REVIEW 4 major objections 5 minor 97 references
Quantum Approximate Optimisation Applied to Graph Similarity
T0 review · 4 major / 5 minor · reviewed 2026-08-11 · deepseek-v4-flash
Pith's one-line read A compact qubit encoding lets QAOA beat random sampling on small graph-similarity problems, but the zero-cost infeasible tail erodes the advantage on larger instances.
desk verdict A careful QAOA-for-graph-similarity study whose own larger-graph results refute its central claim, made worse by a metric defined with contradictory sign conventions. 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 mechanism is the permutation-index encoding: rather than labelling each candidate vertex mapping with $V^2$ unary qubits, the paper orders all $V!$ permutations and stores only the index of a permutation in $q=\lceil\log_2(V!)\rceil$ qubits. The diagonal cost Hamiltonian $\hat C$ gives each index its edge-overlap value, and the unused tail of $2^q-V!$ bit-strings is set to zero. The mixing Hamiltonian $\hat B=\sum_i\sigma^x_i$ makes the search walk over all bit-strings, and $\hat U_B$ is applied through a polynomial expansion of the matrix exponential rather than a full dense exponentiation. Everything that matters for the result follows from that tail: it is what saves memory, and it is what makes the cost landscape hard for the classical optimiser.
What would settle it
Run the exact simulation on randomly deformed graph pairs of increasing vertex count with a fixed evaluation budget, and compare the QAOA expectation value with the expectation of uniform random sampling under the same cost function. The paper's Section 4.4.1 already reports a negative quantum-versus-classical ratio on the larger directed test cases; any negative ratio at a given graph size falsifies the unifying hypothesis for that size, and a consistently positive ratio across the tested range would confirm it.
Extended reading notes
Core claim
The paper's central claim is that graph similarity by maximal edge overlap can be mapped to QAOA with only $q=\lceil\log_2(V!)\rceil$ qubits, and that on small instances the algorithm measurably improves over naive random sampling for a fixed sample count. The unused bit-strings are assigned zero cost, so the algorithm starts from a uniform superposition in which most amplitude sits on infeasible solutions; nevertheless, the reported experiments show QAOA escaping that local minimum and improving with Trotter depth, with undirected graphs performing better than directed ones. At larger graph sizes the improvement reverses: the paper reports that the QAOA expectation value falls below random sampling for all eight classical optimisers tested, which it treats as evidence that the feasible/infeasible mix makes the optimisation problem harder, not as a final refutation of the approach.
Load-bearing premise
The classical optimiser can concentrate enough probability on feasible permutations within its evaluation budget even though the search starts uniformly over a space in which nearly all bit-strings are infeasible zero-cost strings.
Editorial extensions
If this is right
- Small instances and undirected pairs establish a baseline where QAOA with this encoding is a valid laboratory for comparing mixers, cost Hamiltonians, and classical optimisers.
- Increased Trotter depth $p$ buys better solutions at roughly double the evaluation count, so any real device must budget quantum evaluations against solution quality.
- The zero-cost tail creates a landscape in which all infeasible strings look optimal; any future encoding that shrinks or penalises that tail should directly improve the measured quantum-versus-classical ratio.
- Because the simulator is problem-agnostic apart from the cost function, the same package can be used to validate QAOA on other NP-optimisation problems without building a new simulation stack.
Reading between the lines
- The comparison baseline is random sampling only; comparing QAOA on this encoding against classical heuristic graph-matching would clarify whether the small-instance advantage is real or an artefact of a weak baseline.
- The paper's alternate cost function that penalises infeasible states made things worse, which suggests a subtle nonlinearity: a soft, tuned penalty or a separate feasible-mixing operator might outperform both tested extremes.
- The tail proportion oscillates with $V$ (for example, $2^q/V!$ is 0.65 at $V=8$ and 0.15 at $V=10$ in the paper's Table B.1), so picking graph sizes whose factorial sits close to a power of two could give QAOA an easier landscape and a cleaner test of the algorithm.
- The exact state-vector simulation ceiling of 22 qubits is a memory bound, so adapting the encoding to a restricted mixer or approximate simulation scheme would show whether the trend continues or reverses at larger sizes.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper presents an honours-thesis-style study applying the Quantum Approximate Optimisation Algorithm (QAOA) to graph similarity, specifically to the problem of maximal edge overlap between two directed or undirected unweighted graphs under vertex relabelling. The authors introduce Qolab, a C-based simulation package for QAOA, and propose a compact encoding of permutation problems that uses q = ceil(log2(V!)) qubits rather than a unary V^2 encoding. The stated unifying hypothesis (Section 1.5) is that, for a fixed number of samples, QAOA will produce more correct maximal-edge-overlap solutions than naive random sampling. The paper reports experiments with eight classical optimisers at six decomposition levels, on graphs generated by Erdős–Rényi models with several deformation types, and concludes that the QAOA suffers from infeasible solutions in larger test cases.
Significance. If the central hypothesis were supported, the paper would offer two useful contributions: a general-purpose QAOA simulation tool with a reproducible benchmark suite, and a memory-efficient encoding for permutation-based optimisation problems. The manuscript does make some genuine assets explicit: the source code and result data are promised at public repositories, the study covers a broad sweep of classical optimisers (eight methods at six values of p), and the correctness of the QAOA expectation is checked against brute-force optimal values in Section 4.1. However, the paper's own reported results contradict the unifying hypothesis for larger graphs, and the primary comparison metric is defined with contradictory sign conventions. The contribution is therefore not established by the evidence presented, and the significance of the claimed result is currently undermined.
major comments (4)
- [Section 4.1, 'Classical Comparison' metric] The definition of the 'Quantum / Classical Expectation' metric contains two mutually exclusive clauses: 'A negative value indicates better likelihood of a better solution from the QAOA. Higher is better.' These cannot both hold. If negative means QAOA is better, then higher (less negative) is not better; if higher is better, then negative is worse. The plots in Figures 4.2–4.6 show negative values for larger graphs, and Section 4.4.1 explicitly interprets those negative values as failure ('a lower expectation value versus classical sampling'). This contradiction is load-bearing because the metric is the main evidence for or against the Section 1.5 hypothesis. The paper must state an unambiguous sign convention and then re-interpret the figures consistently.
- [Section 4.4.1 and Section 1.5] Section 4.4.1 states: 'for all algorithms tested the effect of infeasible solutions results in a lower expectation value versus classical sampling in the larger test cases.' This is a direct admission that the central hypothesis of Section 1.5 (QAOA outperforms random sampling) is not supported on larger graphs for any of the eight optimisers tested. If the 'negative is better' reading of the metric is instead adopted, then the same sentence would be inconsistent with the reported sign of the values. Either way, the paper does not present an unambiguous positive result, and the hypothesis as stated is not established. This is not a presentation issue but a fundamental problem with the claimed conclusion.
- [Section 3.2.4, tail mapping and Section 4.4.1] The encoding maps all 2^q − V! infeasible bit-strings to cost zero, and the QAOA starts from a uniform superposition over all 2^q states. As the paper itself notes and as Table B.1 shows, the tail fraction can be substantial (e.g., for V=8, 2^q/V! ≈ 1.63). The paper attributes the degraded expectation values on larger graphs to the 'effect of infeasible solutions,' which is a plausible mechanism, but no evidence is provided that the classical optimiser can overcome this tail within the evaluation budget. Since the Section 1.5 hypothesis depends precisely on the QAOA concentrating probability on feasible permutations, the paper needs either a dedicated analysis of the tail's influence or a revised hypothesis that accounts for the reported negative results.
- [Sections 3.2.2 and 3.2.4, resource claims] The claims of 'O(V^3) operations' for the state-preparation encoding and 'O(n^2 log n) operations and O(n log n) qubits' for the edge-overlap circuit are asserted by citation to [1] and [18] rather than derived in this manuscript. Because the claimed 'significant quantum memory savings at the cost of additional operations' is one of the paper's two headline contributions, the resource trade-off should be either derived or stated with the precise conditions under which the cited bounds apply. As written, the reader cannot verify the resource count without going to the cited works, and the paper's own Appendix E, which is used to justify the Chebyshev truncation for U_B, is informal.
minor comments (5)
- [Figures 4.2–4.6 and Appendix F] The figures do not include error bars, confidence intervals, or any measure of trial-to-trial variance; given that results are aggregated over different deformation types and multiple random instances, the absence of dispersion information makes it difficult to assess the reliability of the reported averages.
- [Section 4.5 and figures] The text in Section 4.5 refers to 'Figure 4.5' for the undirected Subplex results, but the displayed figure caption is 'Figure 4.6: Final performance for the Subplex algorithm (undirected graphs)'. Similar numbering inconsistencies appear across the results chapter and should be corrected.
- [Section 3.4.1] The sentence 'This requires the generation of all q! bit-strings' is confusing: the simulation enumerates all 2^q computational basis states, not q! bit-strings. The subsequent sentence about reducing 'generations of the q! permutations' to the 2^{ceil(log2(v!))} state-space needs to be clarified.
- [Equation (3.7.3)] The Chebyshev expansion is written with 'inf' instead of an infinity symbol, and the summation index n is not defined in the displayed formula; the truncation criterion in Eq. (3.7.8) should also state explicitly that n runs from 1 to the truncation order.
- [Appendix E] The proof of λ_{min,max} = ±q is informal: it uses a real-symmetric-matrix argument with a typo ('R3' instead of R^n), invokes Perron–Frobenius without stating the non-negativity conditions precisely, and the final observation 'for a negative v, λ = −q' does not constitute a proof for all vectors. The paper also contains typos such as 'Schr¨ oidgner' in Section 1.4.1 and 'na ¨ ıve' elsewhere.
Circularity Check
No significant circularity: the central QAOA-vs-random comparison is benchmarked against independent brute-force optimal values and per-instance optimisation, not fitted to its own outputs.
full rationale
The paper's derivation chain is self-contained rather than circular. Section 3.2 constructs the problem Hamiltonian C from the graph edge-overlap objective directly, and Section 3.7-3.8 implements the QAOA state evolution and measurement without any parameter fitted to a target answer. The Section 1.5 hypothesis is tested in Section 4 by comparing the QAOA expectation value with an independent random-sampling baseline and with known brute-force optimal values computed by Algorithm 1; the QAOA angles are optimised per instance by nlopt rather than tuned to reproduce the benchmark results. The only in-group citation, Marsh and Wang [61], supports an optional walk-mask extension and is not load-bearing for the central compact permutation encoding, which is derived from Abrams-Lloyd and Chiew et al. rather than imported from the authors. The problematic sign convention in the Section 4.1 'Classical Comparison' definition ('A negative value indicates better likelihood of a better solution from the QAOA. Higher is better') and the interpretation in Section 4.4.1 that negative values indicate failure are internal-consistency and correctness defects, not circularity, because neither clause makes the reported comparison equal to its inputs by construction.
Assumptions & free parameters
free parameters (3)
- MLSL evaluation cap multiplier S =
200
- Chebyshev truncation tolerance epsilon =
1e-18
- Erdos-Renyi edge probability =
0.5
assumptions (5)
- standard math QAOA with cost Hamiltonian C, mixing Hamiltonian B = sum sigma_x, and p Trotter steps approximately optimizes the expectation value F_p(gamma, beta).
- domain assumption The edge-overlap cost for all V! permutations can be encoded as a diagonal Hamiltonian, Eq. 3.2.2, and implemented as e^{-i gamma C} without ancillas via Welch et al. and Chiew et al.
- ad hoc to paper Mapping all 2^q - V! infeasible bit-strings to cost zero still permits the QAOA to concentrate probability on feasible permutations.
- standard math The mixing Hamiltonian B has eigenvalues exactly +/- q, so the Chebyshev propagator can be scaled without solving an eigenproblem.
- domain assumption Erdos-Renyi graphs with edge probability 0.5 and the five graph deformations are representative difficult instances for graph similarity.
Cite this review
Pith. "Pith review of Quantum Approximate Optimisation Applied to Graph Similarity." pith.science (2026). https://pith.science/paper/JNPWBJNI
@misc{pith2026241217309,
author = {Pith},
title = {Pith review of: Quantum Approximate Optimisation Applied to Graph Similarity},
year = {2026},
howpublished = {\url{https://pith.science/paper/JNPWBJNI}},
note = {Machine review of arXiv:2412.17309}
}
read the original abstract
Quantum computing promises solutions to classically difficult and new-found problems through controlling the subtleties of quantum computing. The Quantum Approximate Optimisation Algorithm (QAOA) is a recently proposed quantum algorithm designed to tackle difficult combinatorial optimisation problems utilising both quantum and classical computation. The hybrid nature, generality and typically low gate-depth make it a strong candidate for near-term implementation in quantum computing. Finding the practical limits of the algorithm is currently an open problem. Until now, no tools to facilitate the design and validation of probabilistic quantum optimisation algorithms such as the QAOA on a non-trivial scale exist. Graph similarity is a long standing classically difficult problem withstanding decades of research from academia and industry. Determining the maximal edge overlap between all possible node label permutations is an NP-Complete task and provides an apt measure of graph similarity. We introduce a novel quantum optimisation simulation package facilitating investigation of all constituent components of the QAOA from desktop to cluster scale using graph similarity as an example. Our simulation provides flexibility and performance. We investigate eight classical optimisation methods each at six levels of decomposition. Moreover an encoding for permutation based problems such as graph similarity through edge overlap to the QAOA allows for significant quantum memory savings at the cost of additional operations. This compromise extends into the classical portion of the algorithm as the inclusion of infeasible solutions creates a challenging cost-function landscape. We present performance analysis of our simulation and of the QAOA setting a precedent for investigating and validating numerous other difficult problems to the QAOA as we move towards realising practical quantum computation.
Figures
Figures from the paper (12 more)
Reference graph
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