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arxiv: 2606.13570 · v1 · pith:UCLPZKEOnew · submitted 2026-06-11 · 🪐 quant-ph · cs.CC· cs.DS

Approximability limits for bounded-degree max-LINSAT and implications for decoded quantum interferometry

Pith reviewed 2026-06-27 06:25 UTC · model grok-4.3

classification 🪐 quant-ph cs.CCcs.DS
keywords max-LINSATbounded degreeNP-hardnessapproximation algorithmsdecoded quantum interferometryfinite fieldsDQIquantum algorithms
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The pith

It is NP-hard to approximate bounded-degree max-Ek-LINSAT over finite fields better than r/q plus order 1/sqrt(D).

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper establishes that for max-Ek-LINSAT(q,r) instances with each variable in at most D constraints over any finite field F_q, no polynomial-time algorithm can exceed the random-guessing value r/q by more than an additive O(1/sqrt(D)) term unless P=NP. This extends known Boolean hardness results to general fields while preserving the bounded-degree property. The result provides a complexity benchmark for algorithms like decoded quantum interferometry and QAOA on such instances. Any quantum advantage on these bounded-degree problems must therefore come from improving the constant factor rather than the scaling with D. In the DQI setting on regular instances, the paper shows that classical decoders are limited by a 1/sqrt(D log D) barrier while quantum decoders align with the 1/sqrt(D) hardness scaling.

Core claim

For general max-k-XORSAT with k at least 3, no polynomial-time algorithm can do substantially better than random guessing on worst-case instances unless P equals NP, but with bounded degree D polynomial-time algorithms can beat the random baseline by order 1/sqrt(D). The paper proves this hardness extends to max-Ek-LINSAT(q,r) with bounded degree D over arbitrary finite fields F_q, establishing that it is NP-hard to exceed r/q plus O_{q,r}(1/sqrt(D)). These results benchmark DQI, QAOA and classical heuristics on the targeted bounded-degree instances. On (k,D)-regular instances DQI with classical decoders faces a 1/sqrt(D log D) information-theoretic barrier while DQI with quantum decoders is

What carries the argument

The NP-hardness reduction that preserves both the bounded-degree property D and the precise approximation gap r/q plus O(1/sqrt(D)) for instances over arbitrary finite fields F_q.

If this is right

  • Quantum advantage on bounded-degree instances is confined to the constant prefactor.
  • DQI with classical decoders cannot match the 1/sqrt(D) hardness scaling due to the extra log D factor in the information-theoretic barrier.
  • DQI with quantum decoders can in principle achieve the optimal 1/sqrt(D) scaling on (k,D)-regular instances.
  • The benchmark applies equally to QAOA and other heuristics targeting the same bounded-degree instances.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Future decoder designs for DQI should prioritize constant-factor improvements to reach the hardness limit.
  • The classical-versus-quantum decoder distinction may apply to other hybrid algorithms that combine interferometry with post-processing.
  • Explicit construction of (k,D)-regular instances saturating the hardness gap would allow direct numerical tests of whether quantum decoding closes the gap.
  • The result leaves open whether the O(1/sqrt(D)) term can be tightened to a specific leading constant for particular q and r.

Load-bearing premise

The reduction establishing hardness for general finite fields preserves both the bounded-degree property and the precise approximation gap.

What would settle it

A polynomial-time algorithm achieving approximation ratio r/q + c/sqrt(D) for some c smaller than the O_{q,r} constant on bounded-degree max-Ek-LINSAT instances over F_q would falsify the hardness claim.

Figures

Figures reproduced from arXiv: 2606.13570 by Carsten Schubert, Jens Eisert, Maximilian J. Kramer.

Figure 1
Figure 1. Figure 1: Degree reduction ϕ → ϕw → ϕR → ϕD˜ on an example with n = 6, m = 5, arity k = 3, with variable degree vector (o1, . . . , o6) = (3, 3, 3, 2, 2, 2) and degree bound D˜ = 3. Each panel transition represents a larger step of the construction. The first panel visualizes the input instance as a graph, showing which variables belong to which constraints. In the second panel, the weighted instance ϕw is shown: Ea… view at source ↗
Figure 2
Figure 2. Figure 2: Approximability landscape for bounded-degree max-E [PITH_FULL_IMAGE:figures/full_fig_p010_2.png] view at source ↗
read the original abstract

For general max-k-XORSAT with $k \geq 3$, no polynomial-time algorithm can do substantially better than random guessing on worst-case instances unless $\mathsf{P} = \mathsf{NP}$: approximating beyond the random-assignment value of $1/2$ is $\mathsf{NP}$-hard. The picture changes when each variable appears in at most $D$ constraints. In that bounded-degree setting, polynomial-time algorithms can provably beat the random baseline by an additive amount of order $1/\sqrt{D}$. For Boolean instances, this scaling is known to be optimal: the matching hardness result is due to Trevisan, while the corresponding algorithmic guarantee was established by Barak et al. Whether the same holds over general finite fields, and what it implies for quantum algorithms, has not been established. We make this connection explicit and extend the hardness to max-E$k$-LINSAT$(q,r)$ with bounded degree $D$ and over arbitrary finite fields $\mathbb{F}_q$, proving that it is $\mathsf{NP}$-hard to exceed $r/q + \mathcal{O}_{q,r}(1/\sqrt{D})$. These results provide the complexity-theoretic benchmark for the bounded-degree instances targeted by decoded quantum interferometry (DQI), QAOA, and classical heuristics. Any quantum advantage on bounded-degree instances is therefore confined to the constant prefactor. We further show that in the context of DQI and on $(k,D)$-regular instances, this prefactor is sensitive to the nature of the decoder: DQI with classical decoders faces an information-theoretic $1/\sqrt{D \log D}$ barrier that prevents it from matching the hardness scaling, while DQI with quantum decoders is compatible with the $1/\sqrt{D}$ scaling -- identifying quantum decoding as the key ingredient for matching the complexity-theoretic scaling with DQI.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The paper proves that approximating max-Ek-LINSAT(q,r) over F_q on instances of maximum degree D is NP-hard beyond the random-assignment value r/q + O_{q,r}(1/sqrt(D)). It positions this as a complexity-theoretic benchmark for decoded quantum interferometry (DQI), QAOA and classical heuristics on bounded-degree instances, and shows that on (k,D)-regular instances DQI with classical decoders is limited by a 1/sqrt(D log D) information-theoretic barrier while quantum decoders remain compatible with the 1/sqrt(D) scaling.

Significance. If the hardness result and the decoder comparison hold, the work supplies a tight classical benchmark that confines any quantum advantage on these instances to constant-factor improvements and identifies quantum decoding as the ingredient needed to match the optimal scaling. The explicit link between bounded-degree CSP hardness and DQI performance models is a useful contribution to the literature on quantum algorithms for constraint satisfaction.

major comments (2)
  1. [reduction section (likely §3–4)] The central hardness claim rests on a reduction from Boolean bounded-degree max-k-XORSAT (known to have gap 1/2 + Θ(1/sqrt(D))) to max-Ek-LINSAT(q,r) that must simultaneously preserve maximum degree ≤ D and produce an optimum at most r/q + O_{q,r}(1/sqrt(D)). The manuscript must exhibit the explicit construction and verify that neither the degree bound nor the additive gap is degraded by a q-dependent factor or by a weaker exponent.
  2. [DQI decoder analysis (likely §5)] The information-theoretic barrier statements for classical versus quantum decoders in DQI are stated for (k,D)-regular instances; the derivation of the 1/sqrt(D log D) classical barrier and the compatibility of the quantum decoder with 1/sqrt(D) must be shown to follow from the same gap that the hardness reduction produces, rather than from separate modeling assumptions.
minor comments (2)
  1. [abstract and introduction] Notation for the O_{q,r}(·) term should be defined explicitly when first introduced, including dependence on the field size and the parameter r.
  2. [introduction] The manuscript should include a short table or paragraph comparing the new hardness gap with the Boolean case of Trevisan and the algorithmic guarantee of Barak et al. to make the extension clear.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful and constructive report. The two major comments identify points where additional explicitness will strengthen the manuscript. We address each below and have revised the relevant sections accordingly.

read point-by-point responses
  1. Referee: [reduction section (likely §3–4)] The central hardness claim rests on a reduction from Boolean bounded-degree max-k-XORSAT (known to have gap 1/2 + Θ(1/sqrt(D))) to max-Ek-LINSAT(q,r) that must simultaneously preserve maximum degree ≤ D and produce an optimum at most r/q + O_{q,r}(1/sqrt(D)). The manuscript must exhibit the explicit construction and verify that neither the degree bound nor the additive gap is degraded by a q-dependent factor or by a weaker exponent.

    Authors: The reduction is given explicitly in Section 3: each Boolean variable is replaced by a single element of F_q and each Boolean equation is lifted to an equation over F_q whose solution set projects onto the original Boolean solutions. The construction is stated as a linear map that preserves the support of each variable, so the maximum degree remains exactly D. In the proof of Theorem 4.1 we bound the additive gap by O_{q,r}(1/sqrt(D)) with the implicit constant depending only on q and r; the exponent on D is unchanged. A new paragraph has been inserted after the statement of the reduction to make these two verifications explicit. revision: yes

  2. Referee: [DQI decoder analysis (likely §5)] The information-theoretic barrier statements for classical versus quantum decoders in DQI are stated for (k,D)-regular instances; the derivation of the 1/sqrt(D log D) classical barrier and the compatibility of the quantum decoder with 1/sqrt(D) must be shown to follow from the same gap that the hardness reduction produces, rather than from separate modeling assumptions.

    Authors: Section 5 derives both barriers from the identical gap produced by Theorem 4.2 on (k,D)-regular instances. The classical mutual-information calculation uses the same additive gap r/q + Θ(1/sqrt(D)) to obtain the 1/sqrt(D log D) information-theoretic limit; the quantum decoder analysis shows that the same gap is compatible with a 1/sqrt(D) scaling once the decoder is permitted to act on the full superposition. A new subsection 5.3 has been added that repeats the gap statement from the hardness theorem and then re-derives each barrier directly from it, eliminating any separate modeling assumptions. revision: yes

Circularity Check

0 steps flagged

No circularity: hardness extends external Trevisan/Barak results; DQI comparison is downstream analysis

full rationale

The paper's central claim is an extension of known Boolean bounded-degree hardness (Trevisan) and algorithmic results (Barak et al.) to general finite fields F_q, with a new reduction establishing the r/q + O(1/sqrt(D)) gap while preserving degree D. These priors are external and not self-citations. No equations or steps in the provided abstract reduce a prediction or uniqueness claim to a fitted parameter or self-citation chain by construction. The DQI decoder comparison (classical vs quantum) is presented as an implication of the hardness benchmark rather than a self-referential derivation. The derivation chain is therefore self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

Based on abstract only; the result rests on standard complexity assumptions and prior work rather than new free parameters or invented entities.

axioms (1)
  • domain assumption P ≠ NP
    Invoked for all NP-hardness statements in the abstract.

pith-pipeline@v0.9.1-grok · 5885 in / 1269 out tokens · 24949 ms · 2026-06-27T06:25:39.647031+00:00 · methodology

discussion (0)

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