REVIEW 4 major objections 4 minor 36 references
A quantum dual logarithmic barrier method for linear optimization
T0 review · 4 major / 4 minor · reviewed 2026-08-11 · deepseek-v4-flash
Pith's one-line read The paper claims that an inexact feasible dual logarithmic barrier method, with Newton systems solved by quantum linear system algorithms and iterative refinement, reaches a $\zeta$-optimal solution in $O(\sqrt{n}\log(n\mu_0/\zeta))$…
desk verdict New dual-only quantum IPM with real potential, but Lemma 3.2's false inequality breaks the central iteration bound; needs major revision before it 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 object carrying the argument is the normal equation system (NES) of the dual logarithmic barrier method, $(AS^{-2}A^T)\Delta y = \mu^{-1} r_p$ with $r_p = b - \mu A S^{-1}e$, together with the proximity measure $\delta(s,\mu)=\|s^{-1}\Delta s\|_2$ that measures how far the current dual slack $s$ is from the $\mu$-center. The algorithm builds the NES at each iteration, uses QSVT to prepare a quantum state proportional to its solution, reads a classical unit vector with quantum tomography, and rescales it by $\lambda_*$ to form the inexact Newton step. The two technical workhorses are Lemma 3.2, which bounds the next proximity measure $\delta(s_+,\mu_+)^2$ in terms of $\delta(s,\mu)^4$ plus error terms, and Theorem 3.3, which turns that bound into the $4\sqrt{n}\log(n\mu_0/\zeta)$ iteration count. Iterative refinement works through a dual-only refining problem (DIR) whose initial feasible point is built from the previous solution, so every re-solve keeps the condition number of the Newton systems bounded by the initial condition number.
What would settle it
Set up a small linear program with $n=2$ and a starting point on its central path with $\delta(s,\mu)=0.5$, add an inexact direction whose relative error is exactly $0.1\delta(s,\mu)$ as the paper allows, and compute $\delta(s_+,\mu_+)$ directly; if any coordinate of $(s^{-1}\Delta s)^2$ comes out below $0.25$, or if $\delta(s_+,\mu_+)$ exceeds $0.5$, the $O(\sqrt{n})$ iteration claim is not supported by the proof as written.
Extended reading notes
Core claim
In the paper's own terms, the central discovery is that the normal equation system $(AS^{-2}A^T)\Delta y = \mu^{-1}(b - \mu A S^{-1} e)$ can be solved inexactly by a quantum linear system algorithm and the resulting direction still drives the dual logarithmic barrier method to the central path quadratically. The proximity measure $\delta(s,\mu)=\|s^{-1}\Delta s\|_2$ is shown to contract quadratically in each step, and with $\theta=1/(4\sqrt{n})$ the proximity bound stays below $0.5$ for the whole run, yielding at most $\lceil 4\sqrt{n}\log(n\mu_0/\zeta)\rceil$ iterations. The rescaling of the unit vector output by quantum tomography is chosen as $\lambda_* = r_p^T \bar{\Delta y}/(\mu \|S^{-1}A^T\bar{\Delta y}\|_2^2)$, and the proof that the resulting error is small enough uses a trigonometric bound involving $\kappa(AS^{-2}A^T)$. Iterative refinement is then used to remove the dependence of the condition number on the final precision, giving the $\widetilde{O}(m\sqrt{n}\kappa_0)$ query bound.
Load-bearing premise
The proof that the method keeps its iterates close to the central path depends on Lemma 3.2's assertion that a certain combination of the squared Newton step and the error term has nonnegative inner product; that assertion rests on the inequality $(s^{-1}\Delta s)^2 \ge 0.25 e$, which the text does not justify.
Editorial extensions
If this is right
- Each quantum iteration needs only a coarse direction solve: the tomography accuracy is tied to the current proximity measure, so the per-iteration quantum cost does not grow as the target accuracy tightens.
- For problems with $m \le \sqrt{n}$, the total QRAM query count $\widetilde{O}(m\sqrt{n}\kappa_0)$ is sublinear in $n$, and the classical cost $O(m n^{1.5}\log(n\mu_0/\zeta))$ beats the plain dual barrier method's $O(n^{2.5})$ for large $n$.
- Because dual feasibility is preserved by the barrier framework, the algorithm avoids the normal-equation preprocessing or orthogonal-subspace modification that earlier quantum interior point methods need to stay primal feasible.
- The dual-only iterative refinement reaches an exact optimal solution through $O(\log\zeta/\log\hat{\zeta})$ low-precision solves, so the quantum cost depends on the initial condition number rather than on $1/\zeta$.
- Under the tallness condition $n \ge \vartheta m^2$, the algorithm improves on the closest prior quantum IPM, which only achieves a sublinear query count under the far stronger condition $n \ge \gamma m^{10}$.
Reading between the lines
- Assuming the main iteration bound holds, the dual-only refinement template should extend to semidefinite optimization, where the dual barrier is a matrix function and the NES becomes a Schur complement; a numerical check would be to run the inexact scheme on small SDO instances and monitor whether $\delta$ remains below $0.5$ with the allowed relative error $0.1\delta$.
- A direct experiment on random tall LOs with $m \le \sqrt{n}$, comparing the quantum-style directions (simulated by adding noise with relative norm $0.1\delta$) against exact Newton directions, would show how much of the theory's allowed error is actually needed in practice; the true tolerance may be much looser.
- The sublinear query claim is contingent on QRAM and block encoding; a concrete prediction is that a fault-tolerant implementation would spend most of its runtime on the $\kappa_0$ factor, so preconditioning the input matrix $A$ appears to be the main lever for a practical speedup.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes an inexact-feasible dual logarithmic barrier method for linear optimization, in which the normal equation system at each iteration is solved by a quantum linear system algorithm (QSVT) followed by quantum tomography, and iterative refinement is used to reduce the dependence on the final precision. The central theoretical claims are an O(sqrt(n) log(n mu0/zeta)) iteration complexity for the inexact method (Theorem 3.3), a per-iteration QRAM query bound based on QSVT/QTA (Theorem 3.7), and a final query complexity tilde-O(m sqrt(n) kappa0) for the iterative-refinement variant (Theorem 3.13). The paper also claims that for problems with n >= m^2 the quantum complexity has a sublinear dependence on dimension.
Significance. If the main results were established, the paper would make a useful contribution to quantum interior-point methods: a dual-only framework that avoids the primal-feasibility modifications needed in some prior QIPMs, a local quadratic convergence result for inexact directions (Theorem 3.1), and a detailed quantitative comparison with existing classical and quantum IPMs in Tables 1 and 2. The iterative-refinement idea is sensible and the presentation of the normal-equation structure is mostly clear. However, the central iteration-complexity theorem rests on Lemma 3.2, whose proof contains a false componentwise inequality; the per-iteration quantum complexity is imported from external sources without proof; and the Section 3.2 accuracy analysis contains algebraic errors. These are load-bearing issues for the main claims, so the paper cannot be accepted in its current form.
major comments (4)
- [Lemma 3.2, Eq. (2)] The proof of Lemma 3.2 contains an invalid lower bound. With a = s^{-1}Delta s and b = s^{-1}E^C_{Delta s}, the hypotheses give |a_i| <= delta(s,mu) <= 0.5 and ||b||_2 <= 0.1 delta(s,mu) <= 0.05, hence a_i^2 <= 0.25, not a_i^2 >= 0.25. Equation (2) nevertheless estimates the vector e - a^2 - (a-e)b componentwise from below and multiplies by (0.5)^2 e, which amounts to assuming a_i^2 >= 0.25. The claimed intermediate conclusion h^T(e-h) >= 0 is false: for n = 1, a = 0.05, b = 0.005, all assumptions hold and h = a^2 + (a-1)b = -0.00225 < 0, so h(e-h) < 0. Lemma 3.2 is the step that keeps delta(s,mu) <= 0.5 in Theorem 3.3 and yields the O(sqrt(n) log(n mu0/zeta)) iteration count; without it the central complexity claim is not proven.
- [Section 3.2, Proposition 3.4] The derivation of the quantum accuracy condition has an algebraic error. Let u = S^{-1}A^T Delta y and v = S^{-1}A^T E^Q_{Delta y}. The exact expression is cos(psi) = (||u||^2/||Delta y|| + u^T v) / (||u|| ||u/||Delta y|| + v||), not the displayed fraction with denominator ||Delta y|| ||u|| ||v|| + ||u||^2. The subsequent sufficient condition 0.005 ||Delta y||_{AS^{-2}A^T} >= 1.995 ||Delta y|| ||S^{-1}A^T Delta y|| ||S^{-1}A^T E^Q_{Delta y}|| and the comparison with 1/sqrt(kappa(AS^{-2}A^T)) do not follow; the displayed equality with denominator ||AS^{-2}A^T|| ||Delta y||_2^2 also omits a square root and has the wrong power of ||Delta y||. The final bound on ||E^Q_{Delta y}|| may be correct, but the proof as written does not establish Proposition 3.4.
- [Theorem 3.7 and Section 3.3] The per-iteration QRAM complexity in Theorem 3.7 is imported from Mohammadisiahroudi [2024] (a PhD thesis) without proof, and the uniform condition-number bound kappa(A(S^{(k)})^{-2}A^T) = O(kappa0) for all iterative-refinement iterations is taken from Mohammadisiahroudi et al. [2023a] (to appear). These external results are load-bearing for the final query complexity tilde-O(m sqrt(n) kappa0) in Theorem 3.13. Since they are not established in this manuscript and at least one is not yet published, the paper should either prove these statements or state them explicitly as assumptions with precise hypotheses.
- [Abstract and Section 3.2] The claim of sublinear query complexity under n >= m^2 is not supported by the stated bound. If m = Theta(sqrt(n)), then tilde-O(m sqrt(n) kappa0) is tilde-O(n kappa0), which is linear in n. Sublinearity requires m = o(sqrt(n)), i.e. n/m^2 -> infinity, not merely n >= m^2. In addition, the phrase 'quadratically more constraints than variables' appears to reverse the roles of n and m; the condition used elsewhere is n >= m^2, meaning variables are quadratically more numerous than constraints. The abstract and Table 2 should be corrected accordingly.
minor comments (4)
- [Theorem 3.3, proof] The termination condition is written as n mu0 (1 - theta)^K >= (1 - theta) epsilon; since 1 - theta < 1, the left side decreases in K, so the inequality should presumably be <=. As written it does not imply the stated ceiling for K.
- [Lemma 3.2] The proof repeatedly uses the loose bound ||s^{-1}E|| <= 0.5 although the hypothesis gives ||s^{-1}E|| <= 0.1 delta(s,mu) <= 0.05; using the tighter bound would make the estimates clearer, though it does not by itself repair Eq. (2).
- [Table 1 and surrounding text] The text states that Apers and Gribling [2023] has 'O(n^7.5) complexity', but Table 1 lists several query counts for their algorithm; the derivation of n^7.5 should be given, or the statement should be removed.
- [Section 2.4] The accuracy parameter epsilon is used for the IPM target, while the QLSP error is denoted epsilon_QLSP; later the QTA error and the quantum-direction error are denoted differently. Using distinct symbols throughout would improve readability.
Circularity Check
The quantum query-complexity claims rest on two load-bearing same-group citations, while the classical DLBM iteration analysis is independent.
-
self citation load bearing
[Section 3.2, proof of Theorem 3.7 (per-iteration QSVT-NES complexity).]
"According to Mohammadisiahroudi [2024], since the normal equation can be rewritten as (AS−1)(AS−1)T ∆y = (AS−1) 1/µ (Sx0−µe), the normal equation systems can be solved using quantum singular value transformation (QSVT) Gilyén [2019], Gilyén et al. [2019] with the complexity of ~On,κ(AS−1),1/ε (κ(AS−1)‖AS−1‖F). Furthermore, iterative refinement can mitigate the linear dependence on ‖AS−1‖F into polylogarithmic dependence Mohammadisiahroudi [2024]."
The paper's per-iteration quantum query count mκ²(AS−1), and hence the total query counts in Theorems 3.8 and 3.13, are not derived in this paper. They are imported verbatim from coauthor Mohammadisiahroudi's PhD thesis, with no proof reproduced and no independent verification supplied. The central quantum speedup claim therefore reduces at its key step to a same-author citation rather than to the present analysis: if that cited complexity were removed, Theorem 3.7 would have no proof in this manuscript.
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self citation load bearing
[Section 3.3, paragraph immediately before Lemma 3.12 (uniform condition-number bound for iterative refinement).]
"Thus, we can conclude that all NES systems in iterations of Algorithm 3 have the O(κ0) uniform condition number bound Mohammadisiahroudi et al. [2023a]."
The headline query complexity ~O(m√n κ0) in Theorem 3.13 is obtained only by replacing per-iteration condition numbers κ(A(S^(k))−2A^T) with the initial κ0. That uniform bound is asserted solely by reference to a same-group paper with overlapping authors, without stating the cited theorem's hypotheses or proving it here. The complexity claim's condition-number dependence is therefore imported from the authors' own prior work; absent that citation, the stated O(m√n κ0) bound does not follow from the equations developed in this paper.
full rationale
The classical inexact-feasible DLBM layer is not definitionally circular: the proximity measure, quadratic convergence analysis, and O(√n log(nµ0/ζ)) iteration bound are developed internally from Roos et al.'s classical theory, and no fitted quantity is renamed as a prediction. The circularity burden is concentrated in the quantum layer, where the QSVT-NES per-iteration complexity and the uniform O(κ0) bound for iterative refinement are load-bearing imports from same-group publications rather than results proved or verified here. These are significant self-citations, but they are not reductions by definition: the cited works are prior theorems, not the present paper's own outputs, and the core iteration analysis retains independent content. Separately, Lemma 3.2's proof appears to use the componentwise lower bound (s−1Δs)² ≥ 0.25e, which does not follow from |s−1Δs| ≤ 0.5; that is a correctness gap in the written derivation of δ(s+,µ+)², but it is not a circularity and is not counted in the score here. Overall, score 4 reflects load-bearing same-group citations while acknowledging the independent classical derivation.
Assumptions & free parameters
free parameters (3)
- θ (barrier update step) =
1/(4√n)
- QLSA/QTA accuracy constant =
0.005/1.995 ≈ 0.0025
- residual error threshold =
0.1
assumptions (5)
- domain assumption Interior Point Condition (Assumption 1): existence of (x0, y0, s0) with Ax0=b, A^T y0+s0=c, (x0,s0)>0
- domain assumption Access to QRAM and efficient block encoding of the Newton matrix
- standard math QSVT and QTA complexities from Gilyén et al. and van Apeldoorn et al. are correct
- ad hoc to paper Uniform condition-number bound O(κ0) for all NES systems during iterative refinement
- ad hoc to paper QSVT-based NES solving complexity from Mohammadisiahroudi [2024]
Cite this review
Pith. "Pith review of A quantum dual logarithmic barrier method for linear optimization." pith.science (2026). https://pith.science/paper/SDHGEF2Q
@misc{pith2026241215977,
author = {Pith},
title = {Pith review of: A quantum dual logarithmic barrier method for linear optimization},
year = {2026},
howpublished = {\url{https://pith.science/paper/SDHGEF2Q}},
note = {Machine review of arXiv:2412.15977}
}
abstract
Quantum computing has the potential to speed up some optimization methods. One can use quantum computers to solve linear systems via Quantum Linear System Algorithms (QLSAs). QLSAs can be used as a subroutine for algorithms that require solving linear systems, such as the dual logarithmic barrier method (DLBM) for solving linear optimization (LO) problems. In this paper, we use a QLSA to solve the linear systems arising in each iteration of the DLBM. To use the QLSA in a hybrid setting, we read out quantum states via a tomography procedure which introduces considerable error and noise. Thus, this paper first proposes an inexact-feasible variant of DLBM for LO problems and then extends it to a quantum version. Our quantum approach has quadratic convergence toward the central path with inexact directions and we show that this method has the best-known $\mathcal{O}(\sqrt{n} \log (n \mu_0 /\zeta))$ iteration complexity, where $n$ is the number of variables, $\mu_0$ is the initial duality gap, and $\zeta$ is the desired accuracy. We further use iterative refinement to improve the time complexity dependence on accuracy. For LO problems with quadratically more constraints than variables, the quantum complexity of our method has a sublinear dependence on dimension.
Reference graph
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