REVIEW 4 major objections 4 minor 42 references
Complexity and multi-functional variants of the Quantum-to-Quantum Bernoulli Factories
T0 review · 4 major / 4 minor · reviewed 2026-08-03 · deepseek-v4-flash
Pith's one-line read The paper proves that quantum-to-quantum Bernoulli factories simulate exactly the rational functions, and that simulating f(z) needs deg(f) input qubits—with an explicit circuit and closed-form maximal success probability.
desk verdict A useful resource-counting paper for QQBFs: the qubit lower bound and matching construction are real additions, but the proof has two repairable gaps and the success-probability claim is stronger than what is shown. 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 key object is the quantum coin state |z> = (z|0>+|1>)/sqrt(1+|z|²), a parametrisation of qubit states by a complex number. The argument is carried by the canonical-form reduction: every QQBF becomes one fixed unitary U acting on n copies of |z> plus m ancillas, followed by all-zero postselection. In that form the output amplitude pair (P(z), Q(z)) is a linear combination of monomials z^j with binomial weights, which directly ties the degree of the rational function to the number n of input qubits. The paper constructs the optimal U from two orthonormal vectors |v0>, |v1> whose overlaps with the symmetric states |s^n_j> store the coefficients of P and Q; a parameter w parametrises the unu
What would settle it
A numerical or experimental search for a circuit with only one copy of |z> that outputs |z²> with nonzero probability for a dense set of z values, using any postselection or adaptive strategy, would falsify the lower bound n ≥ deg(f). On the optimality side, computing the success probability of the paper's degree-one circuit for f(z)=z and finding any input z for which a different circuit exceeds Eq. (26) would falsify the maximal-success-probability claim.
Extended reading notes
Core claim
Any valid QQBF can be reduced to a fixed unitary U acting on n copies of |z> plus m ancillas, followed by computational-basis measurements and acceptance only when all non-output qubits read 0. In that canonical form every output amplitude is a polynomial in z of degree at most n, so the output is |P(z)/Q(z)> and simulable functions are rational. The paper's constructive proof makes the bound tight: given P,Q of degree at most n, two orthonormal rows of U can be chosen with overlaps KP(z) and KQ(z), which fixes all but one free parameter |w|². Since |w|² only lowers the success probability, the optimal choice is w=0, giving a circuit with n=deg(f) qubits, no ancillas for degree at least 2, a
Load-bearing premise
The load-bearing premise is that every valid factory can be rearranged into a single fixed unitary followed by measuring all non-output qubits and accepting only the all-zero pattern; if adaptive mid-circuit measurements, feed-forward, or postselection on other outcome patterns could do the job with fewer than deg(f) copies of |z>, the claimed optimality would collapse.
Editorial extensions
If this is right
- Every rational function f with deg(f) ≥ 2 can be implemented with exactly deg(f) quantum coins and no auxiliary qubits; degree-one functions consume at most one auxiliary qubit.
- The success probability formula (Eq. 26) gives the exact maximum for a fixed qubit budget, so no circuit using the same number of qubits can exceed it.
- Using more than deg(f) copies of |z> can raise the success probability for particular inputs or state ensembles, and the optimal number of qubits depends on the function and on the input distribution.
- Multivariate rational functions of k biases are simulable with per-variable qubit counts equal to their partial degrees, and one auxiliary qubit is needed only when every partial degree is 1.
- Any pair of simulable functions can be combined in a single multifunctional circuit; if the pair satisfies the compatibility conditions (29)–(30), the primary function's success probability is preserved, as when the product operation is added to the sum operation.
Reading between the lines
- The degree-equals-qubit-count result suggests that the minimal circuit complexity is essentially the degree of a rational map on the Riemann sphere; one might therefore compare quantum factory complexity directly with classical Bernoulli factories, which are restricted to rational functions with no zeros or poles on [0,1].
- The compatibility conditions define a partial ordering on rational functions: g1 is compatible with g0 when extra unitary rows can be made orthogonal to g0's rows without touching g0's success probability. A useful next step would be to characterise maximal sets of pairwise compatible functions and schedule many factories in a single circuit.
- The lower bound depends on accepting only the all-zero outcome pattern. A testable extension is to search systematically for adaptive or multi-outcome-postselection factories that simulate degree-d functions with fewer than d copies of |z>; finding one would sharpen the boundary of the canonical-form argument.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This paper studies the Quantum-to-Quantum Bernoulli Factory (QQBF). It claims that any QQBF can be represented by a canonical circuit with n copies of the input state |z⟩, a fixed unitary, computational-basis measurements, and acceptance only when all non-output qubits read 0; from this it derives a lower bound n ≥ deg(f) on the number of required |z⟩ copies. The paper then constructs, for every rational function f = P/Q, a circuit using n = deg(f) copies and no ancillas for n ≥ 2 (with at most one ancilla for degree 1), and gives a closed-form expression for the maximal success probability (Eq. 26). It also formalizes two extensions: multivariate QQBFs and multifunctional QQBFs, with applications to sum and product operations.
Significance. If the main claims hold, the paper resolves the qubit-resource complexity of QQBFs, provides an explicit circuit construction that saturates the degree lower bound, and gives a closed-form success probability. The constructive proof is self-contained and does not merely import the rational-function characterization from prior work; it also provides a unitary-completion algorithm. The multivariate and multifunctional extensions are natural and potentially useful for quantum algorithms and experimental implementations. These are substantial contributions. The main caveat is that the lower-bound proof relies on an incompletely justified canonical-form reduction, and some technical details in the explicit formulas and multifunctional characterization need clarification.
major comments (4)
- [Sec. III.A, Fig. 1, Eq. (8)] The reduction to a single accepted outcome (all non-output qubits read 0) is asserted via the deferred measurement principle. Deferred measurement preserves the number n of |z⟩ copies and moves measurements to the end, but it does not imply that a protocol with several accepted outcome strings can be rewritten as one accepting only the all-zero string. Because a unitary is injective, distinct post-measurement ancilla states generally cannot be mapped to a single |0...0⟩ while keeping the output factorized. Since the lower bound n ≥ deg(f) is derived only for the all-zero branch, the proof of the lower bound is incomplete as written. A repair is available: for a fixed accepted measurement record, the unnormalized output amplitudes are degree-≤n polynomials in z; any branch with nonzero success probability succeeds on an open dense set, so its output ratio is a rational P/Q with deg ≤ n, a
- [Sec. III.C, Eqs. (24)-(26)] The text claims to maximize the success probability for a fixed number of qubits n and treats |w|^2 as the only free parameter. But for n > deg(f), the polynomials P and Q representing f are not unique: multiplying both by any polynomial R of degree at most n−deg(f) leaves f unchanged but changes a, b, c, and the numerator in Eq. (25). Thus Eq. (26) is not the maximum over all implementations with n qubits unless one explicitly restricts to the canonical choice deg(P), deg(Q) ≤ n with no common factor. The central tightness claim for n = deg(f) is unaffected, but the stated generality of the optimization is not proven.
- [Sec. III.B, Eqs. (21)-(23)] The closed-form solution for y contains the factor c/|c|, so it is undefined when c = 0. The paper then uses c = 0 examples (Sec. VI, product and sum operations) and claims the construction is general. A separate treatment of the c = 0 case is needed, e.g., choosing y = e^{iφ} sqrt((l − |w|^2 − a + b)/2) with φ fixed by Eq. (17). Without this, the explicit algorithm does not cover all rational functions as stated.
- [Sec. V, Eqs. (29)-(30); Supplementary V] The proof that Eqs. (29)-(30) are necessary and sufficient for compatibility is sketched rather than completed. The text says the conditions allow deriving a1 and a3, and that orthonormality allows always deriving a2, a4, H, but it does not show explicitly that the remaining orthonormality equations are satisfied, nor that setting a5 = 0 yields the maximum probability. In addition, Supplementary V's unitary-dilation construction appears to give a multifunctional circuit for any two functions, which is a different existential claim; the relationship between that construction and the compatibility conditions should be clarified.
minor comments (4)
- [Supplementary V, Eq. (47)] The definition of M contains a duplicated term |10⟩⟨r| and |10⟩⟨s|; the third term should presumably be |01⟩⟨r| (with the four basis states |00⟩, |10⟩, |01⟩, |11⟩ corresponding to the two output cases and the herald q2). Please correct this typo, as it affects the subsequent dilation argument.
- [Sec. II, Eq. (4)] The notation |s_n_j⟩ is used for symmetric states, but it is not explicitly defined whether j counts zeros or ones in the computational basis. Please define it precisely, as the binomial coefficients in Eq. (4) depend on this convention.
- [Throughout] There are several grammatical and typographical errors: 'In this contest' should be 'In this context' (Sec. II), 'desidered' should be 'desired' (Sec. VII), and 'simulatable' and 'simulable' are used inconsistently. A proofreading pass is recommended.
- [Sec. III.C, Eqs. (27)-(28)] The derivation of the averaged success probabilities over the uniform Bloch-sphere and covariant-state ensembles is not shown. Since these formulas are used to draw conclusions about optimal n, please provide the integration details or a reference where the measures are defined.
Circularity Check
No significant circularity — rational-function characterization and qubit-count bounds are re-derived from unitary linearity; self-citations are not load-bearing.
full rationale
The derivation chain is self-contained. In Sec. III.A the input state is expanded in monomials (Eq. 4); linearity of U forces every computational-basis amplitude to be a polynomial of degree at most n (Eqs. 5–6); postselecting on all-zero ancillas gives an output ratio P/Q with degree at most n (Eq. 7), yielding the lower bound n ≥ deg(f) (Eq. 8). This reconstructs the rational-function characterization rather than importing it from Ref. [22]. The converse is constructive: for any P,Q the paper defines coefficients K,x,y,w satisfying the orthonormality conditions for two rows of a unitary (Eqs. 10–23), and the dimension count 2^n−n−1>0 for n≥2 supplies the needed orthogonal vector |θ0⟩, giving m=0. The success probability (Eqs. 24–26) follows from those normalizations and the maximization over the single free parameter w. No parameter is fitted to a data subset, and no "prediction" is an input renamed. The only self-citations, Refs. [25,26], are used for the |z⟩ parametrization and experimental context; the central theorem is re-proved in the paper. One genuine gap exists, but it is not circular: the "without loss of generality" reduction in Sec. III.A to a single all-zero acceptance branch is asserted via deferred measurement and is not separately proven for protocols that could merge multiple heralded branches. That is a completeness or correctness concern, not an equivalence of outputs to inputs, so it does not raise the circularity score.
Assumptions & free parameters
free parameters (1)
- w (coefficient of |θ1> in |v1>) =
0 (optimal)
assumptions (5)
- domain assumption Deferred measurement principle: all measurements can be postponed to the end and all non-output outcomes relabeled as 0.
- standard math Stereographic parameterization |z> = (z|0>+|1>)/sqrt(1+|z|^2) describes all pure qubit states up to global phase.
- standard math The n-qubit symmetric states |s^n_j> form an orthonormal set.
- standard math Finite-dimensional unitary dilation theorem: any contraction can be embedded as a block of a unitary.
- standard math Householder reflection algorithm completes two orthonormal vectors to a unitary basis.
Cite this review
Pith. "Pith review of Complexity and multi-functional variants of the Quantum-to-Quantum Bernoulli Factories." pith.science (2026). https://pith.science/paper/CD5I426C
@misc{pith2026251210810,
author = {Pith},
title = {Pith review of: Complexity and multi-functional variants of the Quantum-to-Quantum Bernoulli Factories},
year = {2026},
howpublished = {\url{https://pith.science/paper/CD5I426C}},
note = {Machine review of arXiv:2512.10810}
}
read the original abstract
A Bernoulli factory is a model for randomness manipulation that transforms an initial Bernoulli random variable into another Bernoulli variable by applying a predetermined function relating the output bias to the input one. In literature, quantum-to-quantum Bernoulli factory schemes have been proposed, which encode both the input and output variables using qubit amplitudes. This fundamental concept can serve as a subroutine for quantum algorithms that involve Bayesian inference and Monte Carlo methods, or that require data encryption, like in blind quantum computation. In this work, we present a characterisation of the complexity of the quantum-to-quantum Bernoulli factory by providing a lower bound on the required number of qubits needed to implement the protocol, an upper bound on the success probability and the quantum circuit that saturates the bounds. We also formalise and analyse two different variants of the original problem that address the possibility of increasing the number of input biases or the number of functions implemented by the quantum-to-quantum Bernoulli factory. The obtained results can be used as a framework for randomness manipulation via such an approach.
Figures
Figures from the paper (2 more)
Reference graph
Works this paper leans on
-
[22]
Mendo, An asymptotically optimal bernoulli factory for cer- tain functions that can be expressed as power series, Stochastic Processes and their Applications 129, 4366–4384 (2019)
L. Mendo, An asymptotically optimal bernoulli factory for cer- tain functions that can be expressed as power series, Stochastic Processes and their Applications 129, 4366–4384 (2019)
2019
-
[1]
First, the input state, which is composed of n copies of the state |z⟩ and some ancillary qubits in known pure states
Since the hidden information in a Bernoulli factory circuit is provided only by the unknown state |z⟩ we can divide the circuit into three principal components. First, the input state, which is composed of n copies of the state |z⟩ and some ancillary qubits in known pure states. Then, a uni- tary evolution and a measurement are performed, such that at the...
-
[2]
(47) which is lower than the optimal circuit found previously
However, the total success probability will be Pr+(z1,z 2) = |z1 +z2|2 + 1 3(1 + |z1|2)(1 + |z2|2) . (47) which is lower than the optimal circuit found previously. C. Compatibility between the sum and the product operation In this section, we identify a multifunctional QQBF that implements both the product and the sum operation, testing if the sum operati...
-
[3]
Analogously to the approach of Sec
As before, this circuit is the most general one implementing a Multivariate Quantum to Quantum Bernoulli factory. Analogously to the approach of Sec. III, it can be shown that the set of constructible functions g(z1,...z k) is a sub- set of the set of multivariate complex rational functions Rat(z1,...,z k). Moreover, adapting the previous construc- tion m...
-
[4]
Given two simulable functions, it is always possible to construct a Multifunctional Quantum-to-quantum Bernoulli factory (see Supplementary Materials V)
The circuit is identical to the one implementing the QQBF with the difference that the measurement of the second qubit q2 indicates which functiong0(z) org1(z) is implemented by the circuit. Given two simulable functions, it is always possible to construct a Multifunctional Quantum-to-quantum Bernoulli factory (see Supplementary Materials V). However, the...
-
[5]
To ensure that the four vec- tors represent four rows of a unitary transformation they must be orthonormal between each other as ⟨vi|vj⟩ = δi,j
The other two vectors can be expressed in the general forms as: |v2⟩ = n∑ j=0 H r∗ j√ ( n j ) ⏐ ⏐sn j ⟩ +Ha 1 |θ0⟩ +Ha 2 |θ1⟩, (33) and |v3⟩ = n∑ j=0 H s∗ j √ ( n j ) ⏐ ⏐sn j ⟩ +Ha 3 |θ0⟩ +Ha 4 |θ1⟩ + +Ha 5 |θ2⟩, (34) where H is the normalization factor and a1, a2, a3, a4 and a5 are free parameters. To ensure that the four vec- tors represent four rows of...
-
[6]
Previously, all the measures had to return the outcome 0
The difference is at the measurement stage. Previously, all the measures had to return the outcome 0. In this case, if the measure q2 returns 0, the circuit im- plements the function g0(z), while if the outcome is 1 1 the circuit implements the function g1(z). As before, this circuit is the most general one implementing a Multifunctional Quantum to Quantu...
-
[7]
F. B. Gonc ¸alves, K. Łatuszy ´nski, and G. O. Roberts, Exact monte carlo likelihood-based inference for jump-diffusion pro- cesses, Journal of the Royal Statistical Society Series B: Statis- tical Methodology 85, 732–756 (2023)
2023
Show all 42 references
-
[8]
on the required number of bits is tight and no ancillary qubits are required (m = 0) except for the particular case ofn = 1. To accomplish this, for each tuple (P (z),Q (z)) ∈ P (C) × P(C) we construct a circuit implementing a Bernoulli factory with an associated function f (z...
-
[9]
M. S. Keane and G. L. O’Brien, A bernoulli factory, ACM Transactions on Modeling and Computer Simulation 4, 213–219 (1994)
1994
-
[10]
Paes Leme and J
R. Paes Leme and J. Schneider, Multiparameter bernoulli fac- tories, The Annals of Applied Probability 33, 3987 (2023)
2023
-
[11]
Niazadeh, R
R. Niazadeh, R. P . Leme, and J. Schneider, Bernoulli factories for flow-based polytopes, SIAM Journal on Discrete Mathemat- ics 38, 726–742 (2024)
2024
-
[12]
Niazadeh, R
R. Niazadeh, R. Paes Leme, and J. Schneider, Combinatorial bernoulli factories, Bernoulli 29, 1246 (2023)
2023
-
[13]
J. M. Flegal and R. Herbei, Exact sampling for intractable prob- ability distributions via a bernoulli factory, Electronic Journal of Statistics 6, 10 (2012)
2012
-
[14]
V ats, F
D. V ats, F. B. Gonc ¸alves, K. Łatuszy ´nski, and G. O. Roberts, Efficient bernoulli factory markov chain monte carlo for in- tractable posteriors, Biometrika 109, 369–385 (2021)
2021
-
[15]
Herbei and L
R. Herbei and L. M. Berliner, Estimating ocean circulation: An mcmc approach with approximated likelihoods via the bernoulli factory, Journal of the American Statistical Association 109, 944–954 (2014)
2014
-
[16]
Koskela, K
J. Koskela, K. Łatuszy ´nski, and D. Span `o, Bernoulli factories and duality in wright-fisher and allen-cahn models of popula- tion genetics (2023), arXiv:2306.03539 [math]
2023 arXiv
-
[17]
Morina, K
G. Morina, K. Łatuszy ´nski, P . Nayar, and A. Wendland, From the bernoulli factory to a dice enterprise via perfect sampling of markov chains, The Annals of Applied Probability 32, 327 (2022)
2022
-
[18]
Goyal and K
V . Goyal and K. Sigman, On simulating a class of bernstein polynomials, ACM Transactions on Modeling and Computer Simulation 22, 1–5 (2012)
2012
-
[19]
Mossel and Y
E. Mossel and Y . Peres, New coins from old: Computing with unknown bias, Combinatorica 25, 707–724 (2005)
2005
-
[20]
Łatuszy ´nski, I
K. Łatuszy ´nski, I. Kosmidis, O. Papaspiliopoulos, and G. O. Roberts, Simulating events of unknown probabilities via re- verse time martingales, Random Structures & Algorithms 38, 441–452 (2011). 10
2011
-
[21]
Nacu and Y
S. Nacu and Y . Peres, Fast simulation of new coins from old, The Annals of Applied Probability 15, 93 (2005)
2005
-
[23]
Flajolet, M
P . Flajolet, M. Pelletier, and M. Soria, On buffon machines and numbers, in Proceedings of the Twenty-Second Annual ACM- SIAM Symposium on Discrete Algorithms (Society for Indus- trial and Applied Mathematics, 2011)
2011
-
[24]
Holtz, F
O. Holtz, F. Nazarov, and Y . Peres, New coins from old, smoothly, Constructive Approximation 33, 331–363 (2010)
2010
-
[25]
H. Dale, D. Jennings, and T. Rudolph, Provable quantum ad- vantage in randomness processing, Nature Communications 6 (2015)
2015
-
[26]
Dale, Quantum coins and quantum sampling , Ph.D
H. Dale, Quantum coins and quantum sampling , Ph.D. thesis , Imperial College London (2016)
2016
-
[27]
R. B. Patel, T. Rudolph, and G. J. Pryde, An experimental quan- tum bernoulli factory, Science Advances 5, eaau6668 (2019)
2019
-
[28]
Y uan, K
X. Y uan, K. Liu, Y . Xu, W. Wang, Y . Ma, F. Zhang, Z. Y an, R. Vijay, L. Sun, and X. Ma, Experimental quantum random- ness processing using superconducting qubits, Physical Review Letters 117, 010502 (2016)
2016
-
[29]
Jiang, J
J. Jiang, J. Zhang, and X. Sun, Quantum-to-quantum bernoulli factory problem, Physical Review A 97 (2018)
2018
-
[30]
X. Zhan, K. Wang, L. Xiao, Z. Bian, and P . Xue, Experimental demonstration of quantum-to-quantum bernoulli factory, Phys- ical Review A 102, 012605 (2020)
2020
-
[31]
Y . Liu, J. Jiang, P . Zhu, D. Wang, J. Ding, X. Qiang, A. Huang, P . Xu, J. Zhang, G. Tian, X. Fu, M. Deng, C. Wu, X. Sun, X. Y ang, and J. Wu, General quantum bernoulli factory: frame- work analysis and experiments, Quantum Science and Technol- ogy 6, 045025 (2021)
2021
-
[32]
F. Hoch, T. Giordani, L. Castello, G. Carvacho, N. Spagnolo, F. Ceccarelli, C. Pentangelo, S. Piacentini, A. Crespi, R. Osel- lame, E. F. Galv ˜ao, and F. Sciarrino, Modular quantum-to- quantum bernoulli factory in an integrated photonic processor, Nature Photonics 19, 12–19 (2024)
2024
-
[33]
Rodari, F
G. Rodari, F. Hoch, A. Suprano, T. Giordani, E. Negro, G. Carvacho, N. Spagnolo, E. F. Galv ˜ao, and F. Sciarrino, Polarization-encoded photonic quantum-to-quantum bernoulli factory based on a quantum dot source, Science Advances 10, eado6244 (2024)
2024
-
[34]
Kashefi and A
E. Kashefi and A. Pappa, Multiparty delegated quantum com- puting, Cryptography 1, 12 (2017)
2017
-
[35]
J. F. Fitzsimons and E. Kashefi, Unconditionally verifiable blind quantum computation, Physical Review A 96, 012303 (2017)
2017
-
[36]
Polacchi, D
B. Polacchi, D. Leichtle, L. Limongi, G. Carvacho, G. Milani, N. Spagnolo, M. Kaplan, F. Sciarrino, and E. Kashefi, Multi- client distributed blind quantum computation with the qline ar- chitecture, Nature Communications 14, 7743 (2023)
2023
-
[37]
M. A. Nielsen and I. L. Chuang, Quantum Computation and Quantum Information (Cambridge University Press, 2009)
2009
-
[38]
Gurevich and A
Y . Gurevich and A. Blass, Quantum circuits with classical channels and the principle of deferred measurements (2021), arXiv:2107.08324 [quant-ph]
2021 arXiv
-
[39]
T. J. Y oder, Building and bounding quantum bernoulli factories (2015). Supplementary materials: Complexity and multi-functional variants of Quantum-to-Quantum Bernoulli Factories Francesco Hoch, 1 Taira Giordani,1 Gonzalo Carvacho, 1 Nicol`o Spagnolo, 1 and Fabio Sciarrino 1,...
2015
-
[40]
To show that the success probability is not optimal we consider the state |ψT ⟩ after the unitary evolution
It can also be shown that all results for the set of simulatable functions and the number of resources required remain valid. To show that the success probability is not optimal we consider the state |ψT ⟩ after the unitary evolution. For the Schmidt decomposition, we can writ...
-
[41]
W. T. V . William H. Press, Saul A. Teukolsky,Numerical recipes 3rd edition: The art of scientific computing (Cambridge University Press, 2007)
2007
-
[42]
Levy and O
E. Levy and O. M. Shalit, Dilation theory in finite dimensions: The possible, the impossible and the unknown, Rocky Mountain Journal of Mathematics 44, 203 (2014)
2014
Reviewed August 3, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.