This paper presents a novel quantum computational framework for algebraic topology, specifically designed to compute topological invariants like homology and Betti numbers using the more general and powerful combinatorial structures known as simplicial sets. This work significantly extends prior quantum algorithms in topological data analysis, which primarily focused on simplicial complexes.
The significance of this development lies in addressing the inherent limitations of simplicial complexes for modeling complex topological spaces. Simplicial complexes struggle with operations like products and quotients, and cannot naturally incorporate “degenerate” simplices (those whose formal dimension is higher than their effective one). Simplicial sets, by contrast, offer a leaner, more versatile, and combinatorially richer approach, allowing for more compact representations of topological spaces and a broader range of applications, including those beyond pure topology like category theory or directed graphs. By adapting quantum computational methods to this more general setting, the paper paves the way for potentially more efficient and broadly applicable quantum algorithms for topological analysis.
The key innovations introduced in this paper are:
- Simplicial Hilbert Space Encoding: A parafinite simplicial set (one with a finite number of simplices at each dimension) is systematically encoded into a finite-dimensional simplicial Hilbert space. This involves mapping each simplex to an orthonormal basis vector (analogous to qubits), and the simplicial face and degeneracy maps to corresponding linear operators acting on these basis vectors.
- Introduction of Adjoint Operators and Simplicial Hilbert Hodge Laplacians: A critical step is the natural incorporation of adjoint operators for the face and degeneracy maps, leveraging the Hilbert space structure. These adjoints, alongside the original operators, enable the definition of various Simplicial Hilbert Hodge Laplacians. Inspired by classical Hodge theory, the kernels of these Laplacians are shown to directly encode the homology of the underlying simplicial set.
- Normalized Simplicial Hilbert Homology: The framework distinguishes between “raw” and “normalized” simplicial Hilbert homology. Crucially, it demonstrates that the normalized homology, which effectively discards the homologically irrelevant degenerate simplices, is isomorphic to the standard simplicial homology of the space. This normalization is vital for computational efficiency, as it focuses resources on the essential non-degenerate components.
- Formalization of Simplicial Quantum Circuits: The paper defines a new class of simplicial quantum circuits, which are unitary operators whose actions are explicitly compatible with the inherent structure and relations of simplicial sets. This provides a mathematical blueprint for how a quantum computer could intrinsically perform operations on simplicial data.
- Algorithmic Scheme for Homology Computation: An outline is provided for a quantum algorithmic scheme that can compute the simplicial homology spaces and Betti numbers. This scheme combines established quantum algorithms, notably Grover’s search algorithm (for projecting onto relevant subspaces and identifying non-degenerate simplices) and Abrams’ and Lloyd’s algorithm (for finding eigenvalues and eigenvectors, which translates to finding the kernel of the Laplacian).
The main prior ingredients upon which this work builds include:
- Algebraic Topology Fundamentals: A deep understanding of simplicial sets, chain complexes, boundary operators, and homology theory is foundational. The paper explicitly leverages established results such as the Eilenberg-Zilber lemma and normalization theorems from classical algebraic topology.
- Linear Algebra and Functional Analysis: Concepts of Hilbert spaces, linear operators, adjoints, kernels, and orthogonal projections are central to encoding the topological structures into a quantum mechanical setting.
- Finite-Dimensional Hodge Theory: The idea of computing homology groups as the kernels of Laplacian operators is directly borrowed from Hodge theory, which provides a powerful analytical tool for this purpose in finite-dimensional settings.
- Prior Quantum Topological Data Analysis (QTDA): The work is directly inspired by and builds upon seminal research, particularly that of Lloyd et al. [19], which first demonstrated quantum algorithms for computing homology in simplicial complexes. This paper’s core contribution is extending these quantum techniques to the more expressive and complex realm of simplicial sets.
- Standard Quantum Algorithms: The proposed computational scheme relies on well-known quantum algorithms such as Grover’s quantum search (for amplitude amplification and state preparation) and Abrams’ and Lloyd’s quantum algorithm for finding eigenvalues and eigenvectors (a variant of quantum phase estimation), demonstrating how these general-purpose tools can be specialized for topological problems.