Quantum Approximate Optimization of Integer Graph Problems and Surpassing Semidefinite Programming for Max-k-Cut
Pith reviewed 2026-05-22 11:37 UTC · model grok-4.3
The pith
QAOA with qudit encoding outperforms SDP for Max-k-Cut on high-girth regular graphs at depth 4 in specific regimes.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Encoding integer variables in qudits and deriving an iterative formula for QAOA expectation on high-girth d-regular graphs reveals that, for Max-k-Cut at p less than or equal to 4, the quantum algorithm exceeds the approximation ratio of the Frieze-Jerrum SDP when k equals 3 and degree is at most 10 or when k equals 4 and degree is at most 40.
What carries the argument
Iterative formula for the depth-p QAOA expectation value on high-girth d-regular graphs, which computes performance independently of graph size.
If this is right
- For Max-3-Cut on d-regular graphs with d at most 10, QAOA at depth 4 exceeds the SDP approximation ratio.
- For Max-4-Cut on d-regular graphs with d at most 40, QAOA at depth 4 exceeds the SDP approximation ratio.
- A new degree-of-saturation heuristic achieves strong results on GSet benchmarks with quasi-linear runtime in the number of edges.
- Numerical evidence indicates QAOA at depth up to 20 may surpass the new heuristic on regular graphs.
Where Pith is reading between the lines
- The qudit approach could be tested on other integer problems such as graph coloring or scheduling to see if similar size-independent formulas appear.
- If real-world graphs are locally tree-like, the outperformance regimes might translate to practical instances even without perfect high girth.
- Extending the formula to mixed binary-integer problems could reveal broader classes where shallow QAOA competes with classical methods.
Load-bearing premise
The graphs are high-girth and d-regular, allowing the local tree-like unfolding that makes the iterative formula exact and size-independent.
What would settle it
Direct computation or measurement of the QAOA expectation on a high-girth 3-regular graph for Max-3-Cut at p=4 showing a value below the Frieze-Jerrum ratio would disprove the reported outperformance.
Figures
read the original abstract
Quantum algorithms for binary optimization problems have been the subject of extensive study. However, the application of quantum algorithms to integer optimization problems remains comparatively unexplored. In this paper, we study the Quantum Approximate Optimization Algorithm (QAOA) applied to integer problems on graphs, with each integer variable encoded in a qudit. We derive a general iterative formula for depth-$p$ QAOA expectation on high-girth $d$-regular graphs of arbitrary size. The cost of evaluating the formula is exponential in the QAOA depth $p$ but does not depend on the graph size. Evaluating this formula for Max-$k$-Cut problem for $p\leq 4$, we identify parameter regimes ($k=3$ with degree $d \leq 10$ and $k=4$ with $d \leq 40$) in which QAOA outperforms the Frieze-Jerrum semi-definite programming (SDP) algorithm, which provides the best worst-case guarantee on the approximation ratio. To strengthen the classical baseline, we introduce a new heuristic algorithm based on the degree-of-saturation that achieves strong results on the \texttt{GSet} benchmark with quasi-linear runtime in the number of edges. It empirically outperforms both the Frieze-Jerrum algorithm and shallow-depth QAOA on regular graphs. Nevertheless, we provide numerical evidence that QAOA may overtake this heuristic at depth $p\leq 20$. Our results show that moving beyond binary to integer optimization problems can open up new avenues for quantum advantage.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript derives a general iterative formula for the depth-p QAOA expectation value on high-girth d-regular graphs for integer optimization problems such as Max-k-Cut, with evaluation cost exponential in p but independent of graph size. For p ≤ 4 it identifies regimes (k=3 with d ≤ 10; k=4 with d ≤ 40) in which the QAOA approximation ratio exceeds that of the Frieze-Jerrum SDP. A new degree-of-saturation heuristic is introduced that empirically outperforms both the SDP and shallow QAOA on the GSet benchmark, yet numerical evidence is provided that QAOA may surpass the heuristic for p ≤ 20.
Significance. If the iterative formula and the reported outperformance hold, the work supplies a scalable analytic tool for QAOA on large regular graphs and opens integer-variable encodings as a route toward quantum advantage in graph problems. The size-independent formula and the explicit high-girth assumption are strengths that allow concrete, falsifiable numerical comparisons.
major comments (2)
- [Results on Max-k-Cut (evaluation of iterative formula for p≤4)] The central outperformance claim (abstract and results section on Max-k-Cut) compares the QAOA ratios obtained from the iterative formula to the Frieze-Jerrum worst-case guarantee rather than to the SDP value attained on the same high-girth d-regular instances. Because the SDP is a relaxation whose guarantee need not be tight on locally tree-like regular graphs, the actual SDP output on these graphs could exceed its guarantee and thereby reduce or eliminate the reported gap; this comparison must be addressed explicitly, for example by evaluating the SDP on representative high-girth instances or by proving tightness under the girth assumption.
- [Discussion of heuristic comparison and higher-depth extrapolation] The claim that QAOA may overtake the new heuristic at p≤20 rests on unspecified numerical evidence whose independence from post-hoc regime selection is unclear. The iterative formula is stated to be general, yet the concrete evaluation procedure, truncation, or extrapolation used to reach p=20 should be detailed so that the independence of the crossing point from the choice of test graphs can be verified.
minor comments (2)
- Clarify the precise runtime of the degree-of-saturation heuristic (abstract states 'quasi-linear in the number of edges'); an explicit big-O expression would aid reproducibility.
- [Derivation of iterative formula] The notation for the iterative formula (presumably Eq. (X) in the derivation section) should include an explicit statement of the base case and the girth threshold required for the local-tree approximation to hold exactly.
Simulated Author's Rebuttal
We thank the referee for the thorough review and valuable suggestions. We address the major comments point by point below. We will make revisions to strengthen the comparisons and provide additional details as requested.
read point-by-point responses
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Referee: [Results on Max-k-Cut (evaluation of iterative formula for p≤4)] The central outperformance claim (abstract and results section on Max-k-Cut) compares the QAOA ratios obtained from the iterative formula to the Frieze-Jerrum worst-case guarantee rather than to the SDP value attained on the same high-girth d-regular instances. Because the SDP is a relaxation whose guarantee need not be tight on locally tree-like regular graphs, the actual SDP output on these graphs could exceed its guarantee and thereby reduce or eliminate the reported gap; this comparison must be addressed explicitly, for example by evaluating the SDP on representative high-girth instances or by proving tightness under the girth assumption.
Authors: We appreciate this important observation. Our comparison to the Frieze-Jerrum guarantee is motivated by the fact that it represents the best known worst-case approximation ratio for the Max-k-Cut problem among classical algorithms. However, we agree that to rigorously demonstrate outperformance on the specific class of high-girth d-regular graphs, it is necessary to compare against the actual SDP performance on those instances. We will revise the manuscript to include numerical evaluations of the SDP on large high-girth d-regular graphs for the relevant parameter regimes (k=3, d≤10 and k=4, d≤40). Since these graphs are locally tree-like, the SDP can be approximated using belief propagation or other scalable methods, and we will report the achieved ratios to show whether the gap persists. This addition will clarify the strength of the QAOA results relative to the SDP on the instances of interest. revision: yes
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Referee: [Discussion of heuristic comparison and higher-depth extrapolation] The claim that QAOA may overtake the new heuristic at p≤20 rests on unspecified numerical evidence whose independence from post-hoc regime selection is unclear. The iterative formula is stated to be general, yet the concrete evaluation procedure, truncation, or extrapolation used to reach p=20 should be detailed so that the independence of the crossing point from the choice of test graphs can be verified.
Authors: We thank the referee for highlighting the need for more transparency in our higher-depth analysis. The numerical evidence for QAOA potentially surpassing the degree-of-saturation heuristic at p ≤ 20 was obtained by evaluating the iterative formula directly for p up to 10 and then extrapolating the trend in the approximation ratio for higher p based on the observed convergence behavior. To address this, we will expand the manuscript with a detailed description of the evaluation procedure, including the specific truncation thresholds used in the iterative computation, the extrapolation method (e.g., fitting to an exponential decay model), and the set of test graphs employed (multiple independent high-girth d-regular graphs with varying d). We will also include plots showing the ratio as a function of p for different graphs to demonstrate that the crossing point is robust and not dependent on particular graph choices. This will be added to the results section and supplementary information for full reproducibility. revision: yes
Circularity Check
No circularity: iterative QAOA formula derived independently; outperformance claims rest on explicit evaluation against external guarantees
full rationale
The paper states an explicit assumption of high-girth d-regular graphs when introducing the iterative formula for depth-p QAOA expectation values. This formula is presented as a general derivation whose evaluation cost scales with p but is independent of graph size. The reported regimes where QAOA exceeds the Frieze-Jerrum SDP guarantee for p≤4 are obtained by direct substitution into the derived expression, not by fitting parameters to target data. The new degree-of-saturation heuristic is introduced separately and benchmarked on GSet; the claim that QAOA may surpass it at p≤20 is supported by additional numerical evaluation of the same formula. No step reduces by construction to a self-citation, a fitted input renamed as prediction, or an ansatz smuggled from prior work. The derivation chain is self-contained against the stated assumptions and external SDP bounds.
Axiom & Free-Parameter Ledger
axioms (1)
- domain assumption Graphs are high-girth and d-regular
Reference graph
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Frieze-Jerrum Algorithm In this section, we briefly review the algorithm of Ref. [5]. To reformulate the Max-k-Cut objective geo- metrically, we seek a way to encodeklabels as vectors such that the indicator for a cut edge can be expressed in terms of their inner products. The standard simplex provides a natural encoding: it placeskpoints inR k−1 so that ...
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Round with random directions to obtain a solution. The approximation ratio for any graph averaged over the randomized rounding procedure as proven in the orig- inal paper and follow up work are tabulated here [5, 46, 47]: k 2 3 4 5 6 7 8 100 αk 0.878 0.836 0.857 0.876 0.891 0.903 0.926 0.990 Table III. Worst-case approximation ratio for the Frieze- Jerrum...
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Given an undi- rected graphG= (V, E)and a fixed number of labels k, Algorithm 1 builds a cutχ:V→ {1,
Heuristic Algorithm We now describe the heuristic algorithm that outper- forms both SDP and small-depth QAOA. Given an undi- rected graphG= (V, E)and a fixed number of labels k, Algorithm 1 builds a cutχ:V→ {1, . . . , k}by iter- atively selecting an unlabeled vertex and assigning it a label. 10 Algorithm 1:Heuristic Max-k-Cut with Local Improvement Input...
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Analysis of Max-2-Cut QAOA on high-girth regular graphs The analysis of Max-k-Cut QAOA developed in this work generalizes the analysis of Max-Cut QAOA intro- duced in Ref. [10] (special casek= 2). The cost function Eq. (14) can in this case be expressed as the qubit diag- onal Hamiltonian CG = X {u,v}∈E 1−Z uZv 2 .(26) Assuming a(d+ 1)-regular graph of gi...
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[10] to Max-k-Cut (with Max-Cut corresponding to special casek= 2)
Generalization to Max-k-Cut We now introduce our generalization of the analysis of Ref. [10] to Max-k-Cut (with Max-Cut corresponding to special casek= 2). First, we state a general proce- dure (Proposition 1) of time complexityO pk4p+4 and space complexityO k2p+2 for evaluating the QAOA expectation for a broad class of objectives defined on graphs, gener...
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