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arxiv: 2506.13534 · v3 · pith:3HULVF5Snew · submitted 2025-06-16 · 🪐 quant-ph

Quantum algorithm for solving generalized eigenvalue problems with application to the Schr\"odinger equation

Pith reviewed 2026-05-22 13:28 UTC · model grok-4.3

classification 🪐 quant-ph
keywords quantum algorithmgeneralized eigenvalue problemSchrödinger equationpseudospectral collocationsingular value spectrumamplitude amplificationphase estimationquantum chemistry
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The pith

A quantum algorithm estimates eigenvalues by locating minima in the singular value spectrum of parameterized matrix families.

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

The paper develops a quantum method to solve generalized eigenvalue problems by scanning a parameter and using amplitude amplification with phase estimation to find where the smallest singular value reaches zero. This is applied to formulate a quantum version of the pseudospectral collocation method for the Schrödinger equation. The approach avoids explicit matrix inversion and its associated instability. Resource estimates show scaling up to square root of the problem size N for well-behaved potentials, compared to linear scaling classically. The method targets computation of multiple eigenvalues in dense spectra for molecular and solid-state systems.

Core claim

The authors claim that eigenvalues of a parameterized matrix family can be identified by using quantum amplitude amplification and phase estimation to minimize the singular value spectrum, with the minimum reaching zero precisely at eigenvalue locations. Applied to the Schrödinger equation, this yields a quantum collocation algorithm whose fault-tolerant resource costs scale as the square root of N rather than linearly for certain potentials, while bypassing the numerical issues of inverting the overlap matrix in the generalized eigenvalue problem.

What carries the argument

Quantum amplitude amplification combined with phase estimation applied to the singular value spectrum of a parameterized matrix family to detect exact zero minima.

If this is right

  • The quantum collocation method achieves up to square-root scaling in problem size N for well-behaved potentials.
  • The algorithm avoids numerical instability by scanning the parameter instead of inverting a matrix.
  • It supports extraction of multiple excited-state energies in systems with dense spectra.
  • The approach may apply to high-dimensional molecular simulations in photodynamics and quasi-continuum regimes.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The same singular-value minimization strategy could extend to other parameterized linear algebra tasks in quantum chemistry beyond the Schrödinger equation.
  • Avoiding inversion may reduce error accumulation when the overlap matrix is ill-conditioned.
  • Small-scale numerical tests on toy molecular potentials could directly check whether the quantum overhead remains favorable.

Load-bearing premise

The parameterized matrix family arising from the pseudospectral collocation method can be prepared and manipulated on a quantum computer so that amplitude amplification and phase estimation locate the singular value minima accurately and with acceptable overhead.

What would settle it

Implementation on a small test instance of the Schrödinger equation where the observed runtime exceeds the claimed square-root scaling or the recovered eigenvalues deviate from known classical values by more than the expected precision.

Figures

Figures reproduced from arXiv: 2506.13534 by Emil Zak, Grzegorz Rajchel-Mieldzio\'c, Szymon Pli\'s.

Figure 1
Figure 1. Figure 1: A sketch of the generic landscape of the eigenval [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: A quantum circuit realizing the search for eigenvalues of a matrix that are close to the target value [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Comparison of the matrix inverse method described in Sec. [PITH_FULL_IMAGE:figures/full_fig_p010_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: The same setup as in Fig [PITH_FULL_IMAGE:figures/full_fig_p010_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Comparison of the matrix inverse method described in Sec. [PITH_FULL_IMAGE:figures/full_fig_p011_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Sketch of the quantum algorithm for searching energies for a given collocation Hamiltonian defined by the collocation [PITH_FULL_IMAGE:figures/full_fig_p011_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: We extend the validity of the cutoff [PITH_FULL_IMAGE:figures/full_fig_p019_7.png] view at source ↗
read the original abstract

Accurate computation of multiple eigenvalues of quantum Hamiltonians is essential in quantum chemistry, materials science, and molecular spectroscopy. Estimating excited-state energies is challenging for classical algorithms due to exponential scaling with system size, posing an even harder problem than ground-state calculations. We present a quantum algorithm for estimating eigenvalues and singular values of parameterized matrix families, including solving generalized eigenvalue problems that frequently arise in quantum simulations. Our method uses quantum amplitude amplification and phase estimation to identify matrix eigenvalues by locating minima in the singular value spectrum. We demonstrate our algorithm by proposing a quantum-computing formulation of the pseudospectral collocation method for the Schr\"odinger equation. We estimate fault-tolerant quantum resource requirements for the quantum collocation method, showing favorable scaling in the size of the problem $N$ (up to $\widetilde{\mathcal{O}}(\sqrt{N})$) compared to classical implementations with $\widetilde{\mathcal{O}}(N)$, for certain well-behaved potentials. Additionally, unlike the standard collocation method, which results in a generalized eigenvalue problem requiring matrix inversion, our algorithm circumvents the associated numerical instability by scanning a parameterized matrix family and detecting eigenvalues through singular value minimization. This approach is particularly effective when multiple eigenvalues are needed or when the generalized eigenvalue problem involves a high condition number. In the fault-tolerant era, our method may thus be useful for simulating high-dimensional molecular systems with dense spectra involving highly excited states, such as those encountered in molecular photodynamics or quasi-continuum regimes in many-body and solid-state systems.

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

1 major / 1 minor

Summary. The paper introduces a quantum algorithm for solving generalized eigenvalue problems by locating minima in the singular value spectrum of parameterized matrix families using amplitude amplification and phase estimation. It applies this framework to a quantum version of the pseudospectral collocation method for the Schrödinger equation, estimating fault-tolerant resource requirements and claiming up to ~O(sqrt(N)) scaling for well-behaved potentials compared to classical ~O(N). The method avoids matrix inversion to improve numerical stability for multiple eigenvalues.

Significance. If the central claims regarding efficient quantum implementation hold, this work could provide a useful tool for computing excited states in high-dimensional quantum systems where classical methods struggle with dense spectra. The avoidance of generalized eigenvalue problem instabilities is a notable strength. Credit is due for framing the approach around standard quantum primitives and providing resource estimates, though the practical advantage hinges on operator implementation costs.

major comments (1)
  1. [Abstract and quantum collocation formulation] The favorable scaling claim of ~O(sqrt(N)) relies on the assumption that the unitary or block-encoding for the parameterized matrix family (incorporating differential operators and potential multiplication) can be implemented with polylog(N) gate cost. For the pseudospectral collocation on an N-point grid, the multiplication by a general potential typically incurs costs that may not be polylogarithmic without additional structure, which could undermine the reported advantage over classical O(N). A detailed complexity analysis or explicit circuit construction for the operator is required to substantiate this.
minor comments (1)
  1. [Abstract] The abstract mentions 'estimating fault-tolerant quantum resource requirements' but does not specify the model (e.g., gate count, depth, or number of qubits) or provide concrete numbers; including a brief summary of these estimates would improve clarity.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their careful reading of the manuscript and for the constructive comment on the complexity assumptions underlying our scaling claims. We address this point directly below and have revised the manuscript to strengthen the discussion of operator implementations and resource estimates.

read point-by-point responses
  1. Referee: [Abstract and quantum collocation formulation] The favorable scaling claim of ~O(sqrt(N)) relies on the assumption that the unitary or block-encoding for the parameterized matrix family (incorporating differential operators and potential multiplication) can be implemented with polylog(N) gate cost. For the pseudospectral collocation on an N-point grid, the multiplication by a general potential typically incurs costs that may not be polylogarithmic without additional structure, which could undermine the reported advantage over classical O(N). A detailed complexity analysis or explicit circuit construction for the operator is required to substantiate this.

    Authors: We agree that the reported scaling advantage is conditional on efficient implementation of the block-encoding or unitary for the parameterized family. The manuscript explicitly qualifies the O(sqrt(N)) claim as holding 'for certain well-behaved potentials,' where the potential multiplication operator admits a polylog(N) implementation (e.g., via low-degree polynomial approximations, smooth functions amenable to quantum arithmetic, or structured potentials such as harmonic oscillators). The differential operators in the pseudospectral collocation are handled via quantum Fourier transforms or equivalent techniques that are known to be polylogarithmic. We have revised the manuscript to include an expanded complexity analysis section that spells out these assumptions, cites standard results on efficient quantum implementations of multiplication and differentiation operators on grids, and clarifies the conditions under which the advantage holds versus cases with unstructured potentials. While a fully explicit gate-by-gate circuit for an arbitrary potential is outside the scope of the present work, the revised text outlines the high-level construction and resource counting used in our fault-tolerant estimates. revision: yes

Circularity Check

0 steps flagged

No circularity in the quantum algorithm derivation

full rationale

The paper constructs a quantum algorithm for generalized eigenvalue problems by directly applying standard primitives (amplitude amplification and phase estimation) to locate singular-value minima in a parameterized matrix family obtained from the pseudospectral collocation discretization of the Schrödinger equation. Resource estimates yielding O(sqrt(N)) scaling for well-behaved potentials follow from the assumed efficient block-encoding of the differential and multiplication operators, which is an external implementability assumption rather than a self-referential definition or fitted parameter. No load-bearing step reduces by construction to the inputs via self-definition, renaming of known results, or self-citation chains; the formulation is self-contained against external quantum and numerical benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim rests on standard quantum computing primitives and the assumption of efficient quantum implementation of the collocation operator family; no free parameters or invented entities are introduced in the abstract.

axioms (1)
  • domain assumption The parameterized matrix family corresponding to the pseudospectral collocation discretization can be efficiently implemented on a quantum computer.
    This is required for the claimed resource scaling to hold when using amplitude amplification and phase estimation.

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Reference graph

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