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REVIEW 4 major objections 5 minor 15 references

Towards Quantum Networks: Characterizing Raman Noise over Metropolitan-scale Fiber Network

T0 review · 4 major / 5 minor · reviewed 2026-07-31 · grok-4.5

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Pith's one-line read Raman noise from O-band classical light dominates C-band quantum SNR on real metro fiber, so channel frequency must be chosen carefully.

desk verdict Clean metro-loop Raman spectra for O→C coexistence, but the quantum-SNR and “networking demo” language outruns the photon-count data. read the letter →

arxiv 2607.27970 v1 pith:DMADJWJY submitted 2026-07-30 quant-ph

Marcello Caleffi , Laura d'Avossa , Italo Ignacio Machuca Flores , Marco Grillo , Elena Montella , Angela Sara Cacciapuoti This is my paper · ORCID
classification quant-ph
keywords quantumcommunicationsInternetentanglementdistributionRamanscatteringclassical-quantumcoexistencemetropolitanfiberC-bandO-band
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

This paper shows that when quantum signals travel in the C-band on the same deployed fiber as ordinary O-band classical traffic, Raman scattering from the classical light is a leading noise source for the quantum channel—larger than background light or detector dark counts. The authors measure the resulting C-band photon counts over a 7 km metropolitan loop using both a commercial transceiver and a narrow-line laser, then compare the spectra with laboratory spools. The overall Raman shape matches the lab, yet the live fiber also produces sharp spectral spikes that appear even with no classical launch and are tied to real-world fiber imperfections. From the clean parts of the spectrum they mark C-band windows that stay quieter, giving concrete guidance on where to park a quantum channel so entanglement distribution can share existing city fiber without being swamped.

What carries the argument

Wavelength-resolved photon-count spectra of C-band light generated by O-band pumps (commercial SFP or CW laser), obtained by cascading WDMs, scanning a 25 GHz tunable filter across the C-band, and detecting with an SNSPD on the live metro loop versus a lab spool; normalization by launch power and effective length isolates the Raman shape and the extra live-fiber peaks.

What would settle it

Repeat the same metro-loop launch while distributing entangled photon pairs (or running QKD) and show that measured fidelity or QBER does not track the reported Raman photon-count spectrum, or that the spectral spikes vanish when the classical O-band source is truly absent and all neighboring fibers are dark.

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Extended reading notes

Core claim

In a real 7 km metropolitan single-mode fiber loop, spontaneous Raman scattering generated by co-propagating O-band classical sources produces a C-band noise floor that is the dominant contribution to quantum signal-to-noise ratio, beyond ambient and detector noise; laboratory Raman profiles remain predictive once the live-fiber spectral anomalies are excluded, and quieter C-band DWDM windows can therefore be identified for quantum allocation.

Load-bearing premise

That raw C-band photon-count spectra measured without actually sending entangled photons or running a quantum protocol are enough to declare Raman the dominant term in quantum SNR and to prescribe which channels are safe.

Editorial extensions

If this is right

  • Quantum channels sharing fiber with O-band classical traffic should be parked in the quieter C-band windows identified by the scan rather than chosen only by ITU grid convenience.
  • Network planners can treat laboratory Raman models as a first-order predictor for metro deployments once live-fiber anomaly maps are subtracted.
  • An operational quantum SNR metric must explicitly fold in Raman counts from co-propagating classical power, not only dark counts and ambient light.
  • Commercial SFP sources and narrow-line lasers produce comparable Raman envelopes after power and length normalization, so either can be used for planning.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • If the unexplained spikes are confirmed as inter-fiber crosstalk inside the shared cable jacket, multi-fiber ducts may need spatial or spectral isolation rules that single-fiber lab models never captured.
  • The same measurement recipe could be turned into a routine pre-deployment audit that maps every new metro span before quantum equipment is installed.
  • Because the C-band is preferred for low loss, the reverse band assignment (quantum in O, classical in C) may remain systematically noisier; the data here already hint that the quieter windows are not uniform across the C-band.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

4 major / 5 minor

Summary. The manuscript reports a field characterization of spontaneous Raman scattering (SpRS) into the C-band produced by O-band classical pumps (commercial SFP at ~−0.8 dBm and a narrowband 1310 nm laser at ~−4.6 dBm) over a ~7 km metropolitan fiber loop (and a 3.5 km one-way path) of the University of Naples Federico II network. Cascaded WDMs isolate bands; a 25 GHz tunable filter steps across 1525–1565 nm; C-band photons are counted on an SNSPD (~80% SDE, ~100 cps dark counts). Loss is tabulated per segment (Table I). Raw and power/length-normalized spectra (Fig. 3) are compared to a no-launch control and to a 5 km lab spool reference from prior work. The authors report overall spectral agreement with the lab after normalization, attribute residual field peaks (present with no launched signal) to fiber imperfections or possible inter-fiber crosstalk, and conclude that Raman noise is a dominant contribution to quantum SNR beyond background/detector noise, so C-band operating frequencies must be chosen carefully and that less-affected spectral regions provide practical guidelines for quantum channel allocation beside O-band classical traffic.

Significance. Field Raman spectra on deployed metro fiber with a commercial SFP, dual path lengths, a no-launch control, and explicit lab–field comparison after standard power/length normalization are a useful incremental contribution to quantum–classical coexistence engineering. The C-band quantum / O-band classical orientation (lowest-loss band for the quantum channel) is less common than the reverse and is practically motivated. If scoped strictly as noise characterization that informs channel planning, the data set has clear value for groups deploying entanglement or QKD over existing plant. The work does not, however, ship protocol-level metrics, machine-checked models, or a closed-form SNR derivation; its significance is empirical and infrastructure-facing rather than foundational.

major comments (4)
  1. [Abstract; §V] Abstract and §V claim that Raman-induced noise is a dominant contribution to quantum SNR beyond background and detector noise, and that the measured spectra yield practical guidelines for optimal C-band quantum channel allocation. The data are O-band-pumped C-band photon-count spectra only (Fig. 3); no co-propagating quantum signal, entangled pairs, weak-coherent channel, coincidences, fidelity, or QBER is measured on the metro loop (§II–IV). Dominance is argued by comparing Raman counts to the no-launch background and ~100 cps dark counts, without a protocol-level SNR (or QBER) formula that folds in expected pair rate, filter bandwidth, timing window, and classical launch power. The allocation guideline therefore rests on the Raman spectral shape alone. Either add a co-propagating quantum/entanglement measurement or a quantitative SNR/QBER model using the reported counts, or substantial
  2. [§I] §I states that, to the authors’ knowledge, this work is “the first experimental demonstration of quantum networking” with C-band quantum and O-band classical (contrasted with reverse configurations [13], [14]). The experiment demonstrates Raman characterization under that band assignment, not entanglement distribution, teleportation, or any quantum networking protocol. That phrasing overstates the result and should be corrected to match what was measured.
  3. [Fig. 3a; §IV–V] Fig. 3a shows localized peaks that remain in the no-launch control and raise counts by up to an order of magnitude; §IV–V attribute them to deployed-fiber imperfections or possible inter-fiber crosstalk (Fig. 4), with analysis “on-going.” §V still presents Raman-shape-based channel selection as the practical guideline. If non-Raman field artifacts can dominate selected DWDM channels (e.g., near 1560 nm / channels 21–22), ranking channels by the smooth Raman profile alone is incomplete for operational allocation. Clarify how anomalies are handled in any recommended channel list, or restrict recommendations to wavelengths where the no-launch control is flat and Raman is the leading excess noise.
  4. [§III–IV; Table I; Fig. 3b] Normalization (Fig. 3b) rescales by launched power and effective length and compares to a 5 km lab spool attenuated at 0.35 dB/km. Table I shows strongly inhomogeneous, wavelength-dependent segment losses (SMF-28 vs SMR; Lab–PoP vs PoP–PoP), and SpRS generation is distributed along the fiber with local pump power. A single effective-length factor can distort the absolute noise level used for the “Raman-dominant SNR” claim even if the spectral shape is preserved. State the effective-length model explicitly (integral of P_pump(z) e^{-α_s z} or equivalent) and report absolute noise spectral density (e.g., cps/nm/mW or photons/s in the 25 GHz filter) before and after normalization so readers can recompute SNR for their launch powers.
minor comments (5)
  1. [Abstract; §I; §IV–V] Abstract and body: repeated grammar/typos — “generated by a classical the O-band signal”; “The main contribution of this study lays in”; “The experimental results shows”; “allo iwng us to identifiy”; “theorethical modelling”; “asses whether”. A full copy-edit pass is needed.
  2. [Fig. 3] Fig. 3 caption and §IV: “in-ab measurements” should be “in-lab”. Axis labels and units (cps vs normalized units) should be stated unambiguously in both panels.
  3. [§II] SFP model is given (Finisar FTLF1321P1BTL) but its optical spectrum / linewidth under the drive conditions used is not shown; a brief OSA trace would strengthen the commercial-vs-narrowband comparison.
  4. [References; §IV] Heavy dependence on the authors’ own preliminary lab preprint [5] for the reference spectrum is fine if the metro data are independent, but the citation should note status (accepted/submitted) and what is new here versus reused.
  5. [§V] DWDM channel numbering (e.g., channels 21–22 at ~1560 nm) should be tied to a standard ITU grid table or explicit center wavelengths so readers can map recommendations without ambiguity.

Circularity Check

1 steps flagged · score 1.0 of 10

Measurement paper with independent metro data; only minor non-load-bearing self-citation to authors' lab baseline [5].

  1. self citation load bearing [§V Discussion; also Abstract and §IV Results (Fig. 3b)]
    "The measurements performed in the urban loop are consistent with previous results obtained in controlled laboratory environments [5]. ... these results provide further confirmation of the accuracy of the theorethical modelling developed from laboratory measurements, such as the one presented in [5], demonstrating their ability to accurately describe Raman noise behaviour even in real urban scenarios."

    Lab–field agreement and endorsement of the authors' own prior modelling rest on self-citation [5] (overlapping authors). This is minor and not load-bearing for the metro spectra themselves, which are independently measured; it slightly props the narrative that lab theory already predicts field Raman behaviour.

full rationale

This is an experimental characterization paper, not a fitted theoretical derivation. C-band photon-count spectra are measured directly on a 7 km metropolitan loop (and 3.5 km path) with commercial SFP and narrowband O-band pumps, then compared to a no-pump background and to a separately acquired 5 km lab spool. Normalization by launched power and effective length is standard metrology and does not force the observed spectral shape or the field anomalies. The central field result—Raman profile plus localized non-Raman peaks in deployed fiber—is acquired independently of any model fit. Self-citation to the authors' prior lab preprint [5] supplies the comparison baseline and is invoked to claim consistency and to endorse prior theoretical modelling, but the metro counts do not reduce to that citation by construction; agreement is an empirical check, not a definitional identity. No uniqueness theorem, ansatz smuggling, or renamed known law appears. Overclaim that count spectra alone demonstrate quantum-SNR dominance and channel-allocation guidelines is a correctness/scope issue, not circularity. Score 1 only for the light self-citation scaffolding around the lab comparison.

Assumptions & free parameters 3 free parameters · 4 assumptions · 0 invented entities

Load-bearing content is experimental, not axiomatic theory. Claims rest on standard fiber Raman physics, the adequacy of SNSPD count rates as a noise proxy, normalization choices, and the interpretation that no-launch peaks are plant artifacts. No new particles or forces; free parameters are experimental set-points (bias current/SDE, filter bandwidth, power scaling) rather than fitted physical constants.

free parameters (3)
  • SNSPD system detection efficiency target (~80%) = ~80% SDE, ~100 cps dark counts
    Bias current is tuned to ~80% SDE with ~100 cps dark counts; absolute count rates and thus claimed noise levels scale with this choice.
  • Tunable filter bandwidth and step (25 GHz / 500 pm) = 25 GHz (200 pm), 500 pm steps
    Spectral resolution and which sub-channel features are resolved depend on this instrument setting; quieter-region identification is resolution-dependent.
  • Power and length normalization factors = Scaled to source power; 7 km / 3.5 km / 5 km spool with 0.35 dB/km
    Datasets are rescaled by source power and effective fiber length (and lab spool by 0.35 dB/km) to overlay spectra; the visual ‘agreement’ depends on these scalings.
assumptions (4)
  • domain assumption Spontaneous Raman scattering from O-band classical light into C-band is the primary co-propagation noise mechanism of interest and is comparable across commercial SMF plant and lab spools once length/power are normalized.
    Stated in Introduction and used throughout Results/Discussion as the reason C-band quantum / O-band classical coexistence needs spectral planning.
  • domain assumption C-band photon counts on an SNSPD after WDM and narrowband filtering are a sufficient proxy for noise that would degrade entanglement distribution and quantum SNR.
    No entanglement source or protocol QBER is measured; §V elevates count spectra to SNR and channel-allocation guidance.
  • ad hoc to paper Spectral peaks that remain with no launched classical signal are not Raman and can be excluded from the Raman profile (attributed to fiber imperfections or inter-fiber crosstalk).
    §IV no-launch control; §V crosstalk hypothesis marked on-going. Central ‘lab agrees with field’ claim depends on treating peaks as non-Raman.
  • domain assumption Standard single-mode fiber loss and Raman behavior in SMF-28/SMR metro plant follow accepted telecom coefficients used for insertion-loss and normalization.
    §III loss characterization and Table I; lab spool attenuation 0.35 dB/km used in §IV.

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Cite this review

Pith. "Pith review of Towards Quantum Networks: Characterizing Raman Noise over Metropolitan-scale Fiber Network." pith.science (2026). https://pith.science/paper/DMADJWJY

@misc{pith2026260727970,
  author       = {Pith},
  title        = {Pith review of: Towards Quantum Networks: Characterizing Raman Noise over Metropolitan-scale Fiber Network},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/DMADJWJY}},
  note         = {Machine review of arXiv:2607.27970}
}
read the original abstract

The coexistence of quantum and classical signals in optical fiber infrastructures represents a major challenge for large-scale quantum networks, as noise sources such as Raman scattering can significantly impact entanglement distribution, and the quantum protocols based on it. In this work, we analyze Raman scattering in the C-band, used for entanglement distribution, generated by a classical the O-band signal. The main contribution of this study lays in investigating these effects in a real metropolitan-fiber network, moving beyond controlled laboratory experiments to deployed telecommunication environments. Measurements are performed over a 7 km metropolitan fiber link using commercial sources and narrowband lasers. The experimental results shows good agreement between the measurements taken under laboratory conditions, although, within the metropolitan-scale loop, localized spectral anomalies are observed in the deployed fibers. Therefore, our results show that, whenever a quantum signal propagates in the C-band alongside an O-band classical channel within the same fiber, careful selection of the operating frequency is required, as Raman scattering and other real-world noise sources can significantly affect the quality and stability of the quantum transmission. In particular, we identify spectral regions that are less affected by Raman noise, thereby providing practical guidelines for optimal quantum channel allocation. We demonstrate that Raman-induced noise constitutes a dominant contribution to the quantum signal-to-noise ratio (SNR) in realistic deployments, beyond background and detector noise. Overall, our findings offer practical insights for deploying quantum communication systems over existing fiber networks, supporting the development of robust and scalable quantum infrastructures.

Figures

Figures reproduced from arXiv: 2607.27970 by the authors.

Figure 1
Figure 1. Scheme of the 7Km urban fiber loop of University of Naples Federico II. The first path, P1, connects Monte Sant’Angelo (MSA) campus to the Engineering Faculty (EF) campus. The loop is closed through the second path, P2, which enables the return transmission of photons back to MSA campus. a) MSA campus hosts the laboratory (Lab), where photons are generated and detected after completing the loop. The campus also cont… view at source ↗
Figure 2
Figure 2. Experimental setup within the National Quantu￾mInternet.it testbed. A classical signal in the O-band, gener￾ated either by a commercial SFP transmitter or by a narrow￾linewidth laser source, is filtered through cascaded WDM modules and launched into the 7Km urban fiber loop. Raman￾scattered photons generated in the C-band are spectrally se￾lected using a tunable narrowband filter and detected by a superconducting na… view at source ↗
Figure 3
Figure 3. Experimental results of Raman scattering spectra. C-band SpRS photons generated from O-band classical sources are measured in terms of photon counts per second as a function of wavelength, corresponding to the ITU DWDM channels. The data include measurements obtained using both a narrowband laser source and a commercial SFP transmitter over the full fiber loop (7 km) and a one-way path (3.5 km). In (a) - Raw data, a… view at source ↗
Figures from the paper (2 more)
Figure 4
Figure 4. Figure 4: Transversal view of the metropolitan fiber. The connection between the two campuses, MSA and EF, is estab￾lished through a bundle of optical fibers. For our experiment, we use one of these channels, consisting of an SMF fiber with a core diameter of approximately 9 µm …
Figure 1
Figure 1. Figure 1: Fig.1. In this case, the narrowband source is used, with the [PITH_FULL_IMAGE:figures/full_fig_p004_1.png]

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Reference graph

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