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arxiv: 2512.18249 · v2 · submitted 2025-12-20 · 🪐 quant-ph

Hermitian Matrix Function Synthesis without Block-Encoding

Pith reviewed 2026-05-16 20:20 UTC · model grok-4.3

classification 🪐 quant-ph
keywords Hermitian matricespolynomial functionsquantum signal processingblock-encodingquantum algorithmsGQSPHamiltonian simulation
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The pith

A method using generalized quantum signal processing implements arbitrary polynomials of Hermitian matrices without block-encoding.

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

The paper shows how to realize polynomial functions on Hermitian matrices by applying the generalized quantum signal processing framework directly. This avoids the need to block-encode the matrix and the associated ancillary qubit overheads and post-selection costs that plague linear combination of unitaries approaches. The resulting success probability remains stable regardless of the polynomial degree. A sympathetic reader would care because this could simplify quantum algorithms for Hamiltonian simulation, linear systems, and state preparation by reducing circuit complexity and resource demands. The approach derives closed-form expressions for symmetric polynomial expansions and uses linear combinations of GQSP circuits.

Core claim

By leveraging the Generalized Quantum Signal Processing (GQSP) framework, arbitrary polynomials of a Hermitian matrix can be implemented through linear combinations of circuits without block-encoding, yielding a stable, degree-independent success probability and closed-form expressions for symmetric expansions.

What carries the argument

Generalized Quantum Signal Processing (GQSP) applied directly to Hermitian matrices, combined with linear combinations of circuits to realize the polynomial transformation.

If this is right

  • Reduces resource overhead by eliminating block-encoding preparation and ancillary qubits.
  • Achieves stable success probability independent of polynomial degree.
  • Opens new pathways for quantum algorithm design in settings where Hermitian operators arise from symmetric combinations of unitaries.
  • Supports applications in Hamiltonian simulation, quantum linear system solving, and high-fidelity state preparation.
  • Derives closed-form expressions for symmetric polynomial expansions.

Where Pith is reading between the lines

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

  • May extend to other matrix functions beyond polynomials if GQSP generalizations exist.
  • Could simplify implementations in quantum machine learning kernels that rely on such functions.
  • Testable by comparing circuit depths and success rates against QSVT-based methods on small Hermitian matrices.
  • Implies potential for lower error rates in noisy intermediate-scale quantum devices due to reduced overhead.

Load-bearing premise

The generalized quantum signal processing framework can be applied directly to Hermitian matrices without introducing new instabilities or additional overheads.

What would settle it

An explicit calculation or simulation showing that the success probability decreases with increasing polynomial degree or that block-encoding is still required for stability.

Figures

Figures reproduced from arXiv: 2512.18249 by Anuradha Mahasinghe, Frederic Cadet, Jingbo Wang, Kaushika De Silva, Peter Chin, Xavier Cadet.

Figure 1
Figure 1. Figure 1: Quantum circuit implementing the polynomial transformation [PITH_FULL_IMAGE:figures/full_fig_p009_1.png] view at source ↗
read the original abstract

Implementing polynomial functions of Hermitian matrices on quantum hardware is a foundational task in quantum computing, critical for accurate Hamiltonian simulation, quantum linear system solving, high-fidelity state preparation, machine learning kernels, and other advanced quantum algorithms. Existing state-of-the-art techniques, including Qubitization, Quantum Singular Value Transformation (QSVT), and Quantum Signal Processing (QSP), rely heavily on block-encoding the Hermitian matrix. These methods are often constrained by the complexity of preparing the block-encoded state, the overhead associated with the required ancillary qubits, or the challenging problem of angle synthesis for the polynomial's phase factors, which limits the achievable circuit depth and overall efficiency. In this work, we propose a novel and resource-efficient approach to implement arbitrary polynomials of a Hermitian matrix by leveraging the Generalized Quantum Signal Processing (GQSP) framework. Our method circumvents the need for block-encoding and avoids the compounding post-selection overheads characteristic of LCU-based constructions, achieving a stable, degree-independent success probability. We derive closed-form expressions for symmetric polynomial expansions and demonstrate how linear combinations of GQSP circuits can realize the desired transformation. This approach reduces resource overhead and opens new pathways for quantum algorithm design for functions of Hermitian matrices, particularly in settings where the Hermitian operator arises naturally from symmetric combinations of unitaries.

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 implementing arbitrary polynomials of Hermitian matrices via the Generalized Quantum Signal Processing (GQSP) framework without block-encoding. It derives closed-form expressions for symmetric polynomials and realizes general polynomials through linear combinations of GQSP circuits, claiming a stable success probability independent of polynomial degree and reduced overhead relative to LCU or standard QSP/QSVT methods.

Significance. If the constructions and probability claims hold, the approach could lower resource costs for Hamiltonian simulation, quantum linear systems, and state preparation by removing block-encoding preparation and ancillary overheads, enabling more efficient circuits in settings where Hermitian operators arise from symmetric unitary combinations.

major comments (2)
  1. [Abstract and the section deriving the linear-combination step] The central claim of degree-independent success probability for linear combinations of GQSP circuits (Abstract) lacks an explicit construction or normalization argument. Preparing a superposition over the individual GQSP circuits to realize the linear combination would bound the post-selection probability by the squared norm of the coefficient vector; for a degree-d polynomial this norm generally varies with d, contradicting the degree-independent claim unless a specific embedding or rescaling is derived.
  2. [Section presenting the closed-form expressions] The closed-form expressions for symmetric polynomial expansions (Abstract) are stated to exist but no explicit formulas, derivation, or verification that they map to the target polynomial without introducing new instabilities are supplied in the provided text; this is load-bearing for the claim that GQSP can be applied directly to Hermitian matrices.
minor comments (2)
  1. [Methods or Results] Add a small-scale numerical example or circuit diagram illustrating the GQSP circuit for a low-degree symmetric polynomial to clarify the construction.
  2. [Introduction] Clarify the precise relationship between GQSP and standard QSP phase-factor synthesis to highlight where the block-encoding avoidance originates.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their careful reading and constructive comments, which help strengthen the presentation of our results. We address each major comment below and confirm that the requested clarifications and explicit derivations will be incorporated in the revised manuscript.

read point-by-point responses
  1. Referee: [Abstract and the section deriving the linear-combination step] The central claim of degree-independent success probability for linear combinations of GQSP circuits (Abstract) lacks an explicit construction or normalization argument. Preparing a superposition over the individual GQSP circuits to realize the linear combination would bound the post-selection probability by the squared norm of the coefficient vector; for a degree-d polynomial this norm generally varies with d, contradicting the degree-independent claim unless a specific embedding or rescaling is derived.

    Authors: We thank the referee for this observation. The linear combination is realized via a rescaled embedding of the coefficient vector that exploits the bounded norm of the symmetric polynomial expansions derived from the GQSP framework; this embedding ensures the squared norm remains bounded by a constant independent of degree d. The post-selection probability is therefore stable. We will add an explicit construction of the embedding together with the normalization argument in the revised manuscript. revision: yes

  2. Referee: [Section presenting the closed-form expressions] The closed-form expressions for symmetric polynomial expansions (Abstract) are stated to exist but no explicit formulas, derivation, or verification that they map to the target polynomial without introducing new instabilities are supplied in the provided text; this is load-bearing for the claim that GQSP can be applied directly to Hermitian matrices.

    Authors: We acknowledge that the initial submission did not include the full explicit formulas and derivation. The closed-form expressions are obtained by expanding the target polynomial in the basis of symmetric Chebyshev polynomials of the first kind and then mapping each term to a GQSP sequence; the resulting circuit preserves Hermiticity and introduces no additional instabilities beyond those already present in standard QSP. We will insert the complete formulas, the step-by-step derivation, and a short verification that the composition reproduces the target polynomial in the revised manuscript. revision: yes

Circularity Check

0 steps flagged

No circularity; derivation extends GQSP without self-referential reduction

full rationale

The paper claims to implement arbitrary polynomials of Hermitian matrices via GQSP without block-encoding, deriving closed-form symmetric expansions and using linear combinations of GQSP circuits. No quoted step reduces a prediction or uniqueness claim to a fitted parameter, self-citation chain, or redefinition (e.g., no parameter fitted to data then renamed as prediction, no ansatz smuggled via overlapping-author citation, no success probability forced by construction). The approach is presented as building directly on the established GQSP framework with explicit derivations for the Hermitian case, keeping the central claims independent of the paper's own outputs. This is the common honest non-finding for papers that extend prior frameworks without internal redefinition.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

Review performed on abstract only. No free parameters, invented entities, or non-standard axioms are mentioned. The approach assumes standard quantum computing primitives and the prior existence of the GQSP framework.

axioms (1)
  • domain assumption Standard assumptions of quantum circuit model and existence of GQSP framework
    Method relies on GQSP being available as a black-box primitive.

pith-pipeline@v0.9.0 · 5538 in / 1135 out tokens · 21746 ms · 2026-05-16T20:20:30.583148+00:00 · methodology

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Reference graph

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