Dear Editor,

thank you for considering our manuscript for publication in Physical Review Applied, as stated in the earlier conversations.

The main issue with our manuscript seemed to be a strong disagreement on the term "quantum sensing" with one of the referees, which we unsuccessfully tried to argue in our earlier reply. In order to avoid further delays, we changed the title and section heading III from "Quantum Sensing" to "Range Sensing" - this should address the concern of both referees, and it unambiguously states what we do.

As for the other comments/suggestions, we address the comments of the reports in
sequence:

[Reviewer A]

"1. While I appreciate the fact that the demonstration in ref. [15] was in-fiber ranging, I do believe that it is important to make the novelty of your current demonstration clearer in the text. To my understanding, the main novelty here is practical - the use of a subthreshold laser diode for an experimental demonstration of thermal light-based range sensing in free space over almost 2 km. This point should be made clear in the text, in respect to previous works (e.g. [15] and https://doi.org/10.1364/OE.486348). The current manuscript in my opinion still slightly implies that ranging using thermal light was not demonstrated/proposed before."

We feel that the statement what we do (range finding using thermal light) is quite clear already in the abstract (sentence 2 and 3). We now state explicitly in the second introduction paragraph that the work of Zhu et al. [Appl. Opt. 51, 4885 (2012); former reference 15, now reference 9) was using "thermal" light to measure the length of an optical fiber (with its intrinsic low losses). We do note that in that experiment, the "thermal" light was generated through the Arecchi method by modulating a laser beam with a rotating ground glass disc as a phase randomizer; such chaotic light is an approximation of thermal light, but has in-principle predictable and a repetitive patterns. We plan to address this difference in further work, as this topic is beyond the scope of the manuscript at hand. 
The optics express work (https://doi.org/10.1364/OE.486348) from this year really is about a classical near distance multimode LIDAR system for automotive applications, and we feel citing this work would really be a stretch in the context of our work.


"2. In the first reply, the higher loss of a free-space link is mentioned: “The return loss in a non- lab based free space range finding measurement is significantly larger…”. What was the loss in your experiment? It would be useful to discuss how it compares to realistic scenarios (for example without a retro-reflector) and what could be further improved in the system to
mitigate additional loss. "

We could not reliably characterize the return loss in our experiment, as this was governed by a combination of hard-to-characterize effects, like absorption, scintillation across the distance, an unspecified loss in the collection optics and possible local detector saturation. A very rough estimation from detector count rates suggests a loss of 80-90dB. However, these effects are equally present in all range sensing methods. Thus, we believe that at the same level of loss, thermal light being several orders of magnitude brighter than spdc light, should lead to correspondingly higher signal-to-noise ratios.


"3. Regarding the use of the term “quantum sensing”. I agree that this term is used extensively in the community. I do suggest at least making it clear in the text what you mean by that. As you suggested: “That said, we do feel that quantum sensing a such has probably a less obvious or widely accepted definition as one would like to in physics, and we hope to perhaps contribute to a necessary discussion with our work”, I feel that having a short description of what you mean by quantum sensing in your context will be beneficial. "

As stated in the introduction, we removed this contentious term. We do stand by our position, though, that the light we use is likely as quantum as that emerging from downconversion processes. However, we feel it is not constructive to have a discussion on the quantumness of our sensing demonstration in this manuscript, and will develop our view on this matter elsewhere. 


"4. I believe that further discussing the advantages of thermal light sensing compared with standard modulated intensity schemes in the text would be useful."

We expanded the introductory paragraphs in this respect.


[Reviewer B]

"Overall, my impression is the same as after reading the first version: not very much novelty, but a very high technical level of the experiment, and several ‘misinterpretations’ in the text. The latter were already mentioned in my first review, but the authors insist on keeping most of their formulations. I strongly disagree with this, and below I give the reasons in more detail.

1. The title calls this technique ‘quantum sensing’. Note that the other reviewer also commented on this: thermal light is not quantum; there are neither quantum states nor quantum effects involved; this phrase is therefore misleading. The authors reply to Reviewer A: ‘. While the thermal fluctuations can be described as classical fluctuations, the origin of the thermal light in our case is a level of spontaneous emission in the laser system, and as such in our view as "quantum" as the spontaneous parametric conversion in SPDC
sources.’ They further refer to this statement in replying to my comment. I would like to stress: indeed, thermal light has quantum origin, but so does any light. This, however, does not make sensing with thermal light ‘quantum’. This method does not use any quantum feature of light. In contrast, range finding with photon pairs is quantum because photon pairs are an example of nonclassical light. I therefore insist that the authors remove the words ‘quantum sensing’ from the title."

We removed the label "quantum sensing" to what we demonstrate in our work in this manuscript, as all we need are temporal photon correlations in a light field. We will try to explain our position in comparison to SPDC light elsewhere.


"2. I brought to the authors’ attention a paper (S. Frick et al., Optics Express 28, 37118 (2020)), where range finding with SPDC has been proposed and implemented. To the best of my knowledge, this is the first work on quantum range finding. The authors still do not cite this work, claiming that the brightness of the SPDC source there, as well as the precision of range finding, was low. I believe they should cite that paper, as the pioneering experiment on SPDC-based range finding. Or, if they know an earlier experiment on SPDC-based range finding, they should cite that one. The technical imperfections of any proof-of-principle experiment should not be a reason for ignoring it."

We added the reference to this work in the introduction.


"3. The authors cite works 5,6 as examples of SPDC-based range finding. But both these papers are on quantum illumination – i.e., on discovering the presence of an object against thermal noise – but not on finding the distance to the object. I mentioned it first time, but the authors ignored it."

We clarified this in the introduction - we were wrongly assuming that quantum illumination gives more than a binary presence information of an object.

Additionally, we adapted figure 1 to correctly reflect the reference ordering, and centered the zoom window indicators in figure 5.

For convenience, we attempted to generate a differential pdf to highlight the changes, but the system seems to get confused with changes in the figures and their caption as well as with reference re-ordering. We still hope it is useful to find text changes quickly.

With these changes in the revised manuscript, we hope to have addressed the concerns of the reviewers, and look forward to a favourable consideration by Physical Review Applied.

With Best Regards on behalf of all authors,

Christian Kurtsiefer