REVIEW 4 major objections 4 minor 4 cited by
Evaluating the performance of quantum processing units at large width and depth
T0 review · 4 major / 4 minor · reviewed 2026-08-08 · deepseek-v4-flash
Pith's one-line read A single fixed-parameter quantum algorithm can rank processors by the depth at which they lose their algorithmic signal, a benchmark the authors apply to 24 QPUs.
desk verdict Broad, open QPU benchmark with real data and a clear pass/fail protocol; the per-backend parameter schedule and best-section selection undercut the 'architecture-neutral' claim but don't sink the main trends. 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 LR-QAOA, a fixed-parameter variant of QAOA in which the variational search is replaced by a deterministic linear ramp: $\beta_k = (1 - k/p)\Delta_{\beta}$ and $\gamma_k = (k+1)/p\,\Delta_{\gamma}$. The circuit alternates problem and mixer unitaries over $p$ layers, and the performance metric is the approximation ratio $r$ relative to the optimal Weighted MaxCut value, compared with a 99.73% random-sampler confidence interval through the normalized effective ratio $r_{\rm eff}$. Because the schedule is fixed, circuits can be generated identically on any device, scaled in width and depth, and mapped onto 1D-chain, native-layout, and fully-connected topologies; the random threshold turns a noisy signal into a binary pass/fail test of coherent algorithmic signal.
What would settle it
Re-run the 56-qubit fully-connected benchmark on Quantinuum H2-1 after a fresh calibration using 100 samples instead of 7; if the approximation ratio falls inside the random-sampler 99.73% interval, the claimed certification was a statistical or calibration artifact.
Extended reading notes
Core claim
On the authors' terms, the central discovery is that a non-variational, depth-scaled QAOA schedule gives a reliable probe of a QPU's coherent signal: the approximation ratio rises with depth under ideal evolution and falls into a noise-dominated regime once decoherence takes over, so the depth at which it crosses the random-sampler threshold measures usable circuit depth. Applying this to 24 QPUs gives a concrete ranking. Quantinuum's H2-1 is the only device that passes the fully connected benchmark at 56 qubits, with $r = 0.872$ and $r_{\rm eff} = 0.079$, the largest certified QAOA runs to date, moving past the exact classical simulation threshold near 50 qubits. Among IBM devices, Heron-generation chips outperform Eagle-generation chips in depth scaling, and fractional ZZ gates cut the number of two-qubit operations in half, which the paper identifies as a major reason for the gain. The paper also finds that error-per-layered-gate values do not consistently predict algorithmic performance, and that the benchmark is stable over time (under 1% variation over two months) and remains decisive even with as few as seven measurement samples.
Load-bearing premise
The rankings assume that selecting the best-performing physical qubit section of each device gives a fair basis for comparing whole processors across vendors.
Editorial extensions
If this is right
- A vendor-neutral, depth-centric benchmark can be standardized for near-term quantum hardware, since LR-QAOA is reproducible, requires no variational optimization, and stays stable over time.
- Device selection for depth-intensive algorithms can be guided by the depth at which each platform leaves the random-sampler region, rather than by gate-fidelity metrics alone.
- Fractional two-qubit gates on Heron-class IBM chips halve the two-qubit gate count and circuit depth per layer, giving a concrete hardware-innovation lever that this benchmark detects.
- The protocol exposes platform-specific bottlenecks: superconducting devices are limited by coherence and crosstalk, while trapped-ion devices are limited by slow gates and lack of parallelism.
- The 56-qubit H2-1 result crosses the exact-classical-simulation threshold, so the benchmark can certify QAOA runs that are not checkable by exact state-vector simulation.
Reading between the lines
- If the section-selection procedure reports only the best-performing physical qubit chain, the cross-platform comparison may overstate whole-device capability; a whole-device or median-section version of the benchmark could rank vendors differently.
- The random-sampler threshold is algorithm-agnostic enough that the same pass/fail logic could be ported to other fixed-depth coherent algorithms, such as quantum simulation or amplitude estimation, to produce a family of depth benchmarks.
- Because $r_{\rm eff}$ normalizes by the random baseline, the benchmark could be extended to track how algorithmic signal scales against noise models, potentially predicting when error mitigation becomes necessary before deployment.
- A functional form for the ramp scale, $\Delta_{\beta,\gamma}(N_q)$, is hinted at but not developed; fitting it empirically could extend the useful depth range of the benchmark across problem sizes.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript introduces a benchmarking protocol based on LR-QAOA, a non-variational QAOA variant with linearly ramped parameters. The protocol measures the approximation ratio r as a function of circuit depth p for Weighted MaxCut instances on three graph topologies (1D chain, native layout, fully connected) and declares a pass when r exceeds a 99.73% random-sampler threshold. The authors apply it to 24 QPUs/emulators from six vendors, with up to 156 qubits and p = 10,000, and report a performance map: Heron-class IBM devices outperform Eagle devices, Quantinuum H2-1 is the only platform passing the 56-qubit FC benchmark, EPLG does not predict algorithmic performance, and results are stable over time. The paper includes gate-count formulas, a SWAP-network routing overhead analysis, and raw-data availability on GitHub.
Significance. If the protocol's architecture-neutrality claim can be sustained, this would be a valuable contribution: it is one of the largest cross-platform algorithmic benchmarking studies to date, uses a clearly defined statistical baseline, provides reproducible data, and directly addresses depth scaling rather than gate-level metrics. The honest discussion of sampling effects (Fig. 6) and the explicit statement that parameter values were not tuned per QPU are strengths. However, the central claim is currently compromised by two methodological choices: per-backend Δβ,γ schedules for FC problems (Supplementary Table III) and best-section selection for 1D-chain/FC benchmarks (Supplementary Sec. 4). These need to be reconciled before the benchmark can be called architecture-neutral.
major comments (4)
- [Supplementary Sec. 1, Table III; Sec. IV] The FC benchmark uses different Δβ,γ values for different backends for Nq > 15, while Sec. IV states 'For a fair comparison, the same set of parameters must be used across all devices.' Since Supplementary Figs. 8(a), 10(b), and 15 show that r(p) is sensitive to Δβ,γ, and reff in Eq. (8) is computed from rmax, a device evaluated at a favorable Δ (e.g., H2-1 at Δ = 0.2 for Nq = 56) may pass the 99.73% threshold when it would fail at another platform's schedule. The claim that values were 'not tuned for performance on individual QPUs' does not resolve this: the selection still follows a per-backend schedule from the authors' prior work [1], and no robustness test shows that the rankings are invariant to Δ within the range used. Please either run all FC experiments with a common Δ(Nq) schedule, or provide a sensitivity analysis demonstrating that every reported pass/fail decision and ranking is unchanged under the full range of Δ values in Table III.
- [Supplementary Sec. 4, Fig. 8(b)-(c)] The 1D-chain and FC benchmarks report the best-performing physical qubit section: 'The section with the highest approximation ratio is highlighted in black... This section selection procedure is used consistently across 1D-chain and FC benchmarks.' This is not equivalent to whole-device performance. The NL benchmark uses all qubits, so cross-benchmark claims such as 'Heron devices outperform Eagle-generation QPUs' and the FC ranking may reflect favorable qubit placement rather than system-wide capability. For FC problems on fixed-layout devices, the SWAP network (Supplementary Sec. 7) maps logical qubits onto a linear chain, so the choice of section directly affects the result; all-to-all trapped-ion devices have no such selection. Please report whole-device (or all-section) results for the 1D-chain and FC benchmarks, or justify why best-section is the appropriate metric for a fair cross-platform comparison.
- [Supplementary Sec. 3, Fig. 6; Sec. III.C] The pass criterion uses n = 7 samples for H2-1 at 56 qubits, n = 50/100 for other Quantinuum/IonQ runs, and n = 1000 for IBM devices. Because the random-sampler threshold is rrand = mean + 3σ over subsets of size n, the threshold stringency varies with n (σ scales as 1/sqrt(n)). While the paper correctly notes that smaller samples demand more signal, the cross-platform comparison is not uniform: a device with n = 7 must clear a higher bar than one with n = 1000, and the reported rmax for H2-1 is itself a single 7-sample estimate with large variance. Please either standardize n across devices or show that the qualitative rankings survive when thresholds are computed at a common n.
- [Sec. II.A; Sec. IV] The architecture-neutrality of the benchmark rests on the assumption that LR-QAOA's approximation-ratio curve is a faithful, hardware-independent probe of algorithmic signal. The Δ schedule is justified by the same authors' prior work [1] rather than by external or independent validation; no comparison with another fixed-parameter algorithm or a controlled experiment isolating the effect of Δ on the pass/fail decision is provided. This is not a circularity in the measured r values, but it is a correctness risk for the benchmark's validity. Please add a test, e.g., a noiseless simulation across architectures showing that the observed r(p) curves track the known coherent signal, or a comparison with a second fixed-parameter schedule, to support the claim that the protocol measures QPU capability rather than the choice of schedule.
minor comments (4)
- [Table II; Sec. III.C] Table II lists reff = 0.020 for ibm fez at Nq = 20, while Sec. III.C reports reff = 0.018 for the same quantity; similarly, H2-1's 56-qubit reff is 0.082 in Table II and 0.079 in the text. Please reconcile these values.
- [Throughout] There are several typographical issues, e.g., 'ref f' with an extra space in Sec. III.C and 'FCNq =20' in the Table II header; a careful proofread is needed.
- [Supplementary Sec. 3, Fig. 6] The caption states that σ7 and σ100 are computed from 200 repeated samplings, while the text says the standard deviation increases by a factor of 3.68; please clarify whether these are standard deviations of single-sample r or of subset means, and define the exact bootstrap procedure.
- [Abstract; Sec. I] The abstract and introduction claim this is 'the most extensive cross-platform quantum benchmarking effort to date'; please provide a citation or comparison with prior multi-device benchmarking studies (e.g., Refs. [29–32]) to substantiate this superlative.
Circularity Check
Empirical QPU measurements are independent; only the LR-QAOA parameter schedule is justified by the authors' prior work, giving a mild self-citation burden.
-
self citation load bearing
[Section II.A (LR-QAOA) and Supplementary Sec. 1, Table III]
"We choose well-conditioned ramp schedules with observable signal growth in noise-free settings and balanced saturation under noise, validated in prior simulations [1]. ... These values were not tuned for performance on individual QPUs. Instead, they follow the observation from [1, Sec. III-B] that the optimal value of ∆γ,β decreases as Nq increases, in order to maintain algorithmic signal."
The benchmark's central control parameter is the linear ramp slope ∆β,γ. Its values are not derived from first principles or from external benchmark data; they are taken from [1], the authors' own earlier LR-QAOA paper, and then assigned per backend and per size in Table III for FC experiments. Since the pass/fail metric reff depends on rmax (Eq. 8), and r(p) is sensitive to ∆ (Supp. Figs. 8a, 10b, 15), the cross-platform rankings are conditional on a schedule whose selection is justified by a self-citation. However, the measured r values on each QPU are new, independently obtained hardware data, so this is a load-bearing self-citation rather than an equivalence-by-construction of inputs and outputs.
full rationale
The core empirical content of the paper is not circular. The approximation ratios r are computed from measured bitstring samples (Eqs. 6-7) against optima obtained from CPLEX, and the random-sampler threshold is a classical statistical baseline (Supp. Sec. 3); no equation makes the QPU output equal to the input parameters by construction. The headline ranking, such as Quantinuum H2-1 passing the 56-qubit FC benchmark, is a direct experimental observation. The only identified circularity burden is the ramp-schedule choice: the paper states that schedules were 'validated in prior simulations [1]' and that per-size FC values follow 'the observation from [1, Sec. III-B]'. Reference [1] is the same group's prior LR-QAOA work, and the schedule affects which devices pass at which depths. This makes part of the protocol's validity rest on self-citation, but it does not reduce the measured hardware results to the prior paper's outputs. The per-backend ∆ values in Table III also raise a fairness concern relative to the Discussion's requirement that 'the same set of parameters must be used across all devices,' but that is an internal-consistency issue rather than a circular derivation. Overall, score 3: some self-citation is load-bearing, yet the central measurement content remains independent.
Assumptions & free parameters
free parameters (3)
- Delta_beta,gamma for 1D-chain and NL benchmarks =
1
- Delta_beta,gamma for FC benchmarks =
0.2 to 0.63 depending on backend and Nq (Table III)
- Random-sampler confidence threshold =
99.73% (3sigma over 100 random subsets)
assumptions (4)
- domain assumption LR-QAOA with linear ramp parameters produces an approximation ratio that grows with depth in the absence of noise and saturates or decays under noise (Fig. 1(d)).
- domain assumption The 99.73% random-sampler interval is a valid statistical definition of meaningful algorithmic signal.
- domain assumption Vendor-reported device parameters (gate errors, durations, layouts) are accurate at the time of the experiments.
- standard math The swap network maps fully connected WMC problems to fixed-layout devices with the stated 3x gate overhead and preserves the algorithmic signal.
Cite this review
Pith. "Pith review of Evaluating the performance of quantum processing units at large width and depth." pith.science (2026). https://pith.science/paper/BAZIRMYC
@misc{pith2026250206471,
author = {Pith},
title = {Pith review of: Evaluating the performance of quantum processing units at large width and depth},
year = {2026},
howpublished = {\url{https://pith.science/paper/BAZIRMYC}},
note = {Machine review of arXiv:2502.06471}
}
read the original abstract
Quantum computers have now surpassed classical simulation limits, yet noise continues to limit their practical utility. As the field shifts from proof-of-principle demonstrations to early deployments, there is no standard method for meaningfully and scalably comparing heterogeneous quantum hardware. Existing benchmarks typically focus on gate-level fidelity or constant-depth circuits, offering limited insight into algorithmic performance at depth. Here we introduce a benchmarking protocol based on the linear ramp quantum approximate optimization algorithm (LR-QAOA), a fixed-parameter, deterministic variant of QAOA. LR-QAOA quantifies a QPU's ability to preserve a coherent signal as circuit depth increases, identifying when performance becomes statistically indistinguishable from random sampling. We apply this protocol to 24 quantum processors from six vendors, testing problems with up to 156 qubits and 10,000 layers across 1D-chains, native layouts, and fully connected topologies. This constitutes the most extensive cross-platform quantum benchmarking effort to date, with circuits reaching a million two-qubit gates. LR-QAOA offers a scalable, unified benchmark across platforms and architectures, making it a tool for tracking performance in quantum computing.
Figures
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For fully connected (FC) problems, the value of ∆ γ,β depends on the problem size
Experimental ∆γ,β used For 1D-chain and native layout (NL) problems, we use a fixed ramp value of ∆ γ,β = 1. For fully connected (FC) problems, the value of ∆ γ,β depends on the problem size. For instances with Nq ≤ 15, we use ∆ γ,β = 0.63 across all QPUs. For Nq > 15, the val...
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For 1D-chain problems on IBM and IQM devices using native CZ gates, the total number of two-qubit gates is N2q = 2p(Nq − 1), and the circuit depth is d = 4p
Circuit Gate Counts and Depth F ormulas This section summarizes analytical expressions for two-qubit gate counts and circuit depth as a function of QAOA layer count p and device architecture. For 1D-chain problems on IBM and IQM devices using native CZ gates, the total number ...
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Noise typically causes degradation in QPU output, and we classify the resulting behavior into three regimes
Random Sampling Limit To determine whether a QPU result is statistically meaningful, we compare the approximation ratio obtained from LR-QAOA with that from a random sampler. Noise typically causes degradation in QPU output, and we classify the resulting behavior into three re...
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[48]
This is motivated by the fact that fully connected (FC) interactions can be constructed from a linear qubit chain using the SW AP strategy described in Sec
1D-chain Benchmarking The 1D-chain benchmark provides a practical diagnostic of coherence and gate fidelity along a device’s longest physical qubit chain. This is motivated by the fact that fully connected (FC) interactions can be constructed from a linear qubit chain using th...
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[49]
Correlations are computed as |Cij| = 1 N NX n=1 sn i sn j , (9) where sn i ∈ {0, 1} is the value of qubit i in sample n, and N is the total number of samples
Correlation in LR-QAOA Figure 9 shows the pairwise correlation structure of bitstring samples for a 10-qubit FC WMC problem solved via LR-QAOA on different QPUs at depths p = 3 and p = 9. Correlations are computed as |Cij| = 1 N NX n=1 sn i sn j , (9) where sn i ∈ {0, 1} is th...
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[50]
Figure 10(a) shows the qubit layout of a 133-qubit Heron r1 device ( ibm torino)
Native Layout Benchmarking Figure 10 presents results for LR-QAOA applied to native layout (NL) WMC problems, where all qubits and native couplers of each device are used. Figure 10(a) shows the qubit layout of a 133-qubit Heron r1 device ( ibm torino). To implement LR-QAOA, t...
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[51]
This strategy, illustrated in Fig
SW AP strategy For fully connected (FC) problems on fixed-layout QPUs, we map the logical connectivity onto a linear topology using a SW AP network [42, 43]. This strategy, illustrated in Fig. 12(a), implements a complete interaction pattern by permuting logical qubit position...
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[52]
Quantinuum Experiments Access to Quantinuum hardware is governed by a quantum cost metric known as hardware quantum cost (HQC), computed as 16 (a) (b) r q18 q30 q31 q32 … … p r (c) (d) p r ibm_fezEagle QPUs p r Δβ,γ FIG. 10. Native layout benchmarking using LR-QAOA for WMC pro...
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Figure 14 compares projected runtime estimates for a fully connected LR-QAOA circuit using p = Nq layers on three representative QPUs
Execution Time Projections Execution time is an important constraint in quantum algorithms, especially for deep circuits with large qubit counts. Figure 14 compares projected runtime estimates for a fully connected LR-QAOA circuit using p = Nq layers on three representative QP...
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It visualizes how the approximation ratio varies as a function of the LR-QAOA depth p and the parameter scale ∆ γ,β
Performance Diagram The performance diagram, introduced in [44], helps identify optimal values of ∆ γ,β for LR-QAOA on a given problem instance. It visualizes how the approximation ratio varies as a function of the LR-QAOA depth p and the parameter scale ∆ γ,β . Figure 15(a) s...
Reviewed August 8, 2026 · model on record in the stance chip above.
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