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REVIEW 3 major objections 4 minor 40 references

Quantum-Classical Coexistence Network Tomography

T0 review · 3 major / 4 minor · reviewed 2026-08-11 · deepseek-v4-flash

Pith's one-line read The paper establishes that all per-link channel parameters of a coexisting fiber—success ratio, loss ratio, and three direction-dependent depolarization ratios—are recoverable in closed form from end-to-end photon counts, and that a…

desk verdict A clean closed-form link tomography result and a genuinely novel gauge-fixing protocol, but the network-level claims rest on a multiplicative Raman composition rule the authors themselves concede is non-physical, and the single-link validation is partly circular; the paper deserves a serious referee but the multi-link claims are not yet established. read the letter →

arxiv 2608.09364 v1 pith:YCKBLGFL submitted 2026-08-10 cs.IT cs.NImath.IT

classification cs.ITcs.NImath.IT MSC 81P4581P50
keywords quantum-classicalcoexistencenetworktomographyclosed-formchannelestimationdirection-dependentdepolarizationRamanscatteringstaridentifiabilityquantummodel
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

The paper tries to establish that a fiber carrying both quantum and classical signals can be fully characterized from end-to-end photon counts, without touching each link. Its channel model splits every input photon into three outcomes—loss, intact success, or depolarization—with the depolarization ratio depending on whether the classical signal is absent, co-propagating, or counter-propagating. The paper derives closed-form estimators for all five per-link parameters on a single fiber, and proves that in a star network the only remaining degeneracy is removed by flipping the classical-signal direction on one link. On experimental testbed data the link estimators track a full process-tomography baseline, so a sympathetic reader would take the framework as a candidate for practical quantum-network monitoring.

What carries the argument

The workhorse is a three-outcome population model: each input photon is either lost (ratio $q$), transmitted with polarization intact (ratio $s$), or depolarized (ratio $d^{(x)}$), with Raman-injected photons making the received ratio $r^{(x)} = s + d^{(x)}$ a per-photon yield that can exceed the loss-limited baseline. From this model the identity $\mathbb{E}[M/T] = s + d^{(x)}/2$ gives the closed-form link estimators, and a recursion on intact versus depolarized populations gives the path-composition rule $R_P/T_P = \prod_{e\in P}(s_e + d_e^{(x_e)})$ together with $(2M_P - R_P)/T_P = \prod_{e\in P} s_e$. The star-network identifiability rests on Lemma 3: the six end-to-end received-ratio equations have rank five with a one-dimensional kernel, so a single flipped-link measurement appends a row with nonzero projection on that kernel and fixes the gauge under the flip-invariance assumption that reversing the classical direction interchanges co- and counter-propagation without introducing new unknowns.

What would settle it

Splice two calibrated coexisting fiber spools, measure each single-link received ratio $r_i = R_i/T_i$ under identical classical traffic, then send a known input population through both and compare the measured end-to-end $R/T$ with the product $r_1 r_2$. If the product systematically disagrees while the downstream-attenuated formula of Remark 5 fits, the multiplicative composition rule that carries the network estimators is falsified.

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Extended reading notes

Core claim

The central claim is that the coexisting fiber is characterized by the success ratio $s$, the loss ratio $q$, and the direction-dependent depolarization ratios $d^{(0)}$, $d^{(1)}$, $d^{(2)}$, and that these are identifiable from the two expected observables $\mathbb{E}[R/T] = s + d^{(x)} = r^{(x)}$ and $\mathbb{E}[M/T] = s + d^{(x)}/2$. The single-link estimators are closed form: $\hat{d}^{(x)} = 2(R^{(x)} - M^{(x)})/T^{(x)}$ for each coexistence scenario and $\hat{s} = \frac{1}{3}\sum_{x=0}^{2}(2M^{(x)} - R^{(x)})/T^{(x)}$, with $\hat{q} = 1 - \hat{s} - \hat{d}^{(0)}$. Along a path the intact-photon count multiplies, $(2M_P - R_P)/T_P = \prod_{e\in P} s_e$, and the received ratio multiplies as $R_P/T_P = \prod_{e\in P}(s_e + d_e^{(x_e)})$, which is what lets star-network success ratios be extracted from pairwise end-node probes. The paper's key identifiability result is that the six received-ratio equations for a three-link star have rank five, with the kernel spanned by the vector that assigns $+1$ to each link's end-to-hub received ratio and $-1$ to each link's hub-to-end received ratio; reversing the classical-signal direction on a single link adds a measured equation that fixes this gauge and identifies all six direction-dependent depolarization ratios.

Load-bearing premise

The load-bearing premise is that per-link received ratios multiply along a path, the injection convention of Proposition 4; if deployed Raman injection is pumped by the classical signal alone and is only attenuated downstream, as Remark 5 notes, then the network-level estimators are biased even when the single-link model is correct.

Editorial extensions

If this is right

  • Single-link tomography recovers depolarization probabilities and process fidelities that closely track the process-tomography baseline across fiber lengths and wavelengths, with the residual gap attributed to the depolarization-only model approximation.
  • Star networks can be fully characterized from edge measurements: per-link success ratios come from $\hat{s}_k = \sqrt{Q_{k,\ell}Q_{k,m}/Q_{\ell,m}}$, and the single-link classical-flip protocol resolves the remaining direction-dependent depolarization ratios.
  • Tree and mesh topologies inherit the same multiplicative observables: peeling gives per-link estimates on trees, and log-linear least squares with a full-column-rank path-by-link incidence matrix gives them on general graphs.
  • Network-level success-ratio estimation is expensive: the required sample size grows super-exponentially with path depth, following $T_{\rm req}(n) = [s^n(1-s^n) + ((s+d)^n - s^n)]/(n^2 s^{2n} \epsilon^2)$.
  • Operationally, the findings favor co-propagating classical traffic, shallow monitored paths, and placing quantum wavelengths away from the classical carrier line.

Reading between the lines

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

  • A decisive next step the paper itself proposes—splicing two calibrated spools and checking the multiplicative law against the downstream-attenuated formula of Remark 5—would settle whether the injection convention holds in practice.
  • The single-flip gauge-fixing trick could transfer to other direction-dependent noise mechanisms, such as polarization-mode dispersion or cross-phase modulation, wherever per-link unknowns pair by propagation direction.
  • If the multiplicative law survives in deployed fibers, regularized or non-negativity-constrained estimators with adaptive probe allocation could attack the super-exponential sample-size bottleneck, which the paper lists as an open direction.
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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

3 major / 4 minor

Summary. The paper develops a tomography framework for quantum-classical coexistence networks (QCNs), in which quantum and classical signals share fiber. It models each coexisting fiber via photon loss, successful transmission, and three direction-dependent depolarization ratios, and derives closed-form estimators for these parameters from end-to-end counts (Section IV). It then extends the approach to star networks using multiplicative path equations, resolving a residual gauge degeneracy by reversing the classical-signal direction on one link (Section V, Lemma 3), and further to trees and meshes via peeling and least-squares methods. Validation consists of single-link experimental data, emulated star networks composed from measured single-link channels, and synthetic Monte-Carlo simulations for general topologies.

Significance. If the central claims hold, the framework would be a useful tool for characterizing coexistence links without per-link access, and the star-network identifiability analysis with the single-link flip protocol is a genuine contribution. The algebraic derivations are self-consistent, the sample-complexity scaling law in Section VI-D is concrete and falsifiable, and the authors are candid about the limits of their validation, explicitly flagging the injection convention in Remark 5 and the missing physical two-link test. The significance of the network-level claim is, however, conditional on a physical multi-link composition test, since the multiplicative path rule is an acknowledged modeling convention rather than a demonstrated property of deployed fibers.

major comments (3)
  1. [§VI-A] The matched-outcome count M is not an independent measurement: the text states that M is obtained from the aggregated counts together with the estimated depolarization probability via M=R(1-p/2), which is exactly the model identity E[M/T]=s+d(x)/2=r(x)(1-p(x)/2). Because the QLT estimators (16) invert this same identity, the QLT-versus-BPT agreement in Figs. 5–6 is circular for the depolarization parameters unless p is obtained from an independent procedure. The manuscript should either reconstruct M directly from per-basis matched-outcome counts without using p, or estimate p from an independent data split, and should then report how the agreement changes.
  2. [§V-A, Proposition 4 and Remark 5] Equation (24) is derived from the recursion (27) in which Raman injection on each link scales with the incoming photon population (the ν_j R(j-1) term). Remark 5 acknowledges that physical spontaneous Raman is pumped by the classical signal alone and is attenuated only downstream, giving R_P/T_P = ∏_j t_j + ∑_j ν_j ∏_{k>j} t_k, and that the two formulas coincide only when one link dominates injection or per-hop transmittances approach unity. In the measured coexistence regime (Table I: s≈1e-3, d in 0.03–0.6) transmittances are not near unity, so on a deployed multi-hop path Eq. (24) is misspecified and the star estimators (21), the peeling estimator (30), and the least-squares solver (31) invert an abstract multiplicative model rather than the physical per-link parameters. The gauge flip of Lemma 3 fixes a degeneracy inside that model only and cannot remove this composition bias. This is the load-bearing gap: the central network-level claim requires a direct physical two-link test of (18)–(19), which is not provided.
  3. [§VI-C] The multi-link validation is emulated by composing single-link channels with exactly the binomial cascade implied by the model under test (Type I), and Type II retains the same received-count model for R while only the matched fraction uses the BPT maps. Figure 10 shows a persistent bias on the 5-km link (s_2≈0.06 against truth 0.0015, with the depolarization estimates displaced by the same amount), which the authors attribute to model mismatch. Consequently the emulations do not establish that physical concatenated links obey (24); they only verify algebraic self-consistency of the estimators under the assumption that the multiplicative law holds. The conclusion lists a direct two-link experiment as future work, but this experiment is necessary to support the paper's central claim rather than optional.
minor comments (4)
  1. [§V, Lemma 3 proof] The displayed ordering of the six unknowns and the null vector v=(+1,-1,-1,+1,-1,+1) are inconsistent with the statement that v assigns +1 to every end-to-hub received ratio and −1 to every hub-to-end ratio; please align the ordering.
  2. [§II and §V-A] The symbol T denotes both the launched-photon count in Section II and the tree (calligraphic T) in Section V-A; although calligraphic T avoids a collision, the notation remains easy to confuse and a rename would improve readability.
  3. [Data and Code Availability] The code and data availability section states that the code will be made available upon publication; for a paper whose numerical claims rest on a custom Monte-Carlo sampler, an archival version should be deposited with the submission.
  4. [§VI-B, Fig. 6] The 'small residual gap' between Coex QLT and Coex BPT is not quantified; reporting the maximum or median fidelity gap across the eight wavelengths and three fiber lengths would strengthen the claim.

Circularity Check

1 steps flagged · score 6.0 of 10

The link-level experimental validation is circular: M is constructed from the very depolarization probability that QLT is then compared against, so the QLT/BPT agreement is algebraic; the network validation is an acknowledged self-consistency check, and the main remaining risk is the Remark 5 injection convention, which is model misspecification rather than circularity.

  1. fitted input called prediction [Section VI-A, 'Deriving the observables T, R, and M' (validation setup); see also §VI-B, Figs. 5–6.]
    "The third is the number of matched-outcome photons M, which we obtain from the same aggregated counts together with the estimated depolarization probability via M=R(1-p/2). This relation is exactly the model identity E[M/T] = s+d(x)/2 = r(x)(1-p(x)/2). Aggregating over bases leaves the link relations intact."

    By the basic model (3), p(x) = d(x)/(s+d(x)) and 2(R-M)/R = [2(s+d/2)-(s+d)]/(s+d) = d/(s+d) = p(x). So the QLT depolarization estimate, p_QLT = 2(R-M)/R, is algebraically identical to the p used to construct M whenever M is defined as R(1-p/2). The paper then presents the QLT-versus-BPT agreement in Figs. 5-6 as evidence that the model and estimators are accurate. But the matched-outcome observable fed to QLT is not an independent raw count: it is built from the already-estimated depolarization probability through the same model identity that QLT inverts.

full rationale

The closed-form link estimators in Eqs. (15)-(16) are algebraically correct inversions of the model definitions in Eq. (3), and the star/general-topology estimators solve well-posed linear systems once Proposition 4 is granted. Those derivations are not circular. The circularity is concentrated in the experimental validation step: the paper reconstructs the matched-outcome observable M from the estimated depolarization probability p via M=R(1-p/2), which is exactly the relation the QLT estimator inverts. Consequently, the reported QLT/BPT agreement in the depolarization probability and process fidelity plots is an identity up to the provenance of p, not independent evidence; the BPT baseline and QLT are not comparing two independent reconstructions from the same raw counts. I therefore score this as a partial circularity of the validation, not of the derivation. The network-level validation is explicitly self-consistent: the emulation and Monte-Carlo simulations compose the same depolarization channels the estimators invert, and the paper concedes this is 'not a test of model fidelity.' That is an honest scope limitation rather than a disguised circular prediction. The reuse of the self-cited progressive-etching algorithm [12] is acknowledged and not load-bearing, since Proposition 4 and the gauge-flip identifiability argument are derived in this paper. Finally, Remark 5's admission that the multiplicative composition law differs from the physically downstream-attenuated Raman formula is a model-risk concern, not circularity; it does not inflate the circularity score but is the main reason the network-level claims would need a physical two-link test.

Assumptions & free parameters 3 free parameters · 7 assumptions · 2 invented entities

The model has five physical parameters per link plus several modeling choices. The depolarization parameters d(1) and d(2) are the most important fitted quantities; the tensor-product ancilla mode and the injection convention are assumptions introduced to make the model and estimators tractable.

free parameters (3)
  • Per-link depolarization ratios d(0), d(1), d(2) = Table I: e.g. d(1)=0.0306, d(2)=0.0967 at 0.5 km; d(1)=0.3355, d(2)=0.5916 at 15 km
    The paper explicitly treats d(1) and d(2) as empirical per-link, per-direction parameters estimated by tomography rather than predicted from first principles; the Raman integrals (13)-(14) are only qualitative.
  • Success ratio s = Table I: ~0.001-0.002
    Estimated from (2M-R)/T counts; every network estimator is a function of s.
  • Loss ratio q = q = 1 - s - d(0)
    Not independent; fixed by the normalization q+s+d(0)=1.
assumptions (7)
  • domain assumption A depolarized photon has uniformly random polarization, so it yields the matched measurement outcome with probability 1/2.
    Used throughout to get E[M/T]=s+d/2; if the depolarization is anisotropic or state-dependent, the estimators are biased.
  • domain assumption The three link outcomes are mutually exclusive and exhaustive with q+s+d(0)=1, and under coexistence q+s+d(x)>=1 because of injected photons.
    Defines the basic model in Section II.B and the normalization used for q.
  • domain assumption The Raman excess d(x)-d(0) lies in [0,1], keeping the injection channel CPTP.
    Required for the CPTP tensor-product form in Section III; not measured or enforced beyond weak-injection plausibility.
  • ad hoc to paper The signal and Raman-collection modes are nearly orthogonal, so ladder operators commute up to a small overlap eta that is neglected.
    The paper states eta<<1 but gives no measurement; this approximation is what allows the tensor-product channel representation.
  • ad hoc to paper Flip invariance: reversing the classical signal direction on a link changes which propagation mode a probe sees but leaves d(1) and d(2) unchanged.
    Assumption 2 is required for the gauge-fixing protocol; the paper gives plausible power-averaging arguments but no dedicated experiment.
  • ad hoc to paper Injection convention: Raman excess on a link scales with the quantum photon population arriving at that link, so path observables multiply.
    Remark 5 acknowledges this is not the physical pump model; it is the assumption under which Proposition 4 and the network estimators are derived.
  • domain assumption Weak injection: multi-photon Raman events are negligible, allowing one-photon truncation of each mode.
    Standard for weak spontaneous Raman; stated in Section III.A.
invented entities (2)
  • Ancilla Raman mode S2 in the tensor-product channel
    purpose: Carries Raman-injected photons so the coexisting fiber can be represented as a normalized CPTP map acting on two modes.
    A bookkeeping mode: the detector observable erases the mode identity, and its main role is to reproduce the basic-model observables. No external measurement distinguishes S2.
  • Two-mode photon-number observable N with spectrum {0,1,2}
    purpose: Accounts for the excess photons delivered by coexistence without making the state unnormalized.
    Constructed so that R/T and M/T reduce exactly to the basic model; it is internal to the model, not an independently observed quantity.

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Pith. "Pith review of Quantum-Classical Coexistence Network Tomography." pith.science (2026). https://pith.science/paper/YCKBLGFL

@misc{pith2026260809364,
  author       = {Pith},
  title        = {Pith review of: Quantum-Classical Coexistence Network Tomography},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/YCKBLGFL}},
  note         = {Machine review of arXiv:2608.09364}
}
read the original abstract

Quantum-classical coexistence networks (QCNs) share optical fiber between quantum and classical signals via wavelength-division multiplexing, offering a practical path to quantum communication over existing telecom infrastructure. However, co- and counter-propagating classical traffic introduce distinct depolarization noise, complicating channel characterization. We develop a tomography framework that infers per-link channel parameters of a QCN from end-to-end measurements alone. We first model each coexisting fiber by decomposing the signal evolution into photon loss, successful transmission, and three direction-dependent depolarization components. We then derive closed-form link-level estimators, and extend the approach to star-topology networks through a system of multiplicative equations across end-node pairs, together with a simple classical-signal-direction-switching protocol that resolves the remaining unknowns. On single-link experimental testbed data, we recover per-link depolarization probabilities accurately, with estimated process fidelities closely tracking the Bayesian-process-tomography baseline across multiple fiber lengths and wavelengths; residual gaps reflect the depolarization-only approximation. Absent a multi-link coexistence testbed, we validate the star-network estimators on emulated paths built from measured single-link channels. We further extend the framework in two directions: (i) a channel model that factorizes the coexisting fiber into a depolarizing-with-loss signal channel and a Raman-noise-injection channel on separate optical modes -- a completely-positive, trace-preserving tensor product -- whose link observables reduce exactly to our basic model; and (ii) a generalization to arbitrary topologies via a peeling algorithm (trees) and a least-squares estimator (meshes), validated by Monte-Carlo simulations on tree and cyclic-mesh networks.

Figures

Figures reproduced from arXiv: 2608.09364 by the authors.

Figure 1
Figure 1. Model of quantum-classical coexistence fiber. The upper block represents the quantum channel with three possible [PITH_FULL_IMAGE:figures/full_fig_p004_1.png] view at source ↗
Figure 2
Figure 2. Channel parameters (top row) and photon numbers [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Link and network tomography illustrations. [PITH_FULL_IMAGE:figures/full_fig_p007_3.png] view at source ↗
Figures from the paper (10 more)
Figure 4
Figure 4. Figure 4: Progressive peeling on a five-link tree (the same network used for the Monte-Carlo simulations in § [PITH_FULL_IMAGE:figures/full_fig_p012_4.png]
Figure 5
Figure 5. Figure 5: Depolarization probability vs. wavelength for different fiber lengths and propagation directions, where (Co) denotes [PITH_FULL_IMAGE:figures/full_fig_p012_5.png]
Figure 6
Figure 6. Figure 6: Process fidelity vs. distance (km) and propagation direction (co- vs. counter-propagating) for six representative [PITH_FULL_IMAGE:figures/full_fig_p013_6.png]
Figure 7
Figure 7. Figure 7: Star network tomography compilation strategies. Each [PITH_FULL_IMAGE:figures/full_fig_p015_7.png]
Figure 8
Figure 8. Figure 8: Estimated channel parameters under Type I compilation (sample size [PITH_FULL_IMAGE:figures/full_fig_p017_8.png]
Figure 9
Figure 9. Figure 9: Estimated channel parameters under Type I compilation (sample size [PITH_FULL_IMAGE:figures/full_fig_p017_9.png]
Figure 10
Figure 10. Figure 10: Estimated channel parameters under Type II compilation (sample size [PITH_FULL_IMAGE:figures/full_fig_p017_10.png]
Figure 11
Figure 11. Figure 11: Per-link success-ratio recovery on a five-link tree [PITH_FULL_IMAGE:figures/full_fig_p018_11.png]
Figure 12
Figure 12. Figure 12: General-graph least-squares recovery on a cyclic mesh ( [PITH_FULL_IMAGE:figures/full_fig_p019_12.png]
Figure 13
Figure 13. Figure 13: Sample-size requirements grow super-exponentially with network depth. [PITH_FULL_IMAGE:figures/full_fig_p019_13.png]

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Pith tools

Reviewed August 11, 2026 · model on record in the stance chip above.