Hermitian Matrix Function Synthesis without Block-Encoding

TL;DR

This paper introduces a resource-efficient method for implementing polynomial functions of Hermitian matrices on quantum hardware without relying on block-encoding, enhancing efficiency and stability in quantum algorithms.

Contribution

It proposes a novel GQSP-based approach that avoids block-encoding and post-selection overheads, enabling stable, degree-independent polynomial implementation.

Findings

Derives closed-form symmetric polynomial expansions.

Demonstrates linear combinations of GQSP circuits for desired transformations.

Reduces resource overhead compared to existing techniques.

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.