REVIEW 4 major objections 4 minor 1 cited by
Quantum Algorithms for Optimal Power Flow
T0 review · 4 major / 4 minor · reviewed 2026-08-11 · deepseek-v4-flash
Pith's one-line read For the small power-flow problems tested, preconditioned HHL and VQLS reproduce the classical optimal cost and iteration counts.
desk verdict A small, honest empirical study that plausibly shows preconditioned VQLS/HHL can match classical OPF costs on tiny cases, but it omits the exact encoding and conditioning details needed to make the result reproducible or to rule out a semiclassical solve. 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 KKT linear system (Eq. 2) that an interior-point method solves at every iteration to obtain the Newton search directions ($\Delta X$, $\Delta Z$, $\Delta \lambda$, $\Delta \mu$). The hybrid method replaces the classical solve of this system with a quantum linear-system algorithm: HHL as a direct quantum solver and VQLS as a variational one. Left preconditioning with incomplete LU factorization is applied before the quantum solve, and the paper argues that this controls the condition number whose growth dominates HHL's complexity. The convergence measure gradcond ties the approximate quantum search directions to the classical convergence criteria.
What would settle it
Rerun one of the reported cases while recording the condition number and residual of the preconditioned KKT system before every interior-point step; if the preconditioned matrix is too ill-conditioned for the quantum error bounds to hold, or if the approximate search direction fails to reduce gradcond on a fresh test case, the parity claim is refuted.
Extended reading notes
Core claim
The paper's central claim is that the KKT linear system arising inside a step-controlled primal-dual interior-point method for optimal power flow can be preconditioned and then solved with HHL or VQLS, and that the resulting hybrid algorithm reaches the same optimal cost as the classical solver on the test cases it could simulate. The reported costs match exactly in every completed run — 746.25 (DC case 3), 2393.31 (case 6ww), 4131.03 (case 9), and 758.21 (AC case 3) — with quantum iteration counts equal or one above the classical counts. The paper presents these runs as a substantial improvement over the earlier load-flow results in [1], and as evidence that the condition-number growth identified in [10] can be contained by ILU preconditioning. It does not claim an asymptotic speedup; the claim is parity and viability at small scale.
Load-bearing premise
The comparison stands on the assumption that an approximate solution read out of HHL or VQLS for the preconditioned KKT system is accurate enough for the interior-point method to keep converging to the true optimum; the paper's own limitations section restricts the tests to three small DC cases and one AC case because of qubit count and numerical instability, and the condition numbers and quantum residuals are never reported.
Editorial extensions
If this is right
- If the parity holds beyond the four test cases, a practical division of labor emerges: classical globalization from the interior-point method plus an approximate quantum KKT solve is enough to reach the true optimum.
- The preconditioning result isolates the condition number as the manageable bottleneck, so better preconditioners may extend the parity to larger DC-OPF cases and to more AC-OPF cases.
- Since iteration counts are equal or within one, the number of quantum solves required is roughly the same as the number of classical solves, so the cost of read-in and read-out, rather than iteration count, becomes the main scaling question.
Reading between the lines
- The paper never reports condition numbers or the precise oracle encoding of the non-Hermitian KKT matrix; measuring both on the same four cases would separate the value of preconditioning from the value of the quantum solver.
- Because VQLS convergence depends on the classical optimizer and the ansatz, the equal costs on tiny cases do not yet reveal how error scales with system size; a controlled test with artificial matrices of known condition number would be the natural next benchmark.
- Interior-point methods for other network optimization problems, such as gas or water networks, have the same KKT structure, so the same preconditioned hybrid pattern is a transferable template if the parity result holds.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes hybrid quantum-classical solvers for optimal power flow (OPF) by replacing the classical linear-system solve inside a step-controlled primal-dual interior-point method (SC-PDIPM) with HHL or VQLS, using ILU left preconditioning. The authors report simulations on small MATPOWER cases (case3, case6ww, case9 for DC-OPF, and case3 for AC-OPF) and claim that the quantum-assisted methods produce the same optimal costs and similar iteration counts as the classical SC-PDIPM solver, with preconditioning controlling the growth of the condition number.
Significance. If substantiated, the result would be a useful empirical step toward hybrid quantum-classical OPF solvers and would directly address the concern raised by Pareek et al. about the poor asymptotic scalability of HHL for power-flow problems. The comparison against MATPOWER's classical solutions is an external benchmark, so the central empirical claim is not circular. The use of standard test cases and the focus on preconditioning are sensible and relevant to the quantum-optimization community. However, the manuscript currently lacks the essential algorithmic details needed to verify that a quantum linear-system solve actually took place: no Hermitian encoding of the KKT system is described, no condition numbers are reported, and no error tolerances or readout procedures are given. As written, the reported agreement in iteration counts and costs is not reproducible and could be explained by a classical solve of the search-direction system.
major comments (4)
- [§V, Algorithm 2 line 9; §IV-B; Eq. (2)] The manuscript never specifies how the KKT linear system in Eq. (2) is encoded as an operator suitable for HHL or VQLS. The KKT matrix is symmetric but indefinite, its (2,2) block [μ/Z] varies by orders of magnitude as the barrier parameter γ changes, and left ILU preconditioning produces the generally non-symmetric matrix M^{-1}A. VQLS, as stated in §IV-B, requires A to be a linear combination of Hermitian unitaries with bounded norm (typically ||A||≤1), and HHL requires a Hermitian or block-encoded operator with bounded norm. The authors must describe the actual encoding used (for example, a doubled Hermitian embedding), the resulting qubit count, the number of Pauli terms, the norm scaling, and the condition number after preconditioning for each test case. Without this information, the equivalence of iteration counts in Table I cannot be attributed to a quantum solve.
- [§VI, Table I; Algorithm 2] No stopping criteria, error tolerances, or readout procedures are reported for the inner HHL/VQLS solves. The SC-PDIPM outer loop requires a search direction Δ = (ΔX, ΔZ, Δλ, Δμ); it is never explained how the quantum state |x⟩ is converted into these classical vectors, nor how approximation error in that readout affects the convergence conditions (feascond, gradcond, compcond, costcond). The paper should report the inner-solve tolerance, the number of VQLS outer iterations, the ansatz depth, the optimizer settings, and the HHL phase-estimation precision for each case, together with a sensitivity study showing how perturbing the search direction changes the reported iteration counts.
- [§VI, Figures 1–4; §V] The claim that preconditioning leads to 'controlled growth in the condition number' is not supported by any numerical evidence. No condition numbers are given for the original KKT matrix or for the preconditioned system, for any test case. Since the stated complexity of HHL is O(log(N) s^2 κ^2/ε) and VQLS performance also degrades with κ, reporting κ before and after ILU preconditioning is essential to evaluate whether the quantum part is feasible at all. The authors should add a table or figure with these values.
- [§VI, Table I and Table II] The empirical basis is very small: only three DC-OPF cases and one AC-OPF case, with HHL missing for case9 and entirely missing for AC-OPF. The conclusion that 'quantum approaches are comparable to classical approaches' is stronger than the data support. Either restrict the conclusion to the tested instances or add more cases and error bars on the reported costs and iteration counts.
minor comments (4)
- [Throughout] There are several typographical errors, including 'quadricatically', 'acutal', 'pseduo-code', 'f alse', and 'SC-PDIM' in place of 'SC-PDIPM'. These should be corrected in a revision.
- [Table I] The dash for HHL in the case9 row is unexplained; the text in §VI discusses Figure 3 only for classical and VQLS, so the reader cannot tell whether HHL failed, was not run, or was omitted for another reason. Please clarify.
- [§VI, Figures 1–4] The figures do not include axis labels or error bars, and the text describing 'slight variations' and 'fluctuations' is qualitative. Adding labeled axes and, where possible, error bars or multiple runs would improve interpretability.
- [Abstract and §VI] The abstract states that the results 'substantially improve the results in [1]', but no quantitative comparison with [1] is provided anywhere in the paper. Either include that comparison or soften the claim.
Circularity Check
No circular step; the empirical comparison is anchored to external MATPOWER classical solutions, with only non-load-bearing self-citations to [1].
full rationale
The derivation chain is: OPF model -> Lagrangian (1) -> KKT linear system (2) -> SC-PDIPM with search directions solved by HHL/VQLS -> costs and iterations compared against classical results from MATPOWER (Tables I-II). None of these steps defines an output in terms of the claimed conclusion. The reported optimal costs (e.g., 746.25 for case 3) are compared with an external classical solver, not constructed from the quantum parameters; no fitted constant or uniqueness theorem forces the match. The paper's references to [1] (same research group) are comparative or expository ('this is already an improvement on the simulations in [1]', 'For an explanation of HHL and its use in solving load flow, see [1]') and are not load-bearing: the central OPF comparison stands on the independently defined MATPOWER benchmarks and the equations derived in this paper. The absence of a Hermitian encoding description, condition numbers, Pauli-term counts, and ansatz details is a reproducibility/correctness risk, not a circularity of the derivation. The explicit Limitations paragraph also confines the claims to small DC-OPF and AC-OPF cases, so the paper does not overstate an internally constructed agreement. No circular step can be exhibited with an equation-to-equation reduction, so the appropriate finding is no significant circularity.
Assumptions & free parameters
free parameters (5)
- VQLS ansatz layer count L
- VQLS optimizer step size alpha
- ILU preconditioner tolerances
- HHL eigenvalue estimation precision
- SC-PDIPM parameters (xi, sigma, gamma_0)
assumptions (4)
- domain assumption The OPF problem can be formulated as in Section III-A with quadratic cost and standard constraints.
- domain assumption SC-PDIPM [19] converges to the OPF optimum for the selected MATPOWER cases when the search direction is solved exactly.
- ad hoc to paper The indefinite KKT matrix in Eq. (2) can be embedded as a Hermitian operator suitable for HHL/VQLS, with bounded norm and manageable condition number after preconditioning.
- ad hoc to paper Errors in the approximate quantum search direction do not prevent SC-PDIPM from converging to the same optimum.
Cite this review
Pith. "Pith review of Quantum Algorithms for Optimal Power Flow." pith.science (2026). https://pith.science/paper/CUMHIO3E
@misc{pith2026241206177,
author = {Pith},
title = {Pith review of: Quantum Algorithms for Optimal Power Flow},
year = {2026},
howpublished = {\url{https://pith.science/paper/CUMHIO3E}},
note = {Machine review of arXiv:2412.06177}
}
read the original abstract
This paper explores the use of quantum computing, specifically the use of HHL and VQLS algorithms, to solve optimal power flow problem in electrical grids. We investigate the effectiveness of these quantum algorithms in comparison to classical methods. The simulation results presented here which substantially improve the results in [1] indicate that quantum approaches yield similar solutions and optimal costs compared to classical methods, suggesting the potential use case of quantum computing for power system optimization.
Figures
Forward citations
Cited by 1 Pith paper
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Quantum Hardware-in-the-Loop for Optimal Power Flow in Renewable-Integrated Power Systems
A hardware-in-the-loop setup running annealer-based power flow and optimal power flow on RTDS matches Newton-Raphson benchmarks on a renewable-integrated nine-bus grid, with OPF solved only by Fujitsu's digital annealer.
Reference graph
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Reviewed August 11, 2026 · model on record in the stance chip above.
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