REVIEW 3 major objections 4 minor 43 references
von Neumann measurement and quantum phase estimation of block-encoded Hamiltonians
T0 review · 3 major / 4 minor · reviewed 2026-08-05 · deepseek-v4-flash
Pith's one-line read This paper establishes that von Neumann's pointer measurement, applied through a QET Hamiltonian simulation subroutine, turns any block-encoding into a qubit-efficient phase and energy estimator.
desk verdict The central resource claim collapses on the pointer normalization: r does not set precision, so the advertised O(1/Δ) query count is wrong. 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 central object is the von Neumann coupling H⊗p, with p a discretized momentum operator on r pointer qubits whose eigenvalues are z/2^r. Simulating e^{-itH⊗p} shifts the pointer by tλ_k, and an inverse QFT turns that shift into a computational-basis answer. The machinery that makes this algorithmic is QSP+ (the polynomial-transformation template over a block-encoding): it prepares e^{-itH} to error ε_HS from a (β,n_anc,ε_BE) block-encoding, and lemma 3's linear error bound dε_BE+(d+1)ε_R lets the whole cascade of encoding, gate, and functional errors be absorbed into a single simulation-error parameter ε_HS=(2d+2)ε_BE.
What would settle it
Run the QET Hamiltonian simulation on a small block-encoded Hamiltonian with known eigenvalues and controllable encoding error ε_BE, compare the prepared e^{-itH} with the exact evolution in operator norm, and check whether the error ever exceeds (2d+2)ε_BE for the chosen polynomial degree d; a violation would break the success probability bound of Theorem 2.
Extended reading notes
Core claim
On the paper's terms, Theorem 2 is the central claim: from a (β, n_anc, ε_BE) block-encoding of H⊗p, an approximate eigenstate with bounded leakage, and rotation gates accurate to ε_R ≤ ε_BE, algorithm 1 returns an ε_vN-precise estimate of λ_k with success probability ≥ 1−δ using n+n_anc+2+O(log2(Δ_k/ε_vN)) qubits. The subroutine evolves e^{-itH⊗p} via QET from the block-encoding, then applies an inverse QFT to the pointer register and reads λ = 2πx/t. Setting the simulation error to ε_HS = δ/(3·4^r), the QSP+ error analysis funnels all encoding and gate errors into (2d+2)ε_BE, which is what buys the 1−δ success probability. Corollaries extend the same recipe to U=e^{iH} oracles and to Pauli
Load-bearing premise
The proof assumes that small errors in the block-encoding grow at most linearly as they pass through the QSP+ simulation circuit; if error growth were faster in the polynomial degree, the promised success probability and query counts would not hold.
Editorial extensions
If this is right
- Any block-encoding, not just an exactly implemented U=e^{iH}, becomes a QPE/QEE input; no controlled powers U, U^2, ..., U^{2^r} are needed.
- For Hamiltonians given by linear combinations of Pauli strings, the Clifford+T depth and gate count are expressed directly in |P|, n, ||H||_1, Δ_k, δ, and ε_vN, making pre-fault-tolerant resource estimation possible.
- The pointer register grows only logarithmically in the demanded resolution (r = 1+⌈log2(Δ_k/ε_vN)⌉), so most of the precision work is done by evolution time, not by qubits.
- The success probability is tied to the simulation error by ε_HS = δ/(3·4^r); any improvement in Hamiltonian simulation accuracy translates immediately into higher QPE success probability or lower δ requirements.
- The linear error-propagation lemma applies to generic QSP/QET/QSVT circuits, so it can be reused outside phase estimation whenever block-encoding error and gate error need to be budgeted together.
Reading between the lines
- Since each run of the pointer measurement samples one eigenvalue, repeated runs could be used to reconstruct an entire spectrum or expectation values; the paper focuses on a single target eigenvalue.
- The pointer-register size r and evolution time t are traded against each other; optimizing that trade-off, or replacing the inverse QFT with an approximate one, may push the query count closer to the phase-estimation lower bound.
- Any future reduction in the polynomial degree d of QSP+ Hamiltonian simulation will automatically shrink the total error budget and query count through the ε_HS=(2d+2)ε_BE relation, so this construction turns simulator improvements into phase-estimation improvements.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes a quantum phase/energy estimation algorithm (vN-QPE) based on von Neumann's pointer measurement scheme. Starting from a block-encoding of a Hamiltonian H, the algorithm uses QSP/QET Hamiltonian simulation to implement e^{-itH⊗p}, where p is a discretized momentum operator on an r-qubit pointer register, then applies an inverse QFT and reads out an estimate λ≈2πx/t. The central Theorem 2 claims an ε_vN-precise eigenvalue estimate with success probability 1−δ using O(β/(δΔ_k)+...) block-encoding calls and a pointer register of size O(log(Δ_k/ε_vN)); the abstract states the pointer has O(1) qubits. Corollary 1 addresses the standard oracle U=e^{iH}, and Corollary 2 addresses Hamiltonians given as linear combinations of Pauli strings, with Clifford+T cost estimates.
Significance. If correct, the paper would give a conceptually simple, low-qubit QPE/QEE algorithm with competitive complexity and would also provide a reusable error-propagation analysis for approximate QSP+ circuits. The paper is clearly written and situates the work in the existing QPE literature. The error-propagation lemma for QSP+ circuits (Lemma 3, Eq. (8)) and the tensor-product block-encoding lemma (Lemma 2) are potentially useful and appear sound. However, the main resource claim is invalid because the pointer normalization removes r from the phase resolution, so the advertised O(β/(δΔ_k)) query complexity cannot deliver ε_vN precision. The central theorem's success-probability algebra is also not justified. These are load-bearing errors, not presentation issues.
major comments (3)
- [§2.3, Eqs. (18)–(32); Theorem 2, Eq. (49)] The main resource claim fails at the pointer normalization. With p|z>=(z/2^r)|z> in Eq. (18), the evolved phase is e^{-itλ z/2^r}. The iQFT amplitude in Eq. (22) peaks at x≈tλ/(2π) mod 2^r, and the final estimate is λ_est=2πx/t. Eq. (27) therefore gives |λ−λ_est|≤2πk/t, which is independent of r. The bit precision k/2^r in Eqs. (30)–(31) concerns x, not λ; after rescaling by 2π/t the resolution is 2π/t, so Eq. (32) cannot deliver ε_vN precision. In Theorem 2, Eq. (49) sets t=π(⌈3/(2δ)⌉+1)/Δ_k, so Eq. (27) gives error 2Δ_k, not ε_vN. To reach error ε_vN one needs t=O(1/ε_vN) and hence query complexity O(β/(δ ε_vN)+...), invalidating the advertised O(β/(δ Δ_k)) bound and the abstract's 'O(1)-qubit pointer' claim.
- [Theorem 2, Eqs. (42)–(47)] The success-probability bound is not derived correctly. From Eq. (37), the relevant factor is (1−Σ_{j≠k}c_j^2)/(1+4rε_HS). With the proof's assignment ε_HS=δ/(3·4^r) and c_j^2≤δ/(3·2^{n−1}), one gets Σ c_j^2≤2δ/3 and 1+4rε_HS≤1+δ/3, so the factor is at best (1−2δ/3)/(1+δ/3), not (1−δ/3)^2/(1+δ/3) as in Eq. (45). For small δ this is strictly smaller than the claimed (1−δ/3)^2/(1+δ/3), and the chain to ≥1−δ in Eq. (47) is not justified. If instead the theorem's stated bound c_j≤δ/(3·2^{n−1}) is intended, the proof's c_j^2≤... bound is a mismatch. Either way, the claimed 1−δ success probability is unsupported.
- [Corollary 1 and Theorem 2] Corollary 1 states r=O(ε_vN/Δ_k), inverting Eq. (32), which gives r=1+log2(Δ_k/ε_vN). This is not a minor typo: it is part of the same confusion between r-limited and t-limited resolution. The abstract's 'pointer system of O(1) qubits' is also inconsistent with Theorem 2's r=O(log(Δ_k/ε_vN)) unless one additionally assumes Δ_k=Θ(ε_vN), which is not the stated promise. The same issue propagates to Corollary 2 and Table 1.
minor comments (4)
- [Theorem 2 statement] The statement bounds c_j≤δ/(3·2^{n−1}), while the proof uses c_j^2≤δ/(3·2^{n−1}); these should be reconciled.
- [Corollary 2 statement] The line '∑_j α = α' should presumably read '∑_l α_l = α' for the LCP coefficients in Eq. (3).
- [Eq. (53)] The definition of ilde δ depends on d(t',ε_BE), and t' itself depends on δ through Eq. (49); the statement δ∈( ilde δ,1) is therefore implicit and should be written as a condition on δ.
- [Corollary 1 proof] The proof's final query complexity, O(||H||_1/(δΔ_k)+log(Δ_k/ε_vN)), does not match the corollary statement's O(||H||_1/(δ ε_vN)+log(Δ_k/ε_vN)); the discrepancy is directly tied to the t-vs-r issue in Major Comment 1.
Circularity Check
No significant circularity: the derivation composes external QSP/QET/block-encoding results with independent error-propagation lemmas; no fitted parameter is renamed as a prediction.
full rationale
The paper's central derivation is not circular. Theorem 2 is proven from an assumed block-encoding oracle for H⊗p together with a QET Hamiltonian-simulation subroutine; the error budget ϵ_HS=(2d+2)ϵ_BE is a chosen allocation, not a fitted input, and the success-probability constraints are solved for the simulation time and error parameters rather than being constructed from the claimed query count. Corollary 1 relies on the external Gilyén et al. block-encoding lemma and a tensor-product lemma proven in the paper, while Corollary 2 uses the external Zhang–Yuan LCP block-encoding construction. No quantity is fitted to a subset of data and then presented as a prediction, and no uniqueness theorem is imported to force the algorithm's form. The only self-citation, [31], appears in a review remark about QSP preprocessing and in a discussion of understood preprocessing techniques; it is not load-bearing for the QPE/QEE claim. A skeptic's concern that the pointer normalization makes the resolution depend on t rather than r is a correctness question about the error analysis, not circularity: the paper's equations do not define ε_vN in terms of the claimed query count, nor does the argument reduce to its own assumptions by construction. Therefore no circular step meeting the quote-and-reduction standard is present.
Assumptions & free parameters
free parameters (3)
- Error budget splits in theorem 2 =
ϵ_QET ≤ ϵ_BE; ϵ_R ≤ ϵ_BE; ϵ_HS = δ/(3·4^r)
- Success-parameter k =
⌈3/(2δ)⌉ + 1
- Pointer register size r =
1 + ⌈log2(Δ_k/ϵ_vN)⌉
assumptions (5)
- standard math Jacobi-Anger truncation bound eq. (15) for approximating e^{-itx} by QSP/QET polynomials
- domain assumption Qubitization construction: any (β,n_anc,ϵ) block-encoding yields a qubitized W(H) with eigenvalues e^{i arccos(λ/β)}
- domain assumption Pointer momentum operator p = Σ 2^{-j}(1−σ_z^{(j)})/2 faithfully reproduces the von Neumann measurement with eigenvalues z/2^r
- ad hoc to paper Eigenstate preparation with bounded coefficients c_j (statement: c_j ≤ δ/(3·2^{n−1}); proof uses c_j² ≤ ...)
- ad hoc to paper Error propagation lemma 3: ||U_QET − Ũ_QET|| ≤ d(ϵ_R + ϵ_BE) + ϵ_R
Cite this review
Pith. "Pith review of von Neumann measurement and quantum phase estimation of block-encoded Hamiltonians." pith.science (2026). https://pith.science/paper/7RCOVCGK
@misc{pith2026250904246,
author = {Pith},
title = {Pith review of: von Neumann measurement and quantum phase estimation of block-encoded Hamiltonians},
year = {2026},
howpublished = {\url{https://pith.science/paper/7RCOVCGK}},
note = {Machine review of arXiv:2509.04246}
}
read the original abstract
We review how to use von Neumann's measurement procedure to estimate a phase, using an efficient Hamiltonian simulation subroutine acts on a block-encoded Hamiltonian. We show that the resulting algorithm can be used to solve quantum phase estimation (QPE) or quantum energy estimation (QEE) {with competitive complexity scaling.} We then use recent results for block-encoding implementations to derive the Clifford + T complexity bound for QPE with respect to model-relevant parameters of the Hamiltonian and the desired precision. With this result, we demonstrate an efficient algorithm for QEE beginning from any linear combinations of Pauli strings. In this way, we argue that a well-understood and long-standing idea retains practical legitimacy for fault-tolerant era algorithms, once the costs of Hamiltonian simulation are accounted for.
Figures
Reference graph
Works this paper leans on
-
[1]
Quantum computation and quantum infor- mation: 10th anniversary edition
Michael A. Nielsen and Isaac L. Chuang. “Quantum computation and quantum infor- mation: 10th anniversary edition”. Cam- bridge University Press. (2010)
2010
-
[2]
Quantum measurements and the Abelian Stabilizer Problem
A. Yu Kitaev. “Quantum measurements and the Abelian Stabilizer Problem” (1995). arXiv:quant-ph/9511026
arXiv 1995
-
[3]
Andrew M. Childs, Enrico Deotto, Edward Farhi, Jeffrey Goldstone, Sam Gutmann, and Andrew J. Landahl. “Quantum search by measurement”. Physical Review A 66, 032314 (2002)
work page 2002
-
[4]
Mathematical Foundations of Quantum Mechanics: New Edition
John von Neumann and Robert T. Beyer. “Mathematical Foundations of Quantum Mechanics: New Edition”. Princeton Uni- versity Press. (2018). New edition
work page 2018
-
[5]
On the relationship be- tween continuous- and discrete-time quan- tum walk
Andrew M. Childs. “On the relationship be- tween continuous- and discrete-time quan- tum walk”. Communications in Mathemati- cal Physics294, 581–603 (2009)
work page 2009
-
[6]
Explicit quantum circuits for block encodings of certain sparse matri- ces
Daan Camps, Lin Lin, Roel Van Beeumen, and Chao Yang. “Explicit quantum circuits for block encodings of certain sparse matri- ces”. SIAM Journal on Matrix Analysis and Applications 45, 801–827 (2024)
work page 2024
-
[7]
András Gilyén, Yuan Su, Guang Hao Low, and Nathan Wiebe. “Quantum singular value transformation and beyond: Exponen- tial improvements for quantum matrix arith- metics”. In Proceedings of the 51st An- nual ACM SIGACT Symposium on Theory of Computing. STOC 2019. Association for Computing Machinery (2019)
work page 2019
-
[8]
Hamiltonian Simulation by Qubitization
Guang Hao Low and Isaac L. Chuang. “Hamiltonian Simulation by Qubitization”. Quantum 3, 163 (2019)
work page 2019
Show all 43 references
-
[9]
Methodology of reso- nant equiangular composite quantum gates
Guang Hao Low, Theodore J. Yoder, and Isaac L. Chuang. “Methodology of reso- nant equiangular composite quantum gates”. Phys. Rev. X6, 041067 (2016)
2016
-
[10]
Encoding electronic spectra in quantum circuits with linear t complexity
Ryan Babbush, Craig Gidney, Dominic W. Berry, Nathan Wiebe, Jarrod McClean, Alexandru Paler, Austin Fowler, and Hart- mut Neven. “Encoding electronic spectra in quantum circuits with linear t complexity”. Phys. Rev. X8, 041015 (2018)
2018
-
[11]
Grand unifica- tion of quantum algorithms
John M. Martyn, Zane M. Rossi, Andrew K. Tan, and Isaac L. Chuang. “Grand unifica- tion of quantum algorithms”. PRX Quantum 2, 040203 (2021)
2021
-
[12]
Faster Coherent Quantum Al- gorithms for Phase, Energy, and Amplitude Estimation
Patrick Rall. “Faster Coherent Quantum Al- gorithms for Phase, Energy, and Amplitude Estimation”. Quantum5, 566 (2021)
2021
-
[13]
Circuit complexity of quantum access models for en- coding classical data
Xiao-Ming Zhang and Xiao Yuan. “Circuit complexity of quantum access models for en- coding classical data”. npj Quantum Infor- mation10 (2024)
2024
-
[14]
Tight Bounds for Quantum Phase Estimation and Related Problems
Nikhil S. Mande and Ronald de Wolf. “Tight Bounds for Quantum Phase Estimation and Related Problems”. In 31st Annual Euro- peanSymposiumonAlgorithms(ESA2023). Volume 274 of Leibniz International Pro- ceedings in Informatics (LIPIcs), pages 81:1– 81:16. Schloss Dagstuhl – Leibn...
2023
-
[15]
Quantum ampli- tude amplification and estimation
Gilles Brassard, Peter Høyer, Michele Mosca, and Alain Tapp. “Quantum ampli- tude amplification and estimation”. Quan- tum Computation and InformationPage 53–74 (2002)
2002
-
[16]
Simpli- fying a classical-quantum algorithm interpo- lation with quantum singular value transfor- mations
Duarte Magano and Miguel Murça. “Simpli- fying a classical-quantum algorithm interpo- lation with quantum singular value transfor- mations”. Physical Review A106 (2022)
2022
-
[17]
A case study against qsvt: assess- ment of quantum phase estimation improved by signal processing techniques
Sean Greenaway, William Pol, and Sukin Sim. “A case study against qsvt: assess- ment of quantum phase estimation improved by signal processing techniques” (2024). arXiv:2404.01396
2024 arXiv
-
[18]
Lecture Notes on Quantum Algo- rithms for Scientific Computation
Lin Lin. “Lecture Notes on Quantum Algo- rithms for Scientific Computation” (2022)
2022
-
[19]
Decomposing dense ma- trices into dense pauli tensors
Tyson Jones. “Decomposing dense ma- trices into dense pauli tensors” (2024). arXiv:2401.16378
2024 arXiv
-
[20]
Tensorized pauli decom- position algorithm
Lukas Hantzko, Lennart Binkowski, and Sabhyata Gupta. “Tensorized pauli decom- position algorithm”. Physica Scripta 99, 085128 (2024)
2024
-
[21]
Paulicomposer: compute tensor 12 products of pauli matrices efficiently
Sebastián Vidal Romero and Juan Santos- Suárez. “Paulicomposer: compute tensor 12 products of pauli matrices efficiently”. Quan- tum Information Processing22 (2023)
2023
-
[22]
Appli- cation of a resource theory for magic states to fault-tolerant quantum computing
Mark Howard and Earl Campbell. “Appli- cation of a resource theory for magic states to fault-tolerant quantum computing”. Phys. Rev. Lett.118, 090501 (2017)
2017
-
[23]
The resource theory of stabilizer quantum computation
Victor Veitch, S A Hamed Mousavian, Daniel Gottesman, and Joseph Emerson. “The resource theory of stabilizer quantum computation”. New Journal of Physics 16, 013009 (2014)
2014
-
[24]
The heisenberg rep- resentation of quantum computers
Daniel Gottesman. “The heisenberg rep- resentation of quantum computers” (1998). arXiv:quant-ph/9807006
1998 arXiv
-
[25]
Methodology for quantum logic gate construction
Xinlan Zhou, Debbie W. Leung, and Isaac L. Chuang. “Methodology for quantum logic gate construction”. Phys. Rev. A 62, 052316 (2000)
2000
-
[26]
Univer- sal quantum computation with ideal clifford gates and noisy ancillas
Sergey Bravyi and Alexei Kitaev. “Univer- sal quantum computation with ideal clifford gates and noisy ancillas”. Phys. Rev. A71, 022316 (2005)
2005
-
[27]
Gen- eralized quantum signal processing
Danial Motlagh and Nathan Wiebe. “Gen- eralized quantum signal processing”. PRX Quantum 5, 020368 (2024)
2024
-
[28]
Product Decomposition of Periodic Functions in Quantum Signal Pro- cessing
Jeongwan Haah. “Product Decomposition of Periodic Functions in Quantum Signal Pro- cessing”. Quantum3, 190 (2019)
2019
-
[29]
Fast phase factor finding for quantum sig- nal processing
Hongkang Ni and Lexing Ying. “Fast phase factor finding for quantum sig- nal processing”. arxiv.2410.06409 (2024) arXiv:2410.06409
2024 arXiv
-
[30]
Robust iterative method for symmetric quantum signal processing in all parameter regimes
Yulong Dong, Lin Lin, Hongkang Ni, and Jiasu Wang. “Robust iterative method for symmetric quantum signal processing in all parameter regimes”. SIAM Journal on Sci- entific Computing46, A2951–A2971 (2024)
2024
-
[31]
The hitchhiker’s guide to qsp pre-processing
S. E. Skelton. “The hitchhiker’s guide to qsp pre-processing” (2025). arXiv:2501.05977
2025 arXiv
-
[32]
El- ementary gates for quantum computation
Adriano Barenco, Charles H. Bennett, Richard Cleve, David P. DiVincenzo, Nor- man Margolus, Peter Shor, Tycho Sleator, John A. Smolin, and Harald Weinfurter. “El- ementary gates for quantum computation”. Physical Review A52, 3457–3467 (1995)
1995
-
[33]
Ap- proximate quantum fourier transform and decoherence
Adriano Barenco, Artur Ekert, Kalle- Antti Suominen, and Päivi Törmä. “Ap- proximate quantum fourier transform and decoherence”. Physical Review A 54, 139–146 (1996)
1996
-
[34]
Approximate quantum Fourier transform with O(n log(n)) T gates
Yunseong Nam, Yuan Su, and Dmitri Maslov. “Approximate quantum Fourier transform with O(n log(n)) T gates”. npj Quantum Inf.6, 26 (2020)
2020
-
[35]
Dou- blingtheefficiencyofhamiltoniansimulation via generalized quantum signal processing
Dominic W. Berry, Danial Motlagh, Gia- como Pantaleoni, and Nathan Wiebe. “Dou- blingtheefficiencyofhamiltoniansimulation via generalized quantum signal processing”. Phys. Rev. A110, 012612 (2024)
2024
-
[36]
Complementary polynomials in quan- tum signal processing
Bjorn K. Berntson and Christoph Sünder- hauf. “Complementary polynomials in quan- tum signal processing”. Communications in Mathematical Physics406 (2025)
2025
-
[37]
Generalized quan- tum singular value transformation
Christoph Sünderhauf. “Generalized quan- tum singular value transformation” (2023). arXiv:2312.00723
2023 arXiv
-
[38]
Fast and efficient exact syn- thesis of single qubit unitaries generated by cliffordandtgates
Vadym Kliuchnikov, Dmitri Maslov, and Michele Mosca. “Fast and efficient exact syn- thesis of single qubit unitaries generated by cliffordandtgates” (2013). arXiv:1206.5236
2013 arXiv
-
[39]
An algorithm for the t-count
David Gosset, Vadym Kliuchnikov, Michele Mosca, and Vincent Russo. “An algorithm for the t-count”. Quantum Info. Comput. 14, 1261–1276 (2014). arXiv:1308.4134
2014 arXiv
-
[40]
Asymptotically optimal ap- proximation of single qubit unitaries by clif- ford andt circuits using a constant number of ancillary qubits
Vadym Kliuchnikov, Dmitri Maslov, and Michele Mosca. “Asymptotically optimal ap- proximation of single qubit unitaries by clif- ford andt circuits using a constant number of ancillary qubits”. Phys. Rev. Lett. 110, 190502 (2013)
2013
-
[41]
Practical approximation of single-qubit unitaries by single-qubit quan- tum clifford and t circuits
Vadym Kliuchnikov, Dmitri Maslov, and Michele Mosca. “Practical approximation of single-qubit unitaries by single-qubit quan- tum clifford and t circuits”. IEEE Transac- tions on Computers65, 161–172 (2016)
2016
-
[42]
Efficient clifford+t approxi- mation of single-qubit operators
Peter Selinger. “Efficient clifford+t approxi- mation of single-qubit operators”. Quantum Info. Comput.15, 159–180 (2015)
2015
-
[43]
A meet- in-the-middle algorithm for fast synthesis of depth-optimal quantum circuits
Matthew Amy, Dmitri Maslov, Michele Mosca, and Martin Roetteler. “A meet- in-the-middle algorithm for fast synthesis of depth-optimal quantum circuits”. Trans. Comp.-Aided Des. Integ. Cir. Sys. 32, 818–830 (2013). 13 A QET oracles Qubitization is the process of taking a block-...
2013
Reviewed August 5, 2026 · model on record in the stance chip above.
Discussion (0). Sign in to comment.