REVIEW 2 major objections 7 minor 28 references
A classical tensor-network baseline for the Lipkin–Meshkov–Glick model up to 1400 particles shows subspace quantum diagonalization staying accurate far longer than variational eigensolvers on present hardware.
Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →
T0 review · grok-4.5
2026-07-31 03:25 UTC pith:WBPXEOEV
load-bearing objection Solid LMG benchmarking package: large DMRG tables plus a clean hardware VQE-vs-SQD comparison that holds inside the exactly solvable window. the 2 major comments →
Benchmarking Quantum Simulations of the Lipkin-Meshkov-Glick Model Using Large Tensor Networks
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Against a DMRG-computed ground-truth energy curve for the Lipkin–Meshkov–Glick Hamiltonian (ε=1, V=1, W=0) that extends to N=1400, variational quantum eigensolver runs on a superconducting processor remain within about one percent error only near six particles, whereas sample-based quantum diagonalization of the same model, using an ensemble of Dicke-state circuits and symmetry-aware post-selection, maintains sub-percent agreement out to roughly seventeen particles before shot-budget limits cause rapid divergence.
What carries the argument
The Density Matrix Renormalization Group (DMRG) representation of the LMG ground state as a matrix-product state, used to produce reference energies that score two NISQ algorithms: compressed VQE and sample-based quantum diagonalization (SQD) that projects the Hamiltonian into the subspace of measured bitstrings from fixed-parity Dicke states.
Load-bearing premise
The claim that the large-N DMRG energies are accurate enough to serve as sole ground truth rests on checks only up to nineteen particles plus smooth scaling and chosen sweep settings, not on an independent certificate once exact diagonalization is impossible.
What would settle it
Recompute selected large-N LMG ground energies (for example N=50, 100, 500 at V=1) with an independent high-precision classical method or substantially tighter DMRG bond-dimension and cutoff schedules; if those energies differ from the published DMRG curve by more than the one-percent success threshold used in the paper, the quantum-method rankings lose their reference.
If this is right
- The released LMG energy tables and phase diagram become a reusable classical yardstick for any future quantum algorithm claiming progress on this Hamiltonian.
- On present noisy hardware, subspace-projection methods that exploit LMG pair-excitation symmetry can reach larger particle numbers at sub-percent accuracy than compressed variational eigensolvers.
- SQD accuracy on this model is gated by whether the shot budget can cover the 2^{N-1}-sized symmetry sector; once that coverage fails, error rises sharply rather than gradually.
- For all-to-all LMG Hamiltonians, classical MPO construction—not the DMRG sweeps—dominates runtime at large N, so classical baselines remain limited by Hamiltonian assembly cost.
- Improved SQD sampling or shallower Dicke preparations would be required before the quantum subspace approach can push the accurate regime past the present shot-budget wall.
Where Pith is reading between the lines
- The same DMRG-versus-SQD comparison pattern is likely to appear for other collective spin models whose eigenstates live in low-dimensional Dicke or fixed-parity sectors.
- Once shot budgets or error-mitigated sampling grow enough to cover larger symmetry sectors, SQD could become a practical cross-check against tensor networks precisely in the intermediate-N window where both remain feasible.
- Hamiltonian libraries aimed at nuclear and condensed-matter applications would benefit from shipping companion large-N tensor-network energy tables as first-class benchmark artifacts alongside the operators themselves.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript generates large classical reference datasets for the Lipkin–Meshkov–Glick (LMG) model (W=0) via DMRG on NERSC Perlmutter—ground-state energies for N up to 1400 at (ε,V,W)=(1,1,0) and a phase diagram over N∈[2,100], V∈[0,1]—and uses them to benchmark VQE and Sample-Based Quantum Diagonalization (SQD) on IBM hardware. Against a 1% relative-energy success rule, VQE stays within threshold only near N=6 while SQD reaches roughly N=17–18 before a shot-budget cliff; the authors conclude that subspace-based NISQ methods currently balance accuracy, depth, and noise better than VQE on this model. Supporting material includes exact-diagonalization checks for N≤19, literature-spectrum replication, SPSA/ansatz details, Dicke-state preparation and error mitigation for SQD, and runtime profiling that attributes DMRG wall time mainly to O(N²) MPO construction.
Significance. Application-centered benchmarks that place NISQ algorithms next to strong classical baselines remain scarce; an openly usable LMG energy library at this scale is a concrete contribution for HamLib/HamPerf-style work and for the nuclear many-body community that already uses LMG as a testbed. The side-by-side VQE vs SQD comparison on the same model, with explicit shot-budget and O(N²) circuit-resource analysis for SQD, is timely and gives a clear, falsifiable ranking inside the exactly solvable window. Strengths include public code/dataset pointers, statistical stability checks of DMRG for small N, and transparent reporting that SQD can beat DMRG absolute error for N∈[3,15] when the symmetry sector is fully sampled. If the large-N DMRG energies are adequately certified and hardware naming/figure issues cleaned up, the paper is a useful reference dataset plus a solid NISQ methods comparison rather than a claim of quantum advantage.
major comments (2)
- [§4.1.1–4.1.2, Appendix B.2] §4.1.1–4.1.2 and Appendix B.2 certify DMRG only for N≤19 (exact diagonalization, >99.99% agreement, X=100 init stability) plus qualitative literature spectra. The central classical claim—accurate ground states through N=1400 and the phase diagram—rests on a fixed empirical schedule (nsweeps=5, maxdim=[10,20,100,100,200], cutoff=1e-10), smooth thermodynamic curves, and the assumption that modest bond dimension captures the all-to-all LMG ground state. For a dataset marketed as a high-precision benchmark, please add load-bearing convergence evidence at representative large N (e.g. N=100,500,1400): energy vs max bond dimension, discarded weight / truncation error, and if feasible a variance or two-site residual. Without that, large-N energies should be labeled provisional reference values, not unqualified ground truth. Note that the NISQ ranking itself is already supported inside N≤19 (Figs
- [§3.2, §4.2, §4.4, Abstract] VQE is run exclusively on the W=0 Gray-encoded compressed Hamiltonian of dimension d=J+1 (§3.2, Eqs. 11–12), whereas SQD samples Dicke sectors of the N-qubit formulation and DMRG builds the full Pauli/MPO form (Eq. 4). The paper states the W=0 restriction once, but the abstract and combined comparison (§4.4) read as a head-to-head on “the LMG model.” Please state explicitly in the abstract, §4.2, and §4.4 that VQE solves the symmetry-reduced problem, report the effective qubit counts, and avoid implying identical resource scalings. The comparative conclusion can stand if framed as method-plus-encoding pairs rather than pure algorithm ranking.
minor comments (7)
- [Abstract, §5] Abstract says “IBM Eagle”; Conclusion §5 says “IBM Heron processor.” Align the device name everywhere and specify the backend used for each of VQE and SQD.
- [§4.2, Figure 5] Figure 5 caption reads “Iterations to convergence across four problem sizes…” while the surrounding text describes VQE energies vs DMRG. Figure 17 in the appendix appears to be the convergence plot. Fix the Fig. 5 caption and cross-references.
- [Abstract] Abstract: “exceedingthatthresholdforallothervalueswhile” — missing spaces (also elsewhere, e.g. “NoisyIntermediate-ScaleQuantum”). Copy-edit for concatenated words.
- [§4] Success is defined as “within 1% of the true ground state” (§4) without motivation. A short sentence on why 1% (nuclear phenomenology, prior LMG VQE papers, or hardware noise floor) would help readers interpret Figs. 6, 8, 12.
- [§4.3.2, Figure 10] Figure 10 notes unexplained local oscillations in SQD error vs N; either a brief hypothesis (parity sectors, weighted shot allocation, recovery bias) or a clearer “left open” statement would suffice.
- [Appendix A, §5] Dataset access is “upon request” in the appendix while GitHub is cited for code [26]. Prefer a stable archival link (Zenodo/Figshare) for the N≤1400 energy tables so the benchmark is fully reproducible without gatekeeping.
- [Appendix B.4, Table 1] Table 1: GPU-DMRG sweep times exceed CPU-DMRG (e.g. N=1400: 73s vs 11s). A one-line explanation (transfer overhead, small bond dim, ITensor GPU maturity) would prevent misreading the profiling message.
Circularity Check
No significant circularity: DMRG baseline is independent of VQE/SQD; quantum errors are measured against external references.
full rationale
This is a benchmarking paper, not a first-principles derivation of a physical law or a fitted predictive model. The load-bearing classical reference is DMRG/MPS variational minimization of the LMG Hamiltonian (Eqs. 5–9), constructed from the standard quasi-spin form (Eqs. 1–4) and validated against exact sparse diagonalization for N≤19 plus published spectra. VQE and SQD energies are then compared to that external baseline (and to exact diagonalization in the overlapping window N≤19, Figs. 11–12). Nothing in the quantum pipelines defines or fits the DMRG energies, and the comparative claim (SQD reaches ~N=17–18 within 1% while VQE does not) is independently supported inside the exactly solvable regime. Empirical DMRG sweep schedules, Gray encoding, and Dicke post-selection are methodological choices, not predictions that reduce to their inputs by construction. No self-definitional loop, fitted-input-as-prediction, load-bearing self-citation uniqueness theorem, or renaming of a known result appears. Score 0 is the honest finding.
Axiom & Free-Parameter Ledger
free parameters (5)
- DMRG sweep schedule (nsweeps, maxdim ladder, cutoff) =
nsweeps=5; maxdim=[10,20,100,100,200]; cutoff=1e-10
- Success threshold of 1% relative energy error =
1%
- SQD total shot budget and weighted Dicke allocation =
~10^6 total shots; max 8192 shots noted as limiting
- VQE ansatz layer pattern and SPSA settings =
SPSA; 8192 shots/eval; selected 3-layer real RY/CNOT-style ansatz
- Hamiltonian parameters for main dataset (ε,V,W) =
ε=1, V=1, W=0 (main); V∈[0,1] step 0.1 for phase diagram
axioms (6)
- domain assumption The LMG Hamiltonian in quasi-spin/Pauli form (Eqs. 1–4) correctly encodes the intended two-level collective model with W=0 for the main study.
- domain assumption An MPS with the chosen bond-dimension schedule can represent LMG ground states to high accuracy even for all-to-all interactions at large N.
- standard math Variational principle: circuit or MPS expectation values upper-bound the true ground energy.
- domain assumption For W=0, pair excitations yield a block structure (even/odd M) allowing Gray-encoded compression of VQE to dimension ~N/2+1.
- domain assumption LMG eigenstates live in fixed Hamming-weight parity sectors well approximated by superpositions of Dicke states, so ensemble Dicke sampling plus weight recovery yields a faithful SQD subspace.
- ad hoc to paper NISQ sampling noise can be adequately mitigated for benchmarking by symmetry post-selection and configuration recovery without biasing the energy comparison unfairly.
read the original abstract
As quantum computing matures, it is critical to benchmark its real-world problem solving performance against competitive classical methods, such as tensor networks. In this work, we leverage the Density Matrix Renormalization Group (DMRG) algorithm to compute ground state energies of the Lipkin Meshkov Glick (LMG) model as a comparative benchmark against popular noisy intermediate-scale (NISQ) algorithms like the Variational Quantum Eigensolver (VQE) and Sample-Based Quantum Diagonalization (SQD) method. By running DMRG on the NERSC Perlmutter supercomputer, we provide one of the largest LMG ground state energy datasets in literature, containing accurate ground state energies for systems up to 1400 particles. We compare these results with VQE and SQD implementations on an IBM Eagle quantum computer for comparison. VQE achieved results within 1 percent error for 6 particles, while exceeding that threshold for all other values while SQD extended that range to 17 particles, suggesting that in a noisy intermediate scale quantum era, subspace-based approaches may strike the best balance between accuracy, circuit depth, and noise resilience.
Figures
Reference graph
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