REVIEW 3 major objections 6 minor 1 cited by
Optimizing sparse quantum state preparation with measurement and feedforward
T0 review · 3 major / 6 minor · reviewed 2026-08-05 · deepseek-v4-flash
Pith's one-line read Two circuits prepare arbitrary sparse quantum states; with measurement and feedforward the depth drops to O(n) using O(d) ancillas.
desk verdict A plausible new depth-ancilla trade-off for sparse QSP, but the O(n)-depth MaF theorem is not yet proven: fan-out reuse and BST uncomputation both need repair. 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 the MaF (measurement and feedforward) class of circuits and, inside it, the constant-depth quantum fan-out gate: a fan-out over k target qubits is realized by a layer of Hadamards and CNOTs, a mid-circuit measurement, a classical XOR computation, and a final layer of X/Z gates. Because the algorithm needs fan-out only on the one-hot register and because a parity-controlled X gate is just a fan-out conjugated by Hadamards (Corollary 1), every parallel step in the one-hot encoding, permutation, and garbage elimination can be compressed to constant depth. The one-hot encoding and the binary-search-tree encoding of the target basis states are what let the n sequential
What would settle it
Take a concrete instance, e.g. n=5, d=4, and explicitly decompose every MaF fan-out gate used in Algorithms 4, 6, and 7 into the constant-depth measurement circuit of [37], tracking qubit identities and resets after each of the n parity-controlled X steps. If the uncomputation of any fan-out leaves the ancillas entangled with the data register, or if resetting/reusing the measured ancillas between steps requires more than O(d) total ancillas or adds more than constant depth, then the size-depth-ancilla claims of Theorem 2 fail on that instance.
Extended reading notes
Core claim
Central claim (Theorem 2): any n-qubit d-sparse state can be prepared in the measurement-and-feedforward (MaF) class with size O(dn), depth O(n), and O(d) ancillas. The circuit prepares the d amplitudes on ⌈log d⌉ qubits, expands them to a one-hot encoding on d ancillas, permutes each amplitude to its target basis state via one parity-controlled X gate per output qubit, and uncomputes the one-hot data through a recorded binary search tree. Lemma 1 makes OR-controlled X equal to parity-controlled X on one-hot controls; parity-controlled X is a fan-out conjugated by Hadamards. Using constant-depth MaF fan-out gates everywhere removes d from the depth; without MaF, the same steps give depth O(n
Load-bearing premise
The construction assumes that a MaF fan-out gate can be applied twice—compute and uncompute—without leaving residual garbage or exceeding O(d) ancillas, and that the measured ancillas can be reused across the n sequential parity gates in Step 3; this is asserted in Algorithms 4, 6, and 7 but never proven.
Editorial extensions
If this is right
- Any n-qubit d-sparse state can be prepared in depth O(n) with O(d) ancillas, so the depth is linear in n and independent of d.
- For d much smaller than 2^n, the O(dn) gate count is near-linear in n, far below the Θ(2^n) gates required for general state preparation.
- Compared with the constant-depth MaF construction in [33], this algorithm shrinks circuit size from O(d^2 log n) to O(dn) and ancillas from O(d^2 log n) to O(d), trading constant depth for linear depth.
- The non-MaF construction (Theorem 1) gives the same size and ancilla counts with depth O(n log d), covering hardware without mid-circuit measurement and feedforward.
- Because many quantum algorithms that call state preparation already run at O(n) depth, an O(n)-depth sparse preparer can be inserted without making preparation the bottleneck.
Reading between the lines
- The same four-step skeleton could generalize to other structured state families (for example, states whose nonzero positions are produced by a classical circuit), wherever an efficient controlled permutation is available; that generalization is not in the paper.
- A natural testable extension is to compile Algorithms 4, 6, and 7 for a small instance, say n=5 and d=4, into native gates, and check whether the MaF fan-out gates can indeed be reused across all n parity steps with only O(d) ancillas and constant reset cost.
- The paper leaves open whether depth Ω(n) is necessary for MaF-based sparse preparation with O(d) ancillas; a matching lower bound would make the O(n) depth of Theorem 2 tight.
- If the fan-out reuse assumption survives, the design suggests a broader recipe for flattening any QSP bottleneck that is fan-out-like: expand to a one-hot register, permute in parallel, then uncompute—which is the paper's insight applied beyond the specific states considered here.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes two sparse quantum state preparation (SQSP) algorithms for arbitrary n-qubit d-sparse states. The first uses O(d) ancilla qubits, circuit size O(dn), and depth O(n log d). The second additionally uses mid-circuit measurement and feedforward (MaF) to reduce the depth to O(n) while keeping size O(dn) and O(d) ancilla. The construction proceeds through four steps: low-depth GQSP on ⌈log d⌉ qubits, one-hot encoding of the d basis indices into ancilla registers, a permutation step that maps |i⟩ to |q_i⟩ using OR- or parity-controlled X gates, and a garbage elimination step that resets the one-hot ancilla using a binary-search-tree recording. Theorems 1 and 2 state the resulting complexity bounds.
Significance. If Theorem 2 is correct, it is a meaningful improvement: it achieves depth O(n) independent of d with only O(d) ancilla, whereas the prior MaF-based construction of [33] uses Θ(1) depth but O(d² log n) size and ancillas, and non-MaF sparse preparation methods typically use either more depth or many more ancillas. The paper is also transparent in using external results (low-depth GQSP [40], constant-depth fan-out [37], Toffoli decompositions [38]) as assumptions rather than deriving them internally, and no fitted parameters are involved. However, the central claim depends on a reusable, clean measurement-based fan-out gadget and on explicit uncomputation protocols, neither of which is established in the manuscript. The result is therefore plausible but not yet rigorously supported.
major comments (3)
- [Section III.B, Algorithms 2/3/4, Eq. (17)] The one-hot encoding is implemented for a B register of size d, but the CSWAP indices run up to 2^{⌈log d⌉}-1. For d not a power of two, these indices exceed the register. Example: d=5, ⌈log d⌉=3, Algorithm 2 with j=1, i=1 references B(4) and B(6), but B(6) does not exist. Since the theorem claims arbitrary d, the algorithm is invalid as written for non-power-of-two d. Either the algorithm must be modified to touch only indices <d (with a proof that the restricted swap network still produces the correct one-hot state), or the theorem must be restricted.
- [Section II.B Theorem 3; Section III.B/C, Algorithms 4, 6, 7] The measurement-based fan-out gate of [37] is treated as a reusable unitary operation whose ancillas can be reused across multiple calls within the O(d) ancilla budget. This is never proved. In Algorithm 4 the same FANOUT is applied twice to the same C register; in Algorithm 6 it is invoked in n consecutive parity gates; and in Algorithm 7 it is called in every layer. A mid-circuit-measurement gadget is not automatically a unitary on data+ancilla and may consume its ancillas. If each invocation requires fresh ancillas, Step 3 alone needs Θ(nd) ancillas, and if each invocation requires resetting ancillas to |0⟩, that reset cost must be counted in the depth. A formal lemma stating the exact action of the gadget on all qubits and specifying the reset/feedforward protocol is load-bearing for Theorem 2.
- [Section IV.D.3, Eq. (21), Algorithm 7] The reset of the C register is asserted to be 'similar to Step 4.1', but the recording operations are not self-inverse. For the first branch, the sequence X(C(0)); CSWAP(A(j), C(0), C(1)) maps |00⟩ to |10⟩ when A(j)=0 and to |01⟩ when A(j)=1. Applying the same sequence to the recorded state |01⟩ with A(j)=1 maps it to |11⟩, not back to |00⟩; the inverse order CSWAP then X is required. The paper does not give the explicit uncompute circuit or its depth. Without a correct uncompute, the C register is not reset and the final state is not |ϕ⟩|0⟩⊗4d. This affects both Theorem 1 and Theorem 2.
minor comments (6)
- [Algorithm 4, line 5] The CSWAP line reads CSWAP(C(j), ...) but it should almost certainly be CSWAP(C(i), ...) to match the loop variable and Algorithm 3.
- [Algorithm 4 output] The output line writes `|ei⟩⊗d`; it should be `|ei⟩` (a single one-hot state).
- [Theorem 2 statement] Typo: 'O(d) acilla qubits' should be 'ancilla qubits'.
- [Section III.D/Table III] The size of the garbage-elimination step is reported inconsistently: Algorithm 8 has d CNOTs, Table III says size O(d), Section III.D.2 says size O(n), and the proof of Theorem 1 says size O(dn). These should be reconciled.
- [Lemma 2 heading] Typo: 'n-Toffoli gat' should be 'n-Toffoli gate'.
- [Definition 2] The MaF model should explicitly specify whether reinitializing a measured qubit to |0⟩ is allowed as part of a classical layer or quantum layer, and how its cost is accounted. This is relevant to the fan-out reuse issue.
Circularity Check
No circularity: Theorem 2 is a composition of external results (low-depth GQSP [40], MaF fan-out [37], Toffoli constructions [38], parity/fan-out duality [41]); the flagged fan-out ancilla-reuse issue is an omitted-proof correctness risk, not a circular reduction.
full rationale
I walked the derivation chain of Theorems 1 and 2, including Algorithms 2–8 and the proofs in Section III.E. Every complexity claim is assembled from external theorems that are taken as assumptions, not derived in this paper: the GQSP subroutine is Theorem 5 from [40], the constant-depth MaF fan-out is Theorem 3 from [37], the n-Toffoli lemmas are from [38], and the parity/fan-out duality is from [41]. None of these is a self-citation of the present authors, and none reduces to the paper's own target quantity. The problem input S defines only the target sparse state; the output states are explicitly checked against Eq. (2). There are no fitted parameters, no numerical data are predicted from fitted values, and no claimed result is defined in terms of the quantity it is supposed to establish. I also considered the passages flagged by the skeptic: the repeated FANOUT calls in Algorithm 4 (lines 3 and 7), Algorithm 7 (lines 16 and 20), and the unproven 'method is similar to Step 4.1' reset claim in Section IV.D.3. These are genuine omissions in the proof that the MaF fan-out gadget can be reused as a clean reversible operation within the O(d) ancilla budget. However, that is a correctness gap or missing proof, not circularity: the fan-out gadget is cited from external work, not constructed from the paper's own conclusions, and the paper does not fit any parameter or rename any known result to produce its complexity bounds. Under the stated external theorems, the derivation is self-contained. The honest finding is no significant circularity; score 0.
Assumptions & free parameters
assumptions (4)
- domain assumption Low-depth GQSP algorithm (Theorem 5 of [40]) prepares an arbitrary n-qubit state in depth O(n) with O(2^n) ancilla; on log d qubits this is depth O(log d), size O(d), ancilla O(d).
- domain assumption Constant-depth fan-out gate via MaF (Theorem 3 of [37]) implements an n-qubit fan-out with n-1 ancilla and a single measurement layer; it is a unitary operation that can be uncomputed by re-applying the same circuit.
- domain assumption Standard gate decompositions: n-Toffoli with one ancilla has depth O(log n) and size O(n) (Lemma 3 of [38]); Fredkin and controlled-Ry have constant depth.
- ad hoc to paper The one-hot encoding construction assumes the B register has at least 2^(ceil(log d)) qubits or that d is a power of two.
Cite this review
Pith. "Pith review of Optimizing sparse quantum state preparation with measurement and feedforward." pith.science (2026). https://pith.science/paper/EB5EKWKT
@misc{pith2026250821346,
author = {Pith},
title = {Pith review of: Optimizing sparse quantum state preparation with measurement and feedforward},
year = {2026},
howpublished = {\url{https://pith.science/paper/EB5EKWKT}},
note = {Machine review of arXiv:2508.21346}
}
abstract
Quantum state preparation (QSP) is a key component in many quantum algorithms. In particular, the problem of sparse QSP (SQSP) $\unicode{x2013}$ the task of preparing the states with only a small number of non-zero amplitudes $\unicode{x2013}$ has garnered significant attention in recent years. In this work, we focus on reducing the circuit depth of SQSP with limited number of ancilla qubits. We present two SQSP algorithms: one with depth $O(n\log d)$, and another that reduces depth to $O(n)$. The latter leverages mid-circuit measurement and feedforward, where intermediate measurement outcomes are used to control subsequent quantum operations. Both constructions have size $O(dn)$ and use $O(d)$ ancilla qubits. Compared to the state-of-the-art SQSP algorithm in arXiv:2108.06150, which allows an arbitrary number of ancilla qubits $m>0$, both of our algorithms achieve lower circuit depth when $m=d$.
Figures
Figures from the paper (12 more)
Forward citations
Cited by 1 Pith paper
-
Performance Model for Hybrid Quantum-Classical Workflows
A two-level runtime model decomposes hybrid quantum-classical cycles into quantum, classical, and communication time, allowing a communication-to-computation ratio to classify workflows as compute- or communication-bound.
Reference graph
Works this paper leans on
-
[33]
In our work, we adopt a permutation-based strategy, as also employed in [23]
O(d2 log n) Θ(1) O(d2 log n) Theorem 2 O(dn) O(n) O(d) many multi-controlled single-target gates [26], while oth- ers employ decision trees to perform the correct rotations on each of the n qubits sequentially [21]. In our work, we adopt a permutation-based strategy, as also employed in [23]. The core idea is to initially produce the cor- rect amplitudes ...
-
[37]
R. Mao, G. Tian, and X. Sun, Phys. Rev. A 110, 032439 (2024)
work page 2024
-
[9]
/ O (n log dn + dn 2 log d n + m ) O (m )
-
[40]
R. Acharya, D. A. Abanin, L. Aghababaie-Beni, I. Aleiner, T. I. Andersen, M. Ansmann, F. Arute, K. Arya, A. Asfaw, N. Astrakhantsev, et al., Nature 638, 920 (2024)
work page 2024
-
[38]
T. M. De Veras, I. C. De Araujo, D. K. Park, and A. J. Da Silva, IEEE Trans. Comput. 70, 2125 (2020)
work page 2020
-
[1]
BST encoding In the first substep, for each basis state |qi⟩ in the super- position, the kth branch node of the BST is recorded in the 2-qubit state |f (qi, k)⟩ in the C register. This pro- cess is performed in parallel over all i, while the records for each k are generated sequentially in the order of branch node indices. Specifically, if |qi⟩ passes thr...
-
[2]
O dn log d O (log dn ) O dn log d
-
[3]
The method is similar to Step 4.1
Reset C register As soon as the garbage state is eliminated, the C register can be rapidly deallocated. The method is similar to Step 4.1. Additionally, the size and depth complexity is the same as in Step 4.1. After completing Step 4, the garbage state is elim- inated and the desired state |ϕ⟩ is obtained. With- out using MaF, the process requires a circ...
Show all 55 references
-
[4]
Woerner and D
S. Woerner and D. J. Egger, Npj Quantum Inf. 5, 15 (2019)
2019
-
[5]
Ω dn log( m + n )+log d + n / O (m )a
-
[6]
Comparison of our result to previous SQSP method with a MaF circuit
Θ dn log dn + n / unlimited number Theorem 1 O (dn ) O (n log d) O (d) a under the condition m = O (dn ) TABLE II. Comparison of our result to previous SQSP method with a MaF circuit. Algorithm Circuit Size Depth #Ancilla
-
[7]
Jennings, M
D. Jennings, M. Lostaglio, S. Pallister, A. T. Sornborger, and Y. Suba¸ sı, Efficient quantum linear solver algorithm with detailed running costs, arXiv:2305.11352v1 [quant- ph]
-
[8]
Alabdulmohsin, H
I. Alabdulmohsin, H. Maennel, and D. Keysers, The im- pact of reinitialization on generalization in convolutional neural networks, arXiv:2109.00267 [quant-ph]
-
[10]
This can be done in parallel and requires only constant depth
Garbage elimination 14 In this substep, we identify the value of |qi⟩ via the C register to flip the correct qubit in the B register. This can be done in parallel and requires only constant depth. It is known that each one-hot encoding|ei⟩ corresponds to a state |qi⟩, and each...
-
[11]
P. W. Shor, SIAM Rev. , 303 (1996)
1996
-
[12]
L. K. Grover, in Proceedings of the twenty-eighth annual ACM symposium on Theory of computing (Assoc. Com- put. Mach., Pennsylvania, 1996) pp. 212–219
1996
-
[13]
Cerezo, A
M. Cerezo, A. Arrasmith, R. Babbush, S. C. Benjamin, S. Endo, K. Fujii, J. R. McClean, K. Mitarai, X. Yuan, L. Cincio, et al. , Nat. Rev. Phys. 3, 625 (2021)
2021
-
[14]
Gisin and H
N. Gisin and H. Bechmann-Pasquinucci, Phys. Lett. A 246, 1 (1998)
1998
-
[15]
Markov, C
V. Markov, C. Stefanski, A. Rao, and C. Gonciulea, A 16 generalized quantum inner product and applications to financial engineering, arXiv:2201.09845 [quant-ph]
-
[16]
A. M. Dalzell, A shortcut to an optimal quantum linear system solver, arXiv:2406.12086v1 [quant-ph]
-
[17]
D¨ ur, G
W. D¨ ur, G. Vidal, and J. I. Cirac, Phys. Rev. A 62, 062314 (2000)
2000
-
[18]
B¨ artschi and S
A. B¨ artschi and S. Eidenbenz, Deterministic preparation of dicke states, in Fundamentals of Computation Theory , edited by L. A. Gasieniec, J. Jansson, and C. Levcopoulos (Springer, Cham, 2019) pp. 126–139
2019
-
[19]
X. Sun, G. Tian, S. Yang, P. Yuan, and S. Zhang, IEEE Trans. CAD 42, 3301 (2023)
2023
-
[20]
Plesch and ˇC
M. Plesch and ˇC. Brukner, Phys. Rev. A 83, 032302 (2011)
2011
-
[21]
Shukla and P
A. Shukla and P. Vedula, Quantum Inf. Process. 23, 38 (2024)
2024
-
[22]
I. F. Araujo, C. Blank, I. C. Ara´ ujo, and A. J. da Silva, IEEE Trans. CAD 43, 161 (2024)
2024
-
[23]
Mozafari, Y
F. Mozafari, Y. Yang, and G. De Micheli, in 27th Asia and South Pacific Design Automation Conference (ASP- DAC),2022 (IEEE, Virtual Conference, 2022) pp. 460– 465
2022
-
[24]
T. M. de Veras, L. D. da Silva, and A. J. da Silva, Quan- tum Inf. Process. 21, 204 (2022)
2022
-
[25]
O (dn log d) Θ(log dn ) O (dn log d)
-
[26]
Frisk Kockum, V
A. Frisk Kockum, V. Macr ` ı, and F. Nori, Phys. Rev. A 98, 062327 (2018)
2018
-
[27]
Cottrell, B
W. Cottrell, B. Freivogel, D. M. Hofman, and S. F. Lokhande, J. High Energy Phys. (online) 2019, 1 (2019)
2019
-
[28]
Gonzales, R
A. Gonzales, R. Herrman, C. Campbell, I. Gaidai, J. Liu, T. Tomesh, and Z. H. Saleem, in 2024 IEEE Interna- tional Conference on Quantum Computing and Engineer- ing (QCE) (IEEE, Montreal, 2024) pp. 616–617
2024
-
[29]
O dn log n + n / 0 [21–24] O (dn ) O (dn ) 1
-
[30]
Chor and B
B. Chor and B. A. Coan, IEEE Trans. Software Eng. SE-11, 531 (1985)
1985
-
[31]
Gleinig and T
N. Gleinig and T. Hoefler, in 2021 58th ACM/IEEE De- sign Automation Conference (DAC) (IEEE, San Fran- cisco, 2021) pp. 433–438
2021
-
[32]
Mozafari, G
F. Mozafari, G. De Micheli, and Y. Yang, Phys. Rev. A 106, 022617 (2022)
2022
-
[34]
N. M. Tubman, C. Mejuto-Zaera, J. M. Epstein, D. Hait, D. S. Levine, W. Huggins, Z. Jiang, J. R. McClean, R. Babbush, M. Head-Gordon, and K. B. Whaley, Post- poning the orthogonality catastrophe: efficient state preparation for electronic structure simulations on quan- tum dev...
-
[35]
Ramacciotti, A
D. Ramacciotti, A. I. Lefterovici, and A. F. Rotundo, Phys. Rev. A 110, 032609 (2024)
2024
-
[36]
Zhang, T
X.-M. Zhang, T. Li, and X. Yuan, Phys. Rev. Lett. 129, 230504 (2022)
2022
-
[39]
Luo and L
J. Luo and L. Li, Nearly optimal circuit size for sparse quantum state preparation, arXiv:2406.16142v2 [quant- ph]
-
[41]
C. H. Bennett, G. Brassard, C. Cr´ epeau, R. Jozsa, A. Peres, and W. K. Wootters, Phys. Rev. Lett. 70, 1895 (1993)
1993
-
[42]
Iqbal, N
M. Iqbal, N. Tantivasadakarn, T. M. Gatterman, J. A. Gerber, K. Gilmore, D. Gresh, A. Hankin, N. Hewitt, C. V. Horst, M. Matheny, et al. , Commun. Phys. 7, 205 (2024)
2024
-
[43]
W. Zi, J. Nie, and X. Sun, Constant-depth quantum cir- cuits for arbitrary quantum state preparation via mea- surement and feedback, arXiv:2503.16208 [quant-ph]
-
[44]
Maslov, Phys
D. Maslov, Phys. Rev. A 76, 052310 (2007)
2007
-
[45]
Ubaru, I
S. Ubaru, I. Y. Akhalwaya, M. S. Squillante, K. L. Clarkson, and L. Horesh, Quantum topological data analysis with linear depth and exponential speedup, arXiv:2108.02811 [quant-ph]
-
[46]
Buhrman, M
H. Buhrman, M. Folkertsma, B. Loff, and N. M. Neu- mann, Quantum 8, 1552 (2024)
2024
-
[47]
BB¨ aumer and S
E. BB¨ aumer and S. Woerner, Phys. Rev. Res.7, 023120 (2025)
2025
-
[48]
J. Nie, W. Zi, and X. Sun, Quantum circuit for multi-qubit toffoli gate with optimal resource (2024), arXiv:2402.05053v1 [quant-ph]
2024 arXiv
-
[49]
Grover and T
L. Grover and T. Rudolph, Creating superpositions that correspond to efficiently integrable probability distribu- tions (2002), arXiv:quant-ph/0208112v1 [quant-ph]
2002 arXiv
-
[50]
K. Gui, A. M. Dalzell, A. Achille, M. Suchara, and F. T. Chong, Quantum 8, 1257 (2024)
2024
-
[51]
Moore, Quantum circuits: Fanout, parity, and count- ing, arXiv:quant-ph/9903046v3 [quant-ph]
C. Moore, Quantum circuits: Fanout, parity, and count- ing, arXiv:quant-ph/9903046v3 [quant-ph]
-
[52]
Chitambar, D
E. Chitambar, D. Leung, L. Manˇ cinska, M. Ozols, and A. Winter, Everything you always wanted to know about locc (but were afraid to ask), in Communications in Mathematical Physics , Vol. 328, edited by L. A. Gasie- niec, J. Jansson, and C. Levcopoulos (Springer, Cham,
-
[53]
Piroli, G
L. Piroli, G. Styliaris, and J. I. Cirac, Phys. Rev. Lett. 133, 230401 (2024)
2024
-
[54]
Y. Yan, M. Ma, Y. Zhou, and X. Ma, Phys. Rev. Lett. 134, 170601 (2025)
2025
-
[55]
Piroli, G
L. Piroli, G. Styliaris, and J. I. Cirac, Phys. Rev. Lett. 127, 220503 (2021)
2021
Reviewed August 5, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.