REVIEW 4 major objections 6 minor 31 references
Quantum multi-output Gaussian Processes based Machine Learning for Line Parameter Estimation in Electrical Grids
T0 review · 4 major / 6 minor · reviewed 2026-08-12 · deepseek-v4-flash
Pith's one-line read This paper demonstrates a quantum multi-output Gaussian process that uses HHL to invert a 32x32 kernel matrix and estimates power-grid line parameters on a real IBM quantum computer.
desk verdict Real 13-qubit HHL kernel inversion on hardware, but the rescaling step in §IV C makes the optimizer minimize the wrong objective—needs a major fix. 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 machinery is HHL as a subroutine inside GP hyper-parameter optimization: the term $y^T (K + \sigma_n^2 I)^{-1} y$ in the negative log marginal likelihood is rewritten as a norm of the solution to a linear system and evaluated by measuring the probability that HHL's auxiliary qubit is in $|1\rangle$. Because the raw kernel has a condition number around $O(2^{30})$, the authors add strong regularization and then scale the kernel matrix by a diagonal matrix $D = \mathrm{diag}(\kappa_{11}^{-1/2}, \ldots, \kappa_{nn}^{-1/2})$ to bring the condition number down to $O(2^9)$; the inner product is then rescaled by a single factor $d_{\min}$, the minimum entry of $D^{-1}$. To make the circuit executable, Approximate Quantum Compiling (AQC) replaces the QPE block by a shallow approximation with a bounded number of CNOT gates, using at most $E = 8$ evaluation qubits. The method therefore rests on three interacting approximations: regularization, diagonal scaling, and approximate compiling.
What would settle it
Take any positive-definite kernel matrix with unequal diagonal entries, compute $y^T (K + \sigma_n^2 I)^{-1} y$ classically, then compute the same quantity with the paper's D-scaling and $d_{\min}$ rescaling; if the two disagree by more than HHL's approximation error for a well-conditioned matrix, the rescaling step does not recover the original inner product. A concrete test is a 2x2 matrix with diagonal entries 1 and 10 and the paper's conditioning recipe.
Extended reading notes
Core claim
On the paper's own terms, the central discovery is that a Gaussian process with multiple coupled outputs can be made to run on present-day quantum hardware by replacing the single expensive inversion $(K + \sigma_n^2 I)^{-1}$ with HHL, provided the ill-conditioned kernel is first regularized and scaled, and the quantum phase estimation section of HHL is approximated by Approximate Quantum Compiling. With this pipeline the authors implemented a 32x32 kernel inversion using up to 13 qubits on IBM Auckland and obtained R and L estimates in the same decimal range as the true values (approximately 0.089 $\Omega$ versus 0.064 $\Omega$ for R, and approximately $5.63 \times 10^{-5}$ H versus $2.64 \times 10^{-5}$ H for L), with the noise-less simulator and the noisy hardware giving close results. The paper presents this as larger than earlier HHL demonstrations on real quantum hardware, and as evidence that QGP-based line parameter estimation is feasible as a proof-of-concept.
Load-bearing premise
The whole demonstration depends on the assumption that the diagonal rescaling of the kernel matrix can be undone by multiplying the HHL output by a single number $d_{\min}$, which is exact only when every diagonal entry of the kernel is equal; for real measurement kernels with varying diagonal entries the rescaling silently changes the quantity being computed.
Editorial extensions
If this is right
- A 13-qubit HHL circuit for a 32x32 kernel inversion can be executed on a real IBM quantum device, going beyond the 5-qubit, 2x2 HHL demonstrations cited in the paper.
- The QGP optimizer estimates the line parameters R and L in the same decimal range as the true values, with the noisy hardware result close to the noise-less simulator, indicating the pipeline tolerates hardware noise at this scale.
- With the hyper-parameters returned by QGP, the multi-output GP models can impute voltage and current signals over a full cycle, so the trained model retains predictive value even when production hardware would not be used online.
- The reported execution time is far from practical for real-world deployment, so the authors position QGP as a proof-of-concept and expect near-term quantum computers to aid classical computers for offline applications.
Reading between the lines
- If the diagonal-scaling rescaling is corrected or removed, the same pipeline could be tested on kernel matrices with strongly varying diagonal entries; a direct numerical comparison of the HHL-estimated quadratic form against exact classical values on such matrices would separate the conditioning approximation from the AQC approximation.
- Because HHL natively returns only scalar expectations $\langle x|M|x\rangle$, the QGP approach is naturally suited to objective functions like the log marginal likelihood, but not to applications that require full posterior covariance samples; extending it would require a different quantum linear-algebra primitive.
- The claimed speedup is asymptotic and assumes sparse, well-conditioned matrices; for dense 32x32 kernel matrices the classical $O(N^3)$ inversion is trivial, so the practical value of QGP will appear only at much larger N with sparse kernel approximations, a regime not tested here.
- A testable extension is to use the same conditioning-and-AQC recipe on a kernel from a different power-system task, such as state estimation or load forecasting, to see whether the parameter errors stay within the same range as reported.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This paper proposes a quantum Gaussian process (QGP) for multi-output regression, replacing the kernel-matrix inversion in the GP marginal likelihood (Eq. (4)) with an HHL subroutine whose circuit depth is reduced using Approximate Quantum Compiling (AQC). The method is applied to estimating resistance R and inductance L of a short transmission line from noisy voltage and current measurements, with a 32×32 kernel matrix built from 32 training samples. The authors report results from the IBM Auckland quantum device and a noiseless simulator, compare them with classical GP, and claim the implementation of up to a 13-qubit HHL circuit for the 32×32 kernel inversion.
Significance. If correct, this would be a notable end-to-end NISQ demonstration of HHL-based Gaussian process training on a realistic power-systems task, and the AQC depth-reduction pipeline would be a useful engineering contribution. The paper is candid about hardware limitations and reports quantitative comparisons with classical GP, which is a strength. The work does not provide machine-checked proofs or reproducible code, and its central derivation has a gap that changes the optimized objective; the headline experimental claim is also not documented at the level needed to verify it. These issues preclude acceptance in the current form.
major comments (4)
- [IV C, Step 2] The rescaling step after diagonal conditioning is algebraically invalid. With κ′ = DκD and D = diag(κ_11^{-1/2}, ..., κ_nn^{-1/2}), the quadratic form computed on the conditioned matrix is y^T(κ′)^{-1}y = (D^{-1}y)^T κ^{-1}(D^{-1}y) = Σ_{ij} κ_ii^{1/2} y_i (κ^{-1})_{ij} y_j κ_jj^{1/2}. This is not a scalar multiple of y^T κ^{-1} y unless all diagonal entries of κ are equal. The stated multiplication by dmin, the smallest diagonal entry of D^{-1}, cannot compensate for the per-component weights κ_ii^{1/2}κ_jj^{1/2}. For the 32×32 kernel used in Section IV B, built from RBF kernels and three measurement signals with different noise variances, the diagonal entries are not equal. Therefore the quantity minimized during QGP optimization is not the GP log marginal likelihood of Eq. (4), and the R and L estimates in Table I do not correspond to the claimed QGP-LPE objective. This is a load-bearing error for the central demonstration.
- [IV C, Step 1] The mapping from the HHL norm readout to the quadratic form of Eq. (4) is not demonstrated. The HHL ancilla measurement produces ||x||^2 for x = A^{-1}|b⟩, i.e., ⟨b|A^{-2}|b⟩, not ⟨b|A^{-1}|b⟩. In Step 1 the manuscript asserts that ||z||^2 with z=κ^{-1}|y⟩ equals ⟨y|K|y⟩, where K=(K+σ_n^2I)^{-1}. This identity requires a specific relationship between K and κ, namely K=(κκ†)^{-1}, but the 'Schutz decomposition' is never defined and no proof is provided. Without this algebraic link, the quantity actually measured on the quantum device is not established to be the first term of Eq. (4).
- [V (Table I) and Abstract] The headline claim that 'up to 13-qubit HHL circuit has been implemented for a 32x32 kernel matrix inversion on IBM Quantum hardware' is not supported by the experimental reporting in Section V. The section provides only the final R and L estimates in Table I and graphical predictions in Fig. 6. It does not report the number of qubits used, the number of QPE evaluation qubits, the gate counts or circuit depths after AQC, or the hardware execution parameters. A reader cannot verify that a 13-qubit circuit corresponding to the 32×32 inversion was executed, as opposed to a smaller circuit or a simulator result. This claim should be removed or accompanied by the relevant circuit and execution metadata.
- [IV C, Step 3] The QPE truncation to E=8 evaluation qubits is an uncontrolled approximation. The paper states that when the condition number exceeds a limit, QPE is evaluated with only 8 qubits, but it gives no error analysis for the resulting eigenvalue estimates or for the norm ||x||^2. For the conditioned matrix with condition number near O(2^9), an 8-qubit eigenvalue register may be insufficient to resolve the small eigenvalues that dominate the inverse. The authors acknowledge in Section V that heavy approximations affect accuracy, but without a bound or a numerical study the effect on the QGP objective and on the reported line-parameter estimates is unknown. This is particularly important because the approximation interacts with the conditioning step in Major Comment 1.
minor comments (6)
- [Throughout] The manuscript contains many typos and grammatical errors (e.g., 'algrithm', 'another another', 'challanging', 'it’s', 'Posi tive', 'F ig'); a thorough language edit is needed.
- [IV C, Step 1] The symbol K is reused for the kernel matrix, its inverse, and the conditioned matrix, and κ/κ† are never precisely defined; please introduce distinct notation.
- [IV C, Step 1] The 'Schutz decomposition' is not defined and no reference is given; if it is a standard matrix factorization, the definition and the factorization algorithm should be provided.
- [V, Table I] Table I reports a single run for the quantum results; for NISQ hardware, confidence intervals or repeated-shot statistics are needed to assess the significance of the R and L estimates.
- [References] Reference [17] is incomplete (author list and title missing), and reference [19] lacks the article title; these should be completed.
- [V, Fig. 6] Figure 6 shows prediction curves without quantitative error metrics, so the statement that the hyper-parameters are 'optimized well enough' is not supported by the figure alone.
Circularity Check
No significant circularity: QGP-LPE is benchmarked against classical GP and known R,L values; the self-citations to [2] and [23] are not load-bearing.
full rationale
The claimed derivation chain is self-contained against external benchmarks. The GP negative log marginal likelihood (Eq. 4) is a standard target, HHL is a standard algorithm, and AQC is imported from the external reference [22] (Madden and Simonetto), not from the authors' own prior work. The paper's contribution is the implementation of this pipeline for line-parameter estimation, and the resulting R and L estimates are compared against the actual values and against classical GP (Table I); no fitted parameter is relabeled as an independent prediction. The self-citations present are not load-bearing: [2] supplies the physics-informed multi-output GP construction, but that construction is stated to follow from the physical line equation (22) and the standard linear-operation closure of GPs, so the present result does not reduce to the citation; [23] is cited only as a previous application of AQC, while the AQC method itself is [22]. The rescaling in Section IV.C Step 2, multiplying the HHL-computed inner product by dmin, is algebraically questionable for a kernel with unequal diagonal entries; however, that is a validity and approximation concern rather than circularity, because the computed quantity is not definitionally equal to the target marginal-likelihood term. No uniqueness theorem is imported from the authors, no ansatz is smuggled via citation, and no known result is merely renamed.
Assumptions & free parameters
free parameters (4)
- HHL normalization constant C
- QPE evaluation qubit cap E =
8
- GP hyperparameters (sigma^2, ARD weights w_d, noise variances)
- Optimization iteration count =
100
assumptions (4)
- ad hoc to paper The kernel matrix can be factored as kappa^dagger kappa with kappa Hermitian via a Schutz decomposition (Section IV C Step 1).
- ad hoc to paper Multiplying the scaled inner product by dmin, the minimum diagonal entry of D^{-1}, recovers the original quadratic form y^T kappa^{-1} y (Section IV C Step 2).
- ad hoc to paper AQC with at most E=8 QPE qubits produces a sufficiently faithful approximation of the HHL circuit for the norm estimate (Section IV C Step 3).
- domain assumption The short-line Pi-model vi = R ii + L dii/dt + vj and i.i.d. Gaussian measurement noise hold for the test network (Section IV A).
Cite this review
Pith. "Pith review of Quantum multi-output Gaussian Processes based Machine Learning for Line Parameter Estimation in Electrical Grids." pith.science (2026). https://pith.science/paper/VDC64DWW
@misc{pith2026241109123,
author = {Pith},
title = {Pith review of: Quantum multi-output Gaussian Processes based Machine Learning for Line Parameter Estimation in Electrical Grids},
year = {2026},
howpublished = {\url{https://pith.science/paper/VDC64DWW}},
note = {Machine review of arXiv:2411.09123}
}
read the original abstract
Gaussian process (GP) is a powerful modeling method with applications in machine learning for various engineering and non-engineering fields. Despite numerous benefits of modeling using GPs, the computational complexity associated with GPs demanding immense resources make their practical usage highly challenging. In this article, we develop a quantum version of multi-output Gaussian Process (QGP) by implementing a well-known quantum algorithm called HHL, to perform the Kernel matrix inversion within the Gaussian Process. To reduce the large circuit depth of HHL a circuit optimization technique called Approximate Quantum Compiling (AQC) has been implemented. We further showcase the application of QGP for a real-world problem to estimate line parameters of an electrical grid. Using AQC, up to 13-qubit HHL circuit has been implemented for a 32x32 kernel matrix inversion on IBM Quantum hardware for demonstrating QGP based line parameter estimation experimentally. Finally, we compare its performance against noise-less quantum simulators and classical computation results.
Figures
Figures from the paper (3 more)
Reference graph
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Reviewed August 12, 2026 · model on record in the stance chip above.
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