REVIEW 2 major objections 6 minor 48 references
Experimental demonstration of the Quantum Fourier Transform on up to 100 qubits using a convolutional compilation strategy
T0 review · 2 major / 6 minor · reviewed 2026-08-08 · deepseek-v4-flash
Pith's one-line read A new compilation strategy gives the quantum Fourier transform the same two-qubit gate count on linear qubit chains as on fully connected hardware, and the circuit works up to 100 qubits.
desk verdict A genuinely better LNN QFT compilation, but the experimental fidelity claims conflate AQFT truncation error with hardware error. 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 identity is the commutation of a √X gate through a specific configuration of three CX gates (Fig 1d), which, combined with rewriting H = S·√X·S and moving the diagonal S gates outward, aligns CX pairs at layer boundaries so they cancel as CX² = I. The second mechanism is the convolutional kernel: starting from the Low-CX circuit, one ancilla in |0> and an initial SWAP chain reverse the direction of the long CX chain; because the first CX in each SWAP is controlled by |0>, it can be deleted, and adjacent aligned CXs cancel, leaving a translation-invariant gadget of d+2 qubits that increments along the register. This gadget confines most entangling operations to a brief kernel pass, reducing the average number of causally upstream CX gates per qubit wire.
What would settle it
Run the exact truncated AQFT circuits (threshold π/16) on a statevector simulator and compute the ideal probability of the target bitstring for n=50 and n=80. If that probability is substantially below 1, then the Eq (2) fidelity is biased upward as a measure of hardware performance, and the reported 11.4% and 1.8% figures would need to be divided by the ideal probability to separate synthesis error from device error.
Extended reading notes
Core claim
The central discovery is a compilation of the QFT onto an LNN architecture that exactly matches the CX gate count of an all-to-all architecture. The derivation rewrites each Hadamard as S·√X·S, moves the diagonal S gates to the circuit edges, and pushes each √X leftward through three preceding CX gates using a proven identity; this makes two CX gates that were separated by an H gate adjacent and aligned, so they cancel at every interface between adjacent QFT layers, saving 2n-4 CX gates over the Park-Ahn construction. A further transformation using an ancilla in |0> and an initial SWAP chain reverses the long initial CX chain and compiles it as late as possible, producing the Convolutional QFT/AQFT: a compact kernel gadget spanning d+2 qubits that strides across the register. Experimentally, the mode of the output distribution equals the encoded frequency for every tested circuit up to n=100, and the process fidelity estimator of Eq (2) yields 11.4% at n=50 and 1.8% at n=80 after readout mitigation.
Load-bearing premise
The fidelity calculation assumes the ideal circuit returns the target bitstring with probability exactly 1, yet the tested circuits are approximate QFTs whose noiseless success probability is never reported, so truncation error and hardware error are not separated.
Editorial extensions
If this is right
- QFT and approximate QFT circuits on any LNN device now cost the same number of CX gates as on an all-to-all device, eliminating routing overhead for this subroutine.
- For a fixed approximation threshold d, the gate count scales linearly in n (d(2n - d - 1) + 2 for the convolutional AQFT), making larger circuits practical on noisy hardware.
- The convolutional kernel structure keeps qubits idle outside a brief kernel pass, so dynamical decoupling sequences can protect them from decoherence and crosstalk.
- The experimental demonstration shows that with 4096 shots per circuit, the correct frequency of a periodic quantum state is identifiable up to 100 qubits, the largest QFT execution reported.
- The construction is directly compatible with heavy-hex or square-lattice topologies and with grids of code patches in fault-tolerant architectures.
Reading between the lines
- The same kernel-gadget strategy could likely compile other translation-invariant circuits, such as quantum walks or Trotterized translation-invariant Hamiltonians, onto LNN hardware with similar routing savings.
- Because the reported fidelities use truncated AQFT circuits, the 11.4% and 1.8% figures probably include synthesis error; a noiseless-simulator baseline would reveal how much headroom remains for hardware improvement.
- The linear scaling for fixed d suggests the method could scale to several hundred qubits if the local two-qubit error in the kernel stays low enough.
- The ALAP scheduling that confines gates to the kernel naturally pairs with error-detection or quantum-error-correction patches on a linear lattice, so the compilation may remain useful in fault-tolerant settings.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper introduces two LNN compilation strategies for the QFT: a 'Low-CX' compilation using n^2-n CX gates (matching the all-to-all baseline for the full QFT and for AQFTs at any truncation threshold), and a 'Convolutional' variant using one ancilla and two additional CX gates, realized as a translation-invariant kernel gadget. The authors then report experiments on IBM Quantum hardware for AQFT circuits with truncation at rotations of pi/16 and below, claiming process fidelities of 11.4% at 50 qubits and 1.8% at 80 qubits, and that the target bitstring remains the most frequent outcome up to 100 qubits, which they describe as the largest QFT circuit executed to date.
Significance. The compilation derivation is the main strength: it is constructive, parameter-free, and if correct it removes routing overhead entirely for QFT on LNN hardware, giving a CX count that matches all-to-all connectivity. The convolutional form and the kernel-gadget picture are elegant and potentially useful for hardware-aware scheduling. The experimental observation that the target bitstring is the mode outcome at 50-100 qubits is a notable empirical result. However, the quantitative fidelity claims rest on a questionable application of the estimator in Eq. (2) to truncated AQFT circuits, so the headline fidelity numbers and the 'above 1% threshold' claim need support before the paper's central quantitative conclusions can be accepted.
major comments (2)
- [Section IV, Eq. (2)] The fidelity estimator in Eq. (2) is applied to circuits that are not the full QFT. The paper states that all executed circuits are AQFTs with rotations of pi/16 and below truncated, and explicitly acknowledges that this 'introduces synthesis error compared to the full QFT unitary definition in Eq. (1)'. For such an AQFT, the ideal probability q_k of observing the full-QFT target bitstring is strictly less than 1 for each random frequency. The estimator in Eq. (2), as used here, is only valid when the ideal success probability of each benchmark circuit is 1; otherwise it conflates AQFT truncation error with hardware noise. The manuscript never reports q_k or a noiseless simulation of the executed AQFT circuits, so the quoted process fidelities of 9.6%/11.4% (n=50) and 1.4%/1.8% (n=80) are not established as process fidelities of the executed unitary relative to its own ideal. Please provide noiseless AQFT success probabilities for the same circuits and either re-estimate the fidelity against the ideal AQFT unitary or explicitly separate the truncation contribution from the hardware contribution.
- [Section IV, Figs. 4-5] The claims that 'the success probability remains above the previously articulated ~1% threshold up to 80 qubits' and the abstract's 'process fidelity of 11.4% at 50 qubits, and 1.8% at 80 qubits' inherit the issue in the first major comment. Until the noiseless q_k values are supplied, these quantitative thresholds are unsupported. The mode-bitstring observation, namely that the target is the most frequent outcome in each trial, is a separate and more robust empirical result; it should be presented as the primary evidence for the 100-qubit claim, with the fidelity numbers either corrected or removed from the abstract and conclusion.
minor comments (6)
- [Section IV, Eq. (2)] The typeset expression for Fhat is ambiguous: it is not clear whether the factor m/(m-1) multiplies both terms or only the first; please add brackets to display the formula unambiguously.
- [Section IV, paragraph on AQFT threshold] The sentence 'The chosen threshold maximizes circuit fidelity in the current benchmarking exercise on the selected device' is an unsubstantiated optimization claim; either provide a small threshold scan or soften the wording to indicate that this threshold was selected empirically.
- [Section IV, Figs. 4-5 and Abstract] The paper uses 'process fidelity' to describe the composite 'QFT + measurement' operation and then 'process fidelity of the unitary QFT alone' after readout mitigation; these are different quantities, and the abstract should state explicitly which one is being reported.
- [Section V and Fig. 5 caption] At a nominal width of 100 qubits, the executed circuit actually uses 101 physical qubits because of the ancilla. The caption discloses this, but the abstract and conclusion should state explicitly that the claim is for 100 data qubits plus one ancilla.
- [Section IV, fidelity estimates] No statistical uncertainties are reported for the fidelity estimates derived from 20 random frequencies and 4096 shots; please provide confidence intervals (for example, by bootstrapping over the 20 trials).
- [Fig. 5] The hexadecimal bitstring labels in Fig. 5a and 5b are dense and partially truncated; they are hard to read and should be reformatted or replaced by abbreviated labels with a mapping table.
Circularity Check
No significant circularity: the LNN QFT compilation is derived from explicit circuit identities with no fitted parameters, and the experimental fidelity claims rely on independent measurement.
full rationale
The core compilation claim (n^2 - n CX on an LNN topology, and the convolutional variant with n^2 - n + 2 CX) is derived transparently in Section III from textbook QFT synthesis, Park and Ahn's LNN layer construction [26], and explicit circuit identities (S synthesis of H, commutation of diagonal S with CP layers, and the sqrt(X)-through-three-CX identity in Fig. 1d). No parameter is fitted to the benchmark data and no step in the derivation assumes the result being established; the CX counts in Table I are summations of gates in the displayed circuits. The experimental pipeline uses the authors' own error-suppression tools (Refs. [18,19]), but these are externally published, independently falsifiable workflows and are not the source of the compilation result. The 'chosen threshold maximizes circuit fidelity' statement (Section IV) is a device-aware tuning choice, and the application of the Eq. (2) estimator from Ref. [38] to truncated AQFT circuits raises a validity concern because the ideal AQFT output is not a computational basis state; however, that is a statistical/correctness issue, not a circular reduction, because the fidelity estimate does not enter the derivation of the compilation and the mode-bitstring observation is reported directly from raw counts. No self-citation chain is load-bearing for the central claim. Therefore no enumerated circularity step is present.
Assumptions & free parameters
free parameters (2)
- AQFT truncation threshold (kept angles pi/2, pi/4, pi/8; omitted pi/16 and below) =
pi/16 cutoff (d=3)
- Number of random frequencies per circuit width =
20
assumptions (3)
- standard math H = S * sqrt(X) * S up to global phase, and sqrt(X) commutes through the three-CX configuration of Fig 1d
- domain assumption The process fidelity estimator of Eq (2) (from Ref [38]) is unbiased for estimating the fidelity of the implemented channel to the ideal unitary, even when the compiled circuit is an approximate QFT and the ideal success probability on benchmark inputs is less than 1
- domain assumption Layout selection, dynamical decoupling insertion, and ALAP scheduling do not change the logical unitary of the compiled circuit
Cite this review
Pith. "Pith review of Experimental demonstration of the Quantum Fourier Transform on up to 100 qubits using a convolutional compilation strategy." pith.science (2026). https://pith.science/paper/BB22R3N5
@misc{pith2026260805435,
author = {Pith},
title = {Pith review of: Experimental demonstration of the Quantum Fourier Transform on up to 100 qubits using a convolutional compilation strategy},
year = {2026},
howpublished = {\url{https://pith.science/paper/BB22R3N5}},
note = {Machine review of arXiv:2608.05435}
}
abstract
We present and experimentally validate the `Convolutional QFT': a constructive compilation strategy for the Quantum Fourier Transform (QFT) subroutine on a linear nearest neighbor (LNN) qubit topology. We first introduce a novel strategy that compiles the $n$-qubit QFT onto an LNN topology using only $n^2 - n$ $CX$ gates, matching requirements of a direct compilation on an all-to-all architecture. We then derive the convolutional variant used in our experiments, which requires an additional two $CX$ gates in total, and is realized via a compact, translation-invariant kernel circuit gadget that traverses a quantum register. We demonstrate the power of the convolutional compilation strategy on the IBM Quantum Platform by executing QFT benchmarking circuits. We measure a process fidelity of 11.4% at 50 qubits, and 1.8% at 80 qubits. The correct output state remains clearly distinguishable above background noise up to 100 qubits. These results constitute the largest experimental QFT demonstrated on any quantum computing hardware to date.
Figures
Figures from the paper (2 more)
Reference graph
Works this paper leans on
-
[1]
EachHin the QFT is expressed using the equiva- lent (up to global phase) synthesisS· √ X·S, where Sis the phase gate
-
[2]
The two introduced phase gates on each wire are moved all the way to the front or end of the cir- cuit, respectively. This is permitted sinceShas a diagonal unitary, and commutes with other diag- onal sub-circuits including the layers of CP gates present in the QFT
-
[3]
Each √ Xgate is moved to the left through three precedingCXgates. The third step is underpinned by the circuit identity allowing a √ Xgate to commute through a particular ar- rangement of threeCXgates (Fig. 1d). The resulting circuit compilation for a single layer of the QFT is de- picted in Fig 1e. For each layer within the QFT, twoCX that were previousl...
-
[4]
Preskill, John, Quantum computing in the NISQ era and beyond, Quantum2, 79 (2018)
work page 2018
- [5]
-
[6]
S. Herbert, On the depth overhead incurred when run- ning quantum algorithms on near-term quantum com- puters with limited qubit connectivity, Quantum Infor- mation and Computation20, 0787 (2020)
work page 2020
- [7]
-
[8]
F. Hua, M. Wang, G. Li, B. Peng, C. Liu, M. Zheng, S. Stein, Y. Ding, E. Z. Zhang, T. Humble,et al., QASM- Trans: A QASM based Quantum Transpiler Framework for NISQ Devices, inProceedings of the SC’23 Workshops of the International Conference on High Performance Computing, Network, Storage, and Analysis(2023) pp. 1468–1477
work page 2023
Show all 48 references
-
[9]
Arute, K
F. Arute, K. Arya, R. Babbush, D. Bacon, J. C. Bardin, R. Barends, R. Biswas, S. Boixo, F. G. Brandao, D. A. Buell,et al., Quantum supremacy using a programmable superconducting processor, Nature574, 505 (2019)
2019
-
[10]
Y. Kim, A. Eddins, S. Anand, K. X. Wei, E. Van Den Berg, S. Rosenblatt, H. Nayfeh, Y. Wu, M. Zale- tel, K. Temme,et al., Evidence for the utility of quan- tum computing before fault tolerance, Nature618, 500 (2023)
2023
-
[11]
A. G. Fowler, M. Mariantoni, J. M. Martinis, and A. N. Cleland, Surface codes: Towards practical large-scale quantum computation, Physical Review A—Atomic, Molecular, and Optical Physics86, 032324 (2012)
2012
-
[12]
Litinski, A game of surface codes: Large-scale quan- tum computing with lattice surgery, Quantum3, 128 (2019)
D. Litinski, A game of surface codes: Large-scale quan- tum computing with lattice surgery, Quantum3, 128 (2019)
2019
-
[13]
Bluvstein, H
D. Bluvstein, H. Levine, G. Semeghini, T. T. Wang, S. Ebadi, M. Kalinowski, A. Keesling, N. Maskara, H. Pichler, M. Greiner,et al., A quantum processor based on coherent transport of entangled atom arrays, Nature 604, 451 (2022)
2022
-
[14]
Kielpinski, C
D. Kielpinski, C. Monroe, and D. J. Wineland, Architec- ture for a large-scale ion-trap quantum computer, Nature 417, 709 (2002)
2002
-
[15]
Wille and L
R. Wille and L. Burgholzer, MQT QMAP: Efficient quan- tum circuit mapping, inProceedings of the 2023 Interna- tional Symposium on Physical Design(2023) pp. 198– 204
2023
-
[16]
G. Li, Y. Ding, and Y. Xie, Tackling the qubit mapping problem for NISQ-era quantum devices, inProceedings of the Twenty-Fourth International Conference on Architec- tural Support for Programming Languages and Operating Systems, pages=1001–1014(2019)
2019
-
[17]
Gokhale, T
P. Gokhale, T. Tomesh, M. Suchara, and F. Chong, Faster and more reliable quantum swaps via native gates, inProceedings of the 2024 International Confer- ence on Parallel Architectures and Compilation Tech- niques(2024) pp. 351–362
2024
-
[18]
Karuppasamy, V
K. Karuppasamy, V. Puram, S. Johnson, and J. P. Thomas, A comprehensive review of quantum circuit op- timization: Current trends and future directions, Quan- tum Reports7, 2 (2025)
2025
-
[19]
Hamada, Y
K. Hamada, Y. Suzuki, and Y. Tokunaga, Efficient and high-performance routing of lattice-surgery paths on three-dimensional lattice, Quantum10, 2061 (2026)
2026
-
[20]
L. S. Herzog, L. Berent, A. Kubica, and R. Wille, Exploit- ing Movable Logical Qubits for Lattice Surgery Compi- lation, arXiv preprint arXiv:2512.04169 (2025)
2025
-
[21]
Coote, R
P. Coote, R. Dimov, S. Maity, G. S. Hartnett, M. J. Biercuk, and Y. Baum, Resource-efficient context-aware dynamical decoupling embedding for arbitrary large-scale quantum algorithms, PRX Quantum6, 010332 (2025)
2025
-
[22]
P. S. Mundada, A. Barbosa, S. Maity, Y. Wang, T. Merkh, T. Stace, F. Nielson, A. R. Carvalho, M. Hush, M. J. Biercuk,et al., Experimental benchmarking of an automated deterministic error-suppression workflow for quantum algorithms, Physical Review Applied20, 024034 (2023)
2023
-
[23]
G. S. Hartnett, K. S. Najafi, A. Khindanov, H. Liao, M. Schutzman, M. R. Hush, M. J. Biercuk, and Y. Baum, Fast, accurate, high-resolution simulation of large-scale Fermi-Hubbard models on a digital quantum processor, arXiv preprint arXiv:2605.04025 (2026)
2026 arXiv
-
[24]
H. Liao, G. S. Hartnett, A. Kakkar, A. Tan, M. Hush, P. S. Mundada, M. J. Biercuk, and Y. Baum, Achiev- ing computational gains with quantum error-correction primitives: Generation of long-range entanglement en- hanced by error detection, PRX Quantum6, 020331 (2025)
2025
-
[25]
Sachdeva, G
N. Sachdeva, G. S. Hartnett, S. Maity, S. Marsh, Y. Wang, A. Winick, R. Dougherty, D. Canuto, Y. Q. Chong, G. A. Cox,et al., Integrated error-suppressed pipeline for quantum optimization of nontrivial bi- nary combinatorial optimization problems on gate-model hardware at the 1...
2026
-
[26]
M. A. Nielsen and I. L. Chuang,Quantum Computation and Quantum Information(Cambridge University Press, 2010)
2010
-
[27]
Y. Nam, Y. Su, and D. Maslov, Approximate quantum Fourier transform with O (n log (n)) T gates, NPJ Quan- tum Information6, 26 (2020)
2020
-
[28]
A. G. Fowler, S. J. Devitt, and L. C. Hollenberg, Im- plementation of Shor’s algorithm on a linear nearest neighbour qubit array, arXiv preprint quant-ph/0402196 (2004)
2004 arXiv
-
[29]
Park and D
B. Park and D. Ahn, Reducing CNOT count in quantum Fourier transform for the linear nearest-neighbor archi- tecture, Scientific Reports13, 8638 (2023)
2023
-
[30]
Park and D
B. Park and D. Ahn, Reducing T-count and T-depth in approximate quantum Fourier transform circuits, Scien- tific Reports15, 37199 (2025)
2025
-
[31]
Dreier, C
F. Dreier, C. Fleckenstein, G. Aigner, M. Fellner, P. Aumann, R. Stahn, M. Lanthaler, and W. Lechner, Connectivity-aware synthesis of quantum algorithms, arXiv preprint arXiv:2501.14020 (2025)
2025
-
[32]
Holmes, S
A. Holmes, S. Johri, G. G. Guerreschi, J. S. Clarke, and A. Y. Matsuura, Impact of qubit connectivity on quan- tum algorithm performance, Quantum Science and Tech- nology5, 025009 (2020)
2020
-
[33]
A. G. Fowler and L. C. Hollenberg, Scalability of Shor’s algorithm with a limited set of rotation gates, Physical 9 Review A—Atomic, Molecular, and Optical Physics70, 032329 (2004)
2004
-
[34]
Nam and R
Y. Nam and R. Bl¨ umel, Scaling laws for Shor’s algo- rithm with a banded quantum Fourier transform, Phys- ical Review A—Atomic, Molecular, and Optical Physics 87, 032333 (2013)
2013
-
[35]
Nam and R
Y. Nam and R. Bl¨ umel, Streamlining Shor’s algorithm for potential hardware savings, Physical Review A—Atomic, Molecular, and Optical Physics87, 060304 (2013)
2013
-
[36]
Coppersmith, An approximate Fourier transform useful in quantum factoring, arXiv preprint quant- ph/0201067 (2002)
D. Coppersmith, An approximate Fourier transform useful in quantum factoring, arXiv preprint quant- ph/0201067 (2002)
2002
-
[37]
White, C
G. White, C. Hill, and L. Hollenberg, Truncated phase- based quantum arithmetic: Error propagation and re- source reduction, Physical Review A108, 052608 (2023)
2023
-
[38]
Z. Cai, R. Babbush, S. C. Benjamin, S. Endo, W. J. Hug- gins, Y. Li, J. R. McClean, and T. E. O’Brien, Quantum error mitigation, Reviews of Modern Physics95, 045005 (2023)
2023
-
[39]
Eddins, M
A. Eddins, M. C. Tran, and P. Rall, Lightcone shading for classically accelerated quantum error mitigation, arXiv preprint arXiv:2409.04401 (2024)
2024 arXiv
-
[40]
IBM Quantum, IBM Quantum Platform,https:// quantum.cloud.ibm.com/(2026), accessed: 2026-06-29
2026
-
[41]
B¨ aumer, V
E. B¨ aumer, V. Tripathi, A. Seif, D. Lidar, and D. S. Wang, Quantum Fourier transform using dynamic cir- cuits, Physical Review Letters133, 150602 (2024)
2024
-
[42]
Aumann, M
P. Aumann, M. Fellner, D. Alber, M. Cykiert, C. Fleck- enstein, R. ter Hoeven, L. Stenzel, R. J. Valencia- Tortora, and W. Lechner, Demonstrating Record Fidelity for the Quantum Fourier Transform, arXiv preprint arXiv:2604.12465 (2026)
2026 arXiv
-
[43]
Barenco, C
A. Barenco, C. H. Bennett, R. Cleve, D. P. DiVincenzo, N. Margolus, P. Shor, T. Sleator, J. A. Smolin, and H. Weinfurter, Elementary gates for quantum computa- tion, Physical review A52, 3457 (1995)
1995
-
[44]
Klaver, S
B. Klaver, S. M. Rombouts, M. Fellner, A. Messinger, K. Ender, K. Ludwig, and W. Lechner, Swap-less im- plementation of quantum algorithms, Physical Review A 113, 012443 (2026)
2026
-
[45]
Lubinski, S
T. Lubinski, S. Johri, P. Varosy, J. Coleman, L. Zhao, J. Necaise, C. H. Baldwin, K. Mayer, and T. Proctor, Application-oriented performance benchmarks for quan- tum computing, IEEE Transactions on Quantum Engi- neering4, 1 (2023)
2023
-
[46]
Y. Wang, E. Ginez, J. Friel, Y. Baum, J.-S. Kim, A. Shih, and O. Green, ∆-Motif: Subgraph Isomorphism at Scale via Data-Centric Parallelism, arXiv preprint arXiv:2508.21287 (2025)
2025
-
[47]
F. B. Maciejewski, Z. Zimbor´ as, and M. Oszmaniec, Mit- igation of readout noise in near-term quantum devices by classical post-processing based on detector tomography, Quantum4, 257 (2020)
2020
-
[48]
Maksymov, J
A. Maksymov, J. Nguyen, Y. Nam, and I. Markov, En- hancing quantum computer performance via symmetriza- tion, arXiv preprint arXiv:2301.07233 (2023)
2023 arXiv
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