Re: LN19154 Narrowband four-photon states from spontaneous four-wave mixing by Yifan Li, Justin Yu Xiang Peh, Chang Hoong Chow, et al. Dear Dr. Kurtsiefer, The above manuscript has been reviewed by our referees. The resulting reports include a critique which is sufficiently adverse that we cannot accept your paper on the basis of material now at hand. We append pertinent comments. If you feel that you can convincingly address the concerns, we will give further consideration. With any resubmittal, please include a summary of changes made and a brief response to all recommendations and criticisms. Yours sincerely, Stojan Rebic, Ph.D. Senior Associate Editor Physical Review Letters Email: prl@aps.org https://ddec1-0-en-ctp.trendmicro.com:443/wis/clicktime/v1/query?url=https%3a%2f%2fjournals.aps.org%2fprl%2f&umid=d5673cb9-accd-4967-a35d-73103a9ff9fc&rct=1773148731&auth=8d3ccd473d52f326e51c0f75cb32c9541898e5d5-2598725673f107c9c333cc656cb14512f274da11 NEWS FROM THE PHYSICAL REVIEW JOURNALS PRL now publishes End Matter https://ddec1-0-en-ctp.trendmicro.com:443/wis/clicktime/v1/query?url=https%3a%2f%2fgo.aps.org%2fendmatter&umid=d5673cb9-accd-4967-a35d-73103a9ff9fc&rct=1773148731&auth=8d3ccd473d52f326e51c0f75cb32c9541898e5d5-83fb153a41df7454d66a6971d9cb882ebd0cfbce Announcing PRX Intelligence https://ddec1-0-en-ctp.trendmicro.com:443/wis/clicktime/v1/query?url=https%3a%2f%2fgo.aps.org%2f4pFrs50&umid=d5673cb9-accd-4967-a35d-73103a9ff9fc&rct=1773148731&auth=8d3ccd473d52f326e51c0f75cb32c9541898e5d5-b9286cbc6dac6763dab3a8f137f46b6bea5c3d8d ---------------------------------------------------------------------- Report of Referee A -- LN19154/Li ---------------------------------------------------------------------- The authors demonstrate the generation of time-correlated four-photon states from spontaneous four-wave mixing (SFWM) in a cold atomic ensemble of 87Rb using continuous-wave pumping. They employ Hanbury Brown-Twiss setups to measure higher-order intensity correlations (up to fourth order) and observe a significant four-fold coincidence peak within a 20 ns window. The reported generation rate of 2.5 million cps and the narrow bandwidth (MHz level) are promising for interfacing with atomic quantum networks. However, despite these technical achievements, the manuscript in its current form lacks the necessary rigor to support its central claims regarding the nature of the four-photon state and its significance compared to the state of the art. Thus, I could not recommend its publication in Physical Review Letters. Major Concerns: 1. Insufficient evidence for four-photon entanglement: The manuscript relies heavily on intensity correlation measurements up to the fourth order to characterize the four-photon state. While these measurements confirm temporal correlation and a high degree of bunching, they do not prove quantum entanglement. To claim a resource for quantum information, the authors must demonstrate non-classicality (e.g., through a violation of Cauchy-Schwarz inequalities for two modes) or directly verify entanglement (e.g., through state tomography, entanglement witnesses, or tests of Bell inequalities). 2. Ambiguity in the term "genuinely correlated": The authors state in the abstract that they "verify the presence of genuinely correlated four-photon states." In quantum information, "genuine" typically refers to genuine multipartite entanglement (GME), i.e., entanglement across all parties that cannot be decomposed into mixtures of bi-separable states. The current work does not provide any evidence for GME. If the authors are using "genuine" in a different context, this must be clarified immediately to avoid misinterpretation. 3. Lack of contextualization and comparison with existing platforms: The manuscript does not adequately position its results within the broader field of multi-photon sources. While the introduction cites SPDC and other SFWM works, the discussion section lacks a quantitative comparison with state-of-the-art platforms, such as quantum dot sources (high purity, indistinguishability), Waveguide-based SPDC/SFWM (high brightness, scalability), and other atomic-ensemble-based multiplexed sources. Critical figures of merit for multi-photon sources include heralding efficiency, brightness, and entanglement fidelity. The current work focuses only on count rates and correlation functions. A fair comparison using these metrics is necessary to truly understand the progress this work represents. Furthermore, if the work is to be compared with other "higher-order correlation" studies, the authors should clarify: What new physical insight does this measurement provide that was not already known from previous higher-order SPDC experiments? 4. Unclear quantum advantage or application pathway: The authors suggest the source is suitable for "quantum networking applications," but they do not specify a concrete protocol or figure of merit where this source would provide a quantum advantage over classical light sources or even over simpler pair sources. For example, would these four-photon states enable quantum-enhanced metrology? Can they be used as a resource for a specific quantum repeater or computing protocol (e.g., cluster state generation)? What is the entanglement structure of the state, and how does it enable these tasks? Without a clearer link to an application that genuinely requires four-photon entanglement, the significance of the work remains unclear. ---------------------------------------------------------------------- Report of Referee B -- LN19154/Li ---------------------------------------------------------------------- This manuscript reports the observation of time-correlated four-photon events generated by spontaneous four-wave mixing in a cold rubidium ensemble. The authors claim four-photon state generation and support this claim with third order and fourth order correlation measurements. The results are presented in a clear and understandable way in my opinion, and the figures are helpful to understand these third and fourth order correlation measurements. Finally, the authors present the rates of the photon generation for completeness. According to the authors, the work addresses an important gap relative to conventional SPDC based multiphoton generation, namely operation in a narrowband regime. In my opinion, I find the results about the generation of high multiphoton correlated states to be interesting, and the story of the paper is really well conveyed even for someone not specialist of the field. The science and the data analysis is solid. For these abovementioned reasons, I recommend this paper for publication although I have some major interrogations that I would like to see answered and perhaps discussed in the introduction of the paper : 1. The authors' comparison of their work with multiplexed SFWM, spontaneous Raman, and cascaded approaches, is too brief and non exhaustive. They emphasize direct cw pumping of a single SFWM process. This may indeed be the main advance. But the manuscript should more precisely state what is unprecedented: is it the direct observation of time-correlated quadruplets from a single SFWM process in cold atoms, the cw operation, the narrowband atom-resonant character, the brightness, or the combination of these? At present, the novelty discussion is not exactly clear for the reader. 2. The authors also claim "The large bandwidth of photons from SPDC also limits their use in quantum memory and repeater schemes that require efficient interfacing with material quantum systems.". However, there already exists some experiments with quantum storage of photons from SPDC sources (for example in rare-earth ions quantum memories where the SPDC source is embedded in a cavity, but also with atomic ensemble quantum memories). Maybe the authors should be more precise on how there system would outperform those ones, if it is the case. Also, maybe it could be useful to cite some relevant publications here. 3. There is no discussion on the Raman generation of correlated photons (DLCZ protocol), which is often used for atomic memories, and comes already with narrow bandwidth. Has this protocol been used in the past for multiphoton state generation ? Also, authors claim "their long coherence times, typically in the order of tens of nanoseconds", but does "tens of nanoseconds" really constitute long coherence times ? I think these coherence times can be longer with these DLCZ generated photons. 4. Finally, while I acknowledge that these results demonstrate genuine four photon states, I think the manuscript could benefit from a concise statement of whether the measured correlations violate any classical inequalities (though I understand that this would require additional measurements and would not be feasible by the authors now). Otherwise, maybe a short discussion on the nonclassicality of these states that are generated. After the authors improve a bit the manuscript's introduction, I am confident that this paper can be published in PRL.