Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors consider the relative merits of satellite-based versus fiber-based quantum networks through optimization of hybrid routes between nodal points.  Optimal paths are determined to be those having the largest product of secret key probabilities, which is taken to be roughly proportional to the total secret key rate.  Several simplifying assumptions are made to model this rate as a function of distance, weather, diurnal patterns, and component properties.  The cost difference associated with implementing and maintaining satellite versus ground systems is not considered in the analysis, though the authors point out that the former poses a significant challenge.  Furthermore, the analysis assumes the availability of quantum memory units that are currently not yet ready for quantum networking applications.

Key findings are that ground-based networks are best for short distances, while satellite or hybrid schemes work best for longer distances.  Qualitatively, these results are not surprising, but the authors provide an interesting quantitative methodology and analysis that characterizes that transition.

The paper is well written, clear, and sufficiently detailed to allow reproducibility.  The authors attempt to provide realistic parameter values while keeping simple several aspects of what is in reality a very complex system.  The authors do not mention ground-based, free-space links as an alternative to fiber and should comment on why this was not considered in their analysis.

The abstract and conclusions seem rather vague given the detailed quantitative analysis of the paper.  For example, the abstract states only that “satellite links will play a significant role.”  It would seem that their results would allow more specific conclusions.  The conclusion is only slightly more specific in identifying a “clear regime switch between ground/space segment choices.”  Of course, any quantitative conclusion must be caveated by the modeling assumptions made.

Efficiencies are defined as the probability of a functional unit performing correctly, but it should be clarified what, precisely, this means.  For entanglement sources, the efficiency is taken to be the probability of emitting an entangled pair for a “given attempt.”  Normally, entanglement pair generation is described as a rate, so to define it as a probability one must define what one considers a discrete attempt (a single pulse of the pump laser? a single coherence time interval?).  Implicitly, it is also assumed that this efficiency is basis independent (a common assumption, I know, but not necessarily a valid one).  Such assumptions should at least be made explicit.  Similarly, quantum memory efficiency is defined as the probability of retrieving a photon from an entangled pair “without disturbing its quantum state.”  I believe this is intended to mean the fidelity of the stored state upon retrieval.  This should be clarified in the manuscript.

Overall, the paper is well written, technically sound, and quite interesting.  There are, I believe, just a few implicit assumptions that needed to be stated more explicitly and conclusions that need to be formulated more concretely.  With these relatively minor changes, I think the paper would be suitable for publication.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript introduces an analysis of the entanglement distribution rates in a quantum network accommodating both ground- and satellite-based links. This paper outlines the methodology of breaking down quantum repeater components to fit satellite constraints and deriving distribution rate equations for both types of channels. The analysis incorporates dynamic factors such as weather and satellite orbit, demonstrating the advantages of satellites for long-distance links. This paper complements existing research efforts and discusses the potential for improvement in satellite-based quantum communications, addressing current technology limitations and suggesting avenues for advancement.

 The future quantum internet aims to connect quantum information processing (QIP) nodes by distributing entanglement over long distances, which is crucial for achieving high-rate, high-fidelity transmission. However, balancing these requirements poses challenges, exemplified by current entanglement generation methods. Thus, the manuscript offers valuable results. I would consider recommending that this manuscript be accepted for publication in Photonics. However, there are several areas in which the manuscript could be improved. In particular, the following comments should be taken into consideration:

1.       In the introduction, some background information on quantum information processing is provided. However, for greater comprehensiveness, it would be beneficial to include more recent quantum communication network progress, such as [1] Phys. Rev. Lett. 130, 250801 (2023); [2] Phys. Rev. Applied 20, 024048 (2023); [3] Natl. Sci. Rev. 10, nwac228 (2023); and [4] Nat. Commun. 14, 704 (2023).

2.       The analysis of the entanglement distribution rates between ground- and satellite-based links is a key contribution. However, more details on the methodology used to incorporate dynamic conditions of the satellite link (e.g., weather, stray light, satellite orbit) would enhance the comprehensibility of the work.

3.       In Section 2, the assignment of the Bell State Measurement (BSM) unit to the satellite in an uplink configuration is discussed. However, more explanation is needed regarding the practical implications and challenges associated with this configuration, particularly concerning atmospheric interference and total loss differences between uplink and downlink configurations.

4.       A discussion on the performance of an ARC in terms of the achievable entanglement distribution rate is crucial. However, additional details on how factors such as minimum decoherence processes, dark counts on BSM detectors, and storage times are accounted for in the analysis would strengthen the manuscript's rigor.

5.       The manuscript mentions assigning efficiencies to the source, memory, BSM, and channels. It would be beneficial to provide more information on how these efficiencies are measured or calculated, particularly in the context of the distance-dependent efficiency of channels.

6.       In the conclusion or discussion section, further elaboration on the potential practical applications of the protocol could enhance the attractiveness of the paper. Using Science Advances 10 and eadk3258 (2024) to illustrate its application prospects could be beneficial.

 Once the authors have solved the above issues, I do not think any other problems can prevent the work from being published.

Comments on the Quality of English Language

Minor editing of English language required

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript presents a comprehensive analysis of the performance of an Automated Repeater Chain (ARC) for quantum internet applications, with a focus on incorporating both ground and space segment architectures. The methodology proposed for evaluating the performance of the ARC under static and dynamic constraints is rigorous and well-structured, providing valuable insights into the regime switch between ground and space segment choices. The identification of necessary improvements, such as narrower spatial filtering and satellite constellation deployment, highlights key areas for enhancing overall performance. The inclusion of dynamic connection switching as a crucial feature for future quantum internet networks is aptly emphasized. Additionally, the discussion of future research directions demonstrates a forward-looking approach to advancing the proposed framework. Overall, the manuscript presents a valuable contribution to the field of quantum networking, with opportunities for refinement and expansion in future work. The manuscript can be published in Photonics after suitable revisions.

1. About the utilization of abbreviations, for example, “automated repeater chain (ARC)” is defined in Line 145-146. However, the corresponding abbreviation is not used in Line 416. Please check throughout the manuscript again.

2. Could the authors explain more about the assumption of a coupling efficiency (η_coup) of 0.4?

3. The description of the process for constructing the hybrid ground/space graph could be expanded for clarity. Providing a flowchart of the process would aid readers in understanding how connections between nodes are evaluated and optimized.

 4. Could the authors further discuss more about the impact of uncertainties in atmospheric conditions, satellite positioning, and other environmental factors on the network simulation framework?

Comments on the Quality of English Language

Minor editing of English language required

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

This manuscript can be accepted for publication.