Dear Prof. Kurtsiefer, Your manuscript, referenced below, has been reviewed for Applied Physics Letters. "Symmetrical clock synchronization with time-correlated photon pairs" APL18-AR-11723 The reviewers' comments are included below and/or attached. In view of their recommendations, we cannot accept your paper for publication in its present form. If you revise the paper to meet the reviewers' objections, we will be happy to give it further consideration. Please indicate how the manuscript has been revised in a separate Response Letter file so that the editors can see whether you have complied with the reviewers' comments. Please use add file to upload the Response Letter file, and use replace for any files that have been revised or changed. Revised manuscripts must be submitted through the online submission system. They are not accepted by email. The revised manuscript should be returned to the Editor promptly. Your revision is due by 03-Mar-2019. A manuscript returned more than 60 days from today should be submitted as a new manuscript and will be given a new receipt date. Please go to the URL below to submit the revised version. To meet AIP Production requirements, please provide a separate figure file for each cited figure number (all parts in one file), in addition to your article-text file. https://apl.peerx-press.org/cgi-bin/main.plex (If clicking on the above URL address directly from your mail program is unsuccessful, please copy and paste the complete address into your browser.) Thank you for the opportunity to examine this work. Sincerely yours, Kenjiro Miyano Associate Editor Applied Physics Letters AIP Publishing 1305 Walt Whitman Road Suite 300 Melville, NY 11747-4300 USA phone: +1-516-576-2344 e-mail: apl-edoffice@aip.org ----------------------------------------------------------------- ---- Manuscript #APL18-AR-11723: Reviewer Comments: Reviewer #1 Evaluations: Does this paper meet APL's standards: Yes Is the paper scientifically sound with the assertions made and conclusions drawn well supported: Yes Is the discussion of related work and associated references adequate?: Yes Is the English satisfactory?: Yes Is the title short, interesting, and descriptive of the contents?: Yes Is the paper well organized and understandable?: Yes Reviewer #2 Evaluations: Does this paper meet APL's standards: Yes Is the paper scientifically sound with the assertions made and conclusions drawn well supported: Yes Is the discussion of related work and associated references adequate?: Yes Is the English satisfactory?: Yes Is the title short, interesting, and descriptive of the contents?: Yes Is the paper well organized and understandable?: Yes Reviewer #2 (Remarks to the Author(s):): The paper presents a new way for secure clock synchronization using bidirectional transmission of entangled photon pairs. The paper is well written and easy to understand. At the same time the contribution is very limited. It builds on existing building blocks and basic protocols. The implementation is only focused on the very principle and does not address any special challenge nor apply any additional security measure. However, the solution is new and was thoroughly evaluated. Nevertheless, providing first results on more sophisticated (asymmetrical) attacks would make the article more interesting.