REVIEW 4 major objections 5 minor 27 references
Joint Communication and Sensing over the Lossy Bosonic Quantum Channel
T0 review · 4 major / 5 minor · reviewed 2026-08-12 · deepseek-v4-flash
Pith's one-line read For a noiseless bosonic channel with backreflection, communication and sensing need not compete: the same code achieves the Holevo capacity and the optimal Chernoff detection exponent.
desk verdict Useful closed-form JCAS region for a bosonic channel, but the ν vs ν² parametrization and the missing constant-energy codebook proof need fixing before the exact statement can be trusted. 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 load-bearing object is the bidirectional lossy bosonic channel (BLBC), a classical–quantum channel $W_{\nu,k}(\alpha)=|k\alpha\rangle\otimes|\nu\alpha\rangle$ that couples one coherent input to an estimated backreflection and a forward data beam. The proof machinery combines the Holevo-capacity formula $g(\nu E)$ for the pure-loss bosonic channel; the quantum Chernoff bound for binary coherent-state discrimination, extended to many hypotheses through the multiple Chernoff distance; a finite-dimensional approximation lemma that projects long sequences of coherent states onto $\lfloor\log n\rfloor$ Fock levels, making the finite-dimensional JCAS achievability proof applicable; and continuity of the Chernoff exponent under trace-norm convergence to lift the result back to the infinite-dimensional setting. These pieces fit together so that the same random codebook and the same measurement sequence attain both extremes simultaneously.
What would settle it
Measure the reflected field of a real beam splitter fed by a coherent state: if the reflected state contains thermal noise above shot noise, or if its strength is tied to the forward strength by a passive-splitter relation such as $k=\sqrt{1-\nu^2}$, then the predicted region $g(\nu E)\times \frac{E}{2}\min_{k\ne k'}|k-k'|^2$ will not be observed. A direct laboratory test would prepare $K=\{0,1\}$ with known energy $E$ and use collective measurements on the backreflected beam to see whether the error exponent reaches $E/2$ or stops at the homodyne value $E/8$.
Extended reading notes
Core claim
The central claim is Theorem 2: for a $K$-family of bidirectional lossy bosonic channels, defined by the map $\alpha\mapsto |k\alpha\rangle\otimes|\nu\alpha\rangle$, the achievable communication–detection region is the full rectangle $[0,g(\nu E)]\times[0,D_{\max}]$ with $D_{\max}=\frac{E}{2}\min_{k\ne k'}|k-k'|^2$. The forward communication task is a classical–quantum channel whose capacity is the Holevo quantity, equal to $g(\nu E)$ for a lossy bosonic channel with transmissivity $\nu$ and mean input energy $E$; the sensing task is $|K|$-ary discrimination of the backreflected coherent states $|k\alpha\rangle$, whose optimal error exponent is a multiple quantum Chernoff distance. The region is proved in both directions: achievability by random coding with a Gaussian-like input distribution and a finite-dimensional approximation that projects onto the lowest $\lfloor\log n\rfloor$ Fock levels, importing the multiple-Chernoff bound for general quantum measurements (POVMs), and the converse by noting that the earlier finite-dimensional JCAS argument applies to coherent states. The result is that energy, not any tradeoff, is the single resource limiting both tasks, and optimal codes are constant-energy codes.
Load-bearing premise
The result stands or falls with the assumption that the reflected and forward beams are exactly two independent ideal laser-like quantum states, with no added noise and no required relation between their strengths; if a real reflector is noisy or if the two strengths are forced to obey a passive-splitter constraint, the rectangle and the fourfold advantage can collapse.
Editorial extensions
If this is right
- For a two-reflectivity setup ($K=\{0,1\}$), the sensing exponent is $E/2$ with optimal quantum measurements versus $E/8$ with homodyne detection: a fixed factor of four that does not depend on photon number.
- The communication rate saturates the Holevo capacity $g(\nu E)$, which exceeds the homodyne/Shannon rate $\frac{1}{2}\log(1+\nu E)$ and becomes unboundedly larger as $E\to 0$, so low-photon regimes are where quantum JCAS pays off most.
- No time-sharing between communication-optimal and sensing-optimal codes is needed; there is a single family of constant-energy codes that are optimal for both tasks at once.
- Any attempt to exceed either $g(\nu E)$ or $D_{\max}$ fails: because the region is a rectangle, the two tasks do not compete for resources beyond the shared energy budget.
- The optimal receiver POVM is not constructed and may be arbitrarily complex, so practical implementations will initially fall short of the predicted region until suitable collective measurements are realized.
Reading between the lines
- If the BLBC is realized by a passive beam splitter, unitarity forces $k^2+\nu^2\le 1$ (or one of the two parameters must be read as an intensity transmissivity), whereas the paper leaves $k$ and $\nu$ independent and switches between amplitude and intensity readings of $\nu$; re-deriving the region under the passive-splitter constraint is a direct test of how generic the rectangle is.
- The no-tradeoff result likely depends on the coherent-state, noiseless structure; adding thermal noise to the backreflected mode should produce a genuine rate–exponent tradeoff, because the Chernoff exponent of displaced thermal states depends on temperature and the forward capacity decreases.
- A natural extension is to replace the finite set $K$ by a continuum of reflectivities and use Bayesian or local estimation instead of Chernoff exponents; the expected behavior is that the rectangular region turns into a curve governed by the energy constraint.
- Because the sensing POVM is non-constructive, a practically testable question is how closely heterodyne or other feasible receivers approach $E/2$ for finite blocklengths; the homodyne comparison already sets a factor-of-four gap to close.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript introduces a bidirectional lossy bosonic quantum channel (BLBC) in which coherent-state symbols are sent to a receiver while a backreflected coherent field with reflectivity k is used for sensing. The main result (Theorem 2) claims that the achievable communication–detection region is rectangular: any rate up to the Holevo capacity g(νE) and any detection exponent up to Dmax = E min_{k≠k'} |k−k'|²/2 are simultaneously achievable, with no tradeoff. The proof combines known results on the pure-loss channel capacity, a finite-dimensional truncation argument (Lemmas 1–3), the multiple quantum Chernoff bound of [14], and a converse adapted from [7]. A comparison with a classical AWGN/homodyne model is used to claim a fixed factor-of-4 detection advantage and an unbounded communication advantage.
Significance. If the technical issues are resolved, this would be a useful first continuous-variable result on joint communication and sensing, showing that for this coherent-state model the two tasks do not compete. Strengths of the paper include the assembly of the central region from external theorems with no fitted parameters, an explicit truncation strategy, a concrete falsifiable detection-exponent formula, and an honest statement of limitations (no thermal noise, non-constructive POVM). However, several load-bearing points in the current version prevent the claims from being accepted as stated.
major comments (4)
- [Definition 1, Theorem 2, Section II.A] The parameter ν is used as an amplitude in Definition 1 (forward output |να⟩) but as an intensity transmissivity in the capacity expression g(νE). With the channel as defined, the forward intensity transmissivity is |ν|², so the correct communication bound is g(|ν|²E), not g(νE). The same inconsistency enters Section IV, where the classical SNR is quoted as νE although homodyne detection on |να⟩ gives a signal amplitude ν Re(α) and hence SNR |ν|²E. This is load-bearing because Theorem 2 and the quantitative comparison depend on it; please either replace the forward output by |√ν α⟩ or use g(|ν|²E) throughout.
- [Section V.C.1, Eq. (18), Theorem 2] The coherent-state Chernoff exponent is quoted as |k'α−kα|²/2. With the paper's own definition D(ρ,σ)=sup_s −log Tr(ρ^s σ^{1−s}), the exact value for two pure coherent states is obtained from |⟨kα|k'α⟩|² = exp(−|k−k'|²|α|²), so D = |k−k'|²|α|², not half of that. Consequently Dmax should be E min_{k≠k'}|k−k'|², a factor of 2 larger than stated. This also changes the claimed quantum advantage over the classical D=E/8 from a factor of 4 to a factor of 8 for K={0,1}. Please correct Eq. (18) (or the conversion from the quadrature-space formula in [23]) and propagate the correction through Theorem 1, Theorem 2, and the converse.
- [Section V.C.1, Eqs. (19)-(20), Definition 3] Detection error in Definition 3 is worst-case over messages. The proof obtains the exponent by replacing Σ_x N(α_x|α^n)|α_x|² with its expectation E via the law of large numbers. For an i.i.d. random codebook of size exp(nR), the minimum-energy codeword has per-symbol energy E−δ_R for a positive δ_R (large-deviations lower tail), so the worst-case exponent is strictly below Dmax. The sentence 'the performance of the code is given by the codeword with the least amount of energy' recognizes this, but the proof does not show that a code with all codewords at exactly energy E also achieves the communication rate g(νE). Please provide an explicit constant-composition/constant-energy code construction and prove that its communication rate tends to g(νE).
- [Definition 5, text after Theorem 3] The classical comparison model is internally inconsistent. Definition 5 defines Q_{ν,k} with outputs Y_Ai=x_i+Z_Ai and Y_Bi=k x_i+Z_Bi, and the text calls Y_A the reflected part; but for the BLBC under homodyne detection with ν=1, the reflected output has amplitude k x_i, not x_i, so the labels are interchanged. The region stated after Theorem 3, R≤1/2 log(1+νE) and D≤E/8, does not follow from Definition 5 as written (which would give R≤1/2 log(1+E) and D≤E/8 for k=1, with no ν dependence). The model and parameter mapping need to be restated before the claimed factor-of-4 comparison can be assessed.
minor comments (5)
- [Section II.A] The definition of the Gordon function has a sign typo: g(x) is written with '+ x log x' but Theorem 2 uses the correct '− x log x'.
- [Definition 3] The indices in the decoding and detection POVMs are inconsistent: the text defines {Λ_k}_{j=1}^M and {Π_k}_{k∈K}, while Eqs. (1)-(2) use Λ_m and Π_{k,m}. Please clarify that the detection POVM may depend on the message m and fix the notation.
- [Definitions 1-2] The physical relation between the amplitude coefficients k and ν is never stated. For a passive beam splitter, k²+ν²≤1; if this is intended, it should be stated, since Theorem 2 and the comparison treat them as independent parameters.
- [Section V.A, Remark] The remark on approximating continuous measures by finite discrete measures is very terse; please spell out why the setwise convergence preserves the Holevo information within ε and how the finite alphabet size is chosen.
- [Section V.C.2] The converse is only a paragraph; please expand the adaptation of [7] to infinite-dimensional coherent-state channels, in particular the handling of the energy constraint and the uniform prior over K.
Circularity Check
No significant circularity: the central region combines external capacity and Chernoff results, with self-citations only in auxiliary bounds and comparisons.
full rationale
The paper's central result, Theorem 2, is assembled from external results: the lossy-bosonic Holevo capacity from [11], the multiple-state Chernoff bound from [14], the coherent-state Chernoff exponent from [23], and the finite-dimensional converse framework from [7]. None of these are self-citations, and no fitted parameter is renamed as a prediction. The communication axis is the known capacity g(νE) applied to the forward output of the defined channel, and the detection axis is obtained by applying the known quantum Chernoff exponent per mode and then summing over the empirical type; this is a direct derivation, not a definitional identity. The self-citations that appear are auxiliary: [19] supplies an elementary tail bound used in the finite-dimensional truncation Lemma 2, and [18] supports the separate comparison claim that the Holevo capacity has an unbounded advantage at low received intensity. Neither of these forces the rectangular region or the theorem's content. The amplitude/intensity ambiguity of ν in Definition 1 versus g(νE) is a modeling or correctness concern, not a circularity, because the capacity formula is invoked as an external result rather than derived from the paper's own definitions. Overall, no step reduces to its own input by construction, so the circularity burden is low; the score reflects only the presence of minor, non-load-bearing self-citations.
Assumptions & free parameters
assumptions (6)
- standard math The Holevo capacity of the pure-loss bosonic channel with transmissivity ν and mean input energy E is g(νE), given by the Gordon function.
- standard math Multiple quantum hypothesis testing: there exists a POVM whose average error for r product-state hypotheses is bounded by the pairwise Chernoff bound, up to a subexponential factor (Theorem 4 from [14]).
- standard math The Chernoff exponent of two coherent states |kα⟩ and |k'α⟩ is |k−k'|²|α|²/2; the binary Chernoff bound for coherent states is optimal.
- domain assumption The converse for detection extends from the finite-dimensional setting of [7] to infinite-dimensional coherent-state channels without modification.
- domain assumption A passive lossy optical link is adequately modeled by two independent coherent-state outputs (forward, backward) with no thermal noise, no phase noise, and no energy-conservation constraint linking k and ν.
- standard math Truncation of each mode to dimension ⌊log n⌋ is asymptotically fidelity-1 for the code ensemble (Lemmas 1-3).
Cite this review
Pith. "Pith review of Joint Communication and Sensing over the Lossy Bosonic Quantum Channel." pith.science (2026). https://pith.science/paper/X6PJXT6V
@misc{pith2026241111604,
author = {Pith},
title = {Pith review of: Joint Communication and Sensing over the Lossy Bosonic Quantum Channel},
year = {2026},
howpublished = {\url{https://pith.science/paper/X6PJXT6V}},
note = {Machine review of arXiv:2411.11604}
}
read the original abstract
We study the problem of joint communication and sensing for data transmission systems using optimal quantum instruments in order to transmit data and, at the same time, estimate environmental parameters. In particular we consider the specific but at the same time generic case of a noiseless bosonic classical-quantum channel where part of the transmitted light is reflected back to the transmitter. While sending messages to the receiver, the transmitter tries at the same time to estimate the reflectivity of the channel. Extending earlier results on similar but finite-dimensional systems, we are able to characterize optimal tradeoffs between communication and detection rates. We also compare quantum performance to analogous classical models, quantifying the quantum advantage.
Figures
Reference graph
Works this paper leans on
-
[7]
Joint quantum communication and sensing,
S.-Y . Wang, T. Erdo ˘gan, U. Pereg, and M. R. Bloch, “Joint quantum communication and sensing,” in 2022 IEEE Information Theory Work- shop (ITW), 2022, pp. 506–511
2022
-
[14]
Discriminating quantum states: The multiple chernoff distance,
K. Li, “Discriminating quantum states: The multiple chernoff distance,” The Annals of Statistics , vol. 44, no. 4, Aug. 2016. [Online]. Available: http://dx.doi.org/10.1214/16-AOS1436
-
[23]
J. Calsamiglia, R. Mu ˜noz-Tapia, L. Masanes, A. Acin, and E. Bagan, “Quantum chernoff bound as a measure of distinguishability between density matrices: Application to qubit and gaussian states,” Physical Review A , vol. 77, no. 3, Mar. 2008. [Online]. Available: http: //dx.doi.org/10.1103/PhysRevA.77.032311
-
[1]
Joint design of communication and sensing for beyond 5g and 6g systems,
T. Wild, V . Braun, and H. Viswanathan, “Joint design of communication and sensing for beyond 5g and 6g systems,” IEEE Access , vol. 9, pp. 30 845–30 857, 2021
work page 2021
-
[2]
Enabling joint communication and radar sensing in mo- bile networks—a survey,
J. A. Zhang, M. L. Rahman, K. Wu, X. Huang, Y . J. Guo, S. Chen, and J. Yuan, “Enabling joint communication and radar sensing in mo- bile networks—a survey,” IEEE Communications Surveys & Tutorials , vol. 24, no. 1, pp. 306–345, 2022
2022
-
[3]
K. Wu, J. A. Zhang, and Y . Guo, Joint Communications and Sensing: From Fundamentals to Advanced Techniques , 11 2022
work page 2022
-
[4]
C. L. Degen, F. Reinhard, and P. Cappellaro, “Quantum sensing,” Rev. Mod. Phys. , vol. 89, p. 035002, Jul 2017. [Online]. Available: https://link.aps.org/doi/10.1103/RevModPhys.89.035002
-
[5]
Advances in photonic quantum sensing,
S. Pirandola, B. R. Bardhan, T. Gehring, C. Weedbrook, and S. Lloyd, “Advances in photonic quantum sensing,” Nature Photonics, vol. 12, no. 12, pp. 724–733, Dec 2018. [Online]. Available: https://doi.org/10.1038/s41566-018-0301-6
Show all 27 references
-
[6]
Quantum communication,
N. Gisin and R. Thew, “Quantum communication,” Nature Photonics , vol. 1, no. 3, pp. 165–171, Mar 2007. [Online]. Available: https://doi.org/10.1038/nphoton.2007.22
2007 doi
-
[8]
Quantum integrated sensing and communication via entanglement,
Y .-C. Liu, Y .-B. Cheng, X.-B. Pan, Z.-Z. Sun, D. Pan, and G.-L. Long, “Quantum integrated sensing and communication via entanglement,” 2024
2024
-
[9]
Integrated distributed sensing and quantum communication networks,
Y . Xu, T. Wang, P. Huang, and G. Zeng, “Integrated distributed sensing and quantum communication networks,” Research, 2024
2024
-
[10]
Communication over quantum channels with parameter estimation,
U. Pereg, “Communication over quantum channels with parameter estimation,” 2021. [Online]. Available: https://arxiv.org/abs/2001.00836
2021 arXiv
-
[11]
Ultimate classical communication rates of quantum optical channels,
V . Giovannetti, R. Garc ´ıa-Patr´on, N. J. Cerf, and A. S. Holevo, “Ultimate classical communication rates of quantum optical channels,” Nature Photonics , vol. 8, no. 10, p. 796–800, Sep. 2014. [Online]. Available: http://dx.doi.org/10.1038/nphoton.2014.216
2014 doi
-
[12]
Discriminating states: The quantum chernoff bound,
K. M. R. Audenaert, J. Calsamiglia, R. Mu ˜noz-Tapia, E. Bagan, L. Masanes, A. Acin, and F. Verstraete, “Discriminating states: The quantum chernoff bound,” Physical Review Letters , vol. 98, no. 16, Apr. 2007. [Online]. Available: http://dx.doi.org/10.1103/PhysRevLett. 98.160501
2007 doi
-
[13]
The chernoff lower bound for symmetric quantum hypothesis testing,
M. Nussbaum and A. Szkoła, “The chernoff lower bound for symmetric quantum hypothesis testing,” The Annals of Statistics , vol. 37, no. 2, Apr. 2009. [Online]. Available: http://dx.doi.org/10.1214/08-AOS593
2009 doi
-
[15]
Joint communication and binary state detection,
H. Joudeh and F. M. J. Willems, “Joint communication and binary state detection,” IEEE Journal on Selected Areas in Information Theory , vol. 3, no. 1, pp. 113–124, 2022
2022
-
[16]
Quantum limits in optical communications,
K. Banaszek, L. Kunz, M. Jachura, and M. Jarzyna, “Quantum limits in optical communications,” Journal of Lightwave Technology , vol. 38, no. 10, p. 2741–2754, May 2020. [Online]. Available: http://dx.doi.org/10.1109/JLT.2020.2973890
2020
-
[17]
Continuous-variable optical quantum-state tomography,
A. I. Lvovsky and M. G. Raymer, “Continuous-variable optical quantum-state tomography,” Reviews of Modern Physics , vol. 81, no. 1, p. 299–332, Mar. 2009. [Online]. Available: http://dx.doi.org/10.1103/ RevModPhys.81.299
2009
-
[18]
Operating fiber networks in the quantum limit,
J. N ¨otzel and M. Rosati, “Operating fiber networks in the quantum limit,” 2022. [Online]. Available: https://arxiv.org/abs/2201.12397
2022 arXiv
-
[19]
Data transmission over a bosonic channel under classical noise,
J. N ¨otzel, “Data transmission over a bosonic channel under classical noise,” in IEEE International Symposium on Information Theory. IEEE, Jul 2024
2024
-
[20]
Tight uniform continuity bounds for quantum entropies: conditional entropy, relative entropy distance and energy constraints,
A. Winter, “Tight uniform continuity bounds for quantum entropies: conditional entropy, relative entropy distance and energy constraints,” Communications in Mathematical Physics, vol. 347, no. 1, pp. 291–313, 2016
2016
-
[21]
A. S. Holevo, Quantum Systems, Channels, Information . Berlin, Boston: De Gruyter, 2013. [Online]. Available: https://doi.org/10.1515/ 9783110273403
2013
-
[22]
More on convergence in unitary matrix spaces,
J. Arazy, “More on convergence in unitary matrix spaces,” Proceedings of the American Mathematical Society , vol. 83, no. 1, pp. 44–48, 1981. [Online]. Available: http://www.jstor.org/stable/2043888
1981
-
[24]
Attainment of the multiple quantum chernoff bound for certain ensembles of mixed states,
M. Nussbaum, “Attainment of the multiple quantum chernoff bound for certain ensembles of mixed states,” 2013
2013
-
[25]
The strong converse exponent of discriminating Infinite- Dimensional quantum states,
M. Mosonyi, “The strong converse exponent of discriminating Infinite- Dimensional quantum states,” Communications in Mathematical Physics, vol. 400, no. 1, pp. 83–132, May 2023
2023
-
[26]
Gaussian hypothesis testing and quantum illumination,
M. M. Wilde, M. Tomamichel, S. Lloyd, and M. Berta, “Gaussian hypothesis testing and quantum illumination,” Phys. Rev. Lett. , vol. 119, p. 120501, Sep 2017. [Online]. Available: https://link.aps.org/doi/ 10.1103/PhysRevLett.119.120501
2017 doi
-
[27]
Gaussian quantum information,
C. Weedbrook, S. Pirandola, R. Garc ´ıa-Patr´on, N. J. Cerf, T. C. Ralph, J. H. Shapiro, and S. Lloyd, “Gaussian quantum information,” Reviews of Modern Physics , vol. 84, no. 2, p. 621–669, May 2012. [Online]. Available: http://dx.doi.org/10.1103/RevModPhys.84.621
2012 doi
Reviewed August 12, 2026 · model on record in the stance chip above.
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