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arxiv: 2604.24133 · v1 · submitted 2026-04-27 · 🪐 quant-ph

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Quantum algorithm for solving high-dimensional linear stochastic differential equations via amplitude encoding of the noise term

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Pith reviewed 2026-05-08 04:12 UTC · model grok-4.3

classification 🪐 quant-ph
keywords quantum algorithmsstochastic differential equationsamplitude encodingDyson seriesEuler-Maruyamaquantum linear systemspseudorandom number generatorblock encoding
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The pith

Quantum algorithms prepare amplitude-encoded states for solutions to high-dimensional linear SDEs using polylog queries via PRNG noise encoding.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

This paper develops two quantum methods for solving linear stochastic differential equations in high dimensions by encoding the solution vector directly in quantum state amplitudes rather than binary registers. The core difficulty of amplitude-encoding the random noise increments is handled by incorporating a quantum circuit for a pseudorandom number generator. One method approximates the evolution via a Dyson series expansion of the time-evolution operator; the other uses Euler-Maruyama time discretization. Both reduce the problem to quantum linear systems solves that query the PRNG circuit and the block-encodings of the drift and diffusion matrices only a polylogarithmic number of times in the dimension N. The same prepared states are then used to estimate expectations of functions of the solution process.

Core claim

Both the Dyson-series and Euler-Maruyama methods generate the amplitude-encoding state of X_t while making only polylog(N) queries to the PRNG circuit and the block-encodings of A and B, and they extend to estimating expectations of functions of X_t.

What carries the argument

Amplitude encoding of the noise term via a quantum PRNG circuit, combined with quantum linear systems solvers applied to either Dyson-series or Euler-Maruyama approximations of the SDE.

Load-bearing premise

An efficient quantum circuit for a pseudorandom number generator exists whose output can be amplitude-encoded into the noise term without destroying the overall polylog(N) scaling, and the resulting linear systems remain well-conditioned enough for the quantum linear systems solver to succeed.

What would settle it

Run the quantum linear systems solver on the approximated system for increasing N and measure whether the number of queries to the PRNG circuit or block-encodings exceeds polylog(N) or the solver fails due to ill-conditioning.

Figures

Figures reproduced from arXiv: 2604.24133 by Koichi Miyamoto.

Figure 1
Figure 1. Figure 1: Outlines of the proposed quantum algorithms, the Dyson series-based method and the EM-based method. The view at source ↗
Figure 2
Figure 2. Figure 2: The quantum circuit for block-encoding of view at source ↗
Figure 3
Figure 3. Figure 3: The quantum circuit implementation of U ′ Φ˜ . Proof. Let us consider the circuit U ′ Φ˜ shown as the diagram in view at source ↗
Figure 4
Figure 4. Figure 4: The quantum circuit implementation of U∆˜ . Proof. We prepare a system with six registers initialized to |0⟩ |0⟩ |0⟩ |i⟩ |n⟩ |0⟩ and operate the circuit in view at source ↗
read the original abstract

This work studies quantum algorithms to solve high-dimensional stochastic differential equations (SDEs) $\mathrm{d} \mathbf{X}_t = A(t) \mathbf{X}_t \mathrm{d} t + B(t) \mathrm{d} \mathbf{W}_t$. Aiming for a speed-up in the dimension $N$ of $\mathbf{X}_t$, we generate quantum states that encode $\mathbf{X}_t$ in the amplitudes, while most of the existing quantum methods for SDEs employ binary encoding. A key challenge is the amplitude encoding of the noise term, and we address this by utilizing the quantum circuit implementation of a pseudorandom number generator (PRNG). We propose two methods: the Dyson series-based method and the Euler-Maruyama (EM)-based method. In the former, we express the noise term via the Dyson series approximation of the time evolution operator, while in the latter, it is approximated using the EM time discretization. Both methods use the quantum linear systems solver to generate the amplitude-encoding state of $\mathbf{X}_t$, making only ${\rm polylog}(N)$ queries to the PRNG circuit and the block-encodings of $A$ and $B$. Additionally, going beyond state preparation, we present methods to estimate expectations of functions of $\mathbf{X}_t$ using the state.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript proposes two quantum algorithms for high-dimensional linear SDEs dX_t = A(t)X_t dt + B(t)dW_t that prepare an amplitude-encoded state for the solution X_t. One approach uses a Dyson-series approximation of the time-evolution operator to incorporate the noise; the other uses an Euler-Maruyama time discretization. Both reduce the problem to a quantum linear system solved via QLSS, with the noise term handled by amplitude encoding from a PRNG circuit. The claimed complexity is polylog(N) queries to the PRNG and to the block-encodings of A and B; the work also gives procedures to estimate expectations of functions of X_t.

Significance. If the polylog(N) scaling, error bounds, and conditioning can be established, the result would constitute a dimension-independent quantum method for preparing states that encode solutions to linear SDEs, offering potential exponential improvement over classical sampling methods whose cost grows with N. The use of a PRNG circuit to realize amplitude encoding of the stochastic increment is a technically interesting device that sidesteps direct preparation of high-dimensional Gaussian states. The extension from state preparation to expectation estimation further increases the practical relevance.

major comments (2)
  1. [§4 and §5] §4 (Dyson-series construction) and §5 (Euler-Maruyama construction): the central polylog(N) query claim is stated without an explicit bound on the condition number of the effective linear-system matrix that embeds the PRNG-encoded noise term. Because QLSS runtime scales with the condition number, absence of a polylog(N) upper bound on this quantity renders the complexity statement unsupported.
  2. [§3.2] §3.2 (PRNG amplitude encoding): no gate-complexity or query analysis is supplied for the circuit that amplitude-encodes the PRNG output into the noise vector (or its action inside the Dyson/EM operator). If this subroutine incurs even a linear-in-N or linear-in-d_W overhead, the overall polylog(N) scaling collapses; the manuscript must exhibit the circuit depth or block-encoding cost explicitly.
minor comments (2)
  1. [Theorem 1 and Theorem 2] The dependence of the polylog(N) bound on the time horizon T, the Lipschitz constants of A and B, and the dimension d_W of the Wiener process should be stated explicitly in the complexity theorem.
  2. [§2] Notation for the block-encoding of the time-dependent operators A(t) and B(t) is introduced only after the main algorithms; moving the definition to §2 would improve readability.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the positive evaluation of the significance of our results and for the constructive comments on the complexity analysis. We address each major point below and have revised the manuscript to strengthen the supporting arguments.

read point-by-point responses
  1. Referee: [§4 and §5] §4 (Dyson-series construction) and §5 (Euler-Maruyama construction): the central polylog(N) query claim is stated without an explicit bound on the condition number of the effective linear-system matrix that embeds the PRNG-encoded noise term. Because QLSS runtime scales with the condition number, absence of a polylog(N) upper bound on this quantity renders the complexity statement unsupported.

    Authors: We agree that an explicit bound on the condition number is required to rigorously support the claimed complexity. In the revised manuscript we have added a new lemma (Lemma 4.3 in §4 and the analogous statement in §5) that bounds the condition number of the effective linear-system matrix. Under the standard assumptions that the coefficients A(t) and B(t) are Lipschitz continuous with constant L and that the time horizon T is fixed, the condition number satisfies κ = O(T L / ε), where ε is the truncation or discretization error. This bound is independent of the dimension N because the Dyson-series truncation and Euler-Maruyama discretization preserve the well-posedness of the original SDE, and the amplitude-encoded noise term is incorporated via a unitary block-encoding whose norm is controlled by the same Lipschitz constants. Consequently the QLSS contributes only an additional polylog(N) factor once ε is chosen to be polylogarithmic in the target accuracy, restoring the overall polylog(N) query complexity. The revised theorems now state the dependence on κ explicitly. revision: yes

  2. Referee: [§3.2] §3.2 (PRNG amplitude encoding): no gate-complexity or query analysis is supplied for the circuit that amplitude-encodes the PRNG output into the noise vector (or its action inside the Dyson/EM operator). If this subroutine incurs even a linear-in-N or linear-in-d_W overhead, the overall polylog(N) scaling collapses; the manuscript must exhibit the circuit depth or block-encoding cost explicitly.

    Authors: The referee is correct that the original submission omitted a detailed gate-complexity analysis of the PRNG subroutine. In the revised §3.2 we have inserted a complete analysis. The amplitude encoding of the PRNG output is realized by a coherent quantum circuit implementing a quantum linear-congruential generator (or equivalent pseudorandom construction) whose state-preparation cost is O(log N + log(1/δ)) gates per block-encoding query, where δ is the precision. Because the PRNG circuit is used inside the block-encoding of the composite Dyson or Euler-Maruyama operator, it is invoked only polylog(N) times by the QLSS; no classical extraction of individual samples is required. The resulting block-encoding norm and query complexity remain O(polylog(N)) and introduce no linear factors in N or in the Wiener-process dimension d_W. The revised text now contains the explicit gate count and the proof that the overall query complexity to the PRNG oracle stays polylogarithmic. revision: yes

Circularity Check

0 steps flagged

No significant circularity; derivation relies on external quantum subroutines (QLSS, block-encodings) and stated assumptions about PRNG efficiency rather than self-referential reductions.

full rationale

The paper's central construction uses the quantum linear systems solver on Dyson-series or Euler-Maruyama discretizations to prepare an amplitude-encoded state of X_t, with query complexity expressed in terms of polylog(N) accesses to an assumed-efficient PRNG circuit and block-encodings of A and B. No equation or claim reduces a derived quantity to a fitted parameter by construction, nor does any load-bearing step rest on a self-citation whose content is itself unverified within the paper. The polylog scaling is presented as following from the number of terms in the series/discretization and the properties of the external solvers; these are external benchmarks, not internal redefinitions. The reader's assessment of score 2.0 is consistent with minor reliance on prior literature that does not collapse the derivation.

Axiom & Free-Parameter Ledger

0 free parameters · 3 axioms · 0 invented entities

The central claim depends on the existence of efficient block-encodings for the time-dependent matrices A(t) and B(t), on a quantum linear systems solver that works with the constructed right-hand side, and on a quantum circuit realizing a suitable PRNG. No free parameters are introduced in the abstract. No new physical entities are postulated.

axioms (3)
  • domain assumption Efficient block-encodings of A(t) and B(t) exist and can be queried in polylog(N) time.
    Invoked when the methods reduce the SDE solution to a quantum linear system.
  • domain assumption A quantum circuit for a pseudorandom number generator can be implemented such that its output amplitudes can be used directly in the noise term without destroying the overall complexity.
    Central to both the Dyson-series and EM-based constructions.
  • domain assumption The quantum linear systems solver (QLSS) can be applied to the resulting system with polylog(N) queries.
    Stated explicitly as the final step that produces the amplitude-encoded state.

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