REVIEW 4 major objections 5 minor 35 references
Robustness of WDM technique for the co-propagation of quantum with classical signals in an optical fiber
T0 review · 4 major / 5 minor · reviewed 2026-08-12 · deepseek-v4-flash
Pith's one-line read Two ITU channels of separation make WDM crosstalk negligible
desk verdict The positive-P simulation of WDM co-propagation is new and the qualitative crosstalk trend is plausible, but the '2 ITU channels is sufficient' conclusion rests on an RMS-width metric that is blind to the phase distortions QKD actually suffers. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The central object is the positive-P representation of the generalized nonlinear Schrödinger equation, which converts the quantum evolution of a field in a lossy Kerr fiber into two coupled stochastic differential equations for scaled field variables $\phi(\zeta,\tau)$ and $\phi^+(\zeta,\tau)$. Crosstalk is quantified by $C(\zeta)$, the ratio of the root-mean-square temporal width of the quantum pulse recovered from its ITU channel after spectral filtering to the width the same pulse would have in dark fiber. The stochastic equations are integrated with the Ito semi-implicit midpoint split-step method, with a 16-QAM classical signal at 10 Gbps as the interfering field.
What would settle it
Run the same model with a quantum bit error rate or phase-quadrature variance as the output metric instead of $C(\zeta)$ for a two-channel separation at 1 mW to 10 mW classical launch power; if the QBER stays above the protocol's error threshold while $C(\zeta)\approx 1$, the two-channel separation claim fails.
Extended reading notes
Core claim
The central claim is that the crosstalk between a weak quantum pulse and a strong classical signal co-propagating over 50 km of standard single-mode fiber depends mainly on the wavelength separation and the classical launch power. With the quantum signal fixed in ITU channel 38 and the classical channel varied across the C-band, only the adjacent and next-nearest channels raise the crosstalk metric $C(\zeta)$ above 1, and the effect grows monotonically with launch power from 0.1 mW to 100 mW. For separations of two ITU channels or more, $C(\zeta)$ stays near 1, meaning the quantum pulse is as clean as if it had propagated alone. The authors conclude that two empty ITU channels, 2.39 nm, suffice for negligible crosstalk, and that shorter quantum pulses at fixed photon number reduce crosstalk, which would favour higher-clock-rate QKD.
Load-bearing premise
The central claim assumes that crosstalk is fully captured by the widening of the quantum pulse's root-mean-square temporal width, so a classical signal that adds phase noise, frequency jitter, or spectral contamination without broadening the envelope would be counted as harmless even if it degrades QKD.
Editorial extensions
If this is right
- A separation of two ITU channels on the 100 GHz grid is sufficient for negligible crosstalk, so QKD and classical traffic can share one fiber.
- Crosstalk rises monotonically with classical launch power for adjacent channels, so launch-power budgets must be set per channel rather than globally.
- Narrowing the quantum pulse at constant photon number lowers crosstalk, implying that higher-repetition-rate QKD systems will coexist more easily with WDM traffic.
- Channels beyond the two nearest neighbors can be treated as effectively isolated, simplifying power and wavelength planning in mixed quantum-classical links.
Reading between the lines
- The ratio of RMS temporal widths captures broadening but not phase noise or frequency jitter, so phase-encoded QKD may show degradation even where $C(\zeta)\approx 1$; a calculation of the phase-error variance would settle this.
- The paper's suspected 'sweet spot' in pulse duration is a concrete testable prediction: scanning $t_0$ at fixed channel spacing and photon number should show a minimum of crosstalk before subpicosecond Raman effects begin.
- Adapting the formalism to Fock states would test whether single-photon sources behave differently from weak coherent pulses, since photon-number statistics change how nonlinear cross-phase modulation acts.
- If the two-channel guard band holds in experiment, existing QKD deployments that use a full band separation, for example O-band versus C-band, may be wasting spectrum, and the guard band could be tightened.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This manuscript studies co-propagation of a weak quantum signal, modeled as a multimode coherent state with average photon number μ=0.4, together with a 16-QAM classical signal in standard single-mode fiber, using the positive-P representation of the generalized nonlinear Schrödinger equation with loss and Kerr noise. Crosstalk is quantified in Eq. (10) as the ratio C(ζ) of the RMS temporal width of the spectrally filtered quantum pulse in the presence of the classical signal to the corresponding width in dark fiber. Numerical simulations on a 100 GHz ITU grid in the C-band over a 50 km span show that C is close to 1 when the classical channel is separated from the quantum channel by two empty channels or more, and that C increases with classical launch power and with decreasing quantum pulse width. The paper concludes that a separation of 2 ITU channels is sufficient for negligible crosstalk and that the WDM technique is therefore robust for integrating QKD with classical traffic.
Significance. The question addressed is practically important: what guard band and launch-power constraints are needed for a QKD channel co-propagating with classical WDM traffic. A notable strength of the paper is that it starts from the established quantum GNLSE in the positive-P representation rather than from an ad hoc crosstalk model, and the input parameters are standard fiber and system values rather than parameters fitted to the output curves. If the central claim were fully supported, the result would simplify QKD deployment on existing fiber infrastructure. However, the central claim is currently tied to a pulse-width-ratio metric that is not linked to any QKD performance measure such as QBER or phase error; the authors themselves state in the Outlook that QBER should be used instead of C(ζ). The work is therefore a promising and potentially useful contribution, but the headline conclusion is not yet established.
major comments (4)
- [§III, Eq. (10), and §VI Conclusion] The conclusion that a separation of 2 ITU channels leads to negligible crosstalk is based entirely on the metric C(ζ), the ratio of RMS temporal widths of the filtered quantum pulse intensity. This metric is insensitive to phase and frequency perturbations: cross-phase modulation from the classical field imprints a time-dependent phase and frequency chirp on the weak quantum pulse, and phase-encoded or time-bin QKD can be degraded by such phase errors and timing jitter even when the RMS intensity envelope is unchanged. The spectral filter in Eq. (9) removes out-of-band classical power but not in-band phase distortion. The paper's own Outlook concedes that application-specific metrics like the quantum bit error rate are needed in place of C(ζ). The numerical results may be internally consistent, but they do not establish negligible crosstalk for QKD operation as claimed.
- [§IV, Fig. 2, and §VI Conclusion] There is an inconsistency between the headline 'separation of 2 ITU channels (2.39 nm)' and the standard 100 GHz ITU grid used in the paper. At 1550 nm, one 100 GHz channel spacing is approximately 0.8 nm, so two spacings correspond to about 1.6 nm, whereas 2.39 nm corresponds to three spacings. The text also says that crosstalk is negligible when there are 'at least two empty channels in between'; with the quantum signal at ITU channel 38, this means the classical signal at channel 41 or beyond, i.e., a separation of three ITU spacings, not two. The quantitative headline claim should be corrected and stated unambiguously.
- [§II, Eq. (8)] The initial condition for the quantum pulse contains a dimensional inconsistency. With τ defined in Section II as (t − z/v_g)/t0, the Gaussian factor in Eq. (8), exp(−τ^2/(2 t0^2)), has an exponent with dimensions of inverse time squared; if taken literally, the pulse has an unphysical temporal width rather than the nominal t0. The intended expression is almost certainly exp(−τ^2/2). Since this initial condition determines all reported crosstalk values, the discrepancy must be resolved before the numerical results can be reproduced or trusted.
- [§V, Eqs. (15)–(17), and §IV, Fig. 2] The stochastic equations contain noise terms, but the manuscript does not report the number of stochastic realizations, the step size Δζ, or any statistical error bars for the plotted crosstalk curves. If the curves in Figs. 2, 4, and 5 come from single realizations, the claimed quantitative behavior such as C ≈ 1 for distant channels and the monotonic power dependence is not fully supported. Ensemble averaging or at least error bars, together with convergence checks for the split-step scheme, are needed to substantiate the quantitative conclusions.
minor comments (5)
- [§II, Eq. (2)] The phase factor in Eq. (2), e^{i((k−k0)x+iω0t)}, mixes a spatial variable x with a time variable t and a plus sign; the propagation coordinate should be identified and the sign convention clarified.
- [Table I] The entry 't0 = √2 100 ps' lacks a multiplication symbol and is ambiguous; it should read '√2 × 100 ps' or equivalent.
- [§III, Eqs. (8)–(9)] The notation for frequency is inconsistent: Eq. (8) uses scaled frequencies Ωj and Ωq, while the filter in Eq. (9) is written using ω. This should be harmonized to avoid confusion.
- [§IV, Fig. 2] The caption says the classical channel is varied over the C-band, but the figure does not clearly label the axes or indicate whether the horizontal axis is ITU channel number, wavelength, or frequency; the units and axis labels should be added.
- [§IV, Fig. 3] The density plot caption does not state the color scale or the normalization of the plotted quantity, so the figure cannot be interpreted quantitatively; at minimum the color bar and the definition of the plotted field amplitude should be given.
Circularity Check
No significant circularity; the simulation uses an externally established stochastic GNLSE model with standard parameter values, and the crosstalk metric is an author-defined but independently computed observable.
full rationale
The paper's derivation chain is self-contained against external theory rather than circular. The propagation model in Eqs. (4a)-(4b) is taken from the established positive-P representation of quantum nonlinear fiber optics (ref. [25]), not from the paper's own results. The initial condition in Eq. (8) combines a standard 16-QAM classical signal and a Gaussian weak coherent pulse using standard fiber and system parameters (Table I: dispersion, Kerr coefficient, attenuation, channel spacing, pulse duration, photon number), with no parameter fitted to the crosstalk curves. The crosstalk metric C(ζ) in Eq. (10) is defined as the ratio of RMS temporal widths of the filtered quantum pulse with and without the classical signal, and the reported C values are numerical outputs of the simulation, not inputs used to adjust the model. The central claim that a separation of 2 ITU channels leads to negligible crosstalk therefore follows from the computed C, not from a definition that makes the result true by construction. The strongest concern in the manuscript is not circularity but external validity: the Outlook explicitly concedes that application-specific metrics such as quantum bit error rate should replace C(ζ), meaning the RMS-width measure may be insensitive to phase or frequency distortions that are relevant for QKD. That is a limitation of the chosen metric, not a circular reduction. The only self-citation is ref. [11] (Pathak, Chaudhary, et al.), used as background on phase-encoded QKD clock rates; it is not load-bearing for the derivation. No fitted-input-called-prediction, no imported uniqueness theorem, and no ansatz smuggled via self-citation are present. Accordingly, the paper is not circular and receives the lowest score.
Assumptions & free parameters
free parameters (5)
- Average photon number per quantum pulse =
0.4
- Classical launch power P0 =
0.1 mW to 100 mW
- Quantum pulse duration t0 =
sqrt(2)*100 ps, with variations
- 16-QAM bit rate Rc =
10 Gbps
- Fiber length L =
50 km
assumptions (5)
- standard math The positive-P representation maps the quantum GNLSE to coupled stochastic PDEs with the stated noise correlations (Eqs. 4a, 4b).
- domain assumption Raman scattering is negligible for the pulse widths and spectra considered.
- ad hoc to paper Crosstalk is adequately quantified by the RMS temporal width ratio C defined in Eq. (10).
- domain assumption The quantum channel is recovered with an ideal rectangular spectral filter of width delta-omega (Eq. 9).
- domain assumption Noise from inline amplifiers, polarization mode dispersion, Brillouin scattering, and other fiber impairments is absent.
Cite this review
Pith. "Pith review of Robustness of WDM technique for the co-propagation of quantum with classical signals in an optical fiber." pith.science (2026). https://pith.science/paper/3XIDULIH
@misc{pith2026241116942,
author = {Pith},
title = {Pith review of: Robustness of WDM technique for the co-propagation of quantum with classical signals in an optical fiber},
year = {2026},
howpublished = {\url{https://pith.science/paper/3XIDULIH}},
note = {Machine review of arXiv:2411.16942}
}
read the original abstract
Many quantum communication systems operate based on weak light pulses which by design are assumed to operate in isolation from regular data traffic. With the widespread availability and commercialization of these systems comes a need for seamless integration already at the physical layer. In particular for optical fiber links where wavelength division multiplexing (WDM) is the dominant data transmission technique this results in the propagation of very weak quantum signals against a strong data signal background. With this work, we present a novel theoretical approach that studies the evolution of co-propagating quantum and classical signals that are launched using WDM. The important factors that contribute to crosstalk, such as the launch power of the classical signal and the separation between the two signals in terms of wavelength, are comprehensively analyzed. Interestingly, calculations show that only the first two nearest channels from the classical channel experience noticeable crosstalk whereas other distant channels have negligible crosstalk effect. This reflects the WDM technique is in principle robust in the integration of weak quantum links into classical data traffic.
Figures
Reference graph
Works this paper leans on
-
[1]
Advances in quantum cryptography,
S. Pirandola, U. L. Andersen, L. Banchi, M. Berta, D. Bunandar, R. Colbeck, D. Englund, T. Gehring, C. Lupo, C. Ottaviani, et al. , “Advances in quantum cryptography,”Advances in optics and photonics, vol. 12, no. 4, pp. 1012–1236, 2020
work page 2020
-
[2]
Experimental twin-field quantum key distribution over 1000 km fiber distance,
Y . Liu, W.-J. Zhang, C. Jiang, J.-P. Chen, C. Zhang, W.-X. Pan, D. Ma, H. Dong, J.-M. Xiong, C.-J. Zhang, et al. , “Experimental twin-field quantum key distribution over 1000 km fiber distance,” Physical Review Letters, vol. 130, no. 21, p. 210801, 2023
work page 2023
-
[3]
A. Shamir, “How to share a secret,” Communications of the ACM , vol. 22, p. 612–613, nov 1979
work page 1979
-
[4]
Quantum secret sharing,
M. Hillery, V . Bu ˇzek, and A. Berthiaume, “Quantum secret sharing,” Physical Review A , vol. 59, p. 1829–1834, Mar. 1999
1999
-
[5]
A. Dutta and A. Pathak, “A short review on quantum identity authenti- cation protocols: how would bob know that he is talking with alice?,” Quantum Information Processing , vol. 21, p. 369, 2022
work page 2022
-
[6]
D. Gottesman and I. Chuang, “Quantum digital signatures,” arXiv preprint quant-ph/0105032, 2001
arXiv 2001
-
[7]
P. D. Drummond and M. Hillery, The quantum theory of nonlinear optics. Cambridge University Press, 2014
work page 2014
-
[8]
All-fiber self-compensating polarization encoder for quantum key distribution,
C. Agnesi, M. Avesani, A. Stanco, P. Villoresi, and G. Vallone, “All-fiber self-compensating polarization encoder for quantum key distribution,” Optics letters, vol. 44, no. 10, pp. 2398–2401, 2019
work page 2019
Show all 35 references
-
[9]
Genuine time-bin-encoded quantum key distribution over a turbulent depolarizing free-space channel,
J. Jin, J.-P. Bourgoin, R. Tannous, S. Agne, C. J. Pugh, K. B. Kuntz, B. L. Higgins, and T. Jennewein, “Genuine time-bin-encoded quantum key distribution over a turbulent depolarizing free-space channel,”Optics express, vol. 27, no. 26, pp. 37214–37223, 2019
2019
-
[10]
100 km differential phase shift quantum key distribution experi- ment with low jitter up-conversion detectors,
E. Diamanti, H. Takesue, C. Langrock, M. M. Fejer, and Y . Yamamoto, “100 km differential phase shift quantum key distribution experi- ment with low jitter up-conversion detectors,” Opt. Express , vol. 14, pp. 13073–13082, Dec 2006
2006
-
[11]
Phase encoded quantum key distribution up to 380 km in standard telecom grade fiber enabled by baseline error optimization,
N. K. Pathak, S. Chaudhary, Sangeeta, and B. Kanseri, “Phase encoded quantum key distribution up to 380 km in standard telecom grade fiber enabled by baseline error optimization,” Scientific Reports, vol. 13, no. 1, p. 15868, 2023
2023
-
[12]
Phase encoding schemes for measurement-device-independent quantum key distribution with basis-dependent flaw,
K. Tamaki, H.-K. Lo, C.-H. F. Fung, and B. Qi, “Phase encoding schemes for measurement-device-independent quantum key distribution with basis-dependent flaw,” Phys. Rev. A, vol. 85, p. 042307, Apr 2012
2012
-
[13]
Quantum cryptography: Public key distribution and coin tossing,
C. H. Bennett and G. Brassard, “Quantum cryptography: Public key distribution and coin tossing,” Theoretical Computer Science , vol. 560, p. 7–11, Dec. 2014
2014
-
[14]
Conjugate coding,
S. Wiesner, “Conjugate coding,” SIGACT News, vol. 15, p. 78–88, jan 1983
1983
-
[15]
D. J. Bernstein, Introduction to post-quantum cryptography , pp. 1–14. Berlin, Heidelberg: Springer Berlin Heidelberg, 2009
2009
-
[16]
The private classical capacity and quantum capacity of a quantum channel,
I. Devetak, “The private classical capacity and quantum capacity of a quantum channel,” IEEE Transactions on Information Theory , vol. 51, no. 1, pp. 44–55, 2005
2005
-
[17]
Integrating quantum key distribution with classical communications in backbone fiber network,
Y . Mao, B.-X. Wang, C. Zhao, G. Wang, R. Wang, H. Wang, F. Zhou, J. Nie, Q. Chen, Y . Zhao, et al., “Integrating quantum key distribution with classical communications in backbone fiber network,” Optics express, vol. 26, no. 5, pp. 6010–6020, 2018
2018
-
[18]
In-band quantum key distribution (qkd) on fiber populated by high-speed classical data channels,
T. J. Xia, D. Z. Chen, G. A. Wellbrock, A. Zavriyev, A. C. Beal, and K. M. Lee, “In-band quantum key distribution (qkd) on fiber populated by high-speed classical data channels,” in Optical Fiber Communication Conference, p. OTuJ7, Optica Publishing Group, 2006
2006
-
[19]
First quantum secured 10- gb/s dwdm transmission over the same installed fibre,
I. Choi, Y . R. Zhou, J. Dynes, Z. Yuan, A. Klar, A. Sharpe, A. Plews, M. Lucamarini, C. Radig, J. Neubert, et al., “First quantum secured 10- gb/s dwdm transmission over the same installed fibre,” in 2014 The European Conference on Optical Communication (ECOC) , pp. 1–3, IEEE, 2014
2014
-
[20]
Long-distance copropagation of quantum key distribution and terabit classical optical data channels,
L.-J. Wang, K.-H. Zou, W. Sun, Y . Mao, Y .-X. Zhu, H.-L. Yin, Q. Chen, Y . Zhao, F. Zhang, T.-Y . Chen,et al., “Long-distance copropagation of quantum key distribution and terabit classical optical data channels,” Physical Review A , vol. 95, no. 1, p. 012301, 2017
2017
-
[21]
Ultra- high bandwidth quantum secured data transmission,
J. F. Dynes, W. W. Tam, A. Plews, B. Fr ¨ohlich, A. W. Sharpe, M. Lucamarini, Z. Yuan, C. Radig, A. Straw, T. Edwards, et al., “Ultra- high bandwidth quantum secured data transmission,” Scientific reports, vol. 6, no. 1, p. 35149, 2016
2016
-
[22]
Wavelength division multiplexing of continuous variable quantum key distribution and 18.3 tbit/s data channels,
T. A. Eriksson, T. Hirano, B. J. Puttnam, G. Rademacher, R. S. Lu ´ıs, M. Fujiwara, R. Namiki, Y . Awaji, M. Takeoka, N. Wada, et al. , “Wavelength division multiplexing of continuous variable quantum key distribution and 18.3 tbit/s data channels,” Communications Physics , vo...
2019
-
[23]
Co-propagation of qkd & 6 tb/s (60x100g) dwdm channels with 17 dbm total wdm power in single and multi-span configurations,
P. Gavignet, E. Pincemin, F. Herviou, Y . Loussouarn, F. Mondain, A. Grant, L. Johnson, R. I. Woodward, J. F. Dynes, B. Summers, et al., “Co-propagation of qkd & 6 tb/s (60x100g) dwdm channels with 17 dbm total wdm power in single and multi-span configurations,” Journal of Lig...
2023
-
[24]
First demonstration of 25λ× 10 gb/s c+ l band classical/dv-qkd co-existence over single bidirectional fiber link,
F. Honz, F. Prawits, O. Alia, H. Sakr, T. Bradley, C. Zhang, R. Slav ´ık, F. Poletti, G. Kanellos, R. Nejabati, et al., “First demonstration of 25λ× 10 gb/s c+ l band classical/dv-qkd co-existence over single bidirectional fiber link,” Journal of Lightwave Technology, vol. 41,...
2023
-
[25]
Quantum theory of nonlinear fiber optics: Phase-space representations,
S. J. Carter, “Quantum theory of nonlinear fiber optics: Phase-space representations,” Phys. Rev. A, vol. 51, pp. 3274–3301, Apr 1995
1995
-
[26]
Quantum noise in optical fibers. i. stochastic equations,
P. D. Drummond and J. F. Corney, “Quantum noise in optical fibers. i. stochastic equations,” JOSA B, vol. 18, no. 2, pp. 139–152, 2001
2001
-
[27]
Quantum-field theory of squeezing in solitons,
P. D. Drummond and S. J. Carter, “Quantum-field theory of squeezing in solitons,” JOSA B, vol. 4, no. 10, pp. 1565–1573, 1987
1987
-
[28]
G. P. Agrawal, Fiber-optic communication systems. John Wiley & Sons, 2012
2012
-
[29]
2 ghz clock quantum key distribution over 260 km of standard telecom fiber,
S. Wang, W. Chen, J.-F. Guo, Z.-Q. Yin, H.-W. Li, Z. Zhou, G.-C. Guo, and Z.-F. Han, “2 ghz clock quantum key distribution over 260 km of standard telecom fiber,” Optics letters , vol. 37, no. 6, pp. 1008–1010, 2012
2012
-
[30]
High-speed integrated qkd system,
R. Sax, A. Boaron, G. Boso, S. Atzeni, A. Crespi, F. Gr ¨unenfelder, D. Rusca, A. Al-Saadi, D. Bronzi, S. Kupijai, et al. , “High-speed integrated qkd system,” Photonics Research, vol. 11, no. 6, pp. 1007– 1014, 2023
2023
-
[31]
High-rate quantum key distribution exceeding 110 mb s–1,
W. Li, L. Zhang, H. Tan, Y . Lu, S.-K. Liao, J. Huang, H. Li, Z. Wang, H.- K. Mao, B. Yan, et al., “High-rate quantum key distribution exceeding 110 mb s–1,” Nature Photonics, vol. 17, no. 5, pp. 416–421, 2023
2023
-
[32]
Backscattering limitation for fiber-optic quantum key distribution systems,
D. Subacius, A. Zavriyev, and A. Trifonov, “Backscattering limitation for fiber-optic quantum key distribution systems,” Applied Physics Letters , vol. 86, no. 1, p. 011103, 2005
2005
-
[33]
Numerical approaches for solving the nonlinear schr ¨odinger equation in the nonlinear fiber optics formalism,
H. Ibarra-Villalon, O. Pottiez, A. G ´omez-Vieyra, J. Lauterio-Cruz, and Y . Bracamontes-Rodriguez, “Numerical approaches for solving the nonlinear schr ¨odinger equation in the nonlinear fiber optics formalism,” Journal of Optics , vol. 22, no. 4, p. 043501, 2020
2020
-
[34]
Embedded split-step methods optimized with a step size control for solving the femtosecond pulse propagation problem in the nonlinear fiber optics formalism,
H. Ibarra-Villalon, O. Pottiez, A. G ´omez-Vieyra, J. Lauterio-Cruz, and Y . Bracamontes-Rodriguez, “Embedded split-step methods optimized with a step size control for solving the femtosecond pulse propagation problem in the nonlinear fiber optics formalism,” Physica Scripta ,...
2021
-
[35]
Robust algorithms for solv- ing stochastic partial differential equations,
M. J. Werner and P. D. Drummond, “Robust algorithms for solv- ing stochastic partial differential equations,” Journal of computational physics, vol. 132, no. 2, pp. 312–326, 1997
1997
Reviewed August 12, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.