REVIEW 4 major objections 5 minor 35 references
Measuring Butterfly Velocity in the XY Model on Emerging Quantum Computers
T0 review · 4 major / 5 minor · reviewed 2026-08-12 · deepseek-v4-flash
Pith's one-line read A teleportation-based OTOC protocol with Riemannian-trust-region circuit compilation estimates the XY model's butterfly velocity on a noisy five-qubit simulation, matching the analytic maximum group velocity.
desk verdict A plausible proof-of-concept for YKY+RTR butterfly velocity measurement on a simulator, with an overclaimed NISQ framing and some unfinished edges; the alleged OTOC mapping contradiction does not hold up. 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
Three pieces carry the argument. The YKY teleportation protocol is a doubled-Hilbert-space circuit in which a Bell-basis post-selection probability gives $F_{\mathrm{EPR}}=1/(4\langle\mathrm{OTOC}\rangle)$, converting operator growth into a measurable teleportation fidelity; the paper uses $F_{\mathrm{EPR}}$ to compute $C_j(t)=2-1/(2F_{\mathrm{EPR}})$. The Riemannian trust-region (RTR) optimizer on the product manifold $U(4)^m$ minimizes the Frobenius error $\|E(G)-e^{-iHt}\|_F^2$ over $m$-layer brick-wall circuits, producing a compiled unitary that is shallower than product-formula decompositions. The threshold rule $C_j(t)\ge 0.1$ defines a site-dependent spreading time $t_j$, and the inverse slope of the best-fit line through $t_j$ versus lattice position $j$ is the reported butterfly velocity.
What would settle it
Run the same YKY-RTR measurement on chains of length 7, 9, and 11 qubits, and re-extract the velocity with thresholds $C_j(t)\ge 0.05$, $0.1$, and $0.2$; if the fitted slope changes systematically with chain length or threshold by more than the few-percent scatter reported here, then the finite-chain linear-fit velocity is not the infinite-chain maximum-group-velocity butterfly velocity.
Extended reading notes
Core claim
The central claim is that the butterfly velocity of the one-dimensional anisotropic XY model, defined as the maximum group velocity of its quasiparticles, can be estimated on noisy near-term quantum hardware by combining a teleportation-based OTOC measurement with Riemannian-trust-region (RTR) circuit compilation. Analytically, the paper diagonalizes $H=J\sum_j(\frac{1+r}{2}X_jX_{j+1}+\frac{1-r}{2}Y_jY_{j+1}+hZ_j)$ via Jordan-Wigner, Fourier, and Bogoliubov transformations, obtaining the dispersion $\varepsilon(k)=-2J\sqrt{(h-\cos k)^2+r^2\sin^2 k}$ and group velocity $v_g(k)=-2J[\sin k(h-\cos k)+r^2\sin k\cos k]/\sqrt{(h-\cos k)^2+r^2\sin^2 k}$, so $v_B=\max_k v_g(k)$. Numerically, the YKY teleportation circuit gives the squared commutator $C_j(t)=2-1/(2F_{\mathrm{EPR}})$ from the teleportation fidelity $F_{\mathrm{EPR}}$, the spreading time at site $j$ is $t_j=\min\{t:C_j(t)\ge 0.1\}$, and a least-squares fit to $t_j$ for $j=2,\ldots,5$ has inverse slope equal to the butterfly velocity. The paper reports that these noisy-simulation estimates agree with the analytic $v_B$ to within a few percent, and that RTR compilation uses significantly fewer circuit layers than Lie-Trotter-Suzuki product formulas.
Load-bearing premise
The load-bearing premise is that the inverse slope of a best-fit line through the spreading times at four lattice sites, fixed by an arbitrary $C_j(t)\ge 0.1$ threshold on a five-qubit chain, equals the infinite-chain butterfly velocity obtained from the maximum group velocity.
Editorial extensions
If this is right
- On a noisily simulated five-qubit device, the protocol recovers the isotropic XY butterfly velocity within about 1.4 percent (1.972 versus the analytic 2) and the anisotropic value within about 0.1 percent (3.745 versus 3.75).
- Because the estimator obeys $1/(4F_{\mathrm{EPR}}) \ge \langle\mathrm{OTOC}\rangle$, the protocol inherits a one-sided robustness to decoherence and small coherent errors, so it does not require explicit error mitigation to remain usable near the noise threshold of current devices.
- RTR circuit compilation requires substantially fewer layers than Lie-Trotter-Suzuki splitting at the same target error, which is the step that makes the OTOC circuit short enough to run on noisy hardware.
- The analytic diagonalization provides a closed-form benchmark, $v_B=\max_k v_g(k)$, that future hardware measurements on longer chains can be tested against as quantum devices improve.
Reading between the lines
- One could turn the fixed threshold into a diagnostic: repeat the $t_j$ extraction at several thresholds and check that the fitted velocity is stable; if it is not, the linear-fit estimator should be replaced by a light-cone-edge extrapolation.
- A natural next test would be to apply the same YKY-RTR measurement to a non-integrable or disordered spin chain, where no analytic $v_B$ exists, and compare the output against exact-diagonalization or tensor-network light-cone speeds.
- Because the protocol measures $C_j(t)$ separately at each site, it could be adapted to measure asymmetric or directional butterfly velocities, and to extract the full light cone rather than a single speed, in models with anisotropic interactions.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper combines the Yoshida-Kitaev-Yao (YKY) teleportation protocol with Riemannian trust-region (RTR) Hamiltonian-to-circuit compilation to estimate the operator-averaged out-of-time-order correlation function and, from it, the butterfly velocity of the one-dimensional XY model. The authors derive the maximum group velocity of the XY model analytically via Jordan-Wigner, Fourier, and Bogoliubov transformations, and then compare these values with velocities extracted from 5-qubit simulations of the YKY-RTR protocol. The numerical results are obtained with the IBM FakeTorino noisy simulator and are reported to agree with the analytical butterfly velocities to within a few percent.
Significance. If the measurement pipeline is correct, the paper offers a useful proof-of-concept for estimating butterfly velocities on near-term devices in models that are not analytically solvable. The analytical derivation in Section II and Appendix A is standard and internally consistent, and the comparison with the independently computed maximum group velocity is a genuine consistency check rather than a fit to the target. The RTR-based circuit compilation is a promising tool for reducing circuit depth relative to product formulas. However, the current manuscript has a load-bearing inconsistency in the relation between the measured teleportation probability and the OTOC, and the numerical extraction lacks the error analysis and hardware context needed to support the claimed few-percent agreement.
major comments (4)
- [Section III A, Eq. (15), and Section IV] Eq. (15) states F_EPR = (1/4)⟨OTOC⟩, while the sentence following it and Section IV use ⟨OTOC⟩ = 1/(4F_EPR), with C_j(t) = 2 - 1/(2F_EPR). These two relations are reciprocal and cannot both hold. A direct two-qubit check (e.g., U = CNOT, for which the operator-averaged OTOC in Eq. (10) equals 1/2 and the conditional Bell probability F_EPR equals 1/2) confirms the reciprocal form used in Section IV, indicating that Eq. (15) and the diagrammatic derivation around Eqs. (16)-(18) contain an inverted factor. Because every numerical C_j(t) value in Section IV is derived from F_EPR, the mapping from the measured probability to the OTOC must be corrected and re-derived before the reported velocities are fully supported; the numerical implementation appears to use the correct form, but the paper as written does not establish the mapping.
- [Section IV, Table I, Fig. 4] The butterfly-velocity estimates rest entirely on the arbitrary spreading threshold C_j(t) >= 0.1, a linear fit to spreading times t_j over only j = 2, ..., 5 on a 5-qubit chain, and the identification of the fitted slope with the infinite-chain maximum group velocity of Eq. (9). No error bars are reported for the FakeTorino estimates, no sensitivity of t_j or the fitted slope to the threshold is given, and finite-size effects, including the boundary term dropped in Eq. (4), are not assessed. These omissions are load-bearing because the claimed few-percent agreement in Table I could be coincidental for a single threshold choice.
- [Abstract and Section IV] The abstract claims a "proof-of-concept demonstration of this method to estimate the butterfly velocity on NISQ-devices," but all quantum results in Section IV are obtained with the FakeTorino noisy simulator rather than a physical quantum device. This overstates the experimental content of the paper; the claim should be narrowed to a noisy-simulation demonstration or supported by data from actual hardware.
- [Sections III B and IV] The RTR mapping produces a brick-wall circuit of preselected depth m, but the manuscript never states the value of m, the optimization tolerance, the achieved Frobenius error ||E(G) - U||_F, or the number of shots used in the FakeTorino runs. Without these details the numerical results are not reproducible, and the possible bias of the circuit approximation on C_j(t) cannot be assessed.
minor comments (5)
- [Eq. (9)] The numerator of the group-velocity expression lacks parentheses; as written, it is ambiguous and should be checked against the derivative of Eq. (8).
- [Section III A] The unparenthesized expression "1/4F_EPR" should be written as "1/(4F_EPR)" to avoid ambiguity; this ambiguity is directly related to the inconsistency in Eq. (15).
- [Figure 2 and Section IV] The text alternates between referring to the final Bell measurement on A1,B1 and on A0,B0; the notation should be made consistent across Eq. (15), the circuit description, and the results section.
- [References [24], [25]] The text contains placeholder citations "[ ? ]" for "standard libraries" and for the comparison with product formulas; complete references should be supplied.
- [Table I] The column labels "Numerical 5 qubits" and "Quantum 5 qubits" are not defined in the text; the paper should state whether these are noiseless and FakeTorino results, respectively, and should report the statistical uncertainty of each entry.
Circularity Check
No significant circularity: the numerical butterfly-velocity estimates are benchmarked against an independently computed analytical maximum group velocity, and the YKY-to-OTOC relation is used consistently.
full rationale
The paper's central comparison is between (i) a numerical measurement of the squared commutator C_j(t) obtained by simulating the YKY teleportation circuit with an RTR-optimized Hamiltonian circuit, and (ii) an analytical butterfly velocity v_B = max_k v_g(k; J, r, h) computed from the dispersion relation in Eq. (8). The analytical value is not used to set the spreading threshold, the linear-fit slope, or any parameter of the simulation; it is an independent benchmark. The RTR optimization fits the circuit to U = e^{-iHt} via the cost function f(G) = ||E(G) - U||_F, not to the OTOC or to the butterfly velocity, so the subsequent OTOC measurement is not forced by the fit. The relation between the teleportation probability and the OTOC, F_EPR = 1/(4<OTOC>), is consistently used in Section IV through C_j(t) = 2 - 1/(2 F_EPR) = 2 - 2<OTOC>; the apparent formatting ambiguity in Eqs. (15) and (18) is resolved by the surrounding sentence 'the estimated <OTOC> as 1/(4 F_EPR)' and by the fact that the Section IV formula requires the reciprocal relation. The only imported physical input is the standard identification of the butterfly velocity with the maximum group velocity, cited to [26]; that citation is external, not self-referential, and is used as a benchmark rather than as a constraint on the measured data. No fitted parameter is renamed as a prediction, and no load-bearing step reduces to its own input.
Assumptions & free parameters
free parameters (2)
- spreading threshold =
C_j >= 0.1
- RTR circuit depth m =
not reported
assumptions (4)
- domain assumption The boundary term in the Jordan-Wigner transform is negligible for n=5
- domain assumption Butterfly velocity equals the maximum group velocity of Eq. (9)
- domain assumption YKY protocol is robust to decoherence and small coherent errors
- domain assumption FakeTorino simulator represents a current NISQ device
Cite this review
Pith. "Pith review of Measuring Butterfly Velocity in the XY Model on Emerging Quantum Computers." pith.science (2026). https://pith.science/paper/D4IYASSO
@misc{pith2026241110206,
author = {Pith},
title = {Pith review of: Measuring Butterfly Velocity in the XY Model on Emerging Quantum Computers},
year = {2026},
howpublished = {\url{https://pith.science/paper/D4IYASSO}},
note = {Machine review of arXiv:2411.10206}
}
read the original abstract
The butterfly velocity is commonly used to understand information transport properties in quantum dynamical systems and is related to growth of operators. Here we utilise a quantum teleportation based protocol and Riemannian Trust-Region method to estimate the butterfly velocity via the operator averaged out-of-time-order correlation function. We particularly study the XY model and analytically find the maximum group velocity. We then report a proof-of-concept demonstration of this method to estimate the butterfly velocity on NISQ-devices. The numerical simulation results obtained here are compared with our analytical calculations and found to be in agreement. The quantum algorithmic methods presented here can be more generally utilised to study information transport properties in more complicated lattice models.
Figures
Reference graph
Works this paper leans on
-
[1]
Lieb-robinson bound and the butterfly effect in quantum field theories
Daniel A Roberts and Brian Swingle. Lieb-robinson bound and the butterfly effect in quantum field theories. Physical review letters, 117(9):091602, 2016
work page 2016
-
[2]
Shenker, and Douglas Stan- ford
Juan Maldacena, Stephen H. Shenker, and Douglas Stan- ford. A bound on chaos. Journal of High Energy Physics, 2016(8):106, Aug 2016
work page 2016
-
[3]
The finite group veloc- ity of quantum spin systems
Elliott Lieb and Derek Robinson. The finite group veloc- ity of quantum spin systems. Communications in Math- ematical Physics, 28, 09 1972
work page 1972
-
[4]
Quantum scrambling and state dependence of the butterfly velocity
Xizhi Han and Sean A Hartnoll. Quantum scrambling and state dependence of the butterfly velocity. SciPost Physics, 7(4):045, 2019
work page 2019
-
[5]
Asymmetric parti- cle transport and light-cone dynamics induced by anyonic statistics
Fangli Liu, James R Garrison, Dong-Ling Deng, Zhe- Xuan Gong, and Alexey V Gorshkov. Asymmetric parti- cle transport and light-cone dynamics induced by anyonic statistics. Physical Review Letters, 121(25):250404, 2018
work page 2018
-
[6]
Asymmetric butterfly velocities in Hamiltonian and circuit models
Charles Stahl, Vedika Khemani, and David A Huse. Asymmetric butterfly velocities in hamiltonian and cir- cuit models. arxiv:1812.05589, 2018
work page Pith review arXiv 2018
-
[7]
Asymmet- ric butterfly velocities in 2-local hamiltonians
Yong-Liang Zhang and Vedika Khemani. Asymmet- ric butterfly velocities in 2-local hamiltonians. SciPost Physics, 9(2):024, 2020
work page 2020
-
[8]
Universal charge diffusion and the butterfly effect in holographic theories
Mike Blake. Universal charge diffusion and the butterfly effect in holographic theories. Physical Review Letters, 117(9):091601, 2016
work page 2016
Show all 35 references
-
[9]
Local criticality, diffusion, and chaos in generalized sachdev-ye-kitaev model
Xiao-Liang Qi. Local criticality, diffusion, and chaos in generalized sachdev-ye-kitaev model. Journal of High Energy Physics, 2017(05):125, 2017
2017
-
[10]
Upper bound on diffusivity
Thomas Hartman, Sean A Hartnoll, and Raghu Maha- jan. Upper bound on diffusivity. Physical Review Letters, 119(14):141601, 2017
2017
-
[11]
Constraints on hydrodynamics from many-body quantum chaos
Andrew Lucas. Constraints on hydrodynamics from many-body quantum chaos. arxiv:1710.01005, 2017
2017 arXiv
-
[12]
Scrambling and thermalization in a diffusive quantum many-body system
Annabelle Bohrdt, Christian B Mendl, Manuel Endres, and Michael Knap. Scrambling and thermalization in a diffusive quantum many-body system. New Journal of Physics, 19(6):063001, 2017
2017
-
[13]
Quantum butterfly effect in weakly interacting diffusive metals
Aavishkar A Patel, Debanjan Chowdhury, Subir Sachdev, and Brian Swingle. Quantum butterfly effect in weakly interacting diffusive metals. Physical Review X, 7(3):031047, 2017
2017
-
[14]
An apologia for firewalls
Ahmed Almheiri, Donald Marolf, Joseph Polchinski, Douglas Stanford, and James Sully. An apologia for firewalls. Journal of High Energy Physics, 2013(9):1–32, 2013
2013
-
[15]
https:// online.kitp.ucsb.edu/online/entangled15/kitaev/ https://online.kitp.ucsb.edu/online/entangled15/ kitaev2/
Alexei kitaev, a toy model of holography. https:// online.kitp.ucsb.edu/online/entangled15/kitaev/ https://online.kitp.ucsb.edu/online/entangled15/ kitaev2/. Accessed: 10-11-2024
2024
-
[16]
Separation of out-of-time- ordered correlation and entanglement
Aram W Harrow, Linghang Kong, Zi-Wen Liu, Saeed Mehraban, and Peter W Shor. Separation of out-of-time- ordered correlation and entanglement. PRX Quantum, 2(2):020339, 2021
2021
-
[17]
Quasiclassical method in the theory of superconductivity
AI Larkin and Yu N Ovchinnikov. Quasiclassical method in the theory of superconductivity. Sov Phys JETP, 28(6):1200–1205, 1969
1969
-
[18]
Unscrambling the physics of out-of-time- order correlators
Brian Swingle. Unscrambling the physics of out-of-time- order correlators. Nature Physics, 14(10):988–990, 2018
2018
-
[19]
Efficient decoding for the hayden-preskill protocol
Beni Yoshida and Alexei Kitaev. Efficient decoding for the hayden-preskill protocol. arxiv:1710.03363, 2017
2017 arXiv
-
[20]
Disentangling scram- bling and decoherence via quantum teleportation
Beni Yoshida and Norman Y Yao. Disentangling scram- bling and decoherence via quantum teleportation. Phys- ical Review X, 9(1):011006, 2019
2019
-
[21]
Exponentially tighter bounds on limitations of quantum error mitiga- tion
Yihui Quek, Daniel Stilck Fran¸ ca, Sumeet Khatri, Jo- hannes Jakob Meyer, and Jens Eisert. Exponentially tighter bounds on limitations of quantum error mitiga- tion. Nature Physics, 20(10):1648–1658, 2024
2024
-
[22]
Trust-region methods on riemannian manifolds
P-A Absil, Christopher G Baker, and Kyle A Gallivan. Trust-region methods on riemannian manifolds. Founda- tions of Computational Mathematics, 7:303–330, 2007
2007
-
[23]
An introduction to optimization on smooth manifolds
Nicolas Boumal. An introduction to optimization on smooth manifolds. Cambridge University Press, 2023
2023
-
[24]
Riemannian optimization of isometric tensor net- works
Markus Hauru, Maarten Van Damme, and Jutho Haege- man. Riemannian optimization of isometric tensor net- works. Scipost physics, 10(2):040, 2021
2021
-
[25]
Quantum channels, complex stiefel manifolds, and op- timization
Ivan Russkikh, Boris Volkov, and Alexander Pechen. Quantum channels, complex stiefel manifolds, and op- timization. arxiv:2408.09820, 2024
2024 arXiv
-
[26]
Lieb-robinson cor- relation function for the quantum transverse-field ising model
Brendan Mahoney and Craig Lent. Lieb-robinson cor- relation function for the quantum transverse-field ising model. Physical Review Research, 6, 06 2024
2024
-
[27]
The Road to Reality
Roger Penrose. The Road to Reality. Random house, 2006
2006
-
[28]
Picturing quantum processes: A first course on quantum theory and dia- grammatic reasoning
Bob Coecke and Aleks Kissinger. Picturing quantum processes: A first course on quantum theory and dia- grammatic reasoning. In Diagrammatic Representation and Inference: 10th International Conference, Diagrams 2018, Edinburgh, UK, June 18-22, 2018, Proceedings 10, pages 28–31....
2018
-
[29]
Information scrambling over bipartitions: Equilibration, entropy production, and typicality
Georgios Styliaris, Namit Anand, and Paolo Zanardi. Information scrambling over bipartitions: Equilibration, entropy production, and typicality. Physical Review Let- ters, 126(3):030601, 2021
2021
-
[30]
Princeton University Press, 2008
P-A Absil, Robert Mahony, and Rodolphe Sepulchre.Op- timization algorithms on matrix manifolds. Princeton University Press, 2008
2008
-
[31]
Measuring the scrambling of quan- tum information
Brian Swingle, Gregory Bentsen, Monika Schleier-Smith, and Patrick Hayden. Measuring the scrambling of quan- tum information. Physical Review A, 94:040302, Oct 2016
2016
-
[32]
Measurement of many-body chaos using a quantum clock
Guanyu Zhu, Mohammad Hafezi, and Tarun Grover. Measurement of many-body chaos using a quantum clock. Physical Review A, 94:062329, Dec 2016
2016
-
[33]
Yao, Fabian Grusdt, Brian Swingle, Mikhail D
Norman Y. Yao, Fabian Grusdt, Brian Swingle, Mikhail D. Lukin, Dan M. Stamper-Kurn, Joel E. Moore, and Eugene A. Demler. Interferometric approach to prob- ing fast scrambling, 2016
2016
-
[34]
Geller, Andrew Arrasmith, Zo¨ e Holmes, Bin Yan, Patrick J
Michael R. Geller, Andrew Arrasmith, Zo¨ e Holmes, Bin Yan, Patrick J. Coles, and Andrew Sornborger. Quan- tum simulation of operator spreading in the chaotic ising model. Physical Review E, 105:035302, Mar 2022
2022
-
[35]
In- formation scrambling in quantum circuits
Xiao Mi, Pedram Roushan, Chris Quintana, Salvatore Mandra, Jeffrey Marshall, Charles Neill, Frank Arute, Kunal Arya, Juan Atalaya, Ryan Babbush, et al. In- formation scrambling in quantum circuits. Science, 374(6574):1479–1483, 2021. 8 Appendix A: Analytic Calculation for Butt...
2021
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