2”

Figure 3 caption. The ADev is not sigma of eq. 1 or fractional accuracy? Again simplify the jargon. If not, define it.

Line 351 off-the-shelve

Author Response

Please see the attached file to find our reply to each of your comments. The corresponding changes in the manuscript are done using track-change mode in the revised version as suggested by the Editor. 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The expansion of table 2 with additional references is appreciated. I think many will find this a helpful resource.

I recommend publication with only the following minor comments:

Line 36: should not be ‘ after "their"

Line 58: floating comma in superscript of Al^+

Line 69-70: I would suggest:

“For example, caesium fountains require approximately 20 days for establishing their intercomparison at 10-16 levels of accuracy via MW communication techniques, such as Common View Global Navigation Satellite Systems (CVGNSS) or Two Way Satellite Time and Frequency Transfer (TWSTFT). In the case of optical clocks this same level of accuracy can be reached in ~1000 s and specialized optical communication techniques such as Two-Way Fiber Optic Time Transfer (TWFOTT) enable the intercomparison of remote systems.”

[the ‘etc.’ seemed redundant as I am not aware of any other methods. As worded was slightly misleading as if using optical techniques you could achieve ~1E-16 comparison of caesium fountains in 1000s, which is our course not the case as the limitation is the instability of the caesium fountain and not the comparison channel]

Line 84: ‘….. optical atomic clocks:’ remove ‘are’ (or else reword so not using :)

Line 179-192: Was this accidentally cut and paste here? Is repeated starting at line 233 (where I think it is suppose to be)

Line 233: missing bullet point

Line 264,268: Incorrect to state those experiments have fractional accuracy 2.5E-19, 3.2E-19 respectively. This is the precision [or measurement instability] they demonstrate but not accuracy.

Figure 2: a legend in plot would be helpful. Also it seems there is misunderstanding between reported accuracy and measurement precision, as commented in Line 264,268. The plot in figure 2 has some blue points with fractional inaccuracies below 1E-18 which should not be the case. The most accurate lattice result [53] reports systematic uncertainty of 1.4E-18 (for both their systems).

Author Response

Thanks to the reviewers for thoroughly checking the manuscript, which certainly helped to improve its quality. We have modified the new manuscript following his/ her suggestions. Two of the following comments we like to give specific reply,

(Comment - 1) Line 179-192: Was this accidentally cut and paste here? Is repeated starting at line 233 (where I think it is suppose to be)
Line 233: missing bullet point

Reply: This line actually goes just below table 2 and follows the bullet points. During the insertion of the new Table 2 that too in landscape orientation changed the formatting by mistake. We have corrected it now. 

(Comment - 2) Line 264,268: Incorrect to state those experiments havefractional accuracy 2.5E-19, 3.2E-19 respectively. This is theprecision [or measurement instability] they demonstrate but not accuracy.

Reply: We have corrected the value 3.2E-19 following the Ref. 52. 

In Ref. 98, under the caption of the Fig. 2 the authors have reported "The measurements remain QPN-limited for 1000 experimental repetitions (red square and line), reaching a fractional uncertainty of 2.5 × 10−19...". Therefore in our view this number 2.5E-19 is correctly mentioned in the manuscript following as it is reported. 

Author Response File: Author Response.docx